gdb | view the variable argument list

Question

I as using the bt command to view the stacktrace. The output is

(gdb) bt
#0  0x001ae4cd in Debugger (message=0x1 \"???\\a\") at /SourceCache/xnu         


        

Answer 1

(gdb) frame 8

will put you in the frame of the caller. Examine the arguments that are being passed.

Answer 2

Looks like this is possible to do it for a simple program like this:

#include <stdarg.h>
#include <stdio.h>

void myfunc(const char *fmt, ...)
{
        va_list args;
        va_start(args, fmt);
        vprintf(fmt, args);
        va_end(args);
        return;
}

int main(int argc, char *argv[])
{
        myfunc("test 1: %s %s\n", "one", "two");
        myfunc("test 2: %s %d %c\n", "apple", 222, 'y');
        return 0;
}

Here is sample gdb session:

$ gdb testprog
GNU gdb (GDB) 7.1-debian
[snip]
Reading symbols from /home/user/testprog...done.
(gdb) break myfunc
Breakpoint 1 at 0x400552: file testprog.c, line 7.
(gdb) run
Starting program: /home/user/testprog

Breakpoint 1, myfunc (fmt=0x4006f4 "test 1: %s %s\n") at testprog.c:7
7               va_start(args, fmt);
(gdb) # initialize args to hold correct values:
(gdb) step
8               vprintf(fmt, args);
(gdb) # print first argument in "..." list which we know is a char*:
(gdb) p *(char **)(((char *)args[0].reg_save_area)+args[0].gp_offset)
$1 = 0x4006f0 "one"

I have not tested all of this, look this link for full solution. This blog will be useful also.