Error: No previous prototype for function. Why am I getting this error?

Question

// screen.h

#ifndef screen_h
#define screen_h

#define MAC  1
#define WIN  2
#define LNX  3

#ifdef PLATFORM 
# undef PLATFORM 
#endif

#define PLATFORM MAC
         


        

Answer 1

I just had this problem today.

I defined a function that just used internally

void func(void) {
}

int main(void) {
    func();
}

This will give me that warning. I had to add the prototype at the beginning of the file to get rid of the warning.

void func(void);

void func(void) {
}

int main(void) {
    func();
}