13.2 Printing current location

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

     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, see Invoking m4), and the `f' and `l' flags of debugmode (see Debug Levels), also use this notion of current file and line. Redefining the three location macros has no effect on syncline, debug, or warning message output. Assume this example is run in the checks directory of the GNU M4 package, using `--include=../examples' in the command line to find the file incl.m4 mentioned earlier:

     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
     =>

Currently, all text wrapped with m4wrap (see M4wrap) behaves as though it came from line 0 of the file “”. It is hoped that a future release of m4 can overcome this limitation and remember which file invoked the call to m4wrap.

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 (see Syscmd) to pick the same version of m4 that is currently running, rather than whatever version of m4 happens to be first in PATH.