Inspecting caller frames with gdb - c

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

Answer 2

Did you compile with debug symbols -g? The command should be bt for backtrace, you can also use bt full for a full backtrace.
More infos: https://sourceware.org/gdb/onlinedocs/gdb/Backtrace.html