GDB: Attempt to dereference generic pointer - c

Question

How can I make GDB do extra dereferences in a printing function like
x/s?
When I try explicit dereferences in x/ I get the error "Attempt to
dereference a generic pointer". Using x/ multiple times works, since
each use includes an implicit dereference, but this is annoying since
I have to copy and paste each intermediate result.
Example
Consider the very useful C program, example.c:
#include <stdio.h>
int main(int argc, char **argv) {
printf("argv[0] = %s\n", argv[0]);
}
If I build it and load it into GDB, I see that argv is stored at
0xc(%ebp), since a double dererence of that is passed as the second
argument to printf (i.e. in 0x4(%esp)) on line 26:
$ gcc -o example example.c
$ gdb example
(gdb) disass main
Dump of assembler code for function main:
0x080483e4 <+0>: push %ebp
0x080483e5 <+1>: mov %esp,%ebp
0x080483e7 <+3>: and $0xfffffff0,%esp
0x080483ea <+6>: sub $0x10,%esp
0x080483ed <+9>: mov 0xc(%ebp),%eax
0x080483f0 <+12>: mov (%eax),%edx
0x080483f2 <+14>: mov $0x80484e0,%eax
0x080483f7 <+19>: mov %edx,0x4(%esp)
0x080483fb <+23>: mov %eax,(%esp)
0x080483fe <+26>: call 0x8048300 <printf#plt>
0x08048403 <+31>: leave
0x08048404 <+32>: ret
End of assembler dump.
I break at printf and run the program with arguments first and
second:
(gdb) break *main + 26
Breakpoint 1 at 0x80483fe
(gdb) run first second
Starting program: /var/tmp/SO-attempt-to-dereference-generic-pointer/example first second
I attempt to print argv[0] in GDB, but I get the "generic pointer"
error:
Breakpoint 1, 0x080483e5 in main ()
(gdb) x/s **(0xc + $ebp)
Attempt to dereference a generic pointer.
However, by using 'x/xw' to manually dereference a few times, I'm
eventually able to print argv[0] (and argv[1]):
(gdb) x/xw 0xc + $ebp
0xbfffeba4: 0xbfffec34
(gdb) x/xw 0xbfffec34
0xbfffec34: 0xbfffedc8
(gdb) x/s 0xbfffedc8
0xbfffedc8: "/var/tmp/SO-attempt-to-dereference-generic-pointer/example"
(gdb) x/xw 0xbfffec34 + 4
0xbfffec38: 0xbfffee03
(gdb) x/s 0xbfffee03
0xbfffee03: "first"
(gdb)
But this is annoying and indirect (as pointer programming is wont to be?)

Answer 1

The solution is to cast the pointers before dereferencing them.
For example, picking up where we left off above:
(gdb) x/s **((char ***) (0xc + $ebp))
0xbfffedc8: "/var/tmp/SO-attempt-to-dereference-generic-pointer/example"
(gdb) x/s *(*((char ***) (0xc + $ebp)) + 1)
0xbfffee03: "first"
(gdb) x/s *(*((char ***) (0xc + $ebp)) + 2)
0xbfffee09: "second"
Note that the stack address 0xc + $ebp is itself a pointer to the
contents of that stack location, and so we need char *** and not
char **.