Question

If I have a several header files :lets say 1.h, 2.h, 3.h.
Let’s say the all three of the header files have #include <stdlib.h> and one of the include files in them.
When I have to use all 3 header files in a C file main.c,
it will have 3 copies of #include <stdlib.h> after the preprocessor.
How does the compiler handle this kind of conflict?
Is this an error or does this create any overhead?

If there are no header guards, what will happen?

Answer 1

Another point: You can redeclare a function (or extern variable) a bazillion times and the compiler will accept it:

int printf(const char*, ...);
int printf(const char*, ...);

is perfectly legal and has a small compilation overhead but no runtime overhead.

That’s what happens when an unguarded include file is included more than once.

Note that it is not true for everything in an include file. You can’t redeclare an enum, for example.