Someone explain it... Plz!!!

Question

I'm a bit confused about something in C. I noticed that if I forget to #include <stdio.h>, my code with printf still compiles and runs. I just get a warning.
How does that actually work? I thought it always needed the header file. Why isn't it a hard error, and how does the program figure out where the real printf function is?

https://sololearn.com/compiler-playground/cr6kUJGXqt8x/?ref=app

Answer 1

Ferdous 👾 I believe the compiler is running in C89/C90 compatibility mode by default and the compiler is part of the GNU Compiler Collection (GCC), which historically allowed this as a warning for common cases like printf even in newer modes, though it's discouraged.

Answer 2

No ...
Calling a variadic function without a prototype (relying on implicit declaration) however may cause undefined behavior.

So in C89/C90, are functions like printf or scanf implicit functions?

No - not implicit but default

Answer 3

BroFar So in C89/C90, are functions like printf or scanf implicit functions?