7.6 Multiple directories

So far, we've only dealt with single-directory projects. Automake can also handle projects with many directories. The variable `SUBDIRS' is used to list the subdirectories which should be built. Here is an example from Automake itself:

SUBDIRS = . m4 tests

Automake does not need to know the list of subdirectories statically, so there is no `EXTRA_SUBDIRS' variable. You might think that Automake would use `SUBDIRS' to see which `Makefile.am's to scan, but it actually gets this information from `configure.in'. This means that, if you have a subdirectory which is optionally built, you should still list it unconditionally in your call to AC_OUTPUT and then arrange for it to be substituted (or not, as appropriate) at configure time.

Subdirectories are always built in the order they appear, but cleaning rules (e.g., maintainer-clean) are always run in the reverse order. The reason for this odd reversal is that it is wrong to remove a file before removing all the files which depend on it.

You can put `.' into `SUBDIRS' to control when the objects in the current directory are built, relative to the objects in the subdirectories. In the example above, targets in `.' will be built before subdirectories are built. If `.' does not appear in `SUBDIRS', it is built following all the subdirectories.