C语言基础专题 - 头文件引用

#include <stdio.h>     // 引用头文件
int main(){
   printf("Hello World")
}

#include <file>    //  用于引用名为file的系统头文件，默认在系统目录的标准列表中搜索该文件
#include "file"     //  用于引用名为file的用户头文件 ，默认在包含当前文件的目录中搜索该文件

-I dir
-iquote dir
-isystem dir
-idirafter dir

/* Checking macros for stdio functions.
   Copyright (C) 2004, 2005, 2009 Free Software Foundation, Inc.
This file is part of GCC.
GCC is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free
Software Foundation; either version 3, or (at your option) any later
version.
In addition to the permissions in the GNU General Public License, the
Free Software Foundation gives you unlimited permission to link the
compiled version of this file into combinations with other programs,
and to distribute those combinations without any restriction coming
from the use of this file.  (The General Public License restrictions
do apply in other respects; for example, they cover modification of
the file, and distribution when not linked into a combine
executable.)
GCC is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
for more details.
Under Section 7 of GPL version 3, you are granted additional
permissions described in the GCC Runtime Library Exception, version
3.1, as published by the Free Software Foundation.
You should have received a copy of the GNU General Public License and
a copy of the GCC Runtime Library Exception along with this program;
see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
<http://www.gnu.org/licenses/>.  */
#ifndef _SSP_STDIO_H
#define _SSP_STDIO_H 1
#include <ssp.h>
#include_next <stdio.h>
#if __SSP_FORTIFY_LEVEL > 0
#include <stdarg.h>
#undef sprintf
#undef vsprintf
#undef snprintf
#undef vsnprintf
#undef gets
#undef fgets
extern int __sprintf_chk (char *__restrict__ __s, int __flag, size_t __slen,
        __const char *__restrict__ __format, ...);
extern int __vsprintf_chk (char *__restrict__ __s, int __flag, size_t __slen,
         __const char *__restrict__ __format,
         va_list __ap);
#define sprintf(str, ...) \
  __builtin___sprintf_chk (str, 0, __ssp_bos (str), \
         __VA_ARGS__)
#define vsprintf(str, fmt, ap) \
  __builtin___vsprintf_chk (str, 0, __ssp_bos (str), fmt, ap)
extern int __snprintf_chk (char *__restrict__ __s, size_t __n, int __flag,
         size_t __slen, __const char *__restrict__ __format,
         ...);
extern int __vsnprintf_chk (char *__restrict__ __s, size_t __n, int __flag,
          size_t __slen, __const char *__restrict__ __format,
          va_list __ap);
#define snprintf(str, len, ...) \
  __builtin___snprintf_chk (str, len, 0, __ssp_bos (str), __VA_ARGS__)
#define vsnprintf(str, len, fmt, ap) \
  __builtin___vsnprintf_chk (str, len, 0, __ssp_bos (str), fmt, ap)
extern char *__gets_chk (char *__str, size_t);
extern char *__SSP_REDIRECT (__gets_alias, (char *__str), gets);
extern inline __attribute__((__always_inline__)) char *
gets (char *__str)
{
  if (__ssp_bos (__str) != (size_t) -1)
    return __gets_chk (__str, __ssp_bos (__str));
  return __gets_alias (__str);
}
extern char *__SSP_REDIRECT (__fgets_alias,
           (char *__restrict__ __s, int __n,
            FILE *__restrict__ __stream), fgets);
extern inline __attribute__((__always_inline__)) char *
fgets (char *__restrict__ __s, int __n, FILE *__restrict__ __stream)
{
  if (__ssp_bos (__s) != (size_t) -1 && (size_t) __n > __ssp_bos (__s))
    __chk_fail ();
  return __fgets_alias (__s, __n, __stream);
}
#endif /* __SSP_FORTIFY_LEVEL > 0 */
#endif /* _SSP_STDIO_H */

#if CONDITION_1              // 若第1个条件成立
   # include "headfile_1.h"  // 引入头文件"headfile_1.h"
#elif CONDITION_2            // 若第2个条件成立
   # include "headfile_2.h"  // 引入头文件"headfile_2.h"
   ...
#endif                       // 条件指令的结束

#ifndef A                        //  当没有宏定义A时
#define A  包含所有头文件的文件B   // 宏定义A为包含所有头文件的文件B 
#endif                           // 用于结束一个 #if……#else 条件编译块