### 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 Controlling debugging output) can help isolate the problem.