Double indirection and structures passed into a function

Question

I am curious why this code works:

typedef struct test_struct {
  int id;
} test_struct;

void test_func(test_struct ** my_struct)
{
test_struct my_test_struct;
my_test_struct.id=267;

*my_struct = &my_test_struct;
}

int main ()
{
test_struct * main_struct;
test_func(&main_struct);    
printf("%d\n",main_struct->id);
}

This works, but pointing to the memory address of a functions local variable is a big no-no, right?

But if i used a structure pointer and malloc, that would be the correct way, right?

void test_func(test_struct ** my_struct)
{
test_struct *my_test_struct;
my_test_struct = malloc(sizeof(test_struct));
my_test_struct->id=267;

*my_struct = my_test_struct;
}

int main ()
{
test_struct * main_struct;
test_func(&main_struct);    
printf("%d\n",main_struct->id);
}

Answer 1

You are right, passing a pointer to something that is allocated on the stack (and therefore goes away when the function returns, is wrong).

Passing a pointer to a heap allocated variable is fine.

"It works" is an illusion. You are returning a pointer to a stack-allocated variable in the first code example.

The pointer will point to garbage -- try dereferencing it...

Answer 2

The first version working is just dumb luck. Try randomly calling something else after test_func returns but before you call printf.

The second version is correct. Of course, you didn't free the heap memory, but whether that matters at the end of a program is a matter of debate.