automake-1.16: Wildcards
26.3 Why doesn’t Automake support wildcards?
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Developers are lazy. They would often like to use wildcards in
‘Makefile.am’s, so that they would not need to remember to update
‘Makefile.am’s 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 quite 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
‘$(wildcard ...)’ when ‘automake’ is run. The resulting ‘Makefile.in’s
would be portable since they would list all files and not use
‘$(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. ‘$(wildcard ...)’
is a black box comparable to ‘AC_SUBST’ed variables as far Automake is
concerned.
You can get warnings about ‘$(wildcard ...’) constructs using the
‘-Wportability’ flag.
Info Catalog
automake-1.16: maintainer-mode
automake-1.16: FAQ
automake-1.16: Limitations on File Names