Why am I getting junk values despite using calloc()?

Question

I would like to know why the first two elements are always non-zero. I don't know how more I can describe the question, but this is not allowing me to post the question, so I'm writing this. Not sure if this will work.

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

#define SIZE 3

void printMatrix(int **m)
{
    for (int i = 0; i < SIZE; i++) {
        for (int j = 0; j < SIZE; j++)
            printf("%d ", m[i][j]);
        printf("\n");
    }
}

int main(int argc, char const *argv[])
{
    int **matrix;

    matrix = (int **) calloc(sizeof(int), SIZE);

    for (int i = 0; i < SIZE; ++i)
        matrix[i] = (int *) calloc(sizeof(int), SIZE);

    printf("%s\n", "Matrix initialized.");

    printMatrix(matrix);

    return 0;
}

Output:

Matrix initialized.
1371548192 32653 0 
0 0 0 
0 0 0

Answer 1

You're not allocating enough memory:

matrix = (int **) calloc(sizeof(int), SIZE);

Here you're attempting to create an array of 3 int *, but you're only allocating space for 3 int. If a pointer is larger than an int on your system, which it most likely is, you write past the end of the array when you create the arrays for each row. Writing past the end of allocated memory invokes undefined behavior.

Since you're creating an array of int *, use that for the size of each element:

matrix = calloc(sizeof(int *), SIZE);

Also, don't cast the return value of malloc/realloc/calloc, as that can mask a bug if you forget to #include <stdlib.h>