CC Mode provides some tools to help keep the line continuation backslashes in macros neat and tidy. Their precise action is customized with these variables:
These variables control the alignment columns for line continuation
backslashes in multiline macros. They are used by the functions that
automatically insert or align such backslashes,
e.g. c-backslash-region and c-context-line-break.
c-backslash-column specifies the minimum column for the
backslashes. If any line in the macro goes past this column, then the
next tab stop (i.e. next multiple of tab-width) in that line is
used as the alignment column for all the backslashes, so that they
remain in a single column. However, if any lines go past
c-backslash-max-column then the backslashes in the rest of the
macro will be kept at that column, so that the lines which are too
long “stick out” instead.
Don’t ever set these variables to nil. If you want to disable
the automatic alignment of backslashes, use
c-auto-align-backslashes.
Align automatically inserted line continuation backslashes if
non-nil. When line continuation backslashes are inserted
automatically for line breaks in multiline macros, e.g. by
c-context-line-break, they are aligned with the other
backslashes in the same macro if this flag is set.
If c-auto-align-backslashes is nil, automatically
inserted backslashes are preceded by a single space, and backslashes
get aligned only when you explicitly invoke the command
c-backslash-region (C-c C-\).