Inspecting caller frames with gdb

Question

Suppose I have:

#include <stdlib.h>

int main()
{
    int a = 2, b = 3;
    if (a!=b)
        abort();
}

Compiled with:

 gcc -g c.c

Running this, I'll get a coredump (due to the SIGABRT raised by abort()), which I can debug with:

gdb a.out core

How can I get gdb to print the values of a and b from this context?

Answer 1

Here's the another way to specifically get a and b values by moving to the interested frame and then info locals would give you the values. a.out was compiled with your code. (frame 2 is what you are interested in i.e., main()).

$ gdb ./a.out core
[ removed some not-so-interesting info here ]
Reading symbols from ./a.out...done.
[New LWP 14732]
Core was generated by `./a.out'.
Program terminated with signal SIGABRT, Aborted.
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
51      ../sysdeps/unix/sysv/linux/raise.c: No such file or directory.
(gdb) bt
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
#1  0x00007fac16269f5d in __GI_abort () at abort.c:90
#2  0x00005592862f266d in main () at f.c:7
(gdb) frame 2
#2  0x00005592862f266d in main () at f.c:7
7               abort();
(gdb) info locals
a = 2
b = 3
(gdb) q

You can also use print once frame 2:

(gdb) print a
$1 = 2
(gdb) print b
$2 = 3