c - Why doesn’t putchar require a header?

Question

Reading this answer which explains the polyglot program on page not found on Stack Overflow I was surprised to read putchar was used because you don't need any #include to use it. This seems to be the case, although en.cppreference.com reference and www.cplusplus.com reference show putchar as defined in the stdio.h header.

How can a function be used (correctly) without having a declaration in C? Or is putchar something inbuilt in compiler (like sizeof operator)?

Answer 1

In c, you can use any function without a declaration.

The compiler then assumes, that the function has a return type of int. The parameters are passed to the function as given. Since there is no function declaration, the compiler cannot verify, if the parameters are correct.

putchar is not builtin into the compiler. However, since

The function call putchar(c) shall be equivalent to putc(c,stdout).

it might be defined as a macro, e.g.

#define putchar(c) putc(c, stdout)

In this case, you must include stdio.h to have the correct definition for putchar.