Question

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behaviour of malloc(0)

I’m trying to understand memory allocation in C. So I am experimenting with malloc. I allotted 0 bytes for this pointer but yet it can still hold an integer. As a matter of fact, no matter what number I put into the parameter of malloc, it can still hold any number I give it. Why is this?

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

int main(void)
{
    int *ptr = (int*)malloc(0);

    *ptr = 9;

    printf("%i", *ptr); // 9

    free(ptr);

    return 0;
}

It still prints 9, what’s up with that?

Answer 1

If size is 0, then malloc() returns either NULL, or a unique pointer
value that can later be successfully passed to free().

I guess you are hitting the 2nd case.
Anyway that pointer just by mistake happens to be in an area where you can write without generating segmentation fault, but you are probably writing in the space of some other variable messing up its value.