Example showing matrix addition in c

sqmat_add.c

#include <stdio.h>
#include <stdlib.h>

int* mat_add(int*, int*, int);

int* mat_add(int* a, int* b, int enteries) {
    // enteries * 4 = 2 * 4 = 8 bytes
    int* summation = (int*) calloc(enteries, sizeof(int));
    for (int row = 0; row < enteries; row++) {
        for (int column = 0; column < enteries; column++) {
            int cell = row * enteries + column;
            summation[cell] = a[cell] + b[cell];
        }
    }
    return summation;
}

int main() {
    int a[2][2] = {{1,2}, 
                   {3,4}};
    int b[2][2] = {{3,4}, 
                   {5,6}};
    int* sum = mat_add(a, b, 2);
    
    for (int row = 0; row < 2; row++) {
        for (int column = 0; column < 2; column++) {
            int cell = row * 2 + column;
            printf("%d, ", sum[cell]);
        }
        printf("\n");
    }

    free(sum);
    return 0;
}

Another equivalent example:

#include <stdio.h>
#include <stdlib.h>

int* mat_add(int*, int*, int);

int* mat_add(int* a, int* b, int enteries) {
    // enteries * 4 = 2 * 4 = 8 bytes
    int* summation = (int*) calloc(enteries, sizeof(int));
    for (int row, cell = 0; row < enteries; row++) {
        cell += row;
        for (int column = 0; column < enteries; column++) {
            cell += column;
            summation[cell] = a[cell] + b[cell];
        }
    }
    return summation;
}

int main() {
    int a[2][2] = {{1,2}, 
                   {3,4}};
    int b[2][2] = {{3,4}, 
                   {5,6}};
    int* sum = mat_add(a, b, 2);
    
    for (int row = 0; row < 2; row++) {
        for (int column = 0; column < 2; column++) {
            int cell = row * 2 + column;
            printf("%d, ", sum[cell]);
        }
        printf("\n");
    }

    free(sum);
    return 0;
}

Another simple solution by manipulating arrays in terms of linear memory buffers:

#include <stdio.h>
#include <stdlib.h>

int* mat_add(int*, int*, int);

int* mat_add(int* a, int* b, int enteries) {
    // enteries * 4 = 2 * 4 = 8 bytes
    int* summation = (int*) calloc(enteries, sizeof(int));
    for (int cell = 0; cell < enteries * 2; cell++) {
        summation[cell] = a[cell] + b[cell];
    }
    return summation;
}

int main() {
    int a[2][2] = {{1,2}, 
                   {3,4}};
    int b[2][2] = {{3,4}, 
                   {5,6}};
    int* sum = mat_add(a, b, 2);
    
    for (int row = 0; row < 2; row++) {
        for (int column = 0; column < 2; column++) {
            int cell = row * 2 + column;
            printf("%d, ", sum[cell]);
        }
        printf("\n");
    }

    free(sum);
    return 0;
}