ar keys archive [obj_ files]
The most important keys are r to replace or add modules to
the archive and t to display a table of contents. The archive
is a name composed of lib name.a. For example the
command sequence:
cc -c func1.c
cc -c func2.c
cc -c func3.c
ar r libmy.a func1.o func2.o func3.o
compiles func1.c, func2.c, func3.c and adds the object files
into the libmy.a.