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Asked: 2026-06-11T15:18:08+00:00

This question is to understand the standard behavior for header file inclusion (not the

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This question is to understand the standard behavior for header file inclusion (not the one implemented on my compiler).

I have two header files with same names (but contents are different):

1) /user/include/myheader.h  # In standard system folder
2) /private/myheader.h       # In my private folder

Assume both the headers contains same multiple inclusion prevension macros

#ifndef MYHEADER
#define MYHEADER
...
#endif

I have C file /private/test.c, which includes both the above headers:

#include <myheader.h>  // Includes from standard system folder
#include "myheader.h"  // Includes from the folder where test.c is present

Do the content from both files go into the C file while pre-processing, since each MYHEADER definition has separate name space ? Or the second inclusion will be prevented since MYHEADER is already defined in same namespace ?

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1 Answer

  1. Editorial Team

    N1570:

    6.10.3 Macro replacement

    …
    7 The identifier immediately following the define is called the macro name. There is one
    name space for macro names.     Any white-space characters preceding or following the
    replacement list of preprocessing tokens are not considered part of the replacement list for either form of macro.

    Emphasis mine.

    As written, only the contents of the first myheader.h file will be processed.

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