out.txt

15:37 [snoopjedi@denton ~/playground/cpp/linking_sandbox (master *)]
$ cat foo.h
int foo();
15:37 [snoopjedi@denton ~/playground/cpp/linking_sandbox (master *)]
$ cat foo.cpp
int foo()
{
    return -1;
}
15:37 [snoopjedi@denton ~/playground/cpp/linking_sandbox (master *)]
$ cat bar.cpp
#include "foo.h"  // at this point we know `int foo()` exists, but not how to execute it

int bar()
{
    return 42 + foo();
}

int main(int argc, char** argv)
{
        return bar();
}
15:37 [snoopjedi@denton ~/playground/cpp/linking_sandbox (master *)]
$ gcc -c foo.cpp bar.cpp  # -c means compile only, producing object files (.o)
15:37 [snoopjedi@denton ~/playground/cpp/linking_sandbox (master *)]
$ nm foo.o   # show the symbols in this object file, which is just foo
0000000000000000 T _Z3foov
15:38 [snoopjedi@denton ~/playground/cpp/linking_sandbox (master *)]
$ nm bar.o  # note: this object file *knows about* the symbol foo (because of the #include) but not how to execute it (the difference between declaration and definition)
                 U _GLOBAL_OFFSET_TABLE_
0000000000000000 T _Z3barv
                 U _Z3foov
0000000000000012 T main
15:39 [snoopjedi@denton ~/playground/cpp/linking_sandbox (master *)]
$ ld --shared -o mylib.so foo.o bar.o   # if we link the two into a shared library...
15:39 [snoopjedi@denton ~/playground/cpp/linking_sandbox (master *)]
$ nm mylib.so  # ...we see that both symbols are defined
0000000000003f00 d _DYNAMIC
0000000000004000 d _GLOBAL_OFFSET_TABLE_
000000000000105f T _Z3barv
0000000000001050 T _Z3foov
0000000000001071 T main