Recall that a target that looks like `a(m)' stands for the member named m in 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 file `bar.o' as an intermediate
file. See section Chains of Implicit Rules.
Implicit rules such as this one are written using the automatic variable `$%'. See section 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
%D and %F may be useful.