Save 2D Dynamic Array (Matrix) for Global Access c

Question

I am trying to save a 2D Array that is populated using a function to global memory. Many matrices are processed and upon success, the correct Matrix needs to be saved for access to numerous functions.

Currently, the matrix within the function is dynamically allocated with the following code:

int **M = malloc(m * sizeof(int *));
for(i = 0; i < m; i++)
    M[i] = malloc(n * sizeof(int));

I have a global variable declared (int **M_save). In the main program when the matrix needs to be saved, the global array is initialized using the same procedure:

**M_save = malloc(m * sizeof(int *));
for (i = 0; i < m; i++)
    M_save[i] = malloc(n * sizeof(int));

The function is then notified that it needs to save the matrix before it frees its memory and the following procedure is used in the function to save:

if(saveMatrix == 1) {
for(i = 0; i < m; i++) {
    for(j = 0; j < n; j++) {
        M_save[i][j] = M[i][j];
    }
}

}

When running the program I receive a segmentation fault when the values are being copied (M_save[i][j] = M[i][j]). I am not sure what I am doing wrong.

I checked to make sure than m and n are the same size for both array and after I am done using the matrices they are freed from memory with the following code:

for(i = 0; i < m; i++) {
    free(M[i]);
}
free(M);

I don't believe you can assign complete arrays to each other in that manner using C. To copy values you must iterate through the array and save each value. — milbert
– milbert, Commented Oct 20, 2011 at 19:07

milbert · Accepted Answer · 2011-10-21 13:24:48Z

1

The solution turned out to be very simple. The matrix was defined globally as

int **M_save;

I was assigned it in main using the following:

**M_save = malloc(m * sizeof(int *));
for (i = 0; i < m; i++)
    M_save[i] = malloc(n * sizeof(int));

Instead I should have been using

M_save = malloc(m * sizeof(int *));
for (i = 0; i < m; i++)
    M_save[i] = malloc(n * sizeof(int));

answered Oct 21, 2011 at 13:24

milbert

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