Deallocation of memory in arrays

Question

I've done a program that opens the file (read binary), and saves all the words(in the file) in an array of char (allocated dynamically in base of the length of the word).
This is the code:

char **leggi_stringhe(const char *filename, size_t *size) {
    FILE *f = fopen(filename, "rb");
    if (f == NULL) {
        *size = 0;
        return NULL;
    }

    int x;

    if (fread(&x, 1, 4, f) != 4) {
        *size = 0;
        return NULL;
    }

    char **stringhe = malloc((x) * sizeof(char));

    for (int i = 0; i < x; i++) {
        int z = 0;
        if (fread(&z, 1, 4, f) != 4) {
            *size = 0;
            return NULL;
        }

        stringhe[i] = malloc((z)* sizeof(char));
        if (fread(stringhe[i], 1, z, f) != z) {
            *size = 0;
            return NULL;
        }
        stringhe[i][z] = 0;
    }
    *size = x;
    fclose(f);
    return stringhe;
}

int main(void) {
    size_t t;
    char **a = leggi_stringhe("file1.bin", &t);

    for (int i = 0; i < t; i++)
        free(a[i]);
    free(a);;
}

The program works, but i have problems with memory deallocation. After calling of leggi_stringhe function, the variable a contains:

a[0] = "first"
a[1] = "second"
a[2] = "third"

but when i'm trying to deallocate the whole a variable as i wrote, the debugger stops with a warning.
I was inspired by this question for writing my code Using Dynamic Memory allocation for arrays, but do not understand why I get this error when i try to deallocate.

Answer 1

Your initial call to malloc is wrong. You allocate space for x characters, not for pointers to char.

Your second call inside the loop is wrong to, as you don't allocate space for the terminator.

Lastly, and unrelated to the problems you ask about, but if the fread calls inside the loop fails, you will have memory leaks.