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27.3 Why doesn’t Automake support wildcards?

Developers are lazy. They would often like to use wildcards in Makefile.ams, so that they would not need to remember to update Makefile.ams every time they add, delete, or rename a file.

There are several objections to this:

• When using CVS (or similar) developers need to remember they have to run ‘cvs add’ or ‘cvs rm’ anyway. Updating Makefile.am accordingly quickly becomes a reflex.

  Conversely, if your application doesn’t compile because you forgot to add a file in Makefile.am, it will help you remember to ‘cvs add’ it.

• Using wildcards makes it easy to distribute files by mistake. For instance, some code a developer is experimenting with (a test case, say) that should not be part of the distribution.
• Using wildcards it’s easy to omit some files by mistake. For instance, one developer creates a new file, uses it in many places, but forgets to commit it. Another developer then checks out the incomplete project and is able to run ‘make dist’ successfully, even though a file is missing. By listing files, ‘make dist’ will complain.
• Wildcards are not portable to some non-GNU make implementations, e.g., NetBSD make will not expand globs such as ‘*’ in prerequisites of a target.
• Finally, it’s really hard to forget to add a file to Makefile.am: files that are not listed in Makefile.am are not compiled or installed, so you can’t even test them.

Still, these are philosophical objections, and as such you may disagree, or find enough value in wildcards to dismiss all of them. Before you start writing a patch against Automake to teach it about wildcards, let’s see the main technical issue: portability.

Although ‘$(wildcard ...)’ works with GNU make, it is not portable to other make implementations.

The only way Automake could support $(wildcard ...) is by expanding $(wildcard ...) when automake is run. The resulting Makefile.ins would be portable since they would list all files and not use ‘$(wildcard ...)’. However that means developers would need to remember to run automake each time they add, delete, or rename files.

Compared to editing Makefile.am, this is a very small gain. Sure, it’s easier and faster to type ‘automake; make’ than to type ‘emacs Makefile.am; make’. But nobody bothered enough to write a patch to add support for this syntax. Some people use scripts to generate file lists in Makefile.am or in separate Makefile fragments.

Even if you don’t care about portability, and are tempted to use ‘$(wildcard ...)’ anyway because you target only GNU Make, you should know there are many places where Automake needs to know exactly which files should be processed. As Automake doesn’t know how to expand ‘$(wildcard ...)’, you cannot use it in these places. ‘$(wildcard ...)’ is a black box comparable to AC_SUBSTed variables as far Automake is concerned.

You can get warnings about ‘$(wildcard ...’) constructs using the -Wportability flag.