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(cpp-295.info)Newlines in Args

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Newlines in Macro Arguments
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   Traditional macro processing carries forward all newlines in macro
arguments into the expansion of the macro.  This means that, if some of
the arguments are substituted more than once, or not at all, or out of
order, newlines can be duplicated, lost, or moved around within the
expansion.  If the expansion consists of multiple statements, then the
effect is to distort the line numbers of some of these statements.  The
result can be incorrect line numbers, in error messages or displayed in
a debugger.

   The GNU C preprocessor operating in ANSI C mode adjusts appropriately
for multiple use of an argument--the first use expands all the
newlines, and subsequent uses of the same argument produce no newlines.
But even in this mode, it can produce incorrect line numbering if
arguments are used out of order, or not used at all.

   Here is an example illustrating this problem:

     #define ignore_second_arg(a,b,c) a; c
     
     ignore_second_arg (foo (),
                        ignored (),
                        syntax error);

The syntax error triggered by the tokens `syntax error' results in an
error message citing line four, even though the statement text comes
from line five.