when to free pointer in C and how to know if it is freed

Question

I am new in C, trying to figure out about memory allocation in C that I kinda confused

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

typedef struct
{
    int a;
} struct1_t;

int main()
{
    funct1(); //init pointer
    return 1;
}


int funct2(struct1_t *ptr2struct)
{
    printf("print a is %d\n",ptr2struct->a);
    //free(ptr2struct);
    printf("value of ptr in funct2 is %p\n", ptr2struct);
    return 1; //success
}


int funct1(){

    struct1_t *ptr2struct = NULL;
    ptr2struct = malloc(sizeof(*ptr2struct));
    ptr2struct->a = 5;
    printf("value of ptr before used is %p", ptr2struct);
    if (funct2(ptr2struct) == 0) {
        goto error;
    }
    free(ptr2struct);

    printf("value of ptr in funct1 after freed is is %p\n", ptr2struct);
    return 1;

error:
    if(ptr2struct) free(ptr2struct);
    return 0;
}

I have funct 1 that calls funct 2, and after using the allocated pointer in funct1, I try to free the pointer. And I create a case where if the return value in funct2 is not 1, then try again to free the pointer.

My question is below

which practice is better, if I should free the memory in funct2 (after I pass it) or in funct1 (after I finish getting the return value of funct1) The second thing is whether this is correct to make a goto error, and error:

if(ptr2struct) free(ptr2struct); 

My third question is , how do I check if the allocated value is already freed or not? because after getting the return value, I free the pointer, but if I print it, it shows the same location with the allocated one (so not a null pointer).

Answer 1

Calling free() on a pointer doesn't change it, only marks memory as free. Your pointer will still point to the same location which will contain the same value, but that value can now get overwritten at any time, so you should never use a pointer after it is freed. To ensure that, it is a good idea to always set the pointer to NULL after free'ing it.