Question

I’m experimenting with writing malloc and free wrappers, and I wonder why does the following code gives error pointer being freed was not allocated, why does not delete() work?

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

#define log(v) printf(#v " == %d \n", v)
#define new(n, type) _new((n), sizeof(type), __LINE__, __FILE__)

void *_new(int n, size_t size, int line, char *file)
{
    int *ptr;
    ptr = malloc(n * size);

    if (ptr == NULL) 
    {
        printf("new(): Memory allocation error, file \"%s\", line %d. \n", file, line);
        exit(EXIT_FAILURE);
    }

    return ptr;
}

void delete(int *ptr)
{
    free(*ptr);
    *ptr = NULL;
}


main()
{  
    int *p;

    p = new(1, int);
    log(p);

    delete(&p);
    log(p);
}

Answer 1

Since you,

int *p;
p = new(1, int);
delete(&p);

Then you should

void delete(int** ptr) //two ** here!!
{
    free(*ptr);
    *ptr = NULL;
}