(m4.info)Location

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14.2 Printing current location
==============================

To make it possible to specify the location of an error, three utility
builtins exist:

 -- Builtin: __file__
 -- Builtin: __line__
 -- Builtin: __program__
     Expand to the quoted name of the current input file, the current
     input line number in that file, and the quoted name of the current
     invocation of 'm4'.

     errprint(__program__:__file__:__line__: `input error
     ')
     error->m4:stdin:1: input error
     =>

   Line numbers start at 1 for each file.  If the file was found due to
the '-I' option or 'M4PATH' environment variable, that is reflected in
the file name.  The syncline option ('-s', *note Invoking m4:
Preprocessor features.), and the 'f' and 'l' flags of 'debugmode' (Note:
Debug Levels), also use this notion of current file and line.
Redefining the three location macros has no effect on syncline, debug,
warning, or error message output.

   This example reuses the file 'incl.m4' mentioned earlier (Note:
Include):

     $ m4 -I examples
     define(`foo', ``$0' called at __file__:__line__')
     =>
     foo
     =>foo called at stdin:2
     include(`incl.m4')
     =>Include file start
     =>foo called at examples/incl.m4:2
     =>Include file end
     =>

   The location of macros invoked during the rescanning of macro
expansion text corresponds to the location in the file where the
expansion was triggered, regardless of how many newline characters the
expansion text contains.  As of GNU M4 1.4.8, the location of text
wrapped with 'm4wrap' (Note: M4wrap) is the point at which the
'm4wrap' was invoked.  Previous versions, however, behaved as though
wrapped text came from line 0 of the file "".

     define(`echo', `$@')
     =>
     define(`foo', `echo(__line__
     __line__)')
     =>
     echo(__line__
     __line__)
     =>4
     =>5
     m4wrap(`foo
     ')
     =>
     foo(errprint(__line__
     __line__
     ))
     error->8
     error->9
     =>8
     =>8
     __line__
     =>11
     m4wrap(`__line__
     ')
     =>
     ^D
     =>12
     =>6
     =>6

   The '__program__' macro behaves like '$0' in shell terminology.  If
you invoke 'm4' through an absolute path or a link with a different
spelling, rather than by relying on a 'PATH' search for plain 'm4', it
will affect how '__program__' expands.  The intent is that you can use
it to produce error messages with the same formatting that 'm4' produces
internally.  It can also be used within 'syscmd' (Note: Syscmd) to
pick the same version of 'm4' that is currently running, rather than
whatever version of 'm4' happens to be first in 'PATH'.  It was first
introduced in GNU M4 1.4.6.