Recall that a target that looks like a(m) stands for the member named m in the archive file a.
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
make has envisioned the file 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 this command:
ar r foo.a dir/file.o
which has the effect of copying the file dir/file.o into a member
named file.o. In connection with such usage, the automatic variables
%F may be useful.