c - What will happen if I don't include header files

Question

What will happen if I don't include the header files when running a c program? I know that I get warnings, but the programs runs perfectly.

I know that the header files contain function declarations. Therefore when I don't include them, how does the compiler figure it out? Does it check all the header files?

Answer 1

I know that I get warnings, but the programs runs perfectly.

That is an unfortunate legacy of pre-ANSI C: the language did not require function prototypes, so the standard C allows it to this day (usually, a warning can be produced to find functions called without a prototype).

When you call a function with no prototype, C compiler makes assumptions about the function being called:

• Function's return type is assumed to be int
• All parameters are assumed to be declared (i.e. no ... vararg stuff)
• All parameters are assumed to be whatever you pass after default promotions, and so on.

If the function being called with no prototype fits these assumptions, your program will run correctly; otherwise, it's undefined behavior.