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9.2 Searching for include files

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 (see Invoking m4), 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 (see Debug Levels) can help isolate the problem.