     [file config-local.texi not included]

An Introduction to GCC
**********************

This manual provides an introduction to the GNU C and C++ Compilers, gcc and g++, which are part of the GNU Compiler Collection (GCC).

The development of this manual was funded entirely by Network Theory Ltd (http://www.network-theory.co.uk/).  Copies published by Network Theory Ltd raise money for more free documentation.

Introduction
************

The purpose of this book is to explain the use of the GNU C and C++ compilers, gcc and g++.  After reading this book you should understand how to compile a program, and how to use basic compiler options for optimization and debugging.  This book does not attempt to teach the C or C++ languages themselves, since this material can be found in many other places (see Further reading).

Experienced programmers who are familiar with other systems, but new to the GNU compilers, can skip the early sections of the chapters ``Compiling a C program'', ``Using the preprocessor'' and ``Compiling a C++ program''.  The remaining sections and chapters should provide a good overview of the features of GCC for those already know how to use other compilers.

A brief history of GCC
======================

The original author of the GNU C Compiler (GCC) is Richard Stallman, the founder of the GNU Project.

The GNU Project was started in 1984 to create a complete Unix-like operating system as free software, in order to promote freedom and cooperation among computer users and programmers.  Every Unix-like operating system needs a C compiler, and as there were no free compilers in existence at that time, the GNU Project had to develop one from scratch.  The work was funded by donations from individuals and companies to the Free Software Foundation, a non-profit organization set up to support the work of the GNU Project.

The first release of GCC was made in 1987.  This was a significant breakthrough, being the first portable ANSI C optimizing compiler released as free software.  Since that time GCC has become one of the most important tools in the development of free software.

A major revision of the compiler came with the 2.0 series in 1992, which added the ability to compile C++.  In 1997 an experimental branch of the compiler (EGCS) was created, to improve optimization and C++ support. Following this work, EGCS was adopted as the new main-line of GCC development, and these features became widely available in the 3.0 release of GCC in 2001.

Over time GCC has been extended to support many additional languages, including Fortran, ADA, Java and Objective-C.  The acronym GCC is now used to refer to the ``GNU Compiler Collection''.  Its development is guided by the GCC Steering Committee, a group composed of representatives from GCC user communities in industry, research and academia.

Major features of GCC
=====================

This section describes some of the most important features of GCC.

First of all, GCC is a portable compiler---it runs on most platforms available today, and can produce output for many types of processors.  In addition to the processors used in personal computers, it also supports microcontrollers, DSPs and 64-bit CPUs.

GCC is not only a native compiler---it can also cross-compile any program, producing executable files for a different system from the one used by GCC itself.  This allows software to be compiled for embedded systems which are not capable of running a compiler.  GCC is written in C with a strong focus on portability, and can compile itself, so it can be adapted to new systems easily.

GCC has multiple language frontends, for parsing different languages.  Programs in each language can be compiled, or cross-compiled, for any architecture.  For example, an ADA program can be compiled for a microcontroller, or a C program for a supercomputer.

GCC has a modular design, allowing support for new languages and architectures to be added.  Adding a new language front-end to GCC enables the use of that language on any architecture, provided that the necessary run-time facilities (such as libraries) are available. Similarly, adding support for a new architecture makes it available to all languages.

Finally, and most importantly, GCC is free software, distributed under the GNU General Public License (GNU GPL). (For details see the license file COPYING distributed with GCC.)  This means you have the freedom to use and to modify GCC, as with all GNU software.  If you need support for a new type of CPU, a new language, or a new feature you can add it yourself, or hire someone to enhance GCC for you.  You can hire someone to fix a bug if it is important for your work.

Furthermore, you have the freedom to share any enhancements you make to GCC.  As a result of this freedom you can also make use of enhancements to GCC developed by others.  The many features offered by GCC today show how this freedom to cooperate works to benefit you, and everyone else who uses GCC.

Programming in C and C++
========================

C and C++ are languages that allow direct access to the computer's memory.  Historically, they have been used for writing low-level systems software, and applications where high-performance or control over resource usage are critical.  However, great care is required to ensure that memory is accessed correctly, to avoid corrupting other data-structures.  This book describes techniques that will help in detecting potential errors during compilation, but the risk in using languages like C or C++ can never be eliminated.

In addition to C and C++ the GNU Project also provides other high-level languages, such as GNU Common Lisp (gcl), GNU Smalltalk (gst), the GNU Scheme extension language (guile) and the GNU Compiler for Java (gcj).  These languages do not allow the user to access memory directly, eliminating the possibility of memory access errors.  They are a safer alternative to C and C++ for many applications.

Conventions used in this manual
===============================

This manual contains many examples which can be typed at the keyboard. A command entered at the terminal is shown like this,

     $ command

followed by its output.  For example:

     $ echo "hello world"
     hello world

The first character on the line is the terminal prompt, and should not be typed.  The dollar sign $ is used as the standard prompt in this manual, although some systems may use a different character.

When a command in an example is too long to fit in a single line it is wrapped and then indented on subsequent lines, like this:

     $ echo "an example of a line which is too long to fit
         in this manual"

When entered at the keyboard, the entire command should be typed on a single line.

The example source files used in this manual can be downloaded from the publisher's website, (See http://www.network-theory.co.uk/gcc/intro/) or entered by hand using any text editor, such as the standard GNU editor, emacs. The example compilation commands use gcc and g++ as the names of the GNU C and C++ compilers, and cc to refer to other compilers.  The example programs should work with any version of GCC. Any command-line options which are only available in recent versions of GCC are noted in the text.

The examples assume the use of a GNU operating system---there may be minor differences in the output on other systems.  Some non-essential and verbose system-dependent output messages (such as very long system paths) have been edited in the examples for brevity. The commands for setting environment variables use the syntax of the standard GNU shell (bash), and should work with any version of the Bourne shell.

Compiling a C program
*********************

This chapter describes how to compile C programs using gcc. Programs can be compiled from a single source file or from multiple source files, and may use system libraries and header files.

Compilation refers to the process of converting a program from the textual source code, in a programming language such as C or C++, into machine code, the sequence of 1's and 0's used to control the central processing unit (CPU) of the computer.  This machine code is then stored in a file known as an executable file, sometimes referred to as a binary file.

Compiling a simple C program
============================

The classic example program for the C language is Hello World. Here is the source code for our version of the program:

          [file hello.c not included]

We will assume that the source code is stored in a file called hello.c. To compile the file hello.c with gcc, use the following command:

     $ gcc -Wall hello.c -o hello

This compiles the source code in hello.c to machine code and stores it in an executable file hello.  The output file for the machine code is specified using the -o option.  This option is usually given as the last argument on the command line.  If it is omitted, the output is written to a default file called a.out.

Note that if a file with the same name as the executable file already exists in the current directory it will be overwritten.

The option -Wall turns on all the most commonly-used compiler warnings---it is recommended that you always use this option! There are many other warning options which will be discussed in later chapters, but -Wall is the most important.  GCC will not produce any warnings unless they are enabled.  Compiler warnings are an essential aid in detecting problems when programming in C and C++.

In this case, the compiler does not produce any warnings with the -Wall option, since the program is completely valid.  Source code which does not produce any warnings is said to compile cleanly.

To run the program, type the path name of the executable like this:

     $ ./hello
     Hello, world!

This loads the executable file into memory and causes the CPU to begin executing the instructions contained within it.  The path ./ refers to the current directory, so ./hello loads and runs the executable file hello located in the current directory.

Finding errors in a simple program
==================================

As mentioned above, compiler warnings are an essential aid when programming in C and C++.  To demonstrate this, the program below contains a subtle error: it uses the function printf incorrectly, by specifying a floating-point format %f for an integer value:

          [file bad.c not included]

This error is not obvious at first sight, but can be detected by the compiler if the warning option -Wall has been enabled.

Compiling the program above, bad.c, with the warning option -Wall produces the following message:

     $ gcc -Wall bad.c -o bad
     bad.c: In function 'main':
     bad.c:6:3: warning: format '%f' expects argument of
     type 'double', but argument 2 has type 'int' [-Wformat]

This indicates that a format string has been used incorrectly in the file bad.c at line 6.  The messages produced by GCC always have the form file:line-number:message.  The compiler distinguishes between error messages, which prevent successful compilation, and warning messages which indicate possible problems (but do not stop the program from compiling).

In this case, the correct format specifier should be %d for an integer argument. The allowed format specifiers for printf can be found in any general book on C, such as the GNU C Library Reference Manual (see Further reading).

Without the warning option -Wall the program appears to compile cleanly, but produces incorrect results:

     $ gcc bad.c -o bad
     $ ./bad
     Two plus two is 0.000000    (incorrect output)

The incorrect format specifier causes the output to be corrupted, because the function printf is passed an integer instead of a floating-point number.  Integers and floating-point numbers are stored in different formats in memory, and generally occupy different numbers of bytes, leading to a spurious result.  The actual output shown above may differ, depending on the specific platform and environment.

Clearly, it is very dangerous to develop a program without checking for compiler warnings.  If there are any functions which are not used correctly they can cause the program to crash or produce incorrect results. Turning on the compiler warning option -Wall will catch many of the commonest errors which occur in C programming.

Compiling multiple source files
===============================

A program can be split up into multiple files.  This makes it easier to edit and understand, especially in the case of large programs---it also allows the individual parts to be compiled independently.

In the following example we will split up the program Hello World into three files: main.c, hello_fn.c and the header file hello.h.  Here is the main program main.c:

          [file main.c not included]

The original call to the printf system function in the previous program hello.c has been replaced by a call to a new external function hello, which we will define in a separate file hello_fn.c.

The main program also includes the header file hello.h which will contain the declaration of the function hello.  The declaration is used to ensure that the types of the arguments and return value match up correctly between the function call and the function definition.  We no longer need to include the system header file stdio.h in main.c to declare the function printf, since the file main.c does not call printf directly.

The declaration in hello.h is a single line specifying the prototype of the function hello:

          [file hello1.h not included]

The definition of the function hello itself is contained in the file hello_fn.c:

          [file hello_fn.c not included]

This function prints the message ``Hello, NAME!'' using its argument as the value of NAME.

Incidentally, the difference between the two forms of the include statement #include "FILE.h" and #include <FILE.h> is that the former searches for FILE.h in the current directory before looking in the system header file directories.  The include statement #include <FILE.h> searches the system header files, but does not look in the current directory by default.

To compile these source files with gcc, use the following command:

     $ gcc -Wall main.c hello_fn.c -o newhello

In this case, we use the -o option to specify a different output file for the executable, newhello.  Note that the header file hello.h is not specified in the list of files on the command line.  The directive #include "hello.h" in the source files instructs the compiler to include it automatically at the appropriate points.

To run the program, type the path name of the executable:

     $ ./newhello
     Hello, world!

All the parts of the program have been combined into a single executable file, which produces the same result as the executable created from the single source file used earlier.

Compiling files independently
=============================

If a program is stored in a single file then any change to an individual function requires the whole program to be recompiled to produce a new executable.  The recompilation of large source files can be very time-consuming.

When programs are stored in independent source files, only the files which have changed need to be recompiled after the source code has been modified.  In this approach, the source files are compiled separately and then linked together---a two stage process.  In the first stage, a file is compiled without creating an executable.  The result is referred to as an object file, and has the extension .o when using GCC.

In the second stage, the object files are merged together by a separate program called the linker.  The linker combines all the object files to create a single executable.

An object file contains machine code where any references to the memory addresses of functions (or variables) in other files are left undefined. This allows source files to be compiled without direct reference to each other.  The linker fills in these missing addresses when it produces the executable.

Creating object files from source files
---------------------------------------

The command-line option -c is used to compile a source file to an object file.  For example, the following command will compile the source file main.c to an object file:

     $ gcc -Wall -c main.c

This produces an object file main.o containing the machine code for the main function.  It contains a reference to the external function hello, but the corresponding memory address is left undefined in the object file at this stage (it will be filled in later by linking).

The corresponding command for compiling the hello function in the source file hello_fn.c is:

     $ gcc -Wall -c hello_fn.c

This produces the object file hello_fn.o.

Note that there is no need to use the option -o to specify the name of the output file in this case.  When compiling with -c the compiler automatically creates an object file whose name is the same as the source file, but with .o instead of the original extension.

There is no need to put the header file hello.h on the command line, since it is automatically included by the #include statements in main.c and hello_fn.c.

Creating executables from object files
--------------------------------------

The final step in creating an executable file is to use gcc to link the object files together and fill in the missing addresses of external functions.  To link object files together, they are simply listed on the command line:

     $ gcc main.o hello_fn.o -o hello

This is one of the few occasions where there is no need to use the -Wall warning option, since the individual source files have already been successfully compiled to object code.  Once the source files have been compiled, linking is an unambiguous process which either succeeds or fails (it fails only if there are references which cannot be resolved).

To perform the linking step gcc uses the linker ld, which is a separate program.  On GNU systems the GNU linker, GNU ld, is used.  Other systems may use the GNU linker with GCC, or may have their own linkers.  The linker itself will be discussed later (see How the compiler works).  By running the linker, gcc creates an executable file from the object files.

The resulting executable file can now be run:

     $ ./hello
     Hello, world!

It produces the same output as the version of the program using a single source file in the previous section.

Recompiling and relinking
=========================

To show how source files can be compiled independently we will edit the main program main.c and modify it to print a greeting to everyone instead of world:

          [file main2.c not included]

The updated file main.c can now be recompiled with the following command:

     $ gcc -Wall -c main2.c

This produces a new object file main.o.  There is no need to create a new object file for hello_fn.c, since that file and the related files that it depends on, such as header files, have not changed.

The new object file can be relinked with the hello function to create a new executable file:

     $ gcc main2.o hello_fn.o -o hello

The resulting executable hello now uses the new main function to produce the following output:

     $ ./hello
     Hello, everyone!

Note that only the file main.c has been recompiled, and then relinked with the existing object file for the hello function. If the file hello_fn.c had been modified instead, we could have recompiled hello_fn.c to create a new object file hello_fn.o and relinked this with the existing file main.o. (If the prototype of a function has changed, it is necessary to modify and recompile all of the other source files which use it.)

In a large project with many source files, recompiling only those that have been modified can make a significant saving.  The process of recompiling only the modified files in a project can be automated with the standard Unix program make.

A simple makefile
=================

For those unfamiliar with make, this section provides a simple demonstration of its use.  Make is a program in its own right and can be found on all Unix systems.  To learn more about the GNU version of make you will need to consult the GNU Make manual by Richard M. Stallman and Roland McGrath (see Further reading).

Make reads a description of a project from a makefile (by default, called Makefile in the current directory).  A makefile specifies a set of compilation rules in terms of targets (such as executables) and their dependencies (such as object files and source files) in the following format:

     target: dependencies
             command

For each target, make checks the modification time of the corresponding dependency files to determine whether the target needs to be rebuilt using the corresponding command.  Note that the command lines in a makefile must be indented with a single TAB character, not spaces.

GNU Make contains many default rules, referred to as implicit rules, to simplify the construction of makefiles.  For example, these specify that .o files can be obtained from .c files by compilation, and that an executable can be made by linking together .o files.  Implicit rules are defined in terms of make variables, such as CC (the C compiler) and CFLAGS (the compilation options for C programs), which can be set using VARIABLE=VALUE lines in the makefile.  For C++ the equivalent variables are CXX and CXXFLAGS, while the make variable CPPFLAGS sets the preprocessor options.  The implicit and user-defined rules are automatically chained together as necessary by GNU Make.

A simple Makefile for the project above can be written as follows:

          [file c1makefile2 not included]

The file can be read like this: using the C compiler gcc, with compilation option -Wall, build the target executable main from the object files main.o and hello_fn.o (these, in turn, will be built via implicit rules from main.c and hello_fn.c).  The target clean has no dependencies and simply removes all the compiled files. (This assumes that there is no file called clean in the current directory---see the discussion of ``phony targets'' in the GNU Make manual for details.) The option -f (force) on the rm command suppresses any error messages if the files do not exist.

To use the makefile, type make.  When called with no arguments, the first target in the makefile is built, producing the executable main:

     $ make
     gcc -Wall   -c -o main.o main.c
     gcc -Wall   -c -o hello_fn.o hello_fn.c
     gcc   main.o hello_fn.o   -o main
     $ ./main
     Hello, world!

To rebuild the executable after modifying a source file, simply type make again. By checking the timestamps of the target and dependency files, make identifies the files which have changed and regenerates the corresponding intermediate files needed to update the targets:

     $ emacs main.c  (edit the file)
     $ make
     gcc -Wall   -c -o main.o main.c
     gcc    main.o hello_fn.o   -o main
     $ ./main
     Hello, everyone!

Finally, to remove the generated files, type make clean:

     $ make clean
     rm -f main main.o hello_fn.o

A more sophisticated makefile would usually contain additional targets for installation (make install) and testing (make check).

The examples in the rest of this book are small enough not to need makefiles, but the use of make is recommended for any larger programs.

Linking with external libraries
===============================

A library is a collection of precompiled object files which can be linked into programs.  The most common use of libraries is to provide system functions, such as the square root function sqrt found in the C math library.

Libraries are typically stored in special archive files with the extension .a, referred to as static libraries.  They are created from object files with a separate tool, the GNU archiver ar, and used by the linker to resolve references to functions at compile-time.  We will see later how to create libraries using the ar command (see Compiler-related tools).  For simplicity, only static libraries are covered in this section---dynamic linking at runtime using shared libraries will be described in the next chapter.

The standard system libraries are usually found in the directories /usr/lib and /lib. (On systems supporting both 64 and 32-bit executables the 64-bit versions of the libraries will often be stored in /usr/lib64 and /lib64, with the 32-bit versions in /usr/lib and /lib.  Libraries may sometimes be also found in an architecture-specific directory, such as /usr/lib/i386-linux-gnu/)  For example, the C math library is typically stored in the file /usr/lib/libm.a on Unix-like systems. The corresponding prototype declarations for the functions in this library are given in the header file /usr/include/math.h. The C standard library itself is stored in /usr/lib/libc.a and contains functions specified in the ANSI/ISO C standard, such as printf---this library is linked by default for every C program.

Here is an example program which makes a call to the external function sqrt in the math library libm.a:

          [file calc.c not included]

Trying to create an executable from this source file alone causes the compiler to give an error at the link stage:

     $ gcc -Wall calc.c -o calc
     /tmp/ccbR6Ojm.o: In function `main':
     /tmp/ccbR6Ojm.o(.text+0x19): undefined reference
       to `sqrt'

The problem is that the reference to the sqrt function cannot be resolved without the external math library libm.a.  The function sqrt is not defined in the program or the default library libc.a, and the compiler does not link to the file libm.a unless it is explicitly selected. Incidentally, the file mentioned in the error message /tmp/ccbR60jm.o is a temporary object file created by the compiler from calc.c, in order to carry out the linking process.

To enable the compiler to link the sqrt function to the main program calc.c we need to supply the library libm.a.  One obvious but cumbersome way to do this is to specify it explicitly on the command line:

     $ gcc -Wall calc.c /usr/lib/libm.a -o calc

The library libm.a contains object files for all the mathematical functions, such as sin, cos, exp, log and sqrt.  The linker searches through these to find the object file containing the sqrt function.

Once the object file for the sqrt function has been found, the main program can be linked and a complete executable produced:

     $ ./calc
     The square root of 2.0 is 1.414214

The executable file includes the machine code for the main function and the machine code for the sqrt function, copied from the corresponding object file in the library libm.a.

To avoid the need to specify long paths on the command line, the compiler provides a short-cut option -l for linking against libraries.  For example, the following command,

     $ gcc -Wall calc.c -lm -o calc

is equivalent to the original command above using the full library name /usr/lib/libm.a.

In general, the compiler option -lNAME will attempt to link object files with a library file libNAME.a in the standard library directories.  Additional directories can specified with command-line options and environment variables, to be discussed shortly. A large program will typically use many -l options to link libraries such as the math library, graphics libraries and networking libraries.

Link order of libraries
-----------------------

The traditional behavior of linkers is to search for external functions from left to right in the libraries specified on the command line.  This means that a library containing the definition of a function should appear after any source files or object files which use it.  This includes libraries specified with the short-cut -l option, as shown in the following command:

     $ gcc -Wall calc.c -lm -o calc   (correct order)

With some linkers the opposite ordering (placing the -lm option before the file which uses it) would result in an error,

     $ cc -Wall -lm calc.c -o calc    (incorrect order)
     main.o: In function `main':
     main.o(.text+0xf): undefined reference to `sqrt'

because there is no library or object file containing sqrt after calc.c.  The option -lm should appear after the file calc.c.

When several libraries are being used, the same convention should be followed for the libraries themselves.  A library which calls an external function defined in another library should appear before the library containing the function.

For example, a program data.c using the GNU Linear Programming library libglpk.a, which in turn uses the math library libm.a, should be compiled as,

     $ gcc -Wall data.c -lglpk -lm

since the object files in libglpk.a use functions defined in libm.a.

Most current linkers will search all libraries, regardless of order, but since some do not do this it is best to follow the convention of ordering libraries from left to right.

This is worth keeping in mind if you ever encounter unexpected problems with undefined references, and all the necessary libraries appear to be present on the command line.

Using library header files
==========================

When using a library it is essential to include the appropriate header files, in order to declare the function arguments and return values with the correct types.  Without declarations, the arguments of a function can be passed with the wrong type, causing corrupted results.

The following example shows another program which makes a function call to the C standard library.  In this case, the function strtod is used to convert a string "123" to a floating-point number:

          [file badconv.c not included]

However, the program contains an error---the #include statement for the required header file stdlib.h is missing, so the prototype double strtod (const char * string, char * tail) given there will not be seen by the compiler.

Compiling the program without any warning options will produce an executable file which gives incorrect results:

     $ gcc badconv.c -lm
     $ ./a.out
     Value is 966656.000000 (incorrect result, should be 123.0)

The results are corrupted because the arguments and return value of the call to strtod are passed with incorrect types. (The actual output shown above may differ, depending on the specific platform and environment.)  This can be detected by turning on the warning option -Wall:

     $ gcc -Wall badconv.c -lm
     badconv.c: In function `main':
     badconv.c:6: warning: implicit declaration of
       function `strtod'

This example shows again the importance of using the warning option -Wall to detect serious problems that could otherwise easily be overlooked.

Compilation options
*******************

This chapter describes other commonly-used compiler options available in GCC.  These options control features such as the search paths used for locating libraries and include files, the use of additional warnings and diagnostics, preprocessor macros and C language dialects.

Setting search paths
====================

In the last chapter, we saw how to link to a program with functions in the C math library libm.a, using the short-cut option -lm and the header file math.h.

A common problem when compiling a program using library header files is the error:

     FILE.H: No such file or directory

This occurs if a header file is not present in the standard include file directories used by gcc.  A similar problem can occur for libraries:

     /usr/bin/ld: cannot find LIBRARY

This happens if a library used for linking is not present in the standard library directories used by gcc.

By default, gcc searches the following directories for header files:

     /usr/local/include/
     /usr/include/

and the following directories for libraries:

     /usr/local/lib/
     /usr/lib/

The list of directories for header files is often referred to as the include path, and the list of directories for libraries as the library search path or link path.

The directories on these paths are searched in order, from first to last in the two lists above. (The default search paths may also include additional system-dependent or site-specific directories, and directories in the GCC installation itself. For example, on 64-bit platforms additional lib64 directories may also be searched by default.)  For example, a header file found in /usr/local/include takes precedence over a file with the same name in /usr/include.  Similarly, a library found in /usr/local/lib takes precedence over a library with the same name in /usr/lib.

When additional libraries are installed in other directories it is necessary to extend the search paths, in order for the libraries to be found.  The compiler options -I and -L add new directories to the beginning of the include path and library search path respectively.

Search path example
-------------------

The following example program uses a library that might be installed as an additional package on a system---the GNU Database Management Library (GDBM).  The GDBM Library stores key-value pairs in a DBM file, a type of data file which allows values to be stored and indexed by a key (an arbitrary sequence of characters).  Here is the example program dbmain.c, which creates a DBM file containing a key testkey with the value testvalue:

          [file dbmain.c not included]

The program uses the header file gdbm.h and the library libgdbm.a.  If the library has been installed in the default location of /usr/local/lib, with the header file in /usr/local/include, then the program can be compiled with the following simple command:

     $ gcc -Wall dbmain.c -lgdbm

Both these directories are part of the default gcc include and link paths.

However, if GDBM has been installed in a different location, trying to compile the program will give the following error:

     $ gcc -Wall dbmain.c -lgdbm
     dbmain.c:1: gdbm.h: No such file or directory

For example, if version 1.8.3 of the GDBM package is installed under the directory /opt/gdbm-1.8.3 the location of the header file would be,

     /opt/gdbm-1.8.3/include/gdbm.h

which is not part of the default gcc include path.  Adding the appropriate directory to the include path with the command-line option -I allows the program to be compiled, but not linked:

     $ gcc -Wall -I/opt/gdbm-1.8.3/include dbmain.c -lgdbm
     /usr/bin/ld: cannot find -lgdbm
     collect2: ld returned 1 exit status

The directory containing the library is still missing from the link path. It can be added to the link path using the following option:

     -L/opt/gdbm-1.8.3/lib/

The following command line allows the program to be compiled and linked:

     $ gcc -Wall -I/opt/gdbm-1.8.3/include
        -L/opt/gdbm-1.8.3/lib dbmain.c -lgdbm

This produces the final executable linked to the GDBM library.  Before seeing how to run this executable we will take a brief look at the environment variables that affect the -I and -L options.

Note that you should never place the absolute paths of header files in #include statements in your source code, as this will prevent the program from compiling on other systems.  The -I option or the INCLUDE_PATH variable described below should always be used to set the include path for header files.

Environment variables
---------------------

The search paths for header files and libraries can also be controlled through environment variables in the shell.  These may be set automatically for each session using the appropriate login file, such as .bash_profile in the case of GNU Bash.

Additional directories can be added to the include path using the environment variable C_INCLUDE_PATH (for C header files) or CPLUS_INCLUDE_PATH (for C++ header files).  For example, the following commands will add /opt/gdbm-1.8.3/include to the include path when compiling C programs:

     $ C_INCLUDE_PATH=/opt/gdbm-1.8.3/include
     $ export C_INCLUDE_PATH

and similarly for C++ programs:

     $ CPLUS_INCLUDE_PATH=/opt/gdbm-1.8.3/include
     $ export CPLUS_INCLUDE_PATH

This directory will be searched after any directories specified on the command line with the option -I, and before the standard default directories (such as /usr/local/include and /usr/include).  The shell command export is needed to make the environment variable available to programs outside the shell itself, such as the compiler---it is only needed once for each variable in each shell session, and can also be set in the appropriate login file. (In GNU Bash, the shorter form export VARIABLE=VALUE is also allowed.)

Similarly, additional directories can be added to the link path using the environment variable LIBRARY_PATH.  For example, the following commands will add /opt/gdbm-1.8.3/lib to the link path:

     $ LIBRARY_PATH=/opt/gdbm-1.8.3/lib
     $ export LIBRARY_PATH

This directory will be searched after any directories specified on the command line with the option -L, and before the standard default directories (such as /usr/local/lib and /usr/lib).

With the environment variable settings given above the program dbmain.c can be compiled without the -I and -L options,

     $ gcc -Wall dbmain.c -lgdbm

because the default paths now use the directories specified in the environment variables C_INCLUDE_PATH and LIBRARY_PATH.  The same compilation command with g++ would use the environment variables CPLUS_INCLUDE_PATH and LIBRARY_PATH.

Extended search paths
---------------------

Following the standard Unix convention for search paths, several directories can be specified together in an environment variable as a colon separated list:

     DIR1:DIR2:DIR3:...

The directories are then searched in order from left to right.  A single dot . can be used to specify the current directory. (The current directory can also be specified using an empty path element. For example, :DIR1:DIR2 is equivalent to .:DIR1:DIR2.)

For example, the following settings create default include and link paths for packages installed in the current directory . and the include and lib directories under /opt/gdbm-1.8.3 and /net respectively:

     $ C_INCLUDE_PATH=.:/opt/gdbm-1.8.3/include:/net/include
     $ LIBRARY_PATH=.:/opt/gdbm-1.8.3/lib:/net/lib

For C++ programs, use the environment variable CPLUS_INCLUDE_PATH instead of C_INCLUDE_PATH.

To specify multiple search path directories on the command line, the options -I and -L can be repeated.  For example, the following command,

     $ gcc -I. -I/opt/gdbm-1.8.3/include -I/net/include
        -L. -L/opt/gdbm-1.8.3/lib -L/net/lib .....

is equivalent to the environment variable settings given above.

When environment variables and command-line options are used together the compiler searches the directories in the following order:

 1. command-line options -I and -L, from left to right

 2. directories specified by environment variables, such as C_INCLUDE_PATH (for C programs), CPLUS_INCLUDE_PATH (for C++ programs) and LIBRARY_PATH

 3. default system directories

In day-to-day usage, directories are usually added to the search paths with the options -I and -L.

Shared libraries and static libraries
=====================================

Although the example program above has been successfully compiled and linked, a final step is needed before being able to load and run the executable file.

If an attempt is made to start the executable directly, the following error will occur on most systems:

     $ ./a.out
     ./a.out: error while loading shared libraries:
     libgdbm.so.3: cannot open shared object file:
     No such file or directory

This is because the GDBM package provides a shared library.  This type of library requires special treatment---it must be loaded from disk before the executable will run.

External libraries are usually provided in two forms: static libraries and shared libraries.  Static libraries are the .a files seen earlier.  When a program is linked against a static library, the machine code from the object files for any external functions used by the program is copied from the library into the final executable.

Shared libraries are handled with a more advanced form of linking, which makes the executable file smaller.  They use the extension .so, which stands for shared object.

An executable file linked against a shared library contains only a small table of the functions it requires, instead of the complete machine code from the object files for the external functions.  Before the executable file starts running, the machine code for the external functions is copied into memory from the shared library file on disk by the operating system---a process referred to as dynamic linking.

Dynamic linking makes executable files smaller and saves disk space, because one copy of a library can be shared between multiple programs. Most operating systems also provide a virtual memory mechanism which allows one copy of a shared library in physical memory to be used by all running programs, saving memory as well as disk space.

Furthermore, shared libraries make it possible to update a library without recompiling the programs which use it (provided the interface to the library does not change).

Because of these advantages gcc compiles programs to use shared libraries by default on most systems, if they are available.  Whenever a static library libNAME.a would be used for linking with the option -lNAME the compiler first checks for an alternative shared library with the same name and a .so extension.

In this case, when the compiler searches for the libgdbm library in the link path, it finds the following two files in the directory /opt/gdbm-1.8.3/lib:

     $ cd /opt/gdbm-1.8.3/lib
     $ ls libgdbm.*
     libgdbm.a  libgdbm.so

Consequently, the libgdbm.so shared object file is used in preference to the libgdbm.a static library.

However, when the executable file is started its loader function must find the shared library in order to load it into memory.  By default the loader searches for shared libraries only in a predefined set of system directories, such as /usr/local/lib and /usr/lib.  If the library is not located in one of these directories it must be added to the load path. (Note that the directory containing the shared library can, in principle, be stored (``hard-coded'') in the executable itself using the linker option -rpath, but this is not usually done since it creates problems if the library is moved or the executable is copied to another system.)

The simplest way to set the load path is through the environment variable LD_LIBRARY_PATH.  For example, the following commands set the load path to /opt/gdbm-1.8.3/lib so that libgdbm.so can be found:

     $ LD_LIBRARY_PATH=/opt/gdbm-1.8.3/lib
     $ export LD_LIBRARY_PATH
     $ ./a.out
     Storing key-value pair... done.

The executable now runs successfully, prints its message and creates a DBM file called test containing the key-value pair testkey and testvalue.

To save typing, the LD_LIBRARY_PATH environment variable can be set automatically for each session using the appropriate login file, such as .bash_profile for the GNU Bash shell.

Several shared library directories can be placed in the load path, as a colon separated list DIR1:DIR2:DIR3:...:DIRN.  For example, the following command sets the load path to use the lib directories under /opt/gdbm-1.8.3 and /opt/gtk-1.4:

     $ LD_LIBRARY_PATH=/opt/gdbm-1.8.3/lib:/opt/gtk-1.4/lib
     $ export LD_LIBRARY_PATH

If the load path contains existing entries, it can be extended using the syntax LD_LIBRARY_PATH=NEWDIRS:$LD_LIBRARY_PATH. For example, the following command adds the directory /opt/gsl-1.5/lib to the load path shown above:

     $ LD_LIBRARY_PATH=/opt/gsl-1.5/lib:$LD_LIBRARY_PATH
     $ echo $LD_LIBRARY_PATH
     /opt/gsl-1.5/lib:/opt/gdbm-1.8.3/lib:/opt/gtk-1.4/lib

It is possible for the system administrator to set the LD_LIBRARY_PATH variable for all users, by adding it to a default login script, such as /etc/profile.  On GNU systems, a system-wide path can also be defined in the loader configuration file /etc/ld.so.conf.

Alternatively, static linking can be forced with the -static option to gcc to avoid the use of shared libraries:

     $ gcc -Wall -static -I/opt/gdbm-1.8.3/include/
         -L/opt/gdbm-1.8.3/lib/ dbmain.c -lgdbm

This creates an executable linked with the static library libgdbm.a which can be run without setting the environment variable LD_LIBRARY_PATH or putting shared libraries in the default directories:

     $ ./a.out
     Storing key-value pair... done.

As noted earlier, it is also possible to link directly with individual library files by specifying the full path to the library on the command line.  For example, the following command will link directly with the static library libgdbm.a,

     $ gcc -Wall -I/opt/gdbm-1.8.3/include
         dbmain.c /opt/gdbm-1.8.3/lib/libgdbm.a

and the command below will link with the shared library file libgdbm.so:

     $ gcc -Wall -I/opt/gdbm-1.8.3/include
         dbmain.c /opt/gdbm-1.8.3/lib/libgdbm.so

In the latter case it is still necessary to set the library load path when running the executable.

C language standards
====================

By default, gcc compiles programs using the GNU dialect of the C language, referred to as GNU C.  This dialect incorporates the official ANSI/ISO standard for the C language with several useful GNU extensions, such as nested functions and variable-size arrays.  Most ANSI/ISO programs will compile under GNU C without changes.

There are several options which control the dialect of C used by gcc.  The most commonly-used options are -ansi and -pedantic.  The specific dialects of the C language for each standard can also be selected with the -std option.

ANSI/ISO
--------

Occasionally a valid ANSI/ISO program may be incompatible with the extensions in GNU C.  To deal with this situation, the compiler option -ansi disables those GNU extensions which are in conflict with the ANSI/ISO standard.  On systems using the GNU C Library (glibc) it also disables extensions to the C standard library.  This allows programs written for ANSI/ISO C to be compiled without any unwanted effects from GNU extensions.

For example, here is a valid ANSI/ISO C program which uses a variable called asm:

          [file ansi.c not included]

The variable name asm is valid under the ANSI/ISO standard, but this program will not compile in GNU C because asm is a GNU C keyword extension (it allows native assembly instructions to be used in C functions).  Consequently, it cannot be used as a variable name without giving a compilation error:

     $ gcc -Wall ansi.c
     ansi.c: In function 'main':
     ansi.c:6:14: error: expected identifier or '(' before 'asm'
     ansi.c:7:39: error: expected expression before 'asm'

In contrast, using the -ansi option disables the asm keyword extension, and allows the program above to be compiled correctly:

     $ gcc -Wall -ansi ansi.c
     $ ./a.out
     the string asm is '6502'

For reference, the non-standard keywords and macros defined by the GNU C extensions are asm, inline, typeof, unix and vax.  More details can be found in the GCC Reference Manual ``Using GCC'' (see Further reading).

The next example shows the effect of the -ansi option on systems using the GNU C Library, such as GNU/Linux systems.  The program below prints the value of pi, \pi=3.14159..., from the preprocessor definition M_PI in the header file math.h:

          [file pi.c not included]

The constant M_PI is not part of the ANSI/ISO C standard library (it comes from the BSD version of Unix).  In this case, the program will not compile with the -ansi option:

     $ gcc -Wall -ansi pi.c
     pi.c: In function 'main':
     pi.c:7:38: error: 'M_PI' undeclared (first use in this
      function)
     pi.c:7:38: note: each undeclared identifier is reported
     only once for each function it appears in

The program can be compiled without the -ansi option.  In this case both the language and library extensions are enabled by default:

     $ gcc -Wall pi.c
     $ ./a.out
     the value of pi is 3.141593

It is also possible to compile the program using ANSI/ISO C, by enabling only the extensions in the GNU C Library itself.  This can be achieved by defining special macros, such as _GNU_SOURCE, which enable extensions in the GNU C Library: (The -D option for defining macros will be explained in detail in the next chapter.)

     $ gcc -Wall -ansi -D_GNU_SOURCE pi.c
     $ ./a.out
     the value of pi is 3.141593

The GNU C Library provides a number of these macros (referred to as feature test macros) which allow control over the support for POSIX extensions (_POSIX_C_SOURCE), BSD extensions (_BSD_SOURCE), SVID extensions (_SVID_SOURCE), XOPEN extensions (_XOPEN_SOURCE) and GNU extensions (_GNU_SOURCE).

The _GNU_SOURCE macro enables all the extensions together, with the POSIX extensions taking precedence over the others in cases where they conflict.  Further information about feature test macros can be found in the GNU C Library Reference Manual (see Further reading).

Strict ANSI/ISO
---------------

The command-line option -pedantic in combination with -ansi will cause gcc to reject all GNU C extensions, not just those that are incompatible with the ANSI/ISO standard.  This helps you to write portable programs which follow the ANSI/ISO standard.

Here is a program which uses variable-size arrays, a GNU C extension. The array x[n] is declared with a length specified by the integer variable n.

          [file gnuarray.c not included]

This program will compile with -ansi, because support for variable length arrays does not interfere with the compilation of valid ANSI/ISO programs---it is a backwards-compatible extension:

     $ gcc -Wall -ansi gnuarray.c

However, compiling with -ansi -pedantic reports warnings about violations of the ANSI/ISO standard:

     $ gcc -Wall -ansi -pedantic gnuarray.c
     gnuarray.c: In function `main':
     gnuarray.c:5: warning: ISO C90 forbids variable-size
       array `x'

Note that an absence of warnings from -ansi -pedantic does not guarantee that a program strictly conforms to the ANSI/ISO standard.  The standard itself specifies only a limited set of circumstances that should generate diagnostics, and these are what -ansi -pedantic reports.

Selecting specific standards
----------------------------

The specific language standard used by GCC can be controlled with the -std option.   The following C language standards are supported:

-std=c89 or -std=iso9899:1990
     The original ANSI/ISO C language standard (ANSI X3.159-1989, ISO/IEC 9899:1990).  GCC incorporates the corrections in the two ISO Technical Corrigenda to the original standard.

-std=iso9899:199409
     The ISO C language standard with ISO Amendment 1, published in 1994. This amendment was mainly concerned with internationalization, such as adding support for multibyte characters to the C library.

-std=c99 or -std=iso9899:1999
     The revised ISO C language standard, published in 1999 (ISO/IEC 9899:1999).

The C language standards with GNU extensions can be selected with the options -std=gnu89 and -std=gnu99.

Warning options in -Wall
========================

As described earlier (see Compiling a simple C program), the warning option -Wall enables warnings for many common errors, and should always be used.  It combines a large number of other, more specific, warning options which can also be selected individually.  Here is a summary of these options:

-Wcomment (included in -Wall)
     This option warns about nested comments.  Nested comments typically arise when a section of code containing comments is later commented out:

          /* commented out
          double x = 1.23 ; /* x-position */
          */

     Nested comments can be a source of confusion---the safe way to ``comment out'' a section of code containing comments is to surround it with the preprocessor directive #if 0 ... #endif:

          /* commented out */
          #if 0
          double x = 1.23 ; /* x-position */
          #endif

-Wformat (included in -Wall)
     This option warns about the incorrect use of format strings in functions such as printf and scanf, where the format specifier does not agree with the type of the corresponding function argument.

-Wunused (included in -Wall)
     This option warns about unused variables.  When a variable is declared but not used this can be the result of another variable being accidentally substituted in its place.  If the variable is genuinely not needed it can be removed from the source code.

-Wimplicit (included in -Wall)
     This option warns about any functions that are used without being declared.  The most common reason for a function to be used without being declared is forgetting to include a header file.

-Wreturn-type (included in -Wall)
     This option warns about functions that are defined without a return type but not declared void.  It also catches empty return statements in functions that are not declared void.

     For example, the following program does not use an explicit return value:

               [file main4.c not included]

     The lack of a return value in the code above could be the result of an accidental omission by the programmer---the value returned by the main function is actually the return value of the printf function (the number of characters printed).  To avoid ambiguity, it is preferable to use an explicit value in the return statement, either as a variable or a constant, such as return 0.

The complete set of warning options included in -Wall can be found in the GCC Reference Manual ``Using GCC'' (see Further reading).  The options included in -Wall have the common characteristic that they report constructions which are always wrong, or can easily be rewritten in an unambiguously correct way.  This is why they are so useful---any warning produced by -Wall can be taken as an indication of a potentially serious problem.

Additional warning options
==========================

GCC provides many other warning options that are not included in -Wall but are often useful.  Typically these produce warnings for source code which may be technically valid but is very likely to cause problems.  The criteria for these options are based on experience of common errors---they are not included in -Wall because they only indicate possibly problematic or ``suspicious'' code.

Since these warnings can be issued for valid code it is not necessary to compile with them all the time.  It is more appropriate to use them periodically and review the results, checking for anything unexpected, or to enable them for some programs or files.

-W
     This is a general option similar to -Wall which warns about a selection of common programming errors, such as functions which can return without a value (also known as ``falling off the end of the function body''), and comparisons between signed and unsigned values. For example, the following function tests whether an unsigned integer is negative (which is impossible, of course):

               [file w.c not included]

     Compiling this function with -Wall does not produce a warning,

          $ gcc -Wall -c w.c

     but does give a warning with -W:

          $ gcc -W -c w.c
          w.c: In function 'foo':
          w.c:4:3: warning: comparison of unsigned
          expression < 0 is always false [-Wtype-limits]

     In practice, the options -W and -Wall are normally used together.

-Wconversion
     This option warns about implicit type conversions that could cause unexpected results, such as conversions between floating-point and integer types, between signed and unsigned types and between types of different width (e.g. long and short integers).  Conversions can occur in expressions and assignments, and in calls to functions if the types of the arguments do not match those specified in the prototype.

     For example, the integer absolute value function int abs(int i) is easily confused with the corresponding floating-point function double fabs(double x).  This can lead to incorrect results, as shown in the following program:

               [file wabs.c not included]

     Compiling this function with -Wall does not produce a warning,

          $ gcc -Wall wabs.c
          $ ./a.out
          x = -3.14 |x| = 3  (incorrect)

     but does give a warning with -Wconversion:

          $ gcc -Wall -Wconversion wabs.c
          wabs.c: In function 'main':
          wabs.c:8:3: warning: conversion to 'int' from
          'double' may alter its value [-Wconversion]

     The -Wconversion option also catches errors such as the assignment of a negative value to an unsigned variable, as in the following code,

          unsigned int x = -1;

     This is technically allowed by the ANSI/ISO C standard (with the negative integer being converted to a positive integer, according to the machine representation) but could be a simple programming error. If you need to perform such a conversion you can use an explicit cast, such as (unsigned int)-1, to avoid any warnings from this option.  On two's-complement machines the cast of -1 gives the maximum number that can be represented by an unsigned integer.

-Wshadow
     This option warns about the redeclaration of a variable name in a scope where it has already been declared.  This is referred to as variable shadowing, and causes confusion about which occurrence of the variable corresponds to which value.

     The following function declares a local variable y that shadows the declaration in the body of the function:

               [file shadow.c not included]

     This is valid ANSI/ISO C, where the return value is 1.  The shadowing of the variable y might make it seem (incorrectly) that the return value is x, when looking at the line y = x (especially in a large and complicated function).

     Shadowing can also occur for function names.  For example, the following program attempts to define a variable sin which shadows the standard function sin(x).

               [file shadow2.c not included]

     This error will be detected by the -Wshadow option.

-Wcast-qual
     This option warns about pointers that are cast to remove a type qualifier, such as const.  For example, the following function discards the const qualifier from its input argument, allowing it to be overwritten:

               [file castqual.c not included]

     The modification of the original contents of str is a violation of its const property.  This option will warn about the improper cast of the variable str which allows the string to be modified.

-Wwrite-strings
     This option implicitly gives all string constants defined in the program a const qualifier, causing a compile-time warning if there is an attempt to overwrite them.  The result of modifying a string constant is not defined by the ANSI/ISO standard, and the use of writable string constants is deprecated in GCC.

-Wtraditional
     This option warns about parts of the code which would be interpreted differently by an ANSI/ISO compiler and a ``traditional'' pre-ANSI compiler. (The traditional form of the C language was described in the original C reference manual ``The C Programming Language (First Edition)'' by Kernighan and Ritchie.)  When maintaining legacy software it may be necessary to investigate whether the traditional or ANSI/ISO interpretation was intended in the original code for warnings generated by this option.

The options above produce diagnostic warning messages, but allow the compilation to continue and produce an object file or executable.  For large programs it can be desirable to catch all the warnings by stopping the compilation whenever a warning is generated.  The -Werror option changes the default behavior by converting warnings into errors, stopping the compilation whenever a warning occurs.

Recommended warning options
===========================

The following options are a good choice for finding problems in C and C++ programs:

     $ gcc -ansi -pedantic -Wall -W -Wconversion
         -Wshadow -Wcast-qual -Wwrite-strings

While this list is not exhaustive, regular use of these options will catch many common errors.

Using the preprocessor
**********************

This chapter describes the use of the GNU C preprocessor cpp, which is part of the GCC package.  The preprocessor expands macros in source files before they are compiled.  It is automatically called whenever GCC processes a C or C++ program. (In recent versions of GCC the preprocessor is integrated into the compiler, although a separate cpp command is also provided.)

Defining macros
===============

The following program demonstrates the most common use of the C preprocessor.  It uses the preprocessor conditional #ifdef to check whether a macro is defined:

          [file dtest.c not included]

When the macro is defined, the preprocessor includes the corresponding code up to the closing #endif command.  In this example, the macro which is tested is called TEST, and the conditional part of the source code is a printf statement which prints the message ``Test mode''.

The gcc option -DNAME defines a preprocessor macro NAME from the command line.  If the program above is compiled with the command-line option -DTEST, the macro TEST will be defined and the resulting executable will print both messages:

     $ gcc -Wall -DTEST dtest.c
     $ ./a.out
     Test mode
     Running...

If the same program is compiled without the -D option then the ``Test mode'' message is omitted from the source code after preprocessing, and the final executable does not include the code for it:

     $ gcc -Wall dtest.c
     $ ./a.out
     Running...

Macros are generally undefined, unless specified on the command line with the option -D, or in a source file (or library header file) with #define.  Some macros are automatically defined by the compiler---these typically use a reserved namespace beginning with a double-underscore prefix __.

The complete set of predefined macros can be listed by running the GNU preprocessor cpp with the option -dM on an empty file:

     $ cpp -dM /dev/null
     #define __i586 1
     #define __WINT_MAX__ 4294967295U
     #define __ORDER_LITTLE_ENDIAN__ 1234
     #define __SIZE_MAX__ 4294967295U
     .......

Note that this list includes a small number of system-specific macros defined by gcc which do not use the double-underscore prefix. These non-standard macros can be disabled with the -ansi option of gcc.

Macros with values
==================

In addition to being defined, a macro can also be given a value.  This value is inserted into the source code at each point where the macro occurs.  The following program uses a macro NUM, to represent a number which will be printed:

          [file dtestval.c not included]

Note that macros are not expanded inside strings---only the occurrence of NUM outside the string is substituted by the preprocessor.

To define a macro with a value, the -D command-line option can be used in the form -DNAME=VALUE.  For example, the following command line defines NUM to be 100 when compiling the program above:

     $ gcc -Wall -DNUM=100 dtestval.c
     $ ./a.out
     Value of NUM is 100

This example uses a number, but a macro can take values of any form. Whatever the value of the macro is, it is inserted directly into the source code at the point where the macro name occurs.  For example, the following definition expands the occurrences of NUM to 2+2 during preprocessing:

     $ gcc -Wall -DNUM="2+2" dtestval.c
     $ ./a.out
     Value of NUM is 4

After the preprocessor has made the substitution NUM ==> 2+2 this is equivalent to compiling the following program:

          [file dtestval2.c not included]

Note that it is a good idea to surround macros by parentheses whenever they are part of an expression.  For example, the following program uses parentheses to ensure the correct precedence for the multiplication 10*NUM:

          [file dtestval3.c not included]

With these parentheses, it produces the expected result when compiled with the same command line as above:

     $ gcc -Wall -DNUM="2+2" dtestval3.c
     $ ./a.out
     Ten times NUM is 40

Without parentheses, the program would produce the value 22 from the literal form of the expression 10*2+2 = 22, instead of the desired value 10*(2+2) = 40.

When a macro is defined with -D alone, gcc uses a default value of 1.  For example, compiling the original test program with the option -DNUM generates an executable which produces the following output:

     $ gcc -Wall -DNUM dtestval.c
     $ ./a.out
     Value of NUM is 1

A macro can be defined with an empty value using quotes on the command line, -DNAME="".  Such a macro is still treated as defined by conditionals such as #ifdef, but expands to nothing.

A macro containing quotes can be defined using shell-escaped quote characters.  For example, the command-line option -DMESSAGE='"Hello, World!"' defines a macro MESSAGE which expands to the sequence of characters "Hello, World!".  The outer shell-quotes '...' protect the C-quotes of the string "Hello, World!". For an explanation of the different types of quoting and escaping used in the shell see the ``GNU Bash Reference Manual'', Further reading.

Preprocessing source files
==========================

It is possible to see the effect of the preprocessor on source files directly, using the -E option of gcc.  For example, the file below defines and uses a macro TEST:

          [file test.c not included]

If this file is called test.c the effect of the preprocessor can be seen with the following command line:

     $ gcc -E test.c
     # 1 "test.c"
     # 1 "<built-in>"
     # 1 "<command-line>"
     # 1 "test.c"

     const char str[] = "Hello, World!";

The -E option causes gcc to run the preprocessor, display the expanded output, and then exit without compiling the resulting source code.  The value of the macro TEST is substituted directly into the output, producing the sequence of characters const char str[] = "Hello, World!" ;.

The preprocessor also inserts lines recording the source file and line numbers in the form # LINE-NUMBER "SOURCE-FILE", to aid in debugging and allow the compiler to issue error messages referring to this information.  These lines do not affect the program itself.

The ability to see the preprocessed source files can be useful for examining the effect of system header files, and finding declarations of system functions.  The following program includes the header file stdio.h to obtain the declaration of the function printf:

          [file hello.c not included]

It is possible to see the declarations from the included header file by preprocessing the file with gcc -E:

     $ gcc -E hello.c

On a GNU system, this produces output similar to the following:

     # 1 "hello.c"
     # 1 "/usr/include/stdio.h" 1 3

     extern FILE *stdin;
     extern FILE *stdout;
     extern FILE *stderr;

     extern int fprintf (FILE * __stream,
                         const char * __format, ...)  ;
     extern int printf (const char * __format, ...)  ;

      [ ... additional declarations ... ]

     # 1 "hello.c" 2

     int
     main (void)
     {
       printf ("Hello, world!\n");
       return 0;
     }

The preprocessed system header files usually generate a lot of output.  This can be redirected to a file, or saved more conveniently using the gcc -save-temps option:

     $ gcc -c -save-temps hello.c

After running this command, the preprocessed output will be available in the file hello.i.  The -save-temps option also saves .s assembly files and .o object files in addition to preprocessed .i files.

Compiling for debugging
***********************

Normally, an executable file does not contain any references to the original program source code, such as variable names or line-numbers---the executable file is simply the sequence of machine code instructions produced by the compiler.  This is insufficient for debugging, since there is no easy way to find the cause of an error if the program crashes.

GCC provides the -g debug option to store additional debugging information in object files and executables.  This debugging information allows errors to be traced back from a specific machine instruction to the corresponding line in the original source file.  The execution of a program compiled with -g can also be followed in a debugger, such as the GNU Debugger gdb (for more information, see ``Debugging with GDB: The GNU Source-Level Debugger'', Further reading).  Using a debugger allows the values of variables to be examined while the program is running.

The debug compilation option works by storing the names and source code line-numbers of functions and variables in a symbol table in the object file or executable.

Examining core files
====================

In addition to allowing programs to be run under the debugger, an important benefit of the -g option is the ability to examine the cause of a program crash from a ``core dump''.

When a program exits abnormally (i.e. crashes) the operating system can write out a core file (usually named core) which contains the in-memory state of the program at the time it crashed.  This file is often referred to as a core dump. (The terminology dates back to the time of magnetic core memory.)  Combined with information from the symbol table produced by -g, the core dump can be used to find the line where the program stopped, and the values of its variables at that point.

This is useful both during the development of software and after deployment---it allows problems to be investigated when a program has crashed ``in the field''.

Here is a simple program containing an invalid memory access bug, which we will use to produce a core file:

          [file null.c not included]

The program attempts to dereference a null pointer p, which is an invalid operation.  On most systems, this will cause a crash.  (Historically, a null pointer corresponded to memory location 0, which is typically restricted to the operating system kernel.  In practice this is not always how a null pointer works, but the result is usually the same.)

In order to be able to find the cause of the crash later, we will need to compile the program with the -g option:

     $ gcc -Wall -g null.c

Note that a null pointer will only cause a problem at run-time, so the option -Wall does not produce any warnings.

Running the executable file on an x86 GNU/Linux system will cause the operating system to terminate the program abnormally:

     $ ./a.out
     Segmentation fault (core dumped)

Whenever the error message core dumped is displayed, the operating system should produce a file called core in the current directory. (Some systems, such as FreeBSD and Solaris, can also be configured to write core files in specific directories, e.g. /var/coredumps/, using the sysctl or coreadm commands.)  This core file contains a complete copy of the pages of memory used by the program at the time it was terminated.  Incidentally, the term segmentation fault refers to the fact that the program tried to access a restricted memory ``segment'' outside the area of memory which had been allocated to it.

Some systems are configured not to write core files by default, since the files can be large and rapidly fill up the available disk space on a system.  In the GNU Bash shell the command ulimit -c controls the maximum size of core files.  If the size limit is zero, no core files are produced.  The current size limit can be shown by typing the following command:

     $ ulimit -c
     0

If the result is zero, as shown above, then it can be increased with the following command to allow core files of any size to be written: (This example uses the ulimit command in the GNU Bash shell.  On other systems the usage of the ulimit command may vary, or have a different name (the tcsh shell uses the limit command instead).  The size limit for core files can also be set to a specific value in kilobytes.)

     $ ulimit -c unlimited

Note that this setting only applies to the current shell.  To set the limit for future sessions the command should be placed in an appropriate login file, such as .bash_profile for the GNU Bash shell.

Core files can be loaded into the GNU Debugger gdb with the following command:

     $ gdb EXECUTABLE-FILE CORE-FILE

Note that both the original executable file and the core file are required for debugging---it is not possible to debug a core file without the corresponding executable.  In this example, we can load the executable and core file with the command:

     $ gdb a.out core

The debugger immediately begins printing diagnostic information, and shows a listing of the line where the program crashed (line 13):

     $ gdb a.out core
     Core was generated by `./a.out'.
     Program terminated with signal 11, Segmentation fault.
     Reading symbols from /lib/libc.so.6...done.
     Loaded symbols for /lib/libc.so.6
     Reading symbols from /lib/ld-linux.so.2...done.
     Loaded symbols for /lib/ld-linux.so.2
     #0  0x080483ed in foo (p=0x0) at null.c:13
     13        int y = *p;
     (gdb)

The final line (gdb) is the GNU Debugger prompt---it indicates that further commands can be entered at this point. (The details of the diagnostic output may vary slightly depending on which version of gdb you are using.)

To investigate the cause of the crash, we display the value of the pointer p using the debugger print command:

     (gdb) print p
     $1 = (int *) 0x0

This shows that p is a null pointer (0x0) of type int *, so we know that dereferencing it with the expression *p in this line has caused the crash.

Displaying a backtrace
======================

The debugger can also show the function calls and arguments up to the current point of execution---this is called a stack backtrace and is displayed with the command backtrace:

     (gdb) backtrace
     #0  0x080483ed in foo (p=0x0) at null.c:13
     #1  0x080483d9 in main () at null.c:7

In this case, the backtrace shows that the crash occurred at line 13 after the function foo was called from main with an argument of p=0x0 at line 7 in null.c.  It is possible to move to different levels in the stack trace, and examine their variables, using the debugger commands up and down.

Setting a breakpoint
====================

A breakpoint stops the execution of a program and returns control to the debugger, where its variables and memory can be examined before continuing.  Breakpoints can be set for specific functions, lines or memory locations with the break command.

To set a breakpoint on a specific function, use the command break FUNCTION-NAME.  For example, the following command sets a breakpoint at the start of the main function in the program above:

     $ gdb a.out
     (gdb) break main
     Breakpoint 1 at 0x80483c6: file null.c, line 6.

The debugger will now take control of the program when the function main is called.  Since the main function is the first function to be executed in a C program the program will stop immediately when it is run:

     (gdb) run
     Starting program: a.out
     Breakpoint 1, main () at null.c:6
     6         int *p = 0;   /* null pointer */
     (gdb)

The display shows the line that will be executed next (the line number is shown on the left).  The breakpoint stops the program before the line is executed, so at this stage the pointer p is undefined and has not yet been set to zero.

Stepping through the program
============================

To move forward and execute the line displayed above, use the command step:

     (gdb) step
     7         return foo (p);

After executing line 6, the debugger displays the next line to be executed.  The pointer p will now have been set to zero (null):

     (gdb) print p
     $1 = (int *) 0x0

The command step will follow the execution of the program interactively through any functions that are called in the current line.  If you want to move forward without tracing these calls, use the command next instead.

Modifying variables
===================

To temporarily fix the null pointer bug discovered above, we can change the value of p in the running program using the set variable command.

Variables can be set to a specific value, or to the result of an expression, which may include function calls.  This powerful feature allows functions in a program to be tested interactively through the debugger.

In this case we will interactively allocate some memory for the pointer p using the function malloc, storing the value 255 in the resulting location:

     (gdb) set variable p = malloc(sizeof(int))
     (gdb) print p
     $2 = (int *) 0x40013f98    (address allocated by malloc)
     (gdb) set variable *p = 255
     (gdb) print *p
     $3 = 255

If we now continue stepping through the program with the new value of p the previous segmentation fault will not occur:

     (gdb) step
     foo (p=0x40013f98) at null.c:13
     13        int y = *p;
     (gdb) step
     14        return y;

Continuing execution
====================

The command finish continues execution up to the end of the current function, displaying the return value:

     (gdb) finish
     Run till exit from #0  0x08048400 in foo (p=0x40013f98)
     at null.c:15
     0x080483d9 in main () at null.c:7
     7         return foo (p);
     Value returned is $13 = 255

To continue execution until the program exits (or hits the next breakpoint) use the command continue,

     (gdb) continue
     Continuing.
     Program exited with code 0377.

Note that the exit code is shown in octal (0377 base 8 = 255 in base 10).

More information
================

For simplicity, the examples in this chapter demonstrate how to use gdb on the command-line.  There are more powerful ways to debug a program interactively using tools such as Emacs GDB mode (M-x gdb), DDD or INSIGHT, graphical interfaces to gdb. Links to these programs can be found on the publisher's webpage for this book. (http://www.network-theory.co.uk/gcc/intro/)

A complete description of all the commands available in gdb can be found in the manual ``Debugging with GDB: The GNU Source-Level Debugger'' (see Further reading).

Compiling with optimization
***************************

GCC is an optimizing compiler.  It provides a wide range of options which aim to increase the speed, or reduce the size, of the executable files it generates.

Optimization is a complex process.  For each high-level command in the source code there are usually many possible combinations of machine instructions that can be used to achieve the appropriate final result. The compiler must consider these possibilities and choose among them.

In general, different code must be generated for different processors, as they use incompatible assembly and machine languages.  Each type of processor also has its own characteristics---some CPUs provide a large number of registers for holding intermediate results of calculations, while others must store and fetch intermediate results from memory.  Appropriate code must be generated in each case.

Furthermore, different amounts of time are needed for different instructions, depending on how they are ordered.  GCC takes all these factors into account and tries to produce the fastest executable for a given system when compiling with optimization.

Source-level optimization
=========================

The first form of optimization used by GCC occurs at the source-code level, and does not require any knowledge of the machine instructions. There are many source-level optimization techniques---this section describes two common types: common subexpression elimination and function inlining.

Common subexpression elimination
--------------------------------

One method of source-level optimization which is easy to understand involves computing an expression in the source code with fewer instructions, by reusing already-computed results.  For example, the following assignment:

     x = cos(v)*(1+sin(u/2)) + sin(w)*(1-sin(u/2))

can be rewritten with a temporary variable t to eliminate an unnecessary extra evaluation of the term sin(u/2):

     t = sin(u/2)
     x = cos(v)*(1+t) + sin(w)*(1-t)

This rewriting is called common subexpression elimination (CSE), and is performed automatically when optimization is turned on. (Temporary values introduced by the compiler during common subexpression elimination are only used internally, and do not affect real variables.  The name of the temporary variable t shown above is only used as an illustration.)  Common subexpression elimination is powerful, because it simultaneously increases the speed and reduces the size of the code.

Function inlining
-----------------

Another type of source-level optimization, called function inlining, increases the efficiency of frequently-called functions.

Whenever a function is used, a certain amount of extra time is required for the CPU to carry out the call: it must store the function arguments in the appropriate registers and memory locations, jump to the start of the function (bringing the appropriate virtual memory pages into physical memory or the CPU cache if necessary), begin executing the code, and then return to the original point of execution when the function call is complete.  This additional work is referred to as function-call overhead.  Function inlining eliminates this overhead by replacing calls to a function by the code of the function itself (known as placing the code in-line).

In most cases, function-call overhead is a negligible fraction of the total run-time of a program.  It can become significant only when there are functions which contain relatively few instructions, and these functions account for a substantial fraction of the run-time---in this case the overhead then becomes a large proportion of the total run-time.

Inlining is always favorable if there is only one point of invocation of a function.  It is also unconditionally better if the invocation of a function requires more instructions (memory) than moving the body of the function in-line.  This is a common situation for simple accessor functions in C++, which can benefit greatly from inlining.  Moreover, inlining may facilitate further optimizations, such as common subexpression elimination, by merging several separate functions into a single large function.

The following function sq(x) is a typical example of a function that would benefit from being inlined.  It computes x^2, the square of its argument x:

     double
     sq (double x)
     {
       return x * x;
     }

This function is small, so the overhead of calling it is comparable to the time taken to execute the single multiplication carried out by the function itself.  If this function is used inside a loop, such as the one below, then the function-call overhead would become substantial:

     for (i = 0; i < 1000000; i++)
       {
          sum += sq (i + 0.5);
       }

Optimization with inlining replaces the inner loop of the program with the body of the function, giving the following code:

     for (i = 0; i < 1000000; i++)
       {
         double t = (i + 0.5);  /* temporary variable */
         sum += t * t;
       }

Eliminating the function call and performing the multiplication in-line allows the loop to run with maximum efficiency.

GCC selects functions for inlining using a number of heuristics, such as the function being suitably small.  As an optimization, inlining is carried out only within each object file.  The inline keyword can be used to request explicitly that a specific function should be inlined wherever possible, including its use in other files. (In this case, the definition of the inline function must be made available to the other files (e.g. in a header file).)  The GCC Reference Manual ``Using GCC'' provides full details of the inline keyword, and its use with the static and extern qualifiers to control the linkage of explicitly inlined functions (see Further reading).

Speed-space tradeoffs
=====================

While some forms of optimization, such as common subexpression elimination, are able to increase the speed and reduce the size of a program simultaneously, other types of optimization produce faster code at the expense of increasing the size of the executable.  This choice between speed and memory is referred to as a speed-space tradeoff. Optimizations with a speed-space tradeoff can also be used in reverse to make an executable smaller, at the expense of making it run slower.

Loop unrolling
--------------

A prime example of an optimization with a speed-space tradeoff is loop unrolling.  This form of optimization increases the speed of loops by eliminating the ``end of loop'' condition on each iteration. For example, the following loop from 0 to 7 tests the condition i < 8 on each iteration:

     for (i = 0; i < 8; i++)
       {
         y[i] = i;
       }

At the end of the loop, this test will have been performed 9 times, and a large fraction of the run time will have been spent checking it.

A more efficient way to write the same code is simply to unroll the loop and execute the assignments directly:

     y[0] = 0;
     y[1] = 1;
     y[2] = 2;
     y[3] = 3;
     y[4] = 4;
     y[5] = 5;
     y[6] = 6;
     y[7] = 7;

This form of the code does not require any tests, and executes at maximum speed.  Since each assignment is independent, it also allows the compiler to use parallelism on processors that support it.  Loop unrolling is an optimization that increases the speed of the resulting executable but also generally increases its size (unless the loop is very short, with only one or two iterations, for example).

Loop unrolling is also possible when the upper bound of the loop is unknown, provided the start and end conditions are handled correctly. For example, the same loop with an arbitrary upper bound,

     for (i = 0; i < n; i++)
       {
         y[i] = i;
       }

can be rewritten by the compiler as follows:

     for (i = 0; i < (n % 2); i++)
       {
         y[i] = i;
       }

     for ( ; i + 1 < n; i += 2) /* no initializer */
       {
         y[i] = i;
         y[i+1] = i+1;
       }

The first loop handles the case i = 0 when n is odd, and the second loop handles all the remaining iterations.  Note that the second loop does not use an initializer in the first argument of the for statement, since it continues where the first loop finishes. The assignments in the second loop can be parallelized, and the overall number of tests is reduced by a factor of 2 (approximately). Higher factors can be achieved by unrolling more assignments inside the loop, at the cost of greater code size.

Scheduling
==========

The lowest level of optimization is scheduling, in which the compiler determines the best ordering of individual instructions.  Most CPUs allow one or more new instructions to start executing before others have finished.  Many CPUs also support pipelining, where multiple instructions execute in parallel on the same CPU.

When scheduling is enabled, instructions must be arranged so that their results become available to later instructions at the right time, and to allow for maximum parallel execution.  Scheduling improves the speed of an executable without increasing its size, but requires additional memory and time in the compilation process itself (due to its complexity).

Optimization levels
===================

In order to control compilation-time and compiler memory usage, and the trade-offs between speed and space for the resulting executable, GCC provides a range of general optimization levels, numbered from 0--3, as well as individual options for specific types of optimization.

An optimization level is chosen with the command line option -OLEVEL, where LEVEL is a number from 0 to 3. The effects of the different optimization levels are described below:

-O0 or no -O option (default)
     At this optimization level GCC does not perform any optimization and compiles the source code in the most straightforward way possible.  Each command in the source code is converted directly to the corresponding instructions in the executable file, without rearrangement.  This is the best option to use when debugging a program and is the default if no optimization level option is specified.

-O1 or -O
     This level turns on the most common forms of optimization that do not require any speed-space tradeoffs.  With this option the resulting executables should be smaller and faster than with -O0.  The more expensive optimizations, such as instruction scheduling, are not used at this level.

     Compiling with the option -O1 can often take less time than compiling with -O0, due to the reduced amounts of data that need to be processed after simple optimizations.

-O2
     This option turns on further optimizations, in addition to those used by -O1.  These additional optimizations include instruction scheduling.  Only optimizations that do not require any speed-space tradeoffs are used, so the executable should not increase in size.  The compiler will take longer to compile programs and require more memory than with -O1.  This option is generally the best choice for deployment of a program, because it provides maximum optimization without increasing the executable size.  It is the default optimization level for releases of GNU packages.

-O3
     This option turns on more expensive optimizations, such as function inlining, in addition to all the optimizations of the lower levels -O2 and -O1.  The -O3 optimization level may increase the speed of the resulting executable, but can also increase its size.  Under some circumstances where these optimizations are not favorable, this option might actually make a program slower.

-funroll-loops

     This option turns on loop-unrolling, and is independent of the other optimization options.  It will increase the size of an executable. Whether or not this option produces a beneficial result has to be examined on a case-by-case basis.

-Os
     This option selects optimizations which reduce the size of an executable.  The aim of this option is to produce the smallest possible executable, for systems constrained by memory or disk space.  In some cases a smaller executable will also run faster, due to better cache usage.

It is important to remember that the benefit of optimization at the highest levels must be weighed against the cost.  The cost of optimization includes greater complexity in debugging, and increased time and memory requirements during compilation.  For most purposes it is satisfactory to use -O0 for debugging, and -O2 for development and deployment.

Examples
========

The following program will be used to demonstrate the effects of different optimization levels:

          [file optim.c not included]

The main program contains a loop calling the powern function. This function computes the n-th power of a floating point number by repeated multiplication---it has been chosen because it is suitable for both inlining and loop-unrolling.  The run-time of the program can be measured using the time command in the GNU Bash shell.

Here are some results for the program above, compiled on a 566MHz Intel Celeron with 16KB L1-cache and 128KB L2-cache, using GCC 3.3.1 on a GNU/Linux system:

     $ gcc -Wall -O0 optim.c -lm
     $ time ./a.out
     real    0m13.388s
     user    0m13.370s
     sys     0m0.010s

     $ gcc -Wall -O1 optim.c -lm
     $ time ./a.out
     real    0m10.030s
     user    0m10.030s
     sys     0m0.000s

     $ gcc -Wall -O2 optim.c -lm
     $ time ./a.out
     real    0m8.388s
     user    0m8.380s
     sys     0m0.000s

     $ gcc -Wall -O3 optim.c -lm
     $ time ./a.out
     real    0m6.742s
     user    0m6.730s
     sys     0m0.000s

     $ gcc -Wall -O3 -funroll-loops optim.c -lm
     $ time ./a.out
     real    0m5.412s
     user    0m5.390s
     sys     0m0.000s

The relevant entry in the output for comparing the speed of the resulting executables is the user time, which gives the actual CPU time spent running the process.  The other rows, real and sys, record the total real time for the process to run (including times where other processes were using the CPU) and the time spent waiting for operating system calls.  Although only one run is shown for each case above, the benchmarks were executed several times to confirm the results.

From the results it can be seen in this case that increasing the optimization level with -O1, -O2 and -O3 produces an increasing speedup, relative to the unoptimized code compiled with -O0. The additional option -funroll-loops produces a further speedup.  The speed of the program is more than doubled overall, when going from unoptimized code to the highest level of optimization.

Note that for a small program such as this there can be considerable variation between systems and compiler versions.  For example, on a Mobile 2.0GHz Intel Pentium 4M system the trend of the results using the same version of GCC is similar except that the performance with -O2 is slightly worse than with -O1.  This illustrates an important point: optimizations may not necessarily make a program faster in every case.

Optimization and debugging
==========================

With GCC it is possible to use optimization in combination with the debugging option -g.  Many other compilers do not allow this.

When using debugging and optimization together, the internal rearrangements carried out by the optimizer can make it difficult to see what is going on when examining an optimized program in the debugger. For example, temporary variables are often eliminated, and the ordering of statements may be changed.

However, when a program crashes unexpectedly, any debugging information is better than none---so the use of -g is recommended for optimized programs, both for development and deployment.  The debugging option -g is enabled by default for releases of GNU packages, together with the optimization option -O2.

Optimization and compiler warnings
==================================

When optimization is turned on, GCC can produce additional warnings that do not appear when compiling without optimization.

As part of the optimization process, the compiler examines the use of all variables and their initial values---this is referred to as data-flow analysis.  It forms the basis for other optimization strategies, such as instruction scheduling.  A side-effect of data-flow analysis is that the compiler can detect the use of uninitialized variables.

The -Wuninitialized option (which is included in -Wall) warns about variables that are read without being initialized.  It only works when the program is compiled with optimization, so that data-flow analysis is enabled.  The following function contains an example of such a variable:

          [file uninit.c not included]

The function works correctly for most arguments, but has a bug when x is zero---in this case the return value of the variable s will be undefined.

Compiling the program with the -Wall option alone does not produce any warnings, because data-flow analysis is not carried out without optimization:

     $ gcc -Wall -c uninit.c

To produce a warning, the program must be compiled with -Wall and optimization simultaneously.  In practice, the optimization level -O2 is needed to give good warnings:

     $ gcc -Wall -O2 -c uninit.c
     uninit.c: In function 'sign':
     uninit.c:11:3: warning: 's' may be used uninitialized
       in this function [-Wuninitialized]

This correctly detects the possibility of the variable s being used without being defined.

Note that while GCC will usually find most uninitialized variables, it does so using heuristics which will occasionally miss some complicated cases or falsely warn about others.  In the latter situation, it is often possible to rewrite the relevant lines in a simpler way that removes the warning and improves the readability of the source code.

Compiling a C++ program
***********************

This chapter describes how to use GCC to compile programs written in C++, and the command-line options specific to that language.

The GNU C++ compiler provided by GCC is a true C++ compiler---it compiles C++ source code directly into assembly language.  Some other C++ ``compilers'' are translators which convert C++ programs into C, and then compile the resulting C program using an existing C compiler.  A true C++ compiler, such as GCC, is able to provide better support for error reporting, debugging and optimization.

Compiling a simple C++ program
==============================

The procedure for compiling a C++ program is the same as for a C program, but uses the command g++ instead of gcc.  Both compilers are part of the GNU Compiler Collection.

To demonstrate the use of g++, here is a version of the Hello World program written in C++:

          [file hello.cc not included]

The program can be compiled with the following command line:

     $ g++ -Wall hello.cc -o hello

The C++ frontend of GCC uses many of the same the same options as the C compiler gcc.  It also supports some additional options for controlling C++ language features, which will be described in this chapter.  Note that C++ source code should be given one of the valid C++ file extensions .cc, .cpp, .cxx or .C rather than the .c extension used for C programs.

The resulting executable can be run in exactly same way as the C version, simply by typing its filename:

     $ ./hello
     Hello, world!

The executable produces the same output as the C version of the program, using std::cout instead of the C printf function.  All the options used in the gcc commands in previous chapters apply to g++ without change, as do the procedures for compiling and linking files and libraries (using g++ instead of gcc, of course).  One natural difference is that the -ansi option requests compliance with the C++ standard, instead of the C standard, when used with g++.

Note that programs using C++ object files must always be linked with g++, in order to supply the appropriate C++ libraries. Attempting to link a C++ object file with the C compiler gcc will cause ``undefined reference'' errors for C++ standard library functions:

     $ g++ -Wall -c hello.cc
     $ gcc hello.o       (should use g++)
     hello.o: In function `main':
     hello.o(.text+0x1b): undefined reference to `std::cout'
     .....
     hello.o(.eh_frame+0x11):
       undefined reference to `__gxx_personality_v0'

Undefined references to internal run-time library functions, such as __gxx_personality_v0, are also a symptom of linking C++ object files with gcc instead of g++. (Since these functions are internal, the error shown in this example will be different in other versions of the compiler.)  Linking the same object file with g++ supplies all the necessary C++ libraries and will produce a working executable:

     $ g++ hello.o
     $ ./a.out
     Hello, world!

A point that sometimes causes confusion is that gcc will actually compile C++ source code when it detects a C++ file extension, but cannot then link the resulting object files.

     $ gcc -Wall -c hello.cc   (succeeds, even for C++)
     $ gcc hello.o
     hello.o: In function `main':
     hello.o(.text+0x1b): undefined reference to `std::cout'

To avoid this problem, use g++ consistently for C++ programs and gcc for C programs.

C++ compilation options
=======================

Most GCC options can be used for both C and C++ programs, but there are also a few options which are specific to each language.  This section describes some of the additional options, and enhancements to existing options, that are available in g++.

-Wall and -W
     When compiling with g++, the options -Wall and -W include extra warnings specific to C++ (the warnings relate to member functions and virtual classes).  The use of these options is always recommended while developing a program.

-fno-default-inline
     This option disables the default inlining of member functions defined in the bodies of C++ classes.  GCC normally inlines all such functions when optimization is turned on, even if they do not explicitly use the inline keyword.  Select this option if you prefer to control inlining yourself, or want to set a breakpoint on member functions that would otherwise be inlined (since it is not possible to set a breakpoint on an inlined function).

-Weffc++
     This option warns about C++ code which breaks some of the programming guidelines given in the books ``Effective C++'' and ``More Effective C++'' by Scott Meyers.  For example, a warning will be given if a class which uses dynamically allocated memory does not define a copy constructor and an assignment operator.  Note that the standard library header files do not follow these guidelines, so you may wish to use this option as an occasional test for possible problems in your own code rather than compiling with it all the time.

-Wold-style-cast

     This option highlights any uses of C-style casts in C++ programs.  The C++ language provides the keywords static_cast, dynamic_cast, reinterpret_cast and const_cast for handling casts and these are often preferable (although C-style casts are still allowed).

Using the C++ standard library
==============================

An implementation of the C++ standard library is provided as a part of GCC.  The following program uses the standard library string class to reimplement the Hello World program:

          [file hellostr.cc not included]

The program can be compiled and run using the same commands as above:

     $ g++ -Wall hellostr.cc
     $ ./a.out
     Hello, World!

Note that in accordance with the C++ standard, the header files for the C++ library itself do not use a file extension.  The classes in the library are also defined in the std namespace, so the directive using namespace std is needed to access them, unless the prefix std:: is used throughout (as in the previous section).

Templates
=========

Templates provide the ability to define C++ classes which support generic programming techniques.  Templates can be considered as a powerful kind of macro facility.  When a templated class or function is used with a specific class or type, such as float or int, the corresponding template code is compiled with that type substituted in the appropriate places.

Using C++ standard library templates
------------------------------------

The C++ standard library libstdc++ supplied with GCC provides a wide range of generic container classes such as lists and queues, in addition to generic algorithms such as sorting.  These classes were originally part of the Standard Template Library (STL), which was a separate package, but are now included in the C++ standard library itself.

The following program demonstrates the use of the template library by creating a list of strings with the template list<string>:

          [file string.cc not included]

No special options are needed to use the template classes in the standard library; the command-line options for compiling this program are the same as before:

     $ g++ -Wall string.cc
     $ ./a.out
     List size = 2

Note that the executables created by g++ using the C++ standard library will be linked to the shared library libstdc++, which is supplied as part of the default GCC installation.  There are several versions of this library---if you distribute executables using the C++ standard library you need to ensure that the recipient has a compatible version of libstdc++, or link your program statically using the command-line option -static.

Providing your own templates
----------------------------

In addition to the template classes provided by the C++ standard library you can define your own templates.  The recommended way to use templates with g++ is to follow the inclusion compilation model, where template definitions are placed in header files.  This is the method used by the C++ standard library supplied with GCC itself.  The header files can then be included with #include in each source file where they are needed.

For example, the following template file creates a simple Buffer<T> class which represents a circular buffer holding objects of type T.

          [file buffer.h not included]

The file contains both the declaration of the class and the definitions of the member functions.  This class is only given for demonstration purposes and should not be considered an example of good programming. Note the use of include guards, which test for the presence of the macro BUFFER_H, ensuring that the definitions in the header file are only parsed once if the file is included multiple times in the same context.

The program below uses the templated Buffer class to create a buffer of size 10, storing the floating point values 0.25 and 1.25 in the buffer:

          [file tprog.cc not included]

The definitions for the template class and its functions are included in the source file for the program with #include "buffer.h" before they are used.  The program can then be compiled using the following command line:

     $ g++ -Wall tprog.cc
     $ ./a.out
     stored value = 1.25

At the points where the template functions are used in the source file, g++ compiles the appropriate definition from the header file and places the compiled function in the corresponding object file.

If a template function is used several times in a program it will be stored in more than one object file.  The GNU Linker ensures that only one copy is placed in the final executable.  Other linkers may report ``multiply defined symbol'' errors when they encounter more than one copy of a template function---a method of working with these linkers is described below.

Explicit template instantiation
-------------------------------

To achieve complete control over the compilation of templates with g++ it is possible to require explicit instantiation of each occurrence of a template, using the option -fno-implicit-templates.  This method is not needed when using the GNU Linker---it is an alternative provided for systems with linkers which cannot eliminate duplicate definitions of template functions in object files.

In this approach, template functions are no longer compiled at the point where they are used, as a result of the -fno-implicit-templates option.  Instead, the compiler looks for an explicit instantiation of the template using the template keyword with a specific type to force its compilation (this is a GNU extension to the standard behavior).  These instantiations are typically placed in a separate source file, which is then compiled to make an object file containing all the template functions required by a program.  This ensures that each template appears in only one object file, and is compatible with linkers which cannot eliminate duplicate definitions in object files.

For example, the following file templates.cc contains an explicit instantiation of the Buffer<float> class used by the program tprog.cc given above:

          [file templates.cc not included]

The whole program can be compiled and linked using explicit instantiation with the following commands:

     $ g++ -Wall -fno-implicit-templates -c tprog.cc
     $ g++ -Wall -fno-implicit-templates -c templates.cc
     $ g++ tprog.o templates.o
     $ ./a.out
     stored value = 1.25

The object code for all the template functions is contained in the file templates.o.  There is no object code for template functions in tprog.o when it is compiled with the -fno-implicit-templates option.

If the program is modified to use additional types, then further explicit instantiations can be added to the file templates.cc. For example, the following code adds instantiations for Buffer objects containing double and int values:

          [file templates2.cc not included]

The disadvantage of explicit instantiation is that it is necessary to know which template types are needed by the program.  For a complicated program this may be difficult to determine in advance.  Any missing template instantiations can be determined at link time, however, and added to the list of explicit instantiations, by noting which functions are undefined.

Explicit instantiation can also be used to make libraries of precompiled template functions, by creating an object file containing all the required instantiations of a template function (as in the file templates.cc above).  For example, the object file created from the template instantiations above contains the machine code needed for Buffer classes with float, double and int types, and could be distributed in a library.

The export keyword
------------------

At the time of writing, GCC does not support the new C++ export keyword (GCC 3.4.4).

This keyword was proposed as a way of separating the interface of templates from their implementation.  However it adds its own complexity to the linking process, which can detract from any advantages in practice.

The export keyword is not widely used, and most other compilers do not support it either.  The inclusion compilation model described earlier is recommended as the simplest and most portable way to use templates.

Platform-specific options
*************************

GCC provides a range of platform-specific options for different types of CPUs.  These options control features such as hardware floating-point modes, and the use of special instructions for different CPUs.  They can be selected with the -m option on the command line, and work with all the GCC language frontends, such as gcc and g++.

The following sections describe some of the options available for common platforms.  A complete list of all platform-specific options can be found in the GCC Reference Manual, ``Using GCC'' (see Further reading).  Support for new processors is added to GCC as they become available, therefore some of the options described in this chapter may not be found in older versions of GCC.

Intel and AMD x86 options
=========================

The features of the widely used Intel and AMD x86 families of processors (386, 486, Pentium, etc) can be controlled with GCC platform-specific options.

On these platforms, GCC produces executable code which is compatible with all the processors in the x86 family by default---going all the way back to the 386.  However, it is also possible to compile for a specific processor to obtain better performance. (Also referred to as ``targeting'' a specific processor.)

For example, recent versions of GCC have specific support for newer processors such as the Pentium 4 and AMD Athlon.  These can be selected with the following option for the Pentium 4,

     $ gcc -Wall -march=pentium4 hello.c

and for the Athlon:

     $ gcc -Wall -march=athlon hello.c

A complete list of supported CPU types can be found in the GCC Reference Manual.

Code produced with a specific -march=CPU option will be faster but will not run on other processors in the x86 family.  If you plan to distribute executable files for general use on Intel and AMD processors they should be compiled without any -march options.

As an alternative, the -mcpu=CPU option provides a compromise between speed and portability---it generates code that is tuned for a specific processor, in terms of instruction scheduling, but does not use any instructions which are not available on other CPUs in the x86 family. (In recent versions of GCC this option has been renamed to -mtune.  The older form -mcpu will continue to work.) The resulting code will be compatible with all the CPUs, and have a speed advantage on the CPU specified by -mcpu.  The executables generated by -mcpu cannot achieve the same performance as -march, but may be more convenient in practice.

x86 extensions
--------------

GCC can take advantage of the additional instructions in the MMX, SSE, SSE2, SSE3 and 3dnow extensions of recent Intel and AMD processors.  The options -mmmx, -msse, -msse2, -msse3 and -m3dnow enable the use of these extra instructions, allowing multiple words of data to be processed in parallel.  The resulting executables will only run on processors supporting the appropriate extensions---on other systems they will crash with an Illegal instruction error (or similar). (On GNU/Linux systems, the command cat /proc/cpuinfo will display information about the CPU.)

The option -mfpmath=sse instructs GCC to use the SSE extensions for floating-point arithmetic where possible.  For this option to take effect, the SSE or SSE2 extensions must first be enabled with -msse or -msse2.

Note that the plain SSE extensions only support single precision operations---double precision arithmetic is part of SSE2.  Since most C and C++ programs declare floating-point variables as double rather than float, the combined options -msse2 -mfpmath=sse are usually needed.  On 64-bit processors these options are enabled by default.

x86 64-bit processors
---------------------

AMD has enhanced the 32-bit x86 instruction set to a 64-bit instruction set called x86-64, which is implemented in their AMD64 processors. (Intel has added support for this instruction set as the ``Intel 64-bit enhancements'' on their Xeon CPUs.)  On AMD64 systems GCC generates 64-bit code by default.  The option -m32 allows 32-bit code to be generated instead.

The AMD64 processor has several different memory models for programs running in 64-bit mode.  The default model is the small code model, which allows code and data up to 2GB in size.  The medium code model allows unlimited data sizes and can be selected with -mcmodel=medium.  There is also a large code model, which supports an unlimited code size in addition to unlimited data size.  It is not currently implemented in GCC since the medium code model is sufficient for all practical purposes---executables with sizes greater than 2GB are not encountered in practice.

A special kernel code model -mcmodel=kernel is provided for system-level code, such as the Linux kernel.  An important point to note is that by default on the AMD64 there is a 128-byte area of memory allocated below the stack pointer for temporary data, referred to as the ``red-zone'', which is not supported by the Linux kernel.  Compilation of the Linux kernel on the AMD64 requires the options -mcmodel=kernel -mno-red-zone.

DEC Alpha options
=================

The DEC Alpha processor has default settings which maximize floating-point performance, at the expense of full support for IEEE arithmetic features.

Support for infinity arithmetic and gradual underflow (denormalized numbers) is not enabled in the default configuration on the DEC Alpha processor. Operations which produce infinities or underflows will generate floating-point exceptions (also known as traps), and cause the program to terminate, unless the operating system catches and handles the exceptions (which is, in general, inefficient).  The IEEE standard specifies that these operations should produce special results to represent the quantities in the IEEE numeric format.

In most cases the DEC Alpha default behavior is acceptable, since the majority of programs do not produce infinities or underflows.  For applications which require these features, GCC provides the option -mieee to enable full support for IEEE arithmetic.

To demonstrate the difference between the two cases the following program divides 1 by 0:

          [file alpha.c not included]

In IEEE arithmetic the result of 1/0 is inf (Infinity). If the program is compiled for the Alpha processor with the default settings it generates an exception, which terminates the program:

     $ gcc -Wall alpha.c
     $ ./a.out
     Floating point exception    (on an Alpha processor)

Using the -mieee option ensures full IEEE compliance -- the division 1/0 correctly produces the result inf and the program continues executing successfully:

     $ gcc -Wall -mieee alpha.c
     $ ./a.out
     x/y = inf

Note that programs which generate floating-point exceptions run more slowly when compiled with -mieee, because the exceptions are handled in software rather than hardware.

SPARC options
=============

On the SPARC range of processors the -mcpu=CPU option generates processor-specific code.  The valid options for CPU are v7, v8 (SuperSPARC), Sparclite, Sparclet and v9 (UltraSPARC).  Code produced with a specific -mcpu option will not run on other processors in the SPARC family, except where supported by the backwards-compatibility of the processor itself.

On 64-bit UltraSPARC systems the options -m32 and -m64 control code generation for 32-bit or 64-bit environments.  The 32-bit environment selected by -m32 uses int, long and pointer types with a size of 32 bits.  The 64-bit environment selected by -m64 uses a 32-bit int type and 64-bit long and pointer types.

POWER/PowerPC options
=====================

On systems using the POWER/PowerPC family of processors the option -mcpu=CPU selects code generation for specific CPU models.  The possible values of CPU include power, power2, powerpc, powerpc64 and common, in addition to other more specific model numbers.  Code generated with the option -mcpu=common will run on any of the processors. The option -maltivec enables use of the Altivec vector processing instructions, if the appropriate hardware support is available.

The POWER/PowerPC processors include a combined ``multiply and add'' instruction a * x + b, which performs the two operations simultaneously for speed---this is referred to as a fused multiply and add, and is used by GCC by default.  Due to differences in the way intermediate values are rounded, the result of a fused instruction may not be exactly the same as performing the two operations separately.  In cases where strict IEEE arithmetic is required, the use of the combined instructions can be disabled with the option -mno-fused-madd.

On AIX systems, the option -mminimal-toc decreases the number of entries GCC puts in the global table of contents (TOC) in executables, to avoid ``TOC overflow'' errors at link time. The option -mxl-call makes the linking of object files from GCC compatible with those from IBM's XL compilers. For applications using POSIX threads, AIX always requires the option -pthread when compiling, even when the program will only run in single-threaded mode.

Multi-architecture support
==========================

A number of platforms can execute code for more than one architecture. For example, 64-bit platforms such as AMD64, MIPS64, Sparc64, and PowerPC64 support the execution of both 32-bit and 64-bit code. Similarly, ARM processors support both ARM code and a more compact code called ``Thumb''.  GCC can be built to support multiple architectures on these platforms.  By default, the compiler will generate 64-bit object files, but giving the -m32 option will generate a 32-bit object file for the corresponding architecture. (The options -maix64 and -maix32 are used on AIX.)

Note that support for multiple architectures depends on the corresponding libraries being available.  On 64-bit platforms supporting both 64 and 32-bit executables, the 64-bit libraries are often placed in lib64 directories instead of lib directories, e.g. in /usr/lib64 and /lib64.  The 32-bit libraries are then found in the default lib directories as on other platforms.  This allows both a 32-bit and a 64-bit library with the same name to exist on the same system.  Other systems, such as the IA64/Itanium, use the directories /usr/lib and /lib for 64-bit libraries.  GCC knows about these paths and uses the appropriate path when compiling 64-bit or 32-bit code.

Floating-point issues
=====================

The IEEE-754 standard defines the bit-level behavior of floating-point arithmetic operations on all modern processors.  This allows numerical programs to be ported between different platforms with identical results, in principle.  In practice, there are often minor variations caused by differences in the order of operations (depending on the compiler and optimization level) but these are generally not significant.

However, more noticeable discrepancies can be seen when porting numerical programs between x86 systems and other platforms, because the the x87 floating point unit (FPU) on x86 processors computes results using extended precision internally (the values being converted to double precision only when they are stored to memory).  In contrast, processors such as SPARC, PA-RISC, Alpha, MIPS and POWER/PowerPC work with native double-precision values throughout. (Motorola 68k processors also use extended precision registers, like the x86.)  The differences between these implementations lead to changes in rounding and underflow/overflow behavior, because intermediate values have a greater relative precision and exponent range when computed in extended precision. (For quantities held in the x87 extended-precision
registers the relative precision is 5.42*10^-20 and the exponent
range is 10^(+/-4932).  Standard double precision values have a
relative precision of 2.22*10^-16 and an exponent range of
10^(+/-308).) In particular, comparisons involving extended precision values may fail where the equivalent double precision values would compare equal.

To avoid these incompatibilities, the x87 FPU also offers a hardware double-precision rounding mode.  In this mode the results of each extended-precision floating-point operation are rounded to double precision in the floating-point registers by the FPU. It is important to note that the rounding only affects the precision, not the exponent range, so the result is a hybrid double-precision format with an extended range of exponents.

On BSD systems such as FreeBSD, NetBSD and OpenBSD, the hardware double-precision rounding mode is the default, giving the greatest compatibility with native double precision platforms.  On x86 GNU/Linux systems the default mode is extended precision (with the aim of providing increased accuracy).  To enable the double-precision rounding mode it is necessary to override the default setting on per-process basis using the FLDCW ``floating-point load control-word'' machine instruction. (The operating system saves and restores the control word when switching between processes, so that each process maintains its own setting.)  A simple function which can be called to execute this instruction is shown below.  It uses the GCC extension keyword asm to insert the specified instruction in the assembly language output:

          [file setfpu.c not included]

The appropriate mode setting for double-precision rounding is 0x27F.  The mode value also controls the floating-point exception handling behavior and rounding-direction (see the Intel and AMD processor reference manuals for details).

On x86 GNU/Linux, the function above can be called at the start of any program to disable excess precision.  This will then reproduce the results of native double-precision processors, in the absence of underflows and overflows.

The following program demonstrates the different rounding modes:

          [file fptest2.c not included]

On x86 GNU/Linux systems the comparison c == a / b can produce an unexpected result if c is taken from memory (double precision) while a / b is computed in extended precision, because the fraction 3/7 has different representations in double and extended precision.

     $ gcc -Wall fptest2.c
     $ ./a.out
     unexpecedt result

Setting the hardware rounding mode to double precision prevents this from happening:

     $ gcc -Wall -DDOUBLE fptest2.c
     $ ./a.out
     comparison succeeds

Note that the floating-point control word affects the whole environment of the process, including any C Library functions that are called.  One consequence of this is that long double arithmetic is effectively reduced to double precision, since it relies on extended precision operations.

The floating point control word only affects the behavior of the x87 FPU.  Floating point operations computed with SSE and SSE2 instructions are always carried out in native double precision.  Thus, the combined options

     $ gcc -Wall -msse2 -mfpmath=sse  ...

are often sufficient to remove the effects of extended-precision. However, some operations (such as transcendental functions) are not available in the SSE/SSE2 extensions and will still be computed on the x87 FPU.

Portability of signed and unsigned types
========================================

The C and C++ standards allows the character type char to be signed or unsigned, depending on the platform and compiler.  Most systems, including x86 GNU/Linux and Microsoft Windows, use signed char, but those based on PowerPC and ARM processors typically use unsigned char. (MacOS X (Darwin) on PowerPC uses signed char, for consistency with other Darwin architectures.)  This can lead to unexpected results when porting programs between platforms which have different defaults for the type of char.

The following code demonstrates the difference between platforms with signed and unsigned char types:

          [file signed.c not included]

With an unsigned char, the variable c takes the value 255, but with a signed char it becomes -1.

The correct way to manipulate char variables in C is through the portable functions declared in ctype.h, such as isalpha, isdigit and isblank, rather than by their numerical values.  The behavior of non-portable conditional expressions such as c > 'a' depends on the signedness of the char type.  If the signed or unsigned version of char is explicitly required at certain points in a program, it can be specified using the declarations signed char or unsigned char.

For existing programs which assume that char is signed or unsigned, GCC provides the options -fsigned-char and -funsigned-char to set the default type of char.  Using these options, the example code above compiles cleanly with -Wall -W when char is unsigned:

     $ gcc -Wall -W -funsigned-char signed.c
     $ ./a.out
     char is unsigned (c = 255)

However, when char is signed the value 255 wraps around to -1, giving a warning when compiled with -Wall -W:

     $ gcc -Wall -W -fsigned-char signed.c
     signed.c: In function `main':
     signed.c:7: warning: comparison is always false due to
       limited range of data type
     $ ./a.out
     char is signed (c = -1)

The warning message ``comparison is always true/false due to limited range of data type'' is one symptom of code which assumes a definition of char which is different from the actual type.

The most common problem with code written assuming signed char types occurs with the functions getc, fgetc and getchar (which read a character from a file).  They have a return type of int, not char, and this allows them to use the special value -1 (defined as EOF) to indicate an end-of-file error.  Unfortunately, many programs have been written which incorrectly store this return value straight into a char variable.  Here is a typical example:

          [file testgetc.c not included]

This only works on platforms which default to a signed char type. (There is also a subtle error even on platforms with signed char---the ASCII character 255 is spuriously interpreted as an end of file condition.) On platforms which use an unsigned char the same code will fail, because the value -1 becomes 255 when stored in an unsigned char.  This usually causes an infinite loop because the end of the file cannot be recognized. (If displayed, character code 255 often appears as ÿ.) To be portable, the program should test the return value as an integer before coercing it to a char, as follows:

          [file testgetc2.c not included]

The same considerations described in this section apply to the definitions of bitfields in structs, which can be signed or unsigned by default.  In GCC, the default type of bitfields can be controlled using the options -fsigned-bitfields and -funsigned-bitfields.

Troubleshooting
***************

GCC provides several help and diagnostic options to assist in troubleshooting problems with the compilation process.  All the options described in this chapter work with both gcc and g++.

Help for command-line options
=============================

To obtain a brief reminder of various command-line options, GCC provides a help option which displays a summary of the top-level GCC command-line options:

     $ gcc --help

To display a complete list of options for gcc and its associated programs, such as the GNU Linker and GNU Assembler, use the help option above with the verbose (-v) option:

     $ gcc -v --help

The complete list of options produced by this command is extremely long---you may wish to page through it using the more command, or redirect the output to a file for reference:

     $ gcc -v --help 2>&1 | more

Version numbers
===============

You can find the version number of gcc using the version option:

     $ gcc --version
     gcc (Debian 4.6.1-4) 4.6.1

The version number is important when investigating compilation problems, since older versions of GCC may be missing some features that a program uses.  The version number has the form MAJOR-VERSION.MINOR-VERSION or MAJOR-VERSION.MINOR-VERSION.MICRO-VERSION, where the additional third ``micro'' version number (as shown above) is used for subsequent bug-fix releases in a release series.

More details about the version can be found using -v:

     $ gcc -v
     Using built-in specs.
     COLLECT_GCC=gcc
     COLLECT_LTO_WRAPPER=/usr/lib/i386-linux-gnu/gcc/
       /i486-linux-gnu/4.6.1/lto-wrapper
     Target: i486-linux-gnu
     Configured with: ../src/configure -v
     --with-pkgversion='Debian 4.6.1-4'
     --with-bugurl=file:///usr/share/doc/gcc-4.6/
       /README.Bugs
     --enable-languages=c,c++,fortran,objc,obj-c++,go
     --prefix=/usr --program-suffix=-4.6
     --enable-shared --enable-multiarch
     --with-multiarch-defaults=i386-linux-gnu
     --enable-linker-build-id --with-system-zlib
     --libexecdir=/usr/lib/i386-linux-gnu
     --without-included-gettext --enable-threads=posix
     --with-gxx-include-dir=/usr/include/c++/4.6
     --libdir=/usr/lib/i386-linux-gnu --enable-nls
     --enable-clocale=gnu --enable-libstdcxx-debug
     --enable-libstdcxx-time=yes --enable-plugin
     --enable-objc-gc --enable-targets=all
     --with-arch-32=i586 --with-tune=generic
     --enable-checking=release --build=i486-linux-gnu
     --host=i486-linux-gnu --target=i486-linux-gnu
     Thread model: posix
     gcc version 4.6.1 (Debian 4.6.1-4)

This includes information on the build flags of the compiler itself and the installed configuration file, specs.

Verbose compilation
===================

The -v option can also be used to display detailed information about the exact sequence of commands used to compile and link a program. Here is an example which shows the verbose compilation of the Hello World program:

     $ gcc -v -Wall hello.c Using built-in specs.
     COLLECT_GCC=gcc
     COLLECT_LTO_WRAPPER=/usr/lib/i386-linux-gnu/gcc/
       /i486-linux-gnu/4.6.1/lto-wrapper
     Target: i486-linux-gnu
     Configured with: ../src/configure
     -v --with-pkgversion='Debian 4.6.1-4'
     --with-bugurl=file:///usr/share/doc/gcc-4.6/
       /README.Bugs
     --enable-la nguages=c,c++,fortran,objc,obj-c++,go
     --prefix=/usr --program-suffix=-4.6 --enable-shared
     --enable-multiarch
     --with-multiarch-defaults=i386-linux-gnu
     --enable-linker-build-id --with-system-zlib
     --libexecdir=/usr/lib/i386-linux-gnu
     --without-included-gettext --enable-threads=posix
     --with-gxx-include-dir=/usr/include/c++/4.6
     --libdir=/usr/lib/i386-linux-gnu --enable-nls
     --enable-clocale=gnu --enable-libstdcxx-debug
     --enable-libstdcxx-time=yes --enable-plugin
     --enable-objc-gc --enable-targets=all
     --with-arch-32=i586 --with-tune=generic
     --enable-checking =release --build=i486-linux-gnu
     --host=i486-linux-gnu --target=i486-linux-gnu
     Thread model: posix gcc version 4.6.1
       (Debian 4.6.1-4)
     COLLECT_GCC_OPTIONS='-v' '-Wall' '-mtune=generic'
     '-march=i586'
     /usr/lib/i386-linux-gnu/gcc/i486-linux-gnu/4.6.1/cc1
     -quiet -v hello.c -quiet -dumpbase hello.c
     -mtune=generic -march=i586 -auxbase hello -Wall -version
     -o /tmp/cchpM0t3.s
     GNU C (Debian 4.6.1-4) version 4.6.1
     (i486-linux-gnu) compiled by GNU C version 4.6.1, GMP
     version 5.0.1, MPFR version 3.0.1-p3, MPC version 0.9
     warning: GMP header version 5.0.1 differs from library
     version 5.0.2.
     GGC heuristics: --param ggc-min-expand=46
     --param ggc-min-heapsize=31802
     ignoring nonexistent directory
     "/usr/local/include/i386-linux-gnu"
     ignoring nonexistent directory
     "/usr/lib/i386-linux-gnu/gcc/i486-linux-gnu/4.6.1/../
       /../../../../i486-linux-gnu/include"
     #include "..." search starts here:
     #include <...> search starts here:
     /usr/lib/i386-linux-gnu/gcc/i486-linux-gnu/4.6.1/include
     /usr/local/include
     /usr/lib/i386-linux-gnu/gcc/i486-linux-gnu/4.6.1/
       /include-fixed
     /usr/include/i386-linux-gnu /usr/include
     End of searchlist.
     GNU C (Debian 4.6.1-4) version 4.6.1
     (i486-linux-gnu) compiled by GNU C version 4.6.1, GMP
     version 5.0.1, MPFR version 3.0.1-p3, MPC version 0.9
     warning: GMP header version 5.0.1 differs from library
     version 5.0.2.
     GGC heuristics: --param ggc-min-expand=46
     --param ggc-min-heapsize=31802
     Compiler executable checksum:
     2cfae8623c84fd817bfff483158c4341
     COLLECT_GCC_OPTIONS='-v' '-Wall' '-mtune=generic'
     '-march=i586' as --32 -o /tmp/ccQxW8a5.o /tmp/cchpM0t3.s
     COMPILER_PATH=/usr/lib/i386-linux-gnu/gcc/
       /i486-linux-gnu/4.6.1/:
     /usr/lib/i386-linux-gnu/gcc/i486-linux-gnu/4.6.1/:
     /usr/lib/i386-linux-gnu/gcc/i486-linux-gnu/:
     /usr/lib/i386-linux-gnu/gcc/i486-linux-gnu/4.6.1/:
     /usr/lib/i386-linux-gnu/gcc/i486-linux-gnu/
     LIBRARY_PATH=/usr/lib/i386-linux-gnu/gcc/
       /i486-linux-gnu/4.6.1/:
     /usr/lib/i386-linux-gnu/gcc/i486-linux-gnu/4.6.1/
       /../../../:
     /lib/:/usr/lib/:/usr/lib/i386-linux-gnu/
     COLLECT_GCC_OPTIONS='-v' '-Wall' '-mtune=generic'
     '-march=i586'
     /usr/lib/i386-linux-gnu/gcc/i486-linux-gnu/4.6.1/
       /collect2
     --build-id --no-add-needed --eh-frame-hdr -m elf_i386
     --hash-style=both -dynamic -linker /lib/ld-linux.so.2
     /usr/lib/i386-linux-gnu/gcc/i486-linux-gnu/4.6.1/
       /../../../crt1.o
     /usr/lib/i386-linux-gnu/gcc/i486-linux-gnu/4.6.1/
       /../../../crti.o
     /usr/lib/i386-linux-gnu/gcc/i486-linux-gnu/4.6.1/
       /crtbegin.o
     -L/usr/lib/i386-linux-gnu/gcc/i486-linux-gnu/4.6.1
     -L/usr/lib/i386-linux-gnu/gcc/i486-linux-gnu/4.6.1/
       /../../..
     -L/usr/lib/i386-linux-gnu/tmp/ccQxW8a5.o
     -lgcc --as-needed -lgcc_s --no-as-needed
     -lc -l gcc --as-needed -lgcc_s --no-as-needed
     /usr/lib/i386-linux-gnu/gcc/i486-linux-gnu/4.6.1/
       /crtend.o
     /usr/lib/i386-linux-gnu/gcc/i486-linux-gnu/4.6.1/
       /../../../crtn.o

The output produced by -v can be useful whenever there is a problem with the compilation process itself.  It displays the full directory paths used to search for header files and libraries, the predefined preprocessor symbols, and the object files and libraries used for linking.

Stopping a program in an infinite loop
======================================

A program which goes into an infinite loop or ``hangs'' can be difficult to debug.  On most systems a foreground process can be stopped by hitting Control-C, which sends it an interrupt signal (SIGINT). However, this does not help in debugging the problem---the SIGINT signal terminates the process without producing a core dump.  A more sophisticated approach is to attach to the running process with a debugger and inspect it interactively.

For example, here is a simple program with an infinite loop:

          [file loop.c not included]

In order to attach to the program and debug it, the code should be compiled with the debugging option -g:

     $ gcc -Wall -g loop.c
     $ ./a.out
     (program hangs)

Once the executable is running we need to find its process id (PID).  This can be done from another session with the command ps x:

     $ ps x
     PID   TTY    STAT TIME COMMAND
     ...  .....    ..  ....
     2333 pts/0    R+  0:16 ./a.out

In this case the process id is 2333, and we can now attach to it with gdb.  The debugger should be started in the directory containing the executable and its source code: (Alternatively, the appropriate paths can be set up in gdb using the file and directory commands.)

     $ gdb a.out
     GNU gdb (GDB) 7.3-debian
     ...
     (gdb) attach 2333
     Attaching to program: a.out, process 2333
     Reading symbols from
     /lib/i386-linux-gnu/i686/cmov/libc.so.6...(no
     debugging symbols found)...done.
     Loaded symbols for
     /lib/i386-linux-gnu/i686/cmov/libc.so.6
     Reading symbols from /lib/ld-linux.so.2...(no debugging
     symbols found)...done.
     Loaded symbols for /lib/ld-linux.so.2
     0x080483c5 in main () at loop.c:5
     5         while (1) { i++; };
     (gdb)

The output shows the line that was about to execute at the point when the debugger attached to the process.  The attached program is paused but still ``live''---it can be examined interactively and continued or terminated (with the kill command) if necessary:

     (gdb) print i
     $1 = 1960534048
     (gdb) kill
     Kill the program being debugged? (y or n) y
     (gdb)

If you want to stop a process immediately and create a core dump, the shell command kill -3 PID (where PID is the process id) will send it a SIGQUIT signal.  The SIGQUIT signal does trigger a core dump, unlike SIGINT.  Note that if core dumps were disabled when the process was started, no core file will be produced (see Examining core files).

Preventing excessive memory usage
=================================

Sometimes a programming error will cause a process to allocate huge amounts of memory, consuming all the RAM on a system.  To prevent this, the GNU Bash command ulimit -v LIMIT can be used to restrict the amount of virtual memory available to each process.  The limit is measured in kilobytes and applies to new processes started in the current shell.  For example,

     $ ulimit -v 4096

will limit subsequent processes to 4 megabytes of virtual memory (4096k).  By default the limit cannot be increased in the same session once it has been applied, so it is best to start a separate shell for reduced ulimit operations.  Alternatively, you can set a soft limit (which can be undone) with the options -S -v.

In addition to preventing run-away processes, limiting the amount of memory a program is allowed to allocate also provides a way to test how robustly out of memory conditions are handled.  An artificially low limit can be used to simulate running out of memory---a well-written program should not crash in this case.

The ulimit command supports other options including -p, which restricts the number of child processes that can be created, and -t, which places a limit on the number of CPU seconds that a process can run for.  The complete list of settings can be shown with the command ulimit -a.  To display more information about the ulimit command, type help ulimit at the Bash prompt.

Compiler-related tools
**********************

This chapter describes a number of tools which are useful in combination with GCC.  These include the GNU archiver ar, for creating libraries, and the GNU profiling and coverage testing programs, gprof and gcov.

Creating a library with the GNU archiver
========================================

The GNU archiver ar combines a collection of object files into a single archive file, also known as a library.  An archive file is simply a convenient way of distributing a large number of related object files together (as described earlier in Linking with external libraries).

To demonstrate the use of the GNU archiver we will create a small library libhello.a containing two functions hello and bye.

The first object file will be generated from the source code for the hello function, in the file hello_fn.c seen earlier:

          [file hello_fn.c not included]

The second object file will be generated from the source file bye_fn.c, which contains the new function bye:

          [file bye_fn.c not included]

Both functions use the header file hello.h, now with a prototype for the function bye():

          [file hello.h not included]

The source code can be compiled to the object files hello_fn.o and bye_fn.o using the commands:

     $ gcc -Wall -c hello_fn.c
     $ gcc -Wall -c bye_fn.c

These object files can be combined into a static library using the following command line:

     $ ar cr libhello.a hello_fn.o bye_fn.o

The option cr stands for ``create and replace''. (Note that ar does not require a prefix - for its options.)  If the library does not exist, it is first created.  If the library already exists, any original files in it with the same names are replaced by the new files specified on the command line.  The first argument libhello.a is the name of the library.  The remaining arguments are the names of the object files to be copied into the library.

The archiver ar also provides a ``table of contents'' option t to list the object files in an existing library:

     $ ar t libhello.a
     hello_fn.o
     bye_fn.o

Note that when a library is distributed, the header files for the public functions and variables it provides should also be made available, so that the end-user can include them and obtain the correct prototypes.

We can now write a program using the functions in the newly created library:

          [file main3.c not included]

This file can be compiled with the following command line, as described in Linking with external libraries, assuming the library libhello.a is stored in the current directory:

     $ gcc -Wall main3.c libhello.a -o hello

The main program is linked against the object files found in the library file libhello.a to produce the final executable.

The short-cut library linking option -l can also be used to link the program, without needing to specify the full filename of the library explicitly:

     $ gcc -Wall -L. main3.c -lhello -o hello

The option -L. is needed to add the current directory to the library search path.  The resulting executable can be run as usual:

     $ ./hello
     Hello, everyone!
     Goodbye!

It displays the output from both the hello and bye functions defined in the library.

Using the profiler gprof
========================

The GNU profiler gprof is a useful tool for measuring the performance of a program---it records the number of calls to each function and the amount of time spent there, on a per-function basis. Functions which consume a large fraction of the run-time can be identified easily from the output of gprof.  Efforts to speed up a program should concentrate first on those functions which dominate the total run-time.

We will use gprof to examine the performance of a small numerical program which computes the lengths of sequences occurring in the unsolved Collatz conjecture in mathematics. (American Mathematical Monthly, Volume 92 (1985), 3--23) The Collatz conjecture involves sequences defined by the rule:

     x_{n+1}  <=    x_{n} / 2  if x_{n} is even
                  3 x_{n} + 1  if x_{n} is odd

The sequence is iterated from an initial value x_0 until it terminates with the value 1.  According to the conjecture, all sequences do terminate eventually---the program below displays the longest sequences as x_0 increases.  The source file collatz.c contains three functions: main, nseq and step:

          [file collatz.c not included]

To use profiling, the program must be compiled and linked with the -pg profiling option:

     $ gcc -Wall -c -pg collatz.c
     $ gcc -Wall -pg collatz.o

This creates an instrumented executable which contains additional instructions that record the time spent in each function.

If the program consists of more than one source file then the -pg option should be used when compiling each source file, and used again when linking the object files to create the final executable (as shown above).  Forgetting to link with the option -pg is a common error, which prevents profiling from recording any useful information.

The executable must be run to create the profiling data:

     $ ./a.out
     (normal program output is displayed)

While running the instrumented executable, profiling data is silently written to a file gmon.out in the current directory.  It can be analyzed with gprof by giving the name of the executable as an argument:

     $ gprof a.out
     Flat profile:
     Each sample counts as 0.01 seconds.
      %     cumul.    self             self   total
     time  seconds seconds    calls us/call us/call name
     68.59    2.14    2.14 62135400    0.03    0.03 step
     31.09    3.11    0.97   499999    1.94    6.22 nseq
      0.32    3.12    0.01                          main

The first column of the data shows that the program spends most of its time (almost 70%) in the function step, and 30% in nseq. Consequently efforts to decrease the run-time of the program should concentrate on the former.  In comparison, the time spent within the main function itself is completely negligible (less than 1%).

The other columns in the output provide information on the total number of function calls made, and the time spent in each function.  Additional output breaking down the run-time further is also produced by gprof but not shown here.  Full details can be found in the manual ``GNU gprof---The GNU Profiler'', by Jay Fenlason and Richard Stallman.

Coverage testing with gcov
==========================

The GNU coverage testing tool gcov analyses the number of times each line of a program is executed during a run.  This makes it possible to find areas of the code which are not used, or which are not exercised in testing.  When combined with profiling information from gprof the information from coverage testing allows efforts to speed up a program to be concentrated on specific lines of the source code.

We will use the example program below to demonstrate gcov.  This program loops overs the integers 1 to 9 and tests their divisibility with the modulus (%) operator.

          [file cov.c not included]

To enable coverage testing the program must be compiled with the following options:

     $ gcc -Wall -fprofile-arcs -ftest-coverage cov.c

This creates an instrumented executable which contains additional instructions that record the number of times each line of the program is executed.  The option -ftest-coverage adds instructions for counting the number of times individual lines are executed, while -fprofile-arcs incorporates instrumentation code for each branch of the program.  Branch instrumentation records how frequently different paths are taken through if statements and other conditionals.  The executable must then be run to create the coverage data:

     $ ./a.out
     3 is divisible by 3
     6 is divisible by 3
     9 is divisible by 3

The data from the run is written to several files with the extensions .bb .bbg and .da respectively in the current directory.  This data can be analyzed using the gcov command and the name of a source file:

     $ gcov cov.c
      88.89% of 9 source lines executed in file cov.c
     Creating cov.c.gcov

The gcov command produces an annotated version of the original source file, with the file extension .gcov, containing counts of the number of times each line was executed:

          [file cov_c_gcov not included]

The line counts can be seen in the first column of the output.  Lines
which were not executed are marked with hashes ######.  The
command grep '######' *.gcov can be used to find parts of a
program which have not been used.

How the compiler works
**********************

This chapter describes in more detail how GCC transforms source files to an executable file.  Compilation is a multi-stage process involving several tools, including the GNU Compiler itself (through the gcc or g++ frontends), the GNU Assembler as, and the GNU Linker ld.  The complete set of tools used in the compilation process is referred to as a toolchain.

An overview of the compilation process
======================================

The sequence of commands executed by a single invocation of GCC consists of the following stages:

  * preprocessing (to expand macros)

  * compilation  (from source code to assembly language)

  * assembly  (from assembly language to machine code)

  * linking  (to create the final executable)

As an example, we will examine these compilation stages individually using the Hello World program hello.c:

          [file hello.c not included]

Note that it is not necessary to use any of the individual commands described in this section to compile a program.  All the commands are executed automatically and transparently by GCC internally, and can be seen using the -v option described earlier (see Verbose compilation).  The purpose of this chapter is to provide an understanding of how the compiler works.

Although the Hello World program is very simple it uses external header files and libraries, and so exercises all the major steps of the compilation process.

The preprocessor
================

The first stage of the compilation process is the use of the preprocessor to expand macros and included header files.  To perform this stage, GCC executes the following command: (As mentioned earlier, the preprocessor is integrated into the compiler in recent versions of GCC.  Conceptually, the compilation process is the same as running the preprocessor as separate application.)

     $ cpp hello.c > hello.i

The result is a file hello.i which contains the source code with all macros expanded.  By convention, preprocessed files are given the file extension .i for C programs and .ii for C++ programs. In practice, the preprocessed file is not saved to disk unless the -save-temps option is used.

The compiler
============

The next stage of the process is the actual compilation of preprocessed source code to assembly language, for a specific processor.  The command-line option -S instructs gcc to convert the preprocessed C source code to assembly language without creating an object file:

     $ gcc -Wall -S hello.i

The resulting assembly language is stored in the file hello.s. Here is what the Hello World assembly language for an Intel x86 (i686) processor looks like:

     $ cat hello.s
             .file   "hello.c"
             .section        .rodata
     .LC0:
             .string "Hello, world!"
             .text
             .globl  main
             .type   main, @function
     main:
     .LFB0:
             .cfi_startproc
             pushl   %ebp
             .cfi_def_cfa_offset 8
             .cfi_offset 5, -8
             movl    %esp, %ebp
             .cfi_def_cfa_register 5
             andl    $-16, %esp
             subl    $16, %esp
             movl    $.LC0, (%esp)
             call    puts
             movl    $0, %eax
             leave
             .cfi_restore 5
             .cfi_def_cfa 4, 4
             ret
             .cfi_endproc
     .LFE0:
             .size   main, .-main
             .ident  "GCC: (Debian 4.6.1-4) 4.6.1"
             .section        .note.GNU-stack,"",@progbits

Note that the assembly language contains a call to the external function puts, a simpler version of printf for strings which do not contain formatting characters.

The assembler
=============

The purpose of the assembler is to convert assembly language into machine code and generate an object file.  When there are calls to external functions in the assembly source file, the assembler leaves the addresses of the external functions undefined, to be filled in later by the linker.  The assembler can be invoked with the following command line:

     $ as hello.s -o hello.o

As with GCC, the output file is specified with the -o option. The resulting file hello.o contains the machine instructions for the Hello World program, with an undefined reference to puts.

The linker
==========

The final stage of compilation is the linking of object files to create an executable.  In practice, an executable requires many external functions from system and C run-time (crt) libraries. Consequently, the actual link commands used internally by GCC are complicated.  For example, the full command for linking the Hello World program is typically something like this: (The precise command varies depending on the version of gcc and operating system.  Recent versions of gcc use a wrapper called collect2 which does additional work before calling ld.)

     $ ld -dynamic-linker /lib/ld-linux.so.2 /usr/lib/crt1.o /usr/lib/crti.o
     /usr/lib/gcc/i486-linux-gnu/4.3/crtbegin.o -L/usr/lib/gcc/i486-linux-gnu/4.3
     hello.o -lgcc -lgcc_eh -lc /usr/lib/gcc/i486-linux-gnu/4.3/crtend.o
     /usr/lib/crtn.o

Fortunately there is never any need to type the command above directly---the entire linking process is handled transparently by gcc when invoked as follows:

     $ gcc hello.o

This links the object file hello.o to the C standard library, and produces an executable file a.out:

     $ ./a.out
     Hello, world!

An object file for a C++ program can be linked to the C++ standard library in the same way with a single g++ command.

Examining compiled files
************************

This chapter describes several useful tools for examining the contents of executable files and object files.

Identifying files
=================

When a source file has been compiled to an object file or executable the options used to compile it are no longer obvious.  The file command looks at the contents of an object file or executable and determines some of its characteristics, such as whether it was compiled with dynamic or static linking.

For example, here is the result of the file command for a typical executable:

     $ file a.out
     a.out: ELF 32-bit LSB executable, Intel 80386,
       version 1 (SYSV), dynamically linked (uses shared
       libs), not stripped

The output shows that the executable file is dynamically linked, and compiled for the Intel 386 and compatible processors.  A full explanation of the output is shown below:

ELF
     The internal format of the executable file (ELF stands for ``Executable and Linking Format'', other formats such as COFF ``Common Object File Format'' are used on some older operating systems (e.g. MS-DOS)).

32-bit
     The word size (for some platforms this would be 64-bit).

LSB
     Compiled for a platform with least significant byte first word-ordering, such as Intel and AMD x86 processors (the alternative MSB most significant byte first is used by other processors, such as the Motorola 680x0) (The MSB and LSB orderings are also known as big-endian and little-endian respectively (the terms originate from Jonathan Swift's satire ``Gulliver's Travels'', 1727).).  Some processors such as Itanium and MIPS support both LSB and MSB orderings.

Intel 80386
     The processor the executable file was compiled for.

version 1 (SYSV)
     This is the version of the internal format of the file.

dynamically linked
     The executable uses shared libraries (statically linked indicates programs linked statically, for example using the -static option)

not stripped
     The executable contains a symbol table (this can be removed with the strip command).

The file command can also be used on object files, where it gives similar output.  The POSIX standard (POSIX.1 (2003 edition), IEEE Std 1003.1-2003.) for Unix systems defines the behavior of the file command.

Examining the symbol table
==========================

As described earlier in the discussion of debugging, executables and object files can contain a symbol table (see Compiling for debugging).  This table stores the location of functions and variables by name, and can be displayed with the nm command:

     $ nm a.out
     08049534 d _DYNAMIC
     08049608 d _GLOBAL_OFFSET_TABLE_
     ........
     080483e4 T main
              U puts@GLIBC_2.0

Among the contents of the symbol table, the output shows that the start of the main function has the hexadecimal offset 080483e4. Most of the symbols are for internal use by the compiler and operating system.  A T in the second column indicates a function that is defined in the object file, while a U indicates a function which is undefined (and should be resolved by linking against another object file).  A complete explanation of the output of nm can be found in the GNU Binutils manual.

The most common use of the nm command is to check whether a library contains the definition of a specific function, by looking for a T entry in the second column against the function name.

Finding dynamically linked libraries
====================================

When a program has been compiled using shared libraries it needs to load those libraries dynamically at run-time in order to call external functions.  The command ldd examines an executable and displays a list of the shared libraries that it needs.  These libraries are referred to as the shared library dependencies of the executable.

For example, the following commands demonstrate how to find the shared library dependencies of the Hello World program:

     $ gcc -Wall hello.c
     $ ldd a.out
     libc.so.6 => /lib/libc.so.6 (0x40020000)
     /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)

The output shows that the Hello World program depends on the C library libc (shared library version 6) and the dynamic loader library ld-linux (shared library version 2).

If the program uses external libraries, such as the math library, these are also displayed.  For example, the calc program (which uses the sqrt function) generates the following output:

     $ gcc -Wall calc.c -lm -o calc
     $ ldd calc
     libm.so.6 => /lib/libm.so.6 (0x40020000)
     libc.so.6 => /lib/libc.so.6 (0x40041000)
     /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)

The first line shows that this program depends on the math library libm (shared library version 6), in addition to the C library and dynamic loader library.

The ldd command can also be used to examine shared libraries themselves, in order to follow a chain of shared library dependencies.

Common error messages
*********************

This chapter describes the most frequent error and warning messages produced by gcc and g++.  Each case is accompanied by a description of the causes, an example and suggestions of possible solutions.  Note that error messages may vary slightly between different versions of the compiler.  Some messages may only be displayed with options such as -Wall and -W.

Preprocessor error messages
===========================

No such file or directory

     This error occurs if GCC cannot find a requested file on its search path.  The file may have been specified on the command-line, or with a preprocessor #include statement.  Either the filename has been spelled incorrectly or the directory for the file needs to be added to the include path or link path.

     Example:

               [file msg-file.c not included]

     The program above tries to include the non-existent file stdoi.h giving the error stdoi.h: No such file or directory.  The correct filename should be stdio.h.

macro or '#include' recursion too deep
#include nested too deeply

     This error occurs if the preprocessor encounters too many nested #include directives.  It is usually caused by two or more files trying to include each other, leading to an infinite recursion.

     Example:

               [file msg-nest1.h not included]

               [file msg-nest2.h not included]

     The solution to this problem is to ensure that files do not mutually include each other, or to use ``include guards'' (see Providing your own templates for an example).

invalid preprocessing directive #...

     This error indicates that the preprocessor encountered an unrecognized # command.

     Example:

               [file msg-invalidpp.c not included]

     The preprocessor syntax requires #elif for the ``else if'' condition in #if blocks, rather than #elseif.  In the example above an invalid directive error occurs at the incorrect usage #elseif, but only when FOO is defined (otherwise the preprocessor ignores everything up to the #else statement).

Compiler error messages
=======================

`VARIABLE' undeclared (first use in this function)

     In C and C++ variables must be declared before they can be used.  This error message indicates that the compiler has encountered a variable name which does not have a corresponding declaration.  It can be caused by a missing declaration, or a typing error in the name.  Variable names are case-sensitive, so foo and Foo represent different variables.  To keep the output short, only the first use of an undeclared variable is reported.

     Example:

               [file msg-undeclared.c not included]

     The variable j is not declared and will trigger the error `j' undeclared.

parse error before `...'
expected ';' before `...'
syntax error

     These error messages occur when the compiler encounters unexpected input, i.e. sequences of characters which do not follow the syntax of the language.  The error messages can be triggered by a missing close bracket, brace or semicolon preceding the line of the error, or an invalid keyword.

     Example:

               [file msg-parse.c not included]

     There is a missing semicolon after the first call to printf, giving a parse error.

parse error at end of input
expected declaration or statement at end of input

     This error occurs if the compiler encounters the end of a file unexpectedly, such as when it has parsed an unbalanced number of opening and closing braces.  It is often caused by a missing closing brace somewhere.

     Example:

               [file msg-eoi.c not included]

     An additional closing brace is needed in this program to prevent the error expected declaration or statement at end of input.

warning: implicit declaration of function `...'
warning: incompatible implicit declaration of built-in function `...'

     This warning is generated when a function is used without a prototype being declared.  It can be caused by failing to include a header file, or otherwise forgetting to provide a function prototype.

     Example:

               [file msg-implicitdecl.c not included]

     The system header file stdio.h is not included, so the prototype for printf is not declared.  The program needs an initial line #include <stdio.h>.

unterminated string or character constant
missing terminating " character

     This error is caused by an opening string or character quote which does not have a corresponding closing quote.  Quotes must occur in matching pairs, either single quotes 'a' for characters or double quotes "aaa" for strings.

     Example:

               [file msg-unterm.c not included]

     The opening quote for the string in this program does not have a corresponding closing quote, so the compiler will read the rest of the file as part of the string.

character constant too long

     In C and C++ character codes are written using single quotes, e.g. 'a' gives the ASCII code for the letter a (67), and '\n' gives the ASCII code for newline (10).  This error occurs if single quotes are used to enclose more than one character.

     Example:

               [file msg-string2.c not included]

     The program above confuses single-quotes and double-quotes.  A sequence of characters should be written with double quotes, e.g. "Hello World!".  This same problem occurs in the following C++ program,

               [file msg-string2.cc not included]

     This error can also occur if the forward slash and backslash are confused in an escape sequence, e.g. using '/n' instead of '\n'.  The sequence /n consists of two separate characters, / and n.

     Note that according to the C standard there is no limit on the length of a character constant, but the value of a character constant that contains more than one character is implementation-defined.  Recent versions of GCC provide support multi-byte character constants, and instead of an error the warnings multiple-character character constant or warning: character constant too long for its type are generated in this case.

warning: initialization makes integer from pointer without a cast

     This error indicates a misuse of a pointer in an integer context. Technically, it is possible to convert between integer and pointer types, but this is rarely needed outside system-level applications. More often, this warning is the result of using a pointer without dereferencing it (e.g.  writing int i = p instead of int i = *p).

     This warning can also occur with char and char * types, since char is an integer type.

     Example:

               [file msg-char.c not included]

     The variable c has type char, while the string "\n" evaluates to a const char * pointer (to a 2-byte region of memory containing the ASCII value for newline followed by a zero byte '\0', since strings are null-terminated).  The ASCII code for newline can be found using char c = '\n';

     Similar errors can occur with misuse of the macro NULL,

               [file msg-null.c not included]

     In C, the macro NULL is defined as ((void *)0) in stdlib.h and should only be used in a pointer context.

dereferencing pointer to incomplete type

     This error occurs when a program attempts to access the elements of struct through a pointer without the layout of the struct being declared first.  In C and C++ it is possible to declare pointers to structs before declaring their struct layout, provided the pointers are not dereferenced---this is known as forward declaration.

     Example:

               [file msg-derefincomplete.c not included]

     This program has a forward declaration of the btree struct data.  However, the definition of the struct is needed before the pointer can be dereferenced to access individual members.

warning: unknown escape sequence `...'

     This error is caused by an incorrect use of the escape character in a string.  Valid escape sequences are:

     \n   newline
     \t   tab

     \b   backspace
     \r   carriage return

     \f   form feed
     \v   vertical tab

     \a   alert (bell)

     The combinations \\, \', \" and \? can be used for individual characters.  Escape sequences can also use octal codes \0--\377 and hexadecimal codes \0x00--\0xFF.

     Example:

               [file msg-unknownesc.c not included]

     The escape sequence \N in the program above is invalid---the correct escape sequence for a newline is \n.

warning: suggest parentheses around assignment used as truth value

     This warning highlights a potentially serious error, using the assignment operator = instead of the comparison operator == in the test of a conditional statement or other logical expression.  While the assignment operator can be used as part of a logical value, this is rarely the intended behavior.

     Example:

               [file msg-assign.c not included]

     The test above should be written as if (i == 1), otherwise the variable i will be set to 1 by the evaluation of the if statement itself.  The operator = both assigns and returns the value of its right-hand side, causing the variable i to be modified and the unexpected branch taken.  Similar unexpected results occur with if (i = 0) instead of if (i == 0), except that in this case the body of the if statement would never be executed.

     This warning is suppressed if the assignment is enclosed in additional parentheses to indicate that it is being used legitimately.

warning: control reaches end of non-void function

     A function which has been declared with a return type, such as int or double, should always have a return statement returning a value of the corresponding type at all possible end points---otherwise its return value is not well-defined. Functions declared void do not need return statements.

     Example:

               [file msg-control.c not included]

     The program above reaches the end of the display function, which has a return type of int, without a return statement.  An additional line such as return 0; is needed.

     When using gcc the main function of a C program must return a value of type int (the exit status of the program). In C++ the return statement can be omitted from the main function---the return value of the C++ main function defaults to 0 if unspecified.

warning: unused variable `...'
warning: unused parameter `...'

     These warnings indicate that a variable has been declared as a local variable or in the parameters of a function, but has not been used anywhere.  An unused variable can be the result of a programming error, such as accidentally using the name of a different variable in place of the intended one.

     Example:

               [file msg-unused.c not included]

     In this program the variable i and the parameter p are never used.  Note that unused variables are reported by -Wall, while unused parameters are only shown with -Wall -W.

warning: passing arg of ... as ... due to prototype

     This warning occurs when a function is called with an argument of a different type from that specified in the prototype.  The option -Wconversion is needed to enable this warning. See the description of -Wconversion in Additional warning options for an example.

warning: assignment of read-only location
warning: cast discards qualifiers from pointer target type
warning: assignment discards qualifiers ...
warning: initialization discards qualifiers ...
warning: return discards qualifiers ...

     These warnings occur when a pointer is used incorrectly, violating a type qualifier such as const.  Data accessed through a pointer marked as const should not be modified, and the pointer itself can only be assigned to other pointers that are also marked const.

     Example:

               [file msg-const.c not included]

     This program attempts to modify constant data, and to discard the const property of the argument s in the return value.

initializer element is not a constant

     In C, global variables can only be initialized with constants, such as numeric values, NULL or fixed strings.  This error occurs if a non-constant value is used.

     Example:

               [file msg-init.c not included]

     This program attempts to initialize two variables from other variables. In particular, the stream stdout is not required to be a constant by the C standard (although on some systems it is a constant).  Note that non-constant initializers are allowed in C++.

Linker error messages
=====================

file not recognized: File format not recognized

     GCC uses the extension of a file, such as .c or .cc, to determine its content.  If the extension is missing GCC cannot recognize the file type and will give this error.

     Example:

               [file msg-ext not included]

     If the program above is saved in a file hello without any extension then compiling it will give the error:

          $ gcc -Wall hello
          hello: file not recognized: File format not
          recognized
          collect2: ld returned 1 exit status

     The solution is to rename the file to the correct extension, in this case hello.c.

undefined reference to `foo'
collect2: ld returned 1 exit status

     This error occurs when a program uses a function or variable which is not defined in any of the object files or libraries supplied to the linker.  It can be caused by a missing library or the use of an incorrect name. In the error message above, the program collect2 is part of the linker.

     Example:

               [file msg-undef.c not included]

     If this program is compiled without linking to a library or object file containing the function foo() there will be an undefined reference error.

/usr/lib/crt1.o(.text+0x18): undefined reference to `main'

     This error is a special case of the error above, when the missing function is main.  In C and C++, every program must have a main function (where execution starts).  When compiling an individual source file without a main function, use the option -c (see Creating object files from source files).

Runtime error messages
======================

error while loading shared libraries:
cannot open shared object file: No such file or directory

     The program uses shared libraries, but the necessary shared library files cannot be found by the dynamic linker when the program starts. The search path for shared libraries is controlled by the environment variable LD_LIBRARY_PATH (see Shared libraries and static libraries).

Segmentation fault
Bus error
     These runtime messages indicate a memory access error.

     Common causes include:

       * dereferencing a null pointer or uninitialized pointer

       * out-of-bounds array access

       * incorrect use of malloc, free and related functions

       * use of scanf with invalid arguments

     There is a subtle difference between segmentation faults and bus errors. A segmentation fault occurs when a process tries to access memory protected by the operating system.  A bus error occurs when valid memory is accessed in an incorrect way (for example, trying to read an unaligned value on architectures where values must be aligned with 4-byte offsets).

floating point exception

     This runtime error is caused by an arithmetic exception, such as division by zero, overflow, underflow or an invalid operation (e.g. taking the square root of -1).  The operating system determines which conditions produce this error.  On GNU systems, the functions feenableexcept and fedisableexcept can be used to trap or mask each type of exception.

Illegal instruction

     This error is produced by the operating system when an illegal machine instruction is encountered.  It occurs when code has been compiled for one specific architecture and run on another.

Getting help
************

If you encounter a problem not covered by this introduction, there are several reference manuals which describe GCC and language-related topics in more detail (see Further reading).  These manuals contain answers to common questions, and careful study of them will usually yield a solution.

Alternatively, there are many companies and consultants who offer commercial support for programming matters related to GCC on an hourly or ongoing basis.  For businesses this can be a cost-effective way to obtain high-quality support.

A directory of free software support companies and their current rates can be found on the GNU Project website. (http://www.gnu.org/prep/service.html) With free software, commercial support is available in a free market---service companies compete in quality and price, and users are not tied to any particular one.  In contrast, support for proprietary software is usually only available from the original vendor.

A higher-level of commercial support for GCC is available from companies involved in the development of the GNU compiler toolchain itself.  A listing of these companies can be found in the ``Development Companies'' section of the publisher's webpage for this book. (http://www.network-theory.co.uk/gcc/intro/) These companies can provide services such as extending GCC to generate code for new CPUs or fixing bugs in the compiler.

Further reading
***************

The definitive guide to GCC is the official reference manual, ``Using GCC'', published by GNU Press:

     Using GCC (for GCC version 3.3.1) by Richard M. Stallman and the GCC Developer Community (Published by GNU Press, ISBN 1-882114-39-6)

This manual is essential for anyone working with GCC because it describes every option in detail.  Note that the manual is updated when new releases of GCC become available, so the ISBN number may change in the future.

If you are new to programming with GCC you will also want to learn how to use the GNU Debugger GDB, and how to compile large programs easily with GNU Make.  These tools are described in the following manuals:

     Debugging with GDB: The GNU Source-Level Debugger by Richard M. Stallman, Roland Pesch, Stan Shebs, et al. (Published by GNU Press, ISBN 1-882114-88-4)

     GNU Make: A Program for Directing Recompilation by Richard M. Stallman and Roland McGrath (Published by  GNU Press, ISBN 1-882114-82-5)

For effective C programming it is also essential to have a good knowledge of the C standard library.  The following manual documents all the functions in the GNU C Library:

     The GNU C Library Reference Manual by Sandra Loosemore with Richard M. Stallman, et al (2 vols) (Published by GNU Press, ISBN 1-882114-22-1 and 1-882114-24-8)

Be sure to check the website http://www.gnupress.org/ for the latest printed editions of manuals published by GNU Press.  The manuals can be purchased online using a credit card at the FSF website (http://order.fsf.org/) in addition to being available for order through most bookstores using the ISBNs.  Manuals published by GNU Press raise funds for the Free Software Foundation and the GNU Project.

Information about shell commands, environment variables and shell-quoting rules can be found in the following book:

     The GNU Bash Reference Manual by Chet Ramey and Brian Fox (Published by Network Theory Ltd, ISBN 0-9541617-7-7)

Other GNU Manuals mentioned in this book (such as GNU gprof---The GNU Profiler and The GNU Binutils Manual) were not available in print at the time this book went to press.  Links to online copies can be found at the publisher's webpage for this book. (http://www.network-theory.co.uk/gcc/intro/)

The official GNU Project webpage for GCC can be found on the GNU website at http://www.gnu.org/software/gcc/.  This includes a list of frequently asked questions, as well as the GCC bug tracking database and a lot of other useful information about GCC.

There are many books about the C and C++ languages themselves.  Two of the standard references are:

     The C Programming Language (ANSI edition) Brian W. Kernighan, Dennis Ritchie (ISBN 0-13110362-8)

     The C++ Programming Language (3rd edition) Bjarne Stroustrup (ISBN 0-20188954-4)

Anyone using the C and C++ languages in a professional context should obtain a copy of the official language standards, which are also available as printed books:

     The C Standard: Incorporating Technical Corrigendum 1 (Published by Wiley, ISBN 0-470-84573-2)

     The C++ Standard (Published by Wiley, ISBN 0-470-84674-7)

For reference, the C standard number is ISO/IEC 9899:1990, for the original C standard published in 1990 and implemented by GCC.  A revised C standard ISO/IEC 9899:1999 (known as C99) was published in 1999, and this is mostly (but not yet fully) supported by GCC.  The C++ standard is ISO/IEC 14882.

The floating-point arithmetic standard IEEE-754 is important for any programs involving numerical computations.  The standard is available commercially from the IEEE, and is also described in the following book:

     Numerical Computing with IEEE Floating Point Arithmetic by Michael Overton (Published by SIAM, ISBN 0-89871-482-6).

The book includes many examples to illustrate the rationale for the standard.

Acknowledgements
****************

Many people have contributed to this book, and it is important to record their names here:

Thanks to Gerald Pfeifer, for his careful reviewing and numerous suggestions for improving the book.

Thanks to Andreas Jaeger, for information on AMD64 and multi-architecture support, and many helpful comments.

Thanks to David Edelsohn, for information on the POWER/PowerPC series of processors.

Thanks to Jamie Lokier, for research.

Thanks to Martin Leisner, Mario Pernici, Stephen Compall and Nigel Lowry, for helpful corrections.

Thanks to Gerard Jungman, for useful comments.

Thanks to Steven Rubin, for generating the chip layout for the cover with Electric.

And most importantly, thanks to Richard Stallman, founder of the GNU Project, for writing GCC and making it free software.

Free software organizations
***************************

The GNU Compiler Collection is part of the GNU Project, launched in 1984 to develop a complete Unix-like operating system which is free software: the GNU system.

The Free Software Foundation (FSF) is a tax-exempt charity that raises funds for continuing work on the GNU Project.  It is dedicated to promoting the right to use, study, copy, modify, and redistribute computer programs.  One of the best ways to help the development of free software is to become an associate member of the Free Software Foundation, and pay regular dues to support their efforts.

Associate members of the Free Software Foundation receive many benefits, including regular newsletters, admission to the FSF annual meeting, and discounts on books and CDROMs published by GNU Press.  Membership dues are also tax deductible in the USA.  For more information on becoming a member, visit the FSF website at http://www.fsf.org/.

The Free Software Foundation Europe (FSFE) is a sister organisation of the Free Software Foundation.  The FSFE is active in promoting free software at all levels in Europe.  For an annual membership fee, individuals can join FSFE and support its work.  Members receive a personalised GPG-compatible membership smartcard, allowing secure digital authentication of email and files, and gain access to the ``FSFE Fellowship'', an electronic community for software freedom.  For more information, visit the FSFE website at http://www.fsfe.org/.

The Foundation for a Free Information Infrastructure (FFII) is another important organization in Europe.  FFII is not specific to free software, but works to defend the rights of all programmers and computer users against monopolies in the field of computing, such as patents on software.  For more information about FFII, or to support their work with a donation, visit their website at http://www.ffii.org/.