File: m4.info,  Node: Search Path,  Prev: Include,  Up: File Inclusion

9.2 Searching for include files
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GNU ‘m4’ allows included files to be found in other directories than the
current working directory.

   If the ‘--prepend-include’ or ‘-B’ command-line option was provided
(*note Invoking m4: Preprocessor features.), those directories are
searched first, in reverse order that those options were listed on the
command line.  Then ‘m4’ looks in the current working directory.  Next
comes the directories specified with the ‘--include’ or ‘-I’ option, in
the order found on the command line.  Finally, if the ‘M4PATH’
environment variable is set, it is expected to contain a colon-separated
list of directories, which will be searched in order.

   If the automatic search for include-files causes trouble, the ‘p’
debug flag (*note Debug Levels::) can help isolate the problem.