Using #define to force call a function - c

Question

Lets say I have the following code:
void test(void)
{
#define INIT_DONE
//General initialization stuff
}
void test2(void)
{
#ifndef INIT_DONE
#error "Call function test() first!"
#endif
// Specific initialization stuff
}
And then in main() I call these function as follows:
int main(void)
{
test();
test2();
}
And even though I call test() first, and #define INIT_DONE I still get:
"Call function test() first!"
error on the compiler.
So, how can I achieve, that the function test() has to get called first before any other functions. I could do this with some global boolean variable or something, but I am hoping there is a preprocessor way of doing it. Is there?

Answer 1

The preprocessor runs before your code is handled to the compiler. Everything it does happens before your code runs. The prepocessor has no notion of functions or variables, it just copies input to output and expands macros in between (it actually does some more stuff but that's unimportant). For your code, the preprocessor essentially sees this:
gibberish
#define INIT_DONE
// comment
more gibberish
#ifndef INIT_DONE
#error "Call function test() first!"
#endif
// another comment
even more gibberish
The preprocessor walks through that and first sees #define INIT_DONE, so it defines the macro INIT_DONE to 1; every future appearance of INIT_DONE will be replaced by 1 discarded before the compiler sees the code. Then it sees #ifndef INIT_DONE, but INIT_DONE is already defined so it skips the following bit.
The point is that at no point the preprocessor cares about what is being executed. To do what you want to, use something like this:
#include <assert.h>
/* only visible in the source code form where test() and test2() are defined */
static int init_done = 0;
void test(void)
{
init_done = 1;
/* ... */
}
void test2(void)
{
assert(init_done);
/* ... */
}
There is generally no way to do this in the preprocessor since the preprocessor runs before your program runs. You can also leave these checks out and just emphasize that initialization needs to be done in your documentation. Another approach is to not require initialization by the programmer at all, that is useful depending on the circumstances:
static int init_done = 0;
/* same initialization function as before */
void test(void)
{
init_done = 1;
/* ... */
}
void test2(void)
{
if (!init_done)
test();
/* ... */
}