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

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);