accessing command-line arguments with gdb

Question

I am on linux using gdb version 6.8-debian. I have been curious about how the main function in a c-program gets executed and playing around and looking in different places, I learned that the function __libc_start_main is responsiple for this. The arguments to __libc_start_main are, among others: The address of main (like we know from c, the path is always given as argv[0]), next argc which should reside in the register ESI, and next address of argv which should be in ECX.

To play around I made the following simple program, cmdargs.c, which simply outputs the first command-line argument given at start:

#include <stdio.h>
#include <stdlib.h>

int main (int argc, char *argv[])
{
    printf("%s: %s\n", "argv[1]", *++argv);
    return EXIT_SUCCESS;
}

Now I start to debug cmdargs and set a breakpoint on main and __libc_start_main (info from starting gdb removed):

gdb cmdargs

(gdb) b main
Breakpoint 1 at 0x80483d2
(gdb) b __libc_start_main
Breakpoint 2 at 0xb7f3f5a8
(gdb) r qwerty

Here i hit the Breakpoint 2 in __libc_start_main and can view argc and argv[0] with

(gdb) p $esi

and

(gdb) x/s *($ecx)

This works as expected, but how do I access the first non-implicit commandline-argument "qwerty" ? I have tried continuing to the breakpoint at main and stepping in, but argc and argv are not recognised (Why?). Can someone tell me whats going on ?

Breakpoint 1, 0x080483d2 in main ()
(gdb) stepi
0x080483d5 in main () 
(gdb) p argc
No symbol "argc" in current context.
(gdb) p argv
No symbol "argv" in current context.
(gdb)

J C Gonzalez J C Gonzalez · Accepted Answer · 2015-09-18T08:23:10

Yep, your problem is the lack of symbols, not included at compilation time.

To compile with debugging information:

$ gcc -g3 cmdargs.c -o cmdargs

Then:

$ gdb ./cmdargs
...
Reading symbols from ./cmdargs...done.
(gdb) b main
Breakpoint 1 at 0x400545: file cmdargs.c, line 6.
(gdb) r
Starting program: cmdargs 

Breakpoint 1, main (argc=1, argv=0x7fffffffdc28) at cmdargs.c:6
6       printf("%s: %s\n", "argv[1]", *++argv);
(gdb) p argc
$1 = 1
(gdb) p argv
$2 = (char **) 0x7fffffffdc28
(gdb) p *argv
$3 = 0x7fffffffe00c "/home/jcgonzalez/cmdargs"

See, now you get access to the symbols (they are recognized), as well as to the line numbers. As shown by Let_Me_Be, you can access single array elements with array[n] notation, but you can also show all the command line arguments at once (including the [0]-ed one) with the *array@times notation. Note that the first argument in the following example is a quoted string:

(gdb) set args "this is an argument" these are four more 
(gdb) r
Starting program: cmdargs "this is an argument" these are four more

Breakpoint 1, main (argc=6, argv=0x7fffffffdbd8) at cmdargs.c:6
6       printf("%s: %s\n", "argv[1]", *++argv);
(gdb) p argc
$4 = 6
(gdb) p *argv@argc                                    
$5 = {0x7fffffffdfe6 "/home/jcgonzalez/cmdargs", 0x7fffffffdfff "this is an argument", 0x7fffffffe012 "these", 0x7fffffffe017 "are", 0x7fffffffe01b "four", 
  0x7fffffffe020 "more"}
(gdb) p argv[1]
$6 = 0x7fffffffdfff "this is an argument"
(gdb) p argv[2]
$7 = 0x7fffffffe012 "these"

accessing command-line arguments with gdb

3 Answers