stdlib.h 标准库头文件
一、简介
stdlib 头文件即standard library标准库头文件
  stdlib 头文件里包含了C、C++语言的最常用的系统函数
  该文件包含了的C语言标准库函数的定义
stdlib.h里面定义了五种类型、一些宏和通用工具函数。 类型例如size_t、wchar_t、div_t、ldiv_t和lldiv_t； 宏例如EXIT_FAILURE、EXIT_SUCCESS、RAND_MAX和MB_CUR_MAX等等； 常用的函数如malloc()、calloc()、realloc()、free()、system()、atoi()、atol()、rand()、srand()、exit()等等。
二、Stdlib.H具体内容
system的头文件
/***
*stdlib.h - declarations/definitions for commonly used library functions
*
*      Copyright (c) 1985-1997, Microsoft Corporation. All rights reserved.
*
*Purpose:
*      This include file contains the function declarations for commonly
*      used library functions which either don't fit somewhere else, or,
*      cannot be declared in the normal place for other reasons.
*      [ANSI]
*
*      [Public]
*
****/
#if _MSC_VER > 1000
#pragma once
#endif  /* _MSC_VER > 1000 */
#ifndef _INC_STDLIB
#define _INC_STDLIB
#if !defined (_WIN32) && !defined (_MAC)
#error ERROR: Only Mac or Win32 targets supported!
#endif  /* !defined (_WIN32) && !defined (_MAC) */
#ifndef _CRTBLD
/* This version of the header files is NOT for user programs.
* It is intended for use when building the C runtimes ONLY.
* The version intended for public use will not have this message.
*/
#error ERROR: Use of C runtime library internal header file.
#endif  /* _CRTBLD */
#ifdef _MSC_VER
/*
* Currently, all MS C compilers for Win32 platforms default to 8 byte
* alignment.
*/
#pragma pack(push,8)
#endif  /* _MSC_VER */
#ifdef __cplusplus
extern "C" {
#endif  /* __cplusplus */
#ifndef _INTERNAL_IFSTRIP_
#include <cruntime.h>
#endif  /* _INTERNAL_IFSTRIP_ */
/* Define _CRTIMP */
#ifndef _CRTIMP
#ifdef CRTDLL
#define _CRTIMP __declspec(dllexport)
#else  /* CRTDLL */
#ifdef _DLL
#define _CRTIMP __declspec(dllimport)
#else  /* _DLL */
#define _CRTIMP
#endif  /* _DLL */
#endif  /* CRTDLL */
#endif  /* _CRTIMP */
/* Define __cdecl for non-Microsoft compilers */
#if (!defined (_MSC_VER) && !defined (__cdecl))
#define __cdecl
#endif  /* (!defined (_MSC_VER) && !defined (__cdecl)) */
/* Define _CRTAPI1 (for compatibility with the NT SDK) */
#ifndef _CRTAPI1
#if _MSC_VER >= 800 && _M_IX86 >= 300
#define _CRTAPI1 __cdecl
#else  /* _MSC_VER >= 800 && _M_IX86 >= 300 */
#define _CRTAPI1
#endif  /* _MSC_VER >= 800 && _M_IX86 >= 300 */
#endif  /* _CRTAPI1 */
#ifndef _SIZE_T_DEFINED
typedef unsigned int size_t;
#define _SIZE_T_DEFINED
#endif  /* _SIZE_T_DEFINED */
#ifndef _WCHAR_T_DEFINED
typedef unsigned short wchar_t;
#define _WCHAR_T_DEFINED
#endif  /* _WCHAR_T_DEFINED */
/* Define NULL pointer value */
#ifndef NULL
#ifdef __cplusplus
#define NULL    0
#else  /* __cplusplus */
#define NULL    ((void *)0)
#endif  /* __cplusplus */
#endif  /* NULL */
/* Definition of the argument values for the exit() function */
#define EXIT_SUCCESS    0
#define EXIT_FAILURE    1
#ifndef _ONEXIT_T_DEFINED
typedef int (__cdecl * _onexit_t)(void);
#if !__STDC__
/* Non-ANSI name for compatibility */
#define onexit_t _onexit_t
#endif  /* !__STDC__ */
#define _ONEXIT_T_DEFINED
#endif  /* _ONEXIT_T_DEFINED */
/* Data structure definitions for div and ldiv runtimes. */
#ifndef _DIV_T_DEFINED
typedef struct _div_t {
        int quot;
        int rem;
} div_t;
typedef struct _ldiv_t {
        long quot;
        long rem;
} ldiv_t;
#define _DIV_T_DEFINED
#endif  /* _DIV_T_DEFINED */
/* Maximum value that can be returned by the rand function. */
#define RAND_MAX 0x7fff
/*
* Maximum number of bytes in multi-byte character in the current locale
* (also defined in ctype.h).
*/
#ifndef MB_CUR_MAX
#ifndef _INTERNAL_IFSTRIP_
#if defined (_DLL) && defined (_M_IX86)
/* Retained for compatibility with VC++ 5.0 and earlier versions */
_CRTIMP int * __cdecl __p___mb_cur_max(void);
#endif  /* defined (_DLL) && defined (_M_IX86) */
#endif  /* _INTERNAL_IFSTRIP_ */
#define MB_CUR_MAX __mb_cur_max
_CRTIMP extern int __mb_cur_max;
#endif  /* MB_CUR_MAX */
/* Minimum and maximum macros */
#define __max(a,b)  (((a) > (b)) ? (a) : (b))
#define __min(a,b)  (((a) < (b)) ? (a) : (b))
/*
* Sizes for buffers used by the _makepath() and _splitpath() functions.
* note that the sizes include space for 0-terminator
*/
#ifndef _MAC
#define _MAX_PATH  260 /* max. length of full pathname */

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