PlagueHammer PlagueHammer - 24 days ago 6
C Question

GDB question: Pretty-Printing a 2D Array?

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

An 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