I figured out last night that I could use allocate memory within another process for the purposes of using more complicated APIs. In the below example, I attached to one of my shells and queried the groups out of it.
In this snippet, the target shell tells me its process id is 40728.
Last login: Fri Jan 8 16:51:00 on ttys000
w3m211a:~ jjore$ echo $$
I'm calling the (int)getgroups( int, int* ) kernel function. I start by getting the size of an integer because I'm going to allocate some multiple of this.
w3m211a:~ jjore$ gdb -p 40728
(gdb) print sizeof(int)
$1 = 4
Call (int)getgroups(int,int*) once to get the number of groups I'm a member of.
(gdb) print (int)getgroups(0,0)
$2 = 16
Allocate 64 bytes of memory to hold my groups array.
(gdb) print (void*)malloc(64)
$3 = (void *) 0x10010fca0
Call (int)getgroups(int,int*) again but this time the kernel will write to my memory with the group ids.
(gdb) print (int)getgroups($2,$3)
$4 = 16
Print the 16 decimal numbers out of memory
(gdb) x/16d $3
0x10010fca0: 2042662593 401 1612991420 1178352327
0x10010fcb0: 102 800006232 204 100
0x10010fcc0: 98 81 80 79
0x10010fcd0: 62 12 155049148 2133303823
(gdb) print (void)free($3)
$5 = void