#include <> files in different files

Question

if I have several header files: let's 1.h,2.h,3.h say that all three header files have #include<stdlib.h>one of the files included in them. when I have to use all 3 header files in file c main.c it will have 3 copies #include<stdlib.h>after the preprocessor. How the compiler handles such a conflict. Is this a mistake or does it create any overhead?

EDIT: if there are no header protectors, what will happen?

Answer 1

Another point: you can reuse a function (or an external variable) bazillion times, and the compiler will accept it:

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

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Answer 2

C :

#ifndef FOO_H
#define FOO_H

/* Header contents here */

#endif

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#ifndef FOO_H
#include <foo.h>
#endif

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Answer 3

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#ifndef __STDLIB_H
  #include <stdlib.h>
  #define __STDLIB_H
#endif

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Answer 4

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Answer 5

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Answer 6

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Answer 7

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#define H_HEADER_NAME_
/* header contents */
#endif

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#ifndef SZ
#define SZ 1024
#else
#if SZ == 1024
#undef SZ
#define SZ 128
#else
#error "You can include me no more than two times!"
#endif
#endif