function to clear malloc, and make pointer to null - c

Question

in my last question, I've asked how to use function to free an malloc'ed array, I wanted to improve my code so that the function won't just free the memory but also will set the pointer to NULL once it finishes the clearing.
Also I want a single function to do both - setting and clearing, depending on the command I'm passing, this is what I've done so far:
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <stdint-gcc.h>
char **set_arr(int number, char *command);
int command_read(char *command);
void clear_arr(char *arr[], int size);
char set[] = "set";
char clear[] = "clear";
int main() {
int num = // get number from user;
char** my_arr = NULL;
my_arr = set_arr(num, set);
// so far the code works as excepted
set_arr((size_t)&my_arr, clear);
return 0;
}
int command_read(char *command) {
if (strcmp(command, set) == 0)
return 'S';
if (strcmp(command, clear) == 0)
return 'C';
}
char **set_arr(int number, char *command) {
static char **arr = NULL;
static int size;
switch (command_read(command)) {
case 'S':
size = (int)number;
arr = malloc((size + 1) * sizeof(char *));
for (int i = 0; i <= size; i++) {
arr[i] = NULL;
if (i == size)
break;
arr[i] = malloc((string_len) * sizeof(char));
}
break;
case 'C':
clear_arr(arr, size);
free(arr);
uintptr_t value = number;
uint64_t *temp = (void *)value;
*temp = 0x0;
break;
}
return arr;
}
void clear_arr(char *arr[], int size) {
for (int i = 0; i < size; i++) {
free(arr[i]);
arr[i] = NULL;
}
}
I know that there is better methods to clear (and allocate memory?) but my primary question is, did I free all the memory I allocated for the array, and after the clearing, does the pointer my_arr is set correctly to NULL?

Answer 1

Writing a generic function to achieve your goal is not possible in Standard C because pointers to different types of objects may have a different representation so you cannot pass the address of a pointer and expect the function to handle it in a generic manner.
Yet this provision in the C Standard is not used on most current systems today. In particular, the POSIX standard mandates that all pointers have the same representation. Hence your generic function can work on these systems, with some precautions to avoid compilation warnings:
// free an array of allocated things
void free_array(void ***p, size_t count) {
void **array = *p;
for (size_t i = 0; i < count; i++) {
free(array[i]);
array[i] = NULL; // for safety
}
free(array);
*p = NULL;
}
// deal with the non portable conversion with macros
#define FREE_ARRAY(p, n) free_array((void ***)(void *)&(p), n)
// allocate an array of pointers to allocated things of size `size`.
// return a pointer to the array or `NULL` if any allocation failed
void **malloc_array(size_t count, size_t size) {
void **array = malloc(count * sizeof(*array));
if (array) {
for (size_t i = 0; i < count; i++) {
array[i] = calloc(size, 1); // allocate and initialize to all bits zero
if (array[i] == NULL) {
while (i-- > 0) {
free(array[i]);
array[i] = NULL;
}
return NULL;
}
}
}
return array;
}
#define MALLOC_ARRAY(n, type) ((type **)(void *)malloc_array(n, sizeof(type)))
#define MALLOC_2D_ARRAY(n1, n2, type) ((type **)(void *)malloc_array(n1, (n2) * sizeof(type)))
Passing the command as a string is very inefficient. You should use an int or an enum for the command, but you can use the above macros and code in your program this way:
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <stdint-gcc.h>
int main() {
int string_len = 100;
int num = 10; // get number from user;
char **my_arr = MALLOC_2D_ARRAY(num, string_len, char);
FREE_ARRAY(my_arr, num);
return 0;
}