GDB question: Pretty-Printing a 2D Array?

Question

I have a 2d array matrix[10][10] that I'd like to inspect at debug time.

I understand that I can do this in GDB using

p *matrix@10

But it outputs this in one line, making it difficult to read.

Is there a way to have this output formatted in any way, lets say as a matrix?

Answer 1

Important feature of GDB is the ability of execute functions in the debugged code, so you can implement whatever printing you like, for example:

#include <stdio.h>

int matrix[10][10];

void print(int matrix[10][10])
{
    int i, j;
    for (i = 0; i < 10; ++i)
    {
        for (j = 0; j < 10; ++j)
            printf("%d ", matrix[i][j]);
        printf("\n");
    }
}

int main()
{
    int i, j;
    for (i = 0; i < 10; ++i)
        for (j = 0; j < 10; ++j)
            matrix[i][j] = i*10 + j;
    return 0;
}

After compiling this code with -g switch and running under GDB you can use the print function as follows:

(gdb) call print(matrix)
0 1 2 3 4 5 6 7 8 9 
10 11 12 13 14 15 16 17 18 19 
20 21 22 23 24 25 26 27 28 29 
30 31 32 33 34 35 36 37 38 39 
40 41 42 43 44 45 46 47 48 49 
50 51 52 53 54 55 56 57 58 59 
60 61 62 63 64 65 66 67 68 69 
70 71 72 73 74 75 76 77 78 79 
80 81 82 83 84 85 86 87 88 89 
90 91 92 93 94 95 96 97 98 99 

Answer 2

You could always define a function in GDB to do the same thing. If you had the following c code:

#include <stdio.h>
#define ARRAY_SIZE 5

void printArray(int array[ARRAY_SIZE][ARRAY_SIZE]) {
  int y;

  for (y = 0; y < ARRAY_SIZE; y++) {
    printf("[%d,%d,%d,%d,%d]\n",
        array[y][0],
        array[y][1],
        array[y][2],
        array[y][3],
        array[y][4]);
  }
}

int main(int argc, char *argv[]) {
  int matrix[ARRAY_SIZE][ARRAY_SIZE] = {
    {1,2,3,4,5},
    {5,1,2,3,4},
    {4,5,1,2,3},
    {3,4,5,1,2},
    {2,3,4,5,1},
  };

  printArray(matrix);

  return 0;
}

You could then define the following GDB function (or something similar to it):

(gdb) define printMatrix
Type commands for definition of "printmatrix".
End with a line saying just "end".
>set $arr = $arg0
>set $y = 0
>while $y < 5
 >printf "[%d,%d,%d,%d,%d]\n",$arr[$y][0],$arr[$y][1],$arr[$y][2],$arr[$y][3],$arr[$y][4]
 >set $y = $y + 1
 >end
>end

which would result in the following output:

(gdb) printMatrix matrix
[1,2,3,4,5]
[5,1,2,3,4]
[4,5,1,2,3]
[3,4,5,1,2]
[2,3,4,5,1]
(gdb) 

You could just as easily use nested while loops in your GDB function. And, as noted in a previous answer you can always just call the printArray function in your program from within GDB.