undefined reference error in static libraries.

[steven woody]

hi, i am build a program, the structure of the source is below,

main.o
    main.c
    libfoo.a
        foo.c
    libbar.a
        bar.c

and, the foo.c has a reference to function bar() which is defined in bar.c

i can successfully compile and produce libfoo.a and libbar.a. but when
compile main.o, i get an error like following:

gcc -g -O2 -o main.o main.o libfoo.a libbar.a

libfoo.a(libfoo_a-foo.o): in function 'foo':
foo.c: undefined reference to 'bar'


what's the cause and how do i do? thanks.

--
woody