How to inspect the return value of a function in GDB?

Is it possible to inspect the return value of a function in gdb assuming the return value is not assigned to a variable?

Solution 1:

I imagine there are better ways to do it, but the finish command executes until the current stack frame is popped off and prints the return value -- given the program

int fun() {
    return 42;
}

int main( int argc, char *v[] ) {
    fun();
    return 0;
}

You can debug it as such --

(gdb) r
Starting program: /usr/home/hark/a.out 

Breakpoint 1, fun () at test.c:2
2               return 42;
(gdb) finish
Run till exit from #0  fun () at test.c:2
main () at test.c:7
7               return 0;
Value returned is $1 = 42
(gdb) 

The finish command can be abbreviated as fin. Do NOT use the f, which is abbreviation of frame command!

Solution 2:

Yes, just examine the EAX register by typing print $eax. For most functions, the return value is stored in that register, even if it's not used.

The exceptions to this are functions returning types larger than 32 bits, specifically 64-bit integers (long long), doubles, and structs or classes.

The other exception is if you're not running on an Intel architecture. In that case, you'll have to figure out which register is used, if any.

Solution 3:

Here's how todo this with no symbols.

gdb ls
This GDB was configured as "ppc64-yellowdog-linux-gnu"...
(no debugging symbols found)
Using host libthread_db library "/lib64/libthread_db.so.1".

(gdb) break __libc_start_main
Breakpoint 1 at 0x10013cb0
(gdb) r
Starting program: /bin/ls
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
Breakpoint 1 at 0xfdfed3c
(no debugging symbols found)
[Thread debugging using libthread_db enabled]
[New Thread 4160418656 (LWP 10650)]
(no debugging symbols found)
(no debugging symbols found)
[Switching to Thread 4160418656 (LWP 10650)]

Breakpoint 1, 0x0fdfed3c in __libc_start_main () from /lib/libc.so.6
(gdb) info frame
Stack level 0, frame at 0xffd719a0:
 pc = 0xfdfed3c in __libc_start_main; saved pc 0x0
 called by frame at 0x0
 Arglist at 0xffd71970, args:
 Locals at 0xffd71970, Previous frame's sp is 0xffd719a0
 Saved registers:
  r24 at 0xffd71980, r25 at 0xffd71984, r26 at 0xffd71988, r27 at 0xffd7198c,
  r28 at 0xffd71990, r29 at 0xffd71994, r30 at 0xffd71998, r31 at 0xffd7199c,
  pc at 0xffd719a4, lr at 0xffd719a4
(gdb) frame 0
#0  0x0fdfed3c in __libc_start_main () from /lib/libc.so.6
(gdb) info fr
Stack level 0, frame at 0xffd719a0:
 pc = 0xfdfed3c in __libc_start_main; saved pc 0x0
 called by frame at 0x0
 Arglist at 0xffd71970, args:
 Locals at 0xffd71970, Previous frame's sp is 0xffd719a0
 Saved registers:
  r24 at 0xffd71980, r25 at 0xffd71984, r26 at 0xffd71988, r27 at 0xffd7198c,
  r28 at 0xffd71990, r29 at 0xffd71994, r30 at 0xffd71998, r31 at 0xffd7199c,
  pc at 0xffd719a4, lr at 0xffd719a4

Formatting kinda messed up there, note the use of "info frame" to inspect frames, and "frame #" to navigate your context to another context (up and down the stack)

bt also show's an abbreviated stack to help out.