C Structures - Change values

Question

I'm trying to change certain values of a struct pointer.

struct gtaVars_s {
    int godmode;
};

struct gtaVars_s *gtaVars;

void test() {
     gtaVars->godmode = 1;
}

Why does test() fail?

I've tried changing the -> to . but that only gave me errors.

Also I've read various articles about structs but none of them seem to give me an answer about this specific problem.

You never allocate memory for `gtaVars` or initialize the pointer in any other meaningful way? — UnholySheep, May 08 '20 at 15:14
In the snippet posted, `gtaVars` has a value of `0` and so you will try to dereference a `NULL` pointer. — Weather Vane, May 08 '20 at 15:15
You might need, for example `gtaVars = malloc(sizeof *gtaVars);` before `gtaVars->godmode = 1;` — Weather Vane, May 08 '20 at 15:17
@ScottHunter is a static variable, which is automatically initialised to `0`, but my comment presumes that `0` is synonymous with `NULL`. — Weather Vane, May 08 '20 at 15:18
@WeatherVane: I guess I missed the `static` keyword, as well as the part of the spec that says uninitialized variables default to 0. — Scott Hunter, May 08 '20 at 15:21
@ScottHunter what you missed is that `struct gtaVars_s *gtaVars;` is not inside any function and therefore is pre-initialised to `0`, in the absence of explicit initialisation. — Weather Vane, May 08 '20 at 15:22

RobertS supports Monica Cellio · Accepted Answer · 2020-05-09T06:59:43.557

Why does test() fail?

test() seems not to be called from main() or one of the called functions from main(), which is required by the C syntax.

I've tried changing the -> to . but that only gave me errors.

That is because you attempt to access a structure object by a pointer. If you use a pointer to access contents of an object of a structure, only -> is allowed.

Why I said "attempt"?

Because you never allocated memory for the structure pointed to by gtaVars in the provided example, so it shall fail in either case.

What you need to do is for example to allocate memory via malloc() and also call test() from main().

Don´t forget to free() dynamically allocated memory to deallocate the memory previously allocated by malloc().

I also passed the pointer as parameter instead of to use a global pointer regarding readability and maintenance:

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

struct gtaVars_s {
    int godmode;
};

void test(struct gtaVars_s *);

int main(void)
{
   struct gtaVars_s *gtaVars = malloc(sizeof(*gtaVars));
   test(gtaVars);

   printf("godmode is %d.",gtaVars->godmode);

   free(gtaVars);
}

void test(struct gtaVars_s *gtaVars) {
     gtaVars->godmode = 1;
}

Output:

godmode is 1.

Personal side notation:

Hasn´t GTA:V already enough available mod-menus with god mode? :-)

I appreciate the example, that explains and helps a lot in my journey to learn C. About your personal side notation, I see you're a man of culture for knowing what this is about. — Appel Flap, May 08 '20 at 18:26

C Structures - Change values

1 Answers1