It's because you haven't declared outchar before you use it. That means that the compiler will assume it's a function returning an int and taking an undefined number of undefined arguments.
You need to add a prototype pf the function before you use it:
void outchar(char); /* Prototype (declaration) of a function to be called */ int main(void) { ... } void outchar(char ch) { ... } Note the declaration of the main function differs from your code as well. It's actually a part of the official C specification, it must return an int and must take either a void argument or an int and a char** argument.