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Implicit Rule for Archive Member Targets

Recall that a target that looks like ‘a(m)’ stands for the member named min the archive file, a.

When make looks for an implicit rule for such a target, as a special feature, it considers implicit rules that match ‘(m)’ as well as those that match the actual target, ‘a(m)’.

This causes one special rule whose target is ‘(%)’ to match. This rule updates the target, ‘a(m)’, by copying the file, m, into the archive. For example, it will update the archive member target, ‘foo.a(bar.o)’ by copying the file, ‘bar.o’, into the archive, ‘foo.a’, as a member named ‘bar.o’.

When this rule is chained with others, the result is very powerful. Thus, ‘make “foo.a(bar.o)”’ (the quotes are needed to protect the ‘(’ and ‘)’ from being interpreted specially by the shell) in the presence of a file ‘bar.c’ is enough to cause the following commands to be run, even without a makefile:

cc -c bar.c -o bar.o
ar r foo.a bar.o
rm -f bar.o

Here make has envisioned the ‘bar.o’ as an intermediate file. See Chains of Implicit Rules.

Implicit rules such as this one are written using the automatic variable ‘$%’. See Automatic Variables.

An archive member name in an archive cannot contain a directory name, but it may be useful in a makefile to pretend that it does. If you write an archive member target ‘foo.a(dir/file.o)’, make will perform automatic updating with the command, ar r foo.a dir/file.o, having the effect of copying the file, ‘dir/file.o’, into a member named ‘file.o’. In connection with such usage, the automatic variables, %D and %F, may be useful.

See Updating Archive Symbol Directories for more discussion.