ptrs2.c

// Copyright 2022 Eliot Roxbergh. Licensed under AGPLv3 as per separate LICENSE file
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

/* test0 */
void set_var(int **var)
{
    **var = 4;
    *var = NULL;
}
void test0()
{
    printf("\n\ntest0\n");
    int test = 0;
    int *test_ptr = &test;

    // change test_ptr to be NULL, but first change the value it points to
    set_var(&test_ptr);  // NOTE: you may NOT take &&test, this is not a valid operator for this purpose

    // -> &test is valid ptrs, test = 4, but "test_ptr" is NULL
    printf("%p -> %d and %p -> Seg fault \n", (void *)&test, test, (void *)test_ptr);
}

/* test1 */
const int my_secret = 42;
int get_var(int **ret)  // this works
{
    int *a_ptr;
    int sanity;
    /*
    //will not be persistent ofc, variable will be set but value will be "freed"
    int a = my_secret;
    *ret = &a;
    */

    /*
     * Any memory we create on the stack will be gone after this function's scope, naturally.
     * But we may return addresses that point to data we did not initialize (not local to this scope).
     * my_secret is local to this file and its address will be valid for longer (local to this file)
     */
    a_ptr = (int *)&my_secret;

    *ret = a_ptr;  // OK, pass address (to "global" variable)
    //*ret = (int*) &my_secret; //OK (this is basically the same as above)
    //*ret = &a_ptr;  //(this function then needs to take int ***ret as input), Not OK, we don't want to return address
    //to local stack data (in this case the data is another valid address, but it doesn't matter, it is stored in
    //soon-to-be-freed stack memory)
    sanity = *a_ptr;
    /*
     * We don't really do too much on the stack here.
     * We just copy the address we want to local variable a_ptr,
     * then in-turn copy that value to memory provided by caller.
     *
     * We'd only have a problem if we did something like:
     * ret = &a_ptr;
     *
     */
    return sanity;
}
void test1()
{
    printf("\n\ntest1\n");
    int a = 0;
    int *a_ptr = &a;
    int a_backup = 0;

    a_backup = get_var(&a_ptr);
    printf("%p -> %d (expected %d)\n", (void *)a_ptr, *a_ptr, a_backup);
}

int main()
{
    test0();
    test1();
}