c - Is malloc() initializing allocated array to zero?

Question

Here is the code I'm using:

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

int main() {
    int *arr;
    int sz = 100000;
    arr = (int *)malloc(sz * sizeof(int));

    int i;
    for (i = 0; i < sz; ++i) {
        if (arr[i] != 0) {
            printf("OK
");
            break;
        }
    }

    free(arr);
    return 0;
}

The program doesn't print OK. malloc isn't supposed to initialize the allocated memory to zero. Why is this happening?

Answer 1

The man page of malloc says:

The malloc() function allocates size bytes and returns a pointer to the allocated memory. The memory is not initialized. If size is 0, then malloc() returns either NULL, or a unique pointer value that can later be successfully passed to free().

So malloc() returns uninitialized memory, the contents of which is indeterminate.

 if (arr[i] != 0)

In your program, You have tried to access the content of a memory block, which is invoked undefined behavior.