11.2 Implicit Rule for Archive Member Targets

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 recipe 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 recipe:

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.

11.2.1 Updating Archive Symbol Directories

An archive file that is used as a library usually contains a special member named ‘__.SYMDEF’ that contains a directory of the external symbol names defined by all the other members. After you update any other members, you need to update ‘__.SYMDEF’ so that it will summarize the other members properly. This is done by running the ranlib program:

ranlib archivefile

Normally you would put this command in the rule for the archive file, and make all the members of the archive file prerequisites of that rule. For example,

libfoo.a: libfoo.a(x.o) libfoo.a(y.o) …
        ranlib libfoo.a

The effect of this is to update archive members ‘x.o’, ‘y.o’, etc., and then update the symbol directory member ‘__.SYMDEF’ by running ranlib. The rules for updating the members are not shown here; most likely you can omit them and use the implicit rule which copies files into the archive, as described in the preceding section.

This is not necessary when using the GNU ar program, which updates the ‘__.SYMDEF’ member automatically.

This document was generated by Davide Tacchella on November 3, 2010 using texi2html 1.82.