free, make pointer NULL

//-std=c99 -Wall -Wextra -Wpedantic -Wshadow

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

int main()
{

    char *ptr = NULL;

    printf("(1) %p\n", (void*) ptr);

    ptr = malloc(10);

    if (!ptr)
    {
        printf("error: cannot allocate memory.\n");

        return 1;
    }

    printf("(2) %p\n", (void*) ptr);

    free(ptr);

    printf("(3) %p\n", (void*) ptr);

    ptr = NULL;

    printf("(4) %p\n", (void*) ptr);

    return 0;
}

/*
output:

(1) (nil)
(2) 0x557cbfb32670
(3) 0x557cbfb32670
(4) (nil)

*/