Why I've allocated a pointer memory in a function, but it's also NULL?

Question

The code confused me.

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

void create_int(int *p)
{
    p = (int *) malloc(sizeof(int));
}

int main()
{
    int *p = NULL;

    create_int(p);

    assert(p != NULL);  /* failed. why? I've allocated memory for it. */

    return 0;
}

Answer 1

You are not passing the pointer value back from the function. Try:

void create_int(int **p) {
     *p = (int *) malloc(sizeof(int)); 
}  

int main() {
     int *p = NULL;      
     create_int(&p);
     assert(p != NULL);  /* failed. why? I've allocated memory for it. */
     return 0;
} 

Answer 2

The variable p in the function create_int is a copy of the variable p in main. So any changes made to p in the called function does not get reflected in main.

To make the change get reflected in main you need to either:

Return the changed value:

int* create_int(int *p) {
    p = malloc(sizeof(int));
    // err checking
    return p:
}
...
// in main:
p = create_int(p);

Or pass the address of p as:

void create_int(int **p) {
    *p = malloc(sizeof(int));
    // err checking
}
...
// in main:
create_int(&p);