Using Malloc to add two arrays HW in C

Question

This program is supposed to ask the user to enter two matrices and provide the sum of the two. When compiled it does not work as expected, I believe it is due to my use of malloc, if anyone can help this would be greatly appreciated.

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


/*Here we declare and define our function 'matrices', which prompts the
 user for Matrix A and Matrix B and stores their values.*/

int matrices(int rows, int cols){

    int i;

    int** matrixA = malloc(rows * sizeof(**matrixA));

    int** matrixB = malloc(rows * sizeof(**matrixB));

    printf("Enter Matrix A\n");

    for (i = 0; i < rows; i++){
        matrixA[i] = (int *) malloc(cols * sizeof(int));

         scanf("%d", matrixA[i]);

    }

    printf("Enter Matrix B\n");

    for (i = 0; i < rows; i++){
        matrixB[i] = (int *) malloc(cols * sizeof(int));

         scanf("%d", matrixB[i]);

    }

    return 0;
}

/*Here we declare and define our function 'sum', which sums Matrix A and 
Matrix B and provides us with the summation of the two in the 
variable 'matrixSum'*/

int sum(int rows, int cols){

    int i;

    int** matrixA = malloc(rows * sizeof(**matrixA));

    int** matrixB = malloc(rows * sizeof(**matrixB));

    int** matrixSum = malloc(rows * sizeof(**matrixSum));



    printf("A + B =\n");

    for (i = 0; i < rows; i++) {

        matrixA[i] = (int *) malloc(cols * sizeof(int));
        matrixB[i] = (int *) malloc(cols * sizeof(int));
        matrixSum[i] = (int *) malloc(cols * sizeof(int));

           *matrixSum[i] = *matrixA[i] + *matrixB[i];
        printf("%d\t", *matrixSum[i]);


            printf("\n");
    }

    return 0;
}
/*Here we declare and define our main function which works to prompt the user for the number of rows and columns and calls the previously declared functions. */

int main(void){

    int rows, cols;

    int** matrixA = malloc(rows * sizeof(**matrixA));

    int** matrixB = malloc(rows * sizeof(**matrixB));

    int** matrixSum = malloc(rows * sizeof(**matrixSum));

    printf("Please enter the number of rows: ");
    scanf("%d", &rows);
    printf("Please enter the number of columns: ");
    scanf("%d", &cols);


    matrices(rows, cols);
    sum(rows, cols);

    free(matrixA);
    free(matrixB);
    free(matrixSum);

    return 0;
}

Answer 1

• The size of what is pointed by int** matrixA; will be sizeof(*matrixA), not sizeof(**matrixA). In environments in which size of pointers and int differs, using latter will make the buffer too small.
• In the function matrices(), some input are read and then discarded. Memory leak also happens.
• In function sum(), indeterminate values in buffer allocated via malloc() and not initialized are used and undefined behavior is invoked.
• They say you shouldn't cast the result of malloc() in C.

To correct:

• Do proper size calculation.
• Avoid causing memory leak, keep and pass what is read.
• Use loops to read all elements, not only the first elements of each row.

Here is a fixed code with error checking:

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

void matrices(int rows, int cols, int*** matrixA, int*** matrixB){

    int i, j;

    *matrixA = malloc(rows * sizeof(**matrixA));
    if (*matrixA == NULL){
        perror("matrixA malloc");
        exit(1);
    }

    *matrixB = malloc(rows * sizeof(**matrixB));
    if (*matrixB == NULL){
        perror("matrixB malloc");
        exit(1);
    }

    printf("Enter Matrix A\n");

    for (i = 0; i < rows; i++){
        (*matrixA)[i] = malloc(cols * sizeof(int));
        if ((*matrixA)[i] == NULL){
            perror("matrixA[i] malloc");
            exit(1);
        }

        for (j = 0; j < cols; j++){
            if (scanf("%d", &(*matrixA)[i][j]) != 1){
                fputs("matrixA read error\n", stderr);
                exit(1);
            }
        }

    }

    printf("Enter Matrix B\n");

    for (i = 0; i < rows; i++){
        (*matrixB)[i] =  malloc(cols * sizeof(int));
        if ((*matrixB)[i] == NULL){
            perror("matrixB[i] malloc");
            exit(1);
        }

        for (j = 0; j < cols; j++){
            if (scanf("%d", &(*matrixB)[i][j]) != 1){
                fputs("matrixB read error\n", stderr);
                exit(1);
            }
        }

    }

}

void sum(int rows, int cols, int** matrixA, int** matrixB, int*** matrixSum){

    int i, j;

    *matrixSum = malloc(rows * sizeof(**matrixSum));
    if (*matrixSum == NULL){
        perror("matrixSum malloc");
        exit(1);
    }

    printf("A + B =\n");

    for (i = 0; i < rows; i++) {

        (*matrixSum)[i] = malloc(cols * sizeof(int));
        if ((*matrixSum)[i] == NULL){
            perror("matrixSum[i] malloc");
            exit(1);
        }

       for (j = 0; j < cols; j++){
           (*matrixSum)[i][j] = matrixA[i][j] + matrixB[i][j];
           printf("%d\t", (*matrixSum)[i][j]);
       }

        printf("\n");
    }

}
/*Here we declare and define our main function which works to prompt the user for the number of rows and columns and calls the previously declared functions. */

int main(void){

    int rows, cols;

    int i;

    int** matrixA;
    int** matrixB;
    int** matrixSum;

    printf("Please enter the number of rows: ");
    if (scanf("%d", &rows) != 1){
        fputs("rows read error\n", stderr);
        return 1;
    }
    printf("Please enter the number of columns: ");
    if (scanf("%d", &cols) != 1){
        fputs("cols read error\n", stderr);
        return 1;
    }

    matrices(rows, cols, &matrixA, &matrixB);
    sum(rows, cols, matrixA, matrixB, &matrixSum);

    for (i = 0; i < rows; i++) {
        free(matrixA[i]);
        free(matrixB[i]);
        free(matrixSum[i]);
    }

    free(matrixA);
    free(matrixB);
    free(matrixSum);

    return 0;
}

Note that what is pointed by *matrixA is **matrixA, so using sizeof(**matrixA) is now correct.