File: make.info,  Node: Archive Pitfalls,  Next: Archive Suffix Rules,  Prev: Archive Update,  Up: Archives

11.3 Dangers When Using Archives
================================

The built-in rules for updating archives are incompatible with parallel
builds.  These rules (required by the POSIX standard) add each object
file into the archive as it's compiled.  When parallel builds are
enabled this allows multiple 'ar' commands to be updating the same
archive simultaneously, which is not supported.

   If you want to use parallel builds with archives you can override the
default rules by adding these lines to your makefile:

     (%) : % ;
     %.a : ; $(AR) $(ARFLAGS) $@ $?

   The first line changes the rule that updates an individual object in
the archive to do nothing, and the second line changes the default rule
for building an archive to update all the outdated objects ('$?') in one
command.

   Of course you will still need to declare the prerequisites of your
library using the archive syntax:

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

   If you prefer to write an explicit rule you can use:

     libfoo.a: libfoo.a(x.o y.o ...)
             $(AR) $(ARFLAGS) $@ $?