free(): double free detected in tcache 2 Aborted

Question

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
void ffree(char **pp)
{
    if (!pp)
        return;

    char **a = pp;

    while (*pp) {
        free(*pp);  // Free the memory pointed to by pp
        pp++;       // Move to the next pointer
    }

    free(a); // Finally, free the memory for the array of pointers
}

int main() {
    char *str1 = strdup("Hello");
    char *str2 = strdup("World");
    
    char *arr[] = {str1, str2};

    ffree(arr);

    return 0;
}

in the simple code snippet above I'm writing my own array of strings free function where I free the pointer to an array of strings

Is there something wrong with my code?

Answer 1

There are several errors in your program:

First of all, you need to know that you can only free dynamically allocated memory (via malloc() for example), in your code you use strdup() which calls malloc().

char *arr[] = {str1, str2}; is not dynamically allocated, you can't free it.

Secondly, in C, double-dimensional character arrays are delimited by NULL. The following block of code therefore doesn't work, because in arr there is no NULL:

while (*pp) {
    free(*pp);
    pp++;
}

For it to work, you need to add the NULL to the last "line" of your array:

char *arr[] = {str1, str2, NULL};

Also, the malloc() function can fail and you need to systematically check that it doesn't return NULL, at the risk of getting a segfault, so you should do the following:

char *str1, *str2;

if ((str1 = strdup("Hello") == NULL) {
    // Error handling
}

if ((str2 = strdup("World") == NULL) {
    // Error handling
}

Here is a corrected version of your code:

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

void ffree(char **pp)
{
    if (!pp)
        return;

    size_t i = 0;

    while (pp[i]) {
        free(pp[i]);
        i++;
    }

    free(pp);
}

int main(void) {

    char *str1 = strdup("Hello");
    char *str2 = strdup("World");

   if (!str1 || !str2) {
        if (str1) {
            free(str1);
        }
        if (str2) {
            free(str2);
        }
        return (-1)
   }

    char **arr = malloc(3 * sizeof(char *));

    if (!arr) {
        free(str1);
        free(str2);
        return (-1);
    }

    arr[0] = str1;
    arr[1] = str2;
    arr[2] = NULL;

    ffree(arr);

    return (0);
}