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();
}

Answer 2

I met this similar error minutes ago. After i'd added the relatived function declaration in head file, error's gone.
Also, some said that canceling the compile option '-Wmissing-prototypes' should work, but i didn't have tried that. Good luck.

Answer 3

ISO/IEC 9899:TC2 - 6.2.1.2:
A function prototype is a declaration of a function that declares the types of its parameters.

An empty argument list in a function declaration indicates that the number and type of parameters is not known. You must explicitly indicate that the function takes no arguments by using the void keyword. Otherwise your function declaration does not count as a valid prototype.

void screen_init(void);