How can I debug functions in shared object libraries with GDB?

Question

I'm trying to verify the functionality of functions in a shared object library. In programs with a main function, I would simply start the program and gdb would automatically breakpoint on main, but that's obviously not available here.

Let's say I have some add.c:

long add(long x, long y) {
    return x + y;
}

I compile this with gcc -shared -o libadd.so -fPIC add.c and load it into GDB:

(gdb) file libadd.so
Reading symbols from libadd.so...(no debugging symbols found)...done.
(gdb) start
Function "main" not defined.
Make breakpoint pending on future shared library load? (y or [n])
Starting program: /tmp/minimal/libadd.so

Program received signal SIGSEGV, Segmentation fault.
0x0000000000000001 in ?? ()

Preferably, I would like to be able to use gdb similar to below:

(gdb) file libadd.so
Reading symbols from libadd.so...(no debugging symbols found)...done.
(gdb) call (long)add(5,6)
$1 = 11

But this call results in You can't do that without a process to debug.

Can I debug libraries like this in GDB?

Answer 1

You can do so with starti, as shown below:

(gdb) file libadd.so
Reading symbols from libadd.so...(no debugging symbols found)...done.
(gdb) starti
Starting program /tmp/minimal/libadd.so

Program stopped.
0x00007ffff7dfd4a0 in deregister_tm_clones ()
(gdb) call (long)add(5,6)
$1 = 11

You can also do this with binaries containing a main function, as seen in this similar question.