File: autoconf.info,  Node: Comments in Make Macros,  Next: Trailing whitespace in Make Macros,  Prev: Newlines in Make Rules,  Up: Portable Make

12.13 Comments in Make Macros
=============================

Avoid putting comments in macro values as far as possible.  Posix
specifies that the text starting from the ‘#’ sign until the end of the
line is to be ignored, which has the unfortunate effect of disallowing
them even within quotes.  Thus, the following might lead to a syntax
error at compile time:

     CPPFLAGS = "-DCOMMENT_CHAR='#'"

as ‘CPPFLAGS’ may be expanded to ‘"-DCOMMENT_CHAR='’.

   Most ‘make’ implementations disregard this and treat single and
double quotes specially here.  Also, GNU ‘make’ lets you put ‘#’ into a
macro value by escaping it with a backslash, i.e., ‘\#’.  However,
neither of these usages are portable.  *Note Comments in Make Rules::,
for a portable alternative.

   Even without quoting involved, comments can have surprising effects,
because the whitespace before them is part of the variable value:

     foo = bar # trailing comment
     print: ; @echo "$(foo)."

prints ‘bar .’, which is usually not intended, and can expose ‘make’
bugs as described below.