File: autoconf.info,  Node: Special Chars in Names,  Next: Backslash-Newline-Empty,  Prev: Failure in Make Rules,  Up: Portable Make

12.3 Special Characters in Make Macro Names
===========================================

Posix limits macro names to nonempty strings containing only ASCII
letters and digits, ‘.’, and ‘_’.  Many ‘make’ implementations allow a
wider variety of characters, but portable makefiles should avoid them.
It is portable to start a name with a special character, e.g.,
‘$(.FOO)’.

   Some ancient ‘make’ implementations don't support leading underscores
in macro names.  An example is NEWS-OS 4.2R.

     $ cat Makefile
     _am_include = #
     _am_quote =
     all:; @echo this is test
     $ make
     Make: Must be a separator on rules line 2.  Stop.
     $ cat Makefile2
     am_include = #
     am_quote =
     all:; @echo this is test
     $ make -f Makefile2
     this is test

However, this problem is no longer of practical concern.