how to print 2d arry values in gdb

Question

when I use gdb to debug my code, I meet a problem which makes me headache, here is my code snippet:

int getMaxProfits( int *boards, int length, int consecutive )
{
    int optBoards[length+3][length+3];
    memset(optBoards, 0, sizeof( optBoards ) );

    for( int i = length -1; i >= 0; i-- )
    {
        for( int j =  i; j <= length - 1; j++ )
        {
            if( j == i )
            {
                optBoards[i][j] = boards[j];
            }
            else if( j - i < consecutive  )
            {
                optBoards[i][j] = boards[j] + optBoards[i][j-1];
            }
.....

when I tried to print out all the elements in 2-dimensional array "optBoards" by

p optBoards

I found the thing is not as easy as I thought, it gives me

$1 = 0x7fff5fbff330

looks like a memory address, then I tried

p optBoards[0][0]

I got

Cannot perform pointer math on incomplete types, try casting to a known type, or void *.

I keep trying

ptype optBoards

I saw

type = int [][0]

I wildly guess optBoards should be a pointer points to a one-dimension array, hence I tried again

p (int[][0])(*optBoards)[0]

I got a memory address again

$2 = 0x7fff5fbff330

I saw some hope and tried again

p (int[][0])*((*optBoards)[0])

now I get a big 0

$3 = 0x0

I thought I already got the value I want, later I found out after I entering the for loop, optBoards would be assign some value, but no matter what, I always got a big 0 for all the elements of optBoards. I feel lost.

what should I do to print out the correct value for this 2-dimension array?

your help would be greatly appreciated.

score 3 · Accepted Answer · answered Oct 21 '13 at 21:14

You can do it as p ((int (*)[8]) optBoards)[6][2], whereas 8 is length + 3. For example I have:

#pragma GCC diagnostic ignored "-Wunused-variable"
#pragma GCC diagnostic ignored "-Wunused-but-set-variable"

#include <stdio.h>

int foo(int length) {
  int optBoards[length + 3][length + 3];
  int i, j, q;

  for( i = 0, q = 0 ; i < length + 3 ; i++ ) {
    for( j = 0 ; j < length + 3 ; j++ ) {
      optBoards[i][j] = ++q;
      printf("optBoards[%d][%d] = %d\n", i, j, q);
    }
  }

  return 0;
}

int main(int argc, char **argv) {
  foo(5);
  return 0;
}

you can check the array data as:

> gcc -Wall file.c -g -o file.exe

> gdb -q file.exe
Reading symbols from file.exe...done.
(gdb) l 18
13            optBoards[i][j] = ++q;
14            printf("optBoards[%d][%d] = %d\n", i, j, q);
15          }
16        }
17
18        return 0;
19      }
20
21      int main(int argc, char **argv) {
22
(gdb) b 18
Breakpoint 1 at 0x4017eb: file file.c, line 18.
(gdb) run
Starting program: file.exe
[New Thread 2912.0xad8]
optBoards[0][0] = 1
...
optBoards[1][2] = 11
...
optBoards[2][7] = 24
...
optBoards[6][2] = 51
...
optBoards[7][7] = 64

Breakpoint 1, foo (length=5) at file.c:18
18        return 0;
(gdb) p length + 3
$1 = 8
(gdb) p ((int (*)[8]) optBoards)[0][0]
$2 = 1
(gdb) p ((int (*)[8]) optBoards)[1][2]
$3 = 11
(gdb) p ((int (*)[8]) optBoards)[2][7]
$4 = 24
(gdb) p ((int (*)[8]) optBoards)[6][2]
$5 = 51
(gdb) p ((int (*)[8]) optBoards)[7][7]
$6 = 64
(gdb)

thanks a lot. that's really helpful. from you answer, it seems we have to know the length of the array, then we could convert the 'optBoards' to a pointer which would point to fixed length of single dimension array. That's something I don't know before. — xin wei, Oct 22 '13 at 01:14
I can not vote you up, because I dont' have enough reputation. but anyway thanks a lot. — xin wei, Oct 22 '13 at 01:14
I already accepted your answer. sorry for the late, I didn't know how to accept until just now. — xin wei, Oct 28 '13 at 17:51

how to print 2d arry values in gdb

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