Looping an array of 2d arrays in C

Applications Programming
1

Le sigh... Hopefully this will be the last time I have to ask for help on this topic. For a while now I've been working with a 1d array that holds 2d arrays. For reference you can view here. Now I'm just trying to loop through the elements with the following:

#include <stdio.h>

void arrbyptr(int (*ptr[2][2])[2]) {

int result;

    for ( int i = 0; i < 2; ++i ){
        for ( int j = 0; j < 2; ++j ){
            for ( int k = 0; k < 2; ++k ){
                result = (*ptr[j])[k];
                printf("%d\n", result);
            }
        }
    }
}

int main(void)
{
        int arr2d[2][2]={ {1,2},{3,4} };
        int arr2db[2][2]={ {5,6},{7,8} };

        int (*ptr[2][2])[2]= { { arr2d, arr2db } };

        arrbyptr(ptr);
}

It runs momentarily before segfaulting:

$ ./test
1
2
5
6
Segmentation fault

I get no errors or warnings from gcc. And yes, I understand logical errors will not show up. Tried to run this through strace, but can't find where its failing or why.

Any advice much appreciated.

2

Use gdb. We could probably point you to your problem but you need to learn how to use gdb.
the commands you need are:

compile & debug with

gcc filename.c -Wall -g -o filename
gdb filename
gdb>  break main
r 
[boom! it segfaults gdb will give you the exact line in the file]

Have fun.

1 Like
3

You have to follow the method I showed to-the-letter. (*arr[a][b])[c] is not the same as (*arr)[a][b][c].

void arrbyptr(int (*ptr[2][2])[2]) {
        int result, i, j, k;
    for ( i = 0; i < 2; ++i ){
        for ( j = 0; j < 2; ++j ){
            for ( k = 0; k < 2; ++k ){
                result = (*ptr)[j][k];
                printf("%d\n", result);
            }
        }
    }
}
2 Likes
4

Ah... That did the trick! Sorry, I guess I was confused as main called the function with (*ptr[2][2])[2] and the function's parameter was in the same format. Usually I try to brute-force every possible combination before posting anything, but I guess I missed that.

I haven't used gdb in a while, but I will have to start again. I'm just more used to strace from work.

Fortunately this should do me on calling all the arrays in this project. Next time I should be posting about a different topic. Much thanks again for all of your patience!