How to execute a part of code inside a function only once using #ifndef?

Question

//static int initialized;
void print(struct student *arg) {
  #ifndef first_call
  #define first_call 1
  //if (!initialized) {
    //initialized = 1;
    printf("sizeof(*arg1): %lu\n", sizeof(*arg));
  //}
  #endif
  ...
}

I want to execute lines of code within if block only once.

Of course I know how to do that through different way(commented part).

But I want to know why my code doesn't work as I intended.

Thanks.

Answer 1

A preprocessor directive will happen during compilation. Meaning, before your program will run it will take:

//static int initialized;
void print(struct student *arg) {
  #ifndef first_call
  #define first_call 1
  //if (!initialized) {
    //initialized = 1;
    printf("sizeof(*arg1): %lu\n", sizeof(*arg));
  //}
  #endif
  ...
}

And turn it into:

//static int initialized;
void print(struct student *arg) {
  #define first_call 1
  //if (!initialized) {
    //initialized = 1;
    printf("sizeof(*arg1): %lu\n", sizeof(*arg));
  //}
  ...
}

Which means, what you intended is not going to happen. You simply defined first_call as 1.

A temporary variable like initialized would be a good solution to make it run once. Do remember though, local variables are destroyed after you exit this function call.. Hint: Look up static variables..

This would work:

void print(struct student *arg) 
{
    static bool initialized = false;
    if (!initialized) 
    {
        initialized = true;
        printf("sizeof(*arg1): %lu\n", sizeof(*arg));
    }

    ...
}