首先说一下源码到底在哪里找。
我们在文件中包含<cstring>时，如果点击右键打开文档，
会打开cstring,我们会发现路径为：
D:\Program Files\visual studio\VC\include\cstring
这个文件内容如下：
// cstring standard header
#pragma once
#ifndef _CSTRING_
#define _CSTRING_
#include <yvals.h> #ifdef _STD_USING
#undef _STD_USING
#include <string.h>
#define _STD_USING #else /* _STD_USING */
#include <string.h>
#endif /* _STD_USING */ #if _GLOBAL_USING && !defined(RC_INVOKED)
_STD_BEGIN
using _CSTD size_t; using _CSTD memchr; using _CSTD memcmp; using _CSTD memcpy; using _CSTD memmove; using _CSTD memset;
using _CSTD strcat; using _CSTD strchr; using _CSTD strcmp;
using _CSTD strcoll; using _CSTD strcpy; using _CSTD strcspn;
using _CSTD strerror; using _CSTD strlen; using _CSTD strncat;
using _CSTD strncmp; using _CSTD strncpy; using _CSTD strpbrk;
using _CSTD strrchr; using _CSTD strspn; using _CSTD strstr;
using _CSTD strtok; using _CSTD strxfrm;
_STD_END
#endif /* _GLOBAL_USING */ #endif /* _CSTRING_ */ /*
* Copyright (c) 1992-2009 by P.J. Plauger. ALL RIGHTS RESERVED.
* Consult your license regarding permissions and restrictions.
V5.20:0009 */
它没有cpp文件。包含了<string.h>头文件，也在这个目录下，内容如下；
/***
*string.h - declarations for string manipulation functions
*
* Copyright (c) Microsoft Corporation. All rights reserved.
*
*Purpose:
* This file contains the function declarations for the string
* manipulation functions.
* [ANSI/System V]
*
* [Public]
*
****/ #pragma once #ifndef _INC_STRING
#define _INC_STRING #include <crtdefs.h> #ifdef __cplusplus
extern "C" {
#endif #ifndef _NLSCMP_DEFINED
#define _NLSCMPERROR 2147483647 /* currently == INT_MAX */
#define _NLSCMP_DEFINED
#endif /* Define NULL pointer value */
#ifndef NULL
#ifdef __cplusplus
#define NULL 0
#else
#define NULL ((void *)0)
#endif
#endif /* For backwards compatibility */
#define _WConst_return _CONST_RETURN /* Function prototypes */
#ifndef _CRT_MEMORY_DEFINED
#define _CRT_MEMORY_DEFINED
_CRTIMP void * __cdecl _memccpy( _Out_opt_bytecap_(_MaxCount) void * _Dst, _In_ const void * _Src, _In_ int _Val, _In_ size_t _MaxCount);
_Check_return_ _CRTIMP _CONST_RETURN void * __cdecl memchr( _In_opt_bytecount_(_MaxCount) const void * _Buf , _In_ int _Val, _In_ size_t _MaxCount);
_Check_return_ _CRTIMP int __cdecl _memicmp(_In_opt_bytecount_(_Size) const void * _Buf1, _In_opt_bytecount_(_Size) const void * _Buf2, _In_ size_t _Size);
_Check_return_ _CRTIMP int __cdecl _memicmp_l(_In_opt_bytecount_(_Size) const void * _Buf1, _In_opt_bytecount_(_Size) const void * _Buf2, _In_ size_t _Size, _In_opt_ _locale_t _Locale);
_Check_return_ int __cdecl memcmp(_In_opt_bytecount_(_Size) const void * _Buf1, _In_opt_bytecount_(_Size) const void * _Buf2, _In_ size_t _Size);
_CRT_INSECURE_DEPRECATE_MEMORY(memcpy_s) void * __cdecl memcpy(_Out_opt_bytecapcount_(_Size) void * _Dst, _In_opt_bytecount_(_Size) const void * _Src, _In_ size_t _Size);
#if __STDC_WANT_SECURE_LIB__
_CRTIMP errno_t __cdecl memcpy_s(_Out_opt_bytecap_post_bytecount_(_DstSize, _MaxCount) void * _Dst, _In_ rsize_t _DstSize, _In_opt_bytecount_(_MaxCount) const void * _Src, _In_ rsize_t _MaxCount);
#if defined(__cplusplus) && _CRT_SECURE_CPP_OVERLOAD_STANDARD_NAMES_MEMORY
extern "C++"
{
#ifndef _CRT_ENABLE_IF_DEFINED
#define _CRT_ENABLE_IF_DEFINED
template<bool _Enable, typename _Ty>
struct _CrtEnableIf; template<typename _Ty>
struct _CrtEnableIf<true, _Ty>
{
typedef _Ty _Type;
};
#endif
template <size_t _Size, typename _DstType>
inline
typename _CrtEnableIf<(_Size > ), void *>::_Type __cdecl memcpy(_DstType (&_Dst)[_Size], _In_opt_bytecount_(_SrcSize) const void *_Src, _In_ size_t _SrcSize) _CRT_SECURE_CPP_NOTHROW
{
return memcpy_s(_Dst, _Size * sizeof(_DstType), _Src, _SrcSize) == ? _Dst : ;
}
}
#endif
#if defined(__cplusplus) && _CRT_SECURE_CPP_OVERLOAD_SECURE_NAMES_MEMORY
extern "C++"
{
template <size_t _Size, typename _DstType>
inline
errno_t __CRTDECL memcpy_s(_DstType (&_Dst)[_Size], _In_opt_bytecount_(_SrcSize) const void * _Src, _In_ rsize_t _SrcSize) _CRT_SECURE_CPP_NOTHROW
{
return memcpy_s(_Dst, _Size * sizeof(_DstType), _Src, _SrcSize);
}
}
#endif
#endif
void * __cdecl memset(_Out_opt_bytecapcount_(_Size) void * _Dst, _In_ int _Val, _In_ size_t _Size); #if !__STDC__
/* Non-ANSI names for compatibility */
_CRT_NONSTDC_DEPRECATE(_memccpy) _CRTIMP void * __cdecl memccpy(_Out_opt_bytecap_(_Size) void * _Dst, _In_opt_bytecount_(_Size) const void * _Src, _In_ int _Val, _In_ size_t _Size);
_Check_return_ _CRT_NONSTDC_DEPRECATE(_memicmp) _CRTIMP int __cdecl memicmp(_In_opt_bytecount_(_Size) const void * _Buf1, _In_opt_bytecount_(_Size) const void * _Buf2, _In_ size_t _Size);
#endif /* __STDC__ */ #endif _Check_return_wat_ _CRTIMP_ALTERNATIVE errno_t __cdecl _strset_s(_Inout_z_cap_(_DstSize) char * _Dst, _In_ size_t _DstSize, _In_ int _Value);
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_1(errno_t, _strset_s, _Deref_prepost_z_ char, _Dest, _In_ int, _Value)
__DEFINE_CPP_OVERLOAD_STANDARD_FUNC_0_1(char *, __RETURN_POLICY_DST, __EMPTY_DECLSPEC, _strset, _Inout_z_, char, _Dest, _In_ int, _Value)
#if __STDC_WANT_SECURE_LIB__
_Check_return_wat_ _CRTIMP_ALTERNATIVE errno_t __cdecl strcpy_s(_Out_z_cap_(_SizeInBytes) char * _Dst, _In_ rsize_t _SizeInBytes, _In_z_ const char * _Src);
#endif
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_1(errno_t, strcpy_s, _Deref_post_z_ char, _Dest, _In_z_ const char *, _Source)
__DEFINE_CPP_OVERLOAD_STANDARD_FUNC_0_1(char *, __RETURN_POLICY_DST, __EMPTY_DECLSPEC, strcpy, _Pre_cap_for_(_Source) _Post_z_, char, _Dest, _In_z_ const char *, _Source)
#if __STDC_WANT_SECURE_LIB__
_Check_return_wat_ _CRTIMP_ALTERNATIVE errno_t __cdecl strcat_s(_Inout_z_cap_(_SizeInBytes) char * _Dst, _In_ rsize_t _SizeInBytes, _In_z_ const char * _Src);
#endif
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_1(errno_t, strcat_s, _Deref_prepost_z_ char, _Dest, _In_z_ const char *, _Source)
__DEFINE_CPP_OVERLOAD_STANDARD_FUNC_0_1(char *, __RETURN_POLICY_DST, __EMPTY_DECLSPEC, strcat, _Pre_cap_for_(_Source) _Prepost_z_, char, _Dest, _In_z_ const char *, _Source)
_Check_return_ int __cdecl strcmp(_In_z_ const char * _Str1, _In_z_ const char * _Str2);
_Check_return_ size_t __cdecl strlen(_In_z_ const char * _Str);
_Check_return_ _CRTIMP size_t __cdecl strnlen(_In_z_ const char * _Str, _In_ size_t _MaxCount);
#if __STDC_WANT_SECURE_LIB__ && !defined (__midl)
_Check_return_ static __inline size_t __CRTDECL strnlen_s(_In_z_ const char * _Str, _In_ size_t _MaxCount)
{
return (_Str==) ? : strnlen(_Str, _MaxCount);
}
#endif
#if __STDC_WANT_SECURE_LIB__
_Check_return_wat_ _CRTIMP errno_t __cdecl memmove_s(_Out_opt_bytecap_post_bytecount_(_DstSize,_MaxCount) void * _Dst, _In_ rsize_t _DstSize, _In_opt_bytecount_(_MaxCount) const void * _Src, _In_ rsize_t _MaxCount);
#endif #if defined(_M_IA64)
_CRT_INSECURE_DEPRECATE_MEMORY(memmove_s) void * __cdecl memmove(_Out_opt_bytecapcount_(_Size) void * _Dst, _In_opt_bytecount_(_Size) const void * _Src, _In_ size_t _Size);
#else /* defined (_M_IA64) */
_CRTIMP _CRT_INSECURE_DEPRECATE_MEMORY(memmove_s) void * __cdecl memmove(_Out_opt_bytecapcount_(_Size) void * _Dst, _In_opt_bytecount_(_Size) const void * _Src, _In_ size_t _Size);
#endif /* defined (_M_IA64) */ #if defined(_DEBUG) && defined(_CRTDBG_MAP_ALLOC)
#pragma push_macro("_strdup")
#undef _strdup
#endif _Check_return_ _CRTIMP char * __cdecl _strdup(_In_opt_z_ const char * _Src); #if defined(_DEBUG) && defined(_CRTDBG_MAP_ALLOC)
#pragma pop_macro("_strdup")
#endif _Check_return_ _CRTIMP _CONST_RETURN char * __cdecl strchr(_In_z_ const char * _Str, _In_ int _Val);
_Check_return_ _CRTIMP int __cdecl _stricmp(_In_z_ const char * _Str1, _In_z_ const char * _Str2);
_Check_return_ _CRTIMP int __cdecl _strcmpi(_In_z_ const char * _Str1, _In_z_ const char * _Str2);
_Check_return_ _CRTIMP int __cdecl _stricmp_l(_In_z_ const char * _Str1, _In_z_ const char * _Str2, _In_opt_ _locale_t _Locale);
_Check_return_ _CRTIMP int __cdecl strcoll(_In_z_ const char * _Str1, _In_z_ const char * _Str2);
_Check_return_ _CRTIMP int __cdecl _strcoll_l(_In_z_ const char * _Str1, _In_z_ const char * _Str2, _In_opt_ _locale_t _Locale);
_Check_return_ _CRTIMP int __cdecl _stricoll(_In_z_ const char * _Str1, _In_z_ const char * _Str2);
_Check_return_ _CRTIMP int __cdecl _stricoll_l(_In_z_ const char * _Str1, _In_z_ const char * _Str2, _In_opt_ _locale_t _Locale);
_Check_return_ _CRTIMP int __cdecl _strncoll (_In_z_ const char * _Str1, _In_z_ const char * _Str2, _In_ size_t _MaxCount);
_Check_return_ _CRTIMP int __cdecl _strncoll_l(_In_z_ const char * _Str1, _In_z_ const char * _Str2, _In_ size_t _MaxCount, _In_opt_ _locale_t _Locale);
_Check_return_ _CRTIMP int __cdecl _strnicoll (_In_z_ const char * _Str1, _In_z_ const char * _Str2, _In_ size_t _MaxCount);
_Check_return_ _CRTIMP int __cdecl _strnicoll_l(_In_z_ const char * _Str1, _In_z_ const char * _Str2, _In_ size_t _MaxCount, _In_opt_ _locale_t _Locale);
_Check_return_ _CRTIMP size_t __cdecl strcspn(_In_z_ const char * _Str, _In_z_ const char * _Control);
_Check_return_ _CRT_INSECURE_DEPRECATE(_strerror_s) _CRTIMP char * __cdecl _strerror(_In_opt_z_ const char * _ErrMsg);
_Check_return_wat_ _CRTIMP errno_t __cdecl _strerror_s(_Out_z_cap_(_SizeInBytes) char * _Buf, _In_ size_t _SizeInBytes, _In_opt_z_ const char * _ErrMsg);
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_1(errno_t, _strerror_s, char, _Buffer, _In_opt_z_ const char *, _ErrorMessage)
_Check_return_ _CRT_INSECURE_DEPRECATE(strerror_s) _CRTIMP char * __cdecl strerror(_In_ int);
#if __STDC_WANT_SECURE_LIB__
_Check_return_wat_ _CRTIMP errno_t __cdecl strerror_s(_Out_z_cap_(_SizeInBytes) char * _Buf, _In_ size_t _SizeInBytes, _In_ int _ErrNum);
#endif
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_1(errno_t, strerror_s, char, _Buffer, _In_ int, _ErrorMessage)
_Check_return_wat_ _CRTIMP errno_t __cdecl _strlwr_s(_Inout_z_cap_(_Size) char * _Str, _In_ size_t _Size);
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_0(errno_t, _strlwr_s, _Deref_prepost_z_ char, _String)
__DEFINE_CPP_OVERLOAD_STANDARD_FUNC_0_0(char *, __RETURN_POLICY_DST, _CRTIMP, _strlwr, _Inout_z_, char, _String)
_Check_return_wat_ _CRTIMP errno_t __cdecl _strlwr_s_l(_Inout_z_cap_(_Size) char * _Str, _In_ size_t _Size, _In_opt_ _locale_t _Locale);
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_1(errno_t, _strlwr_s_l, _Deref_prepost_z_ char, _String, _In_opt_ _locale_t, _Locale)
__DEFINE_CPP_OVERLOAD_STANDARD_FUNC_0_1_EX(char *, __RETURN_POLICY_DST, _CRTIMP, _strlwr_l, _strlwr_s_l, _Deref_inout_z_cap_c_(_Size) char, _Inout_z_, char, _String, _In_opt_ _locale_t, _Locale)
#if __STDC_WANT_SECURE_LIB__
_Check_return_wat_ _CRTIMP_ALTERNATIVE errno_t __cdecl strncat_s(_Inout_z_cap_(_SizeInBytes) char * _Dst, _In_ rsize_t _SizeInBytes, _In_z_ const char * _Src, _In_ rsize_t _MaxCount);
#endif
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_2(errno_t, strncat_s, _Deref_prepost_z_ char, _Dest, _In_z_ const char *, _Source, _In_ size_t, _Count)
#pragma warning(push)
#pragma warning(disable:6059)
/* prefast noise VSW 489802 */
__DEFINE_CPP_OVERLOAD_STANDARD_NFUNC_0_2_EX(char *, __RETURN_POLICY_DST, _CRTIMP, strncat, strncat_s, _Deref_inout_z_cap_c_(_Size) char, _Inout_z_cap_(_Count), char, _Dest, _In_z_ const char *, _Source, _In_ size_t, _Count)
#pragma warning(pop)
#if defined(_M_IA64)
_Check_return_ int __cdecl strncmp(_In_z_ const char * _Str1, _In_z_ const char * _Str2, _In_ size_t _MaxCount);
#else
_Check_return_ _CRTIMP int __cdecl strncmp(_In_z_ const char * _Str1, _In_z_ const char * _Str2, _In_ size_t _MaxCount);
#endif
_Check_return_ _CRTIMP int __cdecl _strnicmp(_In_z_ const char * _Str1, _In_z_ const char * _Str2, _In_ size_t _MaxCount);
_Check_return_ _CRTIMP int __cdecl _strnicmp_l(_In_z_ const char * _Str1, _In_z_ const char * _Str2, _In_ size_t _MaxCount, _In_opt_ _locale_t _Locale);
#if __STDC_WANT_SECURE_LIB__
_Check_return_wat_ _CRTIMP_ALTERNATIVE errno_t __cdecl strncpy_s(_Out_z_cap_(_SizeInBytes) char * _Dst, _In_ rsize_t _SizeInBytes, _In_z_ const char * _Src, _In_ rsize_t _MaxCount);
#endif
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_2(errno_t, strncpy_s, char, _Dest, _In_z_ const char *, _Source, _In_ size_t, _Count)
__DEFINE_CPP_OVERLOAD_STANDARD_NFUNC_0_2_EX(char *, __RETURN_POLICY_DST, _CRTIMP_NOIA64, strncpy, strncpy_s, _Deref_out_z_cap_c_(_Size) char, _Out_cap_(_Count) _Post_maybez_, char, _Dest, _In_z_ const char *, _Source, _In_ size_t, _Count)
_Check_return_wat_ _CRTIMP_ALTERNATIVE errno_t __cdecl _strnset_s(_Inout_z_cap_(_SizeInBytes) char * _Str, _In_ size_t _SizeInBytes, _In_ int _Val, _In_ size_t _MaxCount);
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_2(errno_t, _strnset_s, _Deref_prepost_z_ char, _Dest, _In_ int, _Val, _In_ size_t, _Count)
__DEFINE_CPP_OVERLOAD_STANDARD_NFUNC_0_2_EX(char *, __RETURN_POLICY_DST, _CRTIMP, _strnset, _strnset_s, _Deref_inout_z_cap_c_(_Size) char, _Inout_z_cap_(_Count), char, _Dest, _In_ int, _Val, _In_ size_t, _Count)
_Check_return_ _CRTIMP _CONST_RETURN char * __cdecl strpbrk(_In_z_ const char * _Str, _In_z_ const char * _Control);
_Check_return_ _CRTIMP _CONST_RETURN char * __cdecl strrchr(_In_z_ const char * _Str, _In_ int _Ch);
_CRTIMP char * __cdecl _strrev(_Inout_z_ char * _Str);
_Check_return_ _CRTIMP size_t __cdecl strspn(_In_z_ const char * _Str, _In_z_ const char * _Control);
_Check_return_ _CRTIMP _CONST_RETURN char * __cdecl strstr(_In_z_ const char * _Str, _In_z_ const char * _SubStr);
_Check_return_ _CRT_INSECURE_DEPRECATE(strtok_s) _CRTIMP char * __cdecl strtok(_Inout_opt_z_ char * _Str, _In_z_ const char * _Delim);
#if __STDC_WANT_SECURE_LIB__
_Check_return_ _CRTIMP_ALTERNATIVE char * __cdecl strtok_s(_Inout_opt_z_ char * _Str, _In_z_ const char * _Delim, _Inout_ _Deref_prepost_opt_z_ char ** _Context);
#endif
_Check_return_wat_ _CRTIMP errno_t __cdecl _strupr_s(_Inout_z_cap_(_Size) char * _Str, _In_ size_t _Size);
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_0(errno_t, _strupr_s, _Deref_prepost_z_ char, _String)
__DEFINE_CPP_OVERLOAD_STANDARD_FUNC_0_0(char *, __RETURN_POLICY_DST, _CRTIMP, _strupr, _Inout_z_, char, _String)
_Check_return_wat_ _CRTIMP errno_t __cdecl _strupr_s_l(_Inout_z_cap_(_Size) char * _Str, _In_ size_t _Size, _locale_t _Locale);
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_1(errno_t, _strupr_s_l, _Deref_prepost_z_ char, _String, _locale_t, _Locale)
__DEFINE_CPP_OVERLOAD_STANDARD_FUNC_0_1_EX(char *, __RETURN_POLICY_DST, _CRTIMP, _strupr_l, _strupr_s_l, _Deref_inout_z_cap_c_(_Size) char, _Inout_z_, char, _String, _In_opt_ _locale_t, _Locale)
_Check_return_opt_ _CRTIMP size_t __cdecl strxfrm (_Out_opt_cap_(_MaxCount) _Post_maybez_ char * _Dst, _In_z_ const char * _Src, _In_ size_t _MaxCount);
_Check_return_opt_ _CRTIMP size_t __cdecl _strxfrm_l(_Out_opt_cap_(_MaxCount) _Post_maybez_ char * _Dst, _In_z_ const char * _Src, _In_ size_t _MaxCount, _In_opt_ _locale_t _Locale); #ifdef __cplusplus
extern "C++" {
#ifndef _CPP_NARROW_INLINES_DEFINED
#define _CPP_NARROW_INLINES_DEFINED
_Check_return_ inline char * __CRTDECL strchr(_In_z_ char * _Str, _In_ int _Ch)
{ return (char*)strchr((const char*)_Str, _Ch); }
_Check_return_ inline char * __CRTDECL strpbrk(_In_z_ char * _Str, _In_z_ const char * _Control)
{ return (char*)strpbrk((const char*)_Str, _Control); }
_Check_return_ inline char * __CRTDECL strrchr(_In_z_ char * _Str, _In_ int _Ch)
{ return (char*)strrchr((const char*)_Str, _Ch); }
_Check_return_ inline char * __CRTDECL strstr(_In_z_ char * _Str, _In_z_ const char * _SubStr)
{ return (char*)strstr((const char*)_Str, _SubStr); }
#endif
#ifndef _CPP_MEMCHR_DEFINED
#define _CPP_MEMCHR_DEFINED
_Check_return_ inline void * __CRTDECL memchr(_In_opt_bytecount_(_N) void * _Pv, _In_ int _C, _In_ size_t _N)
{ return (void*)memchr((const void*)_Pv, _C, _N); }
#endif
}
#endif #if !__STDC__ #if defined(_DEBUG) && defined(_CRTDBG_MAP_ALLOC)
#pragma push_macro("strdup")
#undef strdup
#endif _Check_return_ _CRT_NONSTDC_DEPRECATE(_strdup) _CRTIMP char * __cdecl strdup(_In_opt_z_ const char * _Src); #if defined(_DEBUG) && defined(_CRTDBG_MAP_ALLOC)
#pragma pop_macro("strdup")
#endif /* prototypes for oldnames.lib functions */
_Check_return_ _CRT_NONSTDC_DEPRECATE(_strcmpi) _CRTIMP int __cdecl strcmpi(_In_z_ const char * _Str1, _In_z_ const char * _Str2);
_Check_return_ _CRT_NONSTDC_DEPRECATE(_stricmp) _CRTIMP int __cdecl stricmp(_In_z_ const char * _Str1, _In_z_ const char * _Str2);
_CRT_NONSTDC_DEPRECATE(_strlwr) _CRTIMP char * __cdecl strlwr(_Inout_z_ char * _Str);
_Check_return_ _CRT_NONSTDC_DEPRECATE(_strnicmp) _CRTIMP int __cdecl strnicmp(_In_z_ const char * _Str1, _In_z_ const char * _Str, _In_ size_t _MaxCount);
_CRT_NONSTDC_DEPRECATE(_strnset) _CRTIMP char * __cdecl strnset(_Inout_z_cap_(_MaxCount) char * _Str, _In_ int _Val, _In_ size_t _MaxCount);
_CRT_NONSTDC_DEPRECATE(_strrev) _CRTIMP char * __cdecl strrev(_Inout_z_ char * _Str);
_CRT_NONSTDC_DEPRECATE(_strset) char * __cdecl strset(_Inout_z_ char * _Str, _In_ int _Val);
_CRT_NONSTDC_DEPRECATE(_strupr) _CRTIMP char * __cdecl strupr(_Inout_z_ char * _Str); #endif /* !__STDC__ */ #ifndef _WSTRING_DEFINED /* wide function prototypes, also declared in wchar.h */ #if defined(_DEBUG) && defined(_CRTDBG_MAP_ALLOC)
#pragma push_macro("_wcsdup")
#undef _wcsdup
#endif _Check_return_ _CRTIMP wchar_t * __cdecl _wcsdup(_In_z_ const wchar_t * _Str); #if defined(_DEBUG) && defined(_CRTDBG_MAP_ALLOC)
#pragma pop_macro("_wcsdup")
#endif #if __STDC_WANT_SECURE_LIB__
_Check_return_wat_ _CRTIMP_ALTERNATIVE errno_t __cdecl wcscat_s(_Inout_z_cap_(_SizeInWords) wchar_t * _Dst, _In_ rsize_t _SizeInWords, _In_z_ const wchar_t * _Src);
#endif
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_1(errno_t, wcscat_s, _Deref_prepost_z_ wchar_t, _Dest, _In_z_ const wchar_t *, _Source)
__DEFINE_CPP_OVERLOAD_STANDARD_FUNC_0_1(wchar_t *, __RETURN_POLICY_DST, _CRTIMP, wcscat, _Pre_cap_for_(_Source) _Prepost_z_, wchar_t, _Dest, _In_z_ const wchar_t *, _Source)
_Check_return_ _CRTIMP _CONST_RETURN wchar_t * __cdecl wcschr(_In_z_ const wchar_t * _Str, wchar_t _Ch);
_Check_return_ _CRTIMP int __cdecl wcscmp(_In_z_ const wchar_t * _Str1, _In_z_ const wchar_t * _Str2);
#if __STDC_WANT_SECURE_LIB__
_Check_return_wat_ _CRTIMP_ALTERNATIVE errno_t __cdecl wcscpy_s(_Out_z_cap_(_SizeInWords) wchar_t * _Dst, _In_ rsize_t _SizeInWords, _In_z_ const wchar_t * _Src);
#endif
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_1(errno_t, wcscpy_s, wchar_t, _Dest, _In_z_ const wchar_t *, _Source)
__DEFINE_CPP_OVERLOAD_STANDARD_FUNC_0_1(wchar_t *, __RETURN_POLICY_DST, _CRTIMP, wcscpy, _Pre_cap_for_(_Source) _Post_z_, wchar_t, _Dest, _In_z_ const wchar_t *, _Source)
_Check_return_ _CRTIMP size_t __cdecl wcscspn(_In_z_ const wchar_t * _Str, _In_z_ const wchar_t * _Control);
_Check_return_ _CRTIMP size_t __cdecl wcslen(_In_z_ const wchar_t * _Str);
_Check_return_ _CRTIMP size_t __cdecl wcsnlen(_In_z_ const wchar_t * _Src, _In_ size_t _MaxCount);
#if __STDC_WANT_SECURE_LIB__ && !defined (__midl)
_Check_return_ static __inline size_t __CRTDECL wcsnlen_s(_In_z_ const wchar_t * _Src, _In_ size_t _MaxCount)
{
return (_Src == NULL) ? : wcsnlen(_Src, _MaxCount);
}
#endif
#if __STDC_WANT_SECURE_LIB__
_Check_return_wat_ _CRTIMP_ALTERNATIVE errno_t __cdecl wcsncat_s(_Inout_z_cap_(_SizeInWords) wchar_t * _Dst, _In_ rsize_t _SizeInWords, _In_z_ const wchar_t * _Src, _In_ rsize_t _MaxCount);
#endif
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_2(errno_t, wcsncat_s, _Deref_prepost_z_ wchar_t, _Dest, _In_z_ const wchar_t *, _Source, _In_ size_t, _Count)
#pragma warning(push)
#pragma warning(disable:6059)
__DEFINE_CPP_OVERLOAD_STANDARD_NFUNC_0_2_EX(wchar_t *, __RETURN_POLICY_DST, _CRTIMP, wcsncat, wcsncat_s, _Deref_inout_z_cap_c_(_Size) wchar_t, _Inout_z_cap_(_Count), wchar_t, _Dest, _In_z_ const wchar_t *, _Source, _In_ size_t, _Count)
#pragma warning(pop)
_Check_return_ _CRTIMP int __cdecl wcsncmp(_In_z_ const wchar_t * _Str1, _In_z_ const wchar_t * _Str2, _In_ size_t _MaxCount);
#if __STDC_WANT_SECURE_LIB__
_Check_return_wat_ _CRTIMP_ALTERNATIVE errno_t __cdecl wcsncpy_s(_Out_z_cap_(_SizeInWords) wchar_t * _Dst, _In_ rsize_t _SizeInWords, _In_z_ const wchar_t * _Src, _In_ rsize_t _MaxCount);
#endif
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_2(errno_t, wcsncpy_s, wchar_t, _Dest, _In_z_ const wchar_t *, _Source, _In_ size_t, _Count)
__DEFINE_CPP_OVERLOAD_STANDARD_NFUNC_0_2_EX(wchar_t *, __RETURN_POLICY_DST, _CRTIMP, wcsncpy, wcsncpy_s, _Pre_notnull_ _Post_maybez_ wchar_t, _Out_cap_(_Count) _Post_maybez_, wchar_t, _Dest, _In_z_ const wchar_t *, _Source, _In_ size_t, _Count)
_Check_return_ _CRTIMP _CONST_RETURN wchar_t * __cdecl wcspbrk(_In_z_ const wchar_t * _Str, _In_z_ const wchar_t * _Control);
_Check_return_ _CRTIMP _CONST_RETURN wchar_t * __cdecl wcsrchr(_In_z_ const wchar_t * _Str, _In_ wchar_t _Ch);
_Check_return_ _CRTIMP size_t __cdecl wcsspn(_In_z_ const wchar_t * _Str, _In_z_ const wchar_t * _Control);
_Check_return_ _CRTIMP _CONST_RETURN wchar_t * __cdecl wcsstr(_In_z_ const wchar_t * _Str, _In_z_ const wchar_t * _SubStr);
_Check_return_ _CRT_INSECURE_DEPRECATE(wcstok_s) _CRTIMP wchar_t * __cdecl wcstok(_Inout_opt_z_ wchar_t * _Str, _In_z_ const wchar_t * _Delim);
#if __STDC_WANT_SECURE_LIB__
_Check_return_ _CRTIMP_ALTERNATIVE wchar_t * __cdecl wcstok_s(_Inout_opt_z_ wchar_t * _Str, _In_z_ const wchar_t * _Delim, _Inout_ _Deref_prepost_opt_z_ wchar_t ** _Context);
#endif
_Check_return_ _CRT_INSECURE_DEPRECATE(_wcserror_s) _CRTIMP wchar_t * __cdecl _wcserror(_In_ int _ErrNum);
_Check_return_wat_ _CRTIMP errno_t __cdecl _wcserror_s(_Out_opt_z_cap_(_SizeInWords) wchar_t * _Buf, _In_ size_t _SizeInWords, _In_ int _ErrNum);
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_1(errno_t, _wcserror_s, wchar_t, _Buffer, _In_ int, _Error)
_Check_return_ _CRT_INSECURE_DEPRECATE(__wcserror_s) _CRTIMP wchar_t * __cdecl __wcserror(_In_opt_z_ const wchar_t * _Str);
_Check_return_wat_ _CRTIMP errno_t __cdecl __wcserror_s(_Out_opt_z_cap_(_SizeInWords) wchar_t * _Buffer, _In_ size_t _SizeInWords, _In_z_ const wchar_t * _ErrMsg);
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_1(errno_t, __wcserror_s, wchar_t, _Buffer, _In_z_ const wchar_t *, _ErrorMessage) _Check_return_ _CRTIMP int __cdecl _wcsicmp(_In_z_ const wchar_t * _Str1, _In_z_ const wchar_t * _Str2);
_Check_return_ _CRTIMP int __cdecl _wcsicmp_l(_In_z_ const wchar_t * _Str1, _In_z_ const wchar_t * _Str2, _In_opt_ _locale_t _Locale);
_Check_return_ _CRTIMP int __cdecl _wcsnicmp(_In_z_ const wchar_t * _Str1, _In_z_ const wchar_t * _Str2, _In_ size_t _MaxCount);
_Check_return_ _CRTIMP int __cdecl _wcsnicmp_l(_In_z_ const wchar_t * _Str1, _In_z_ const wchar_t * _Str2, _In_ size_t _MaxCount, _In_opt_ _locale_t _Locale);
_Check_return_wat_ _CRTIMP_ALTERNATIVE errno_t __cdecl _wcsnset_s(_Inout_z_cap_(_SizeInWords) wchar_t * _Dst, _In_ size_t _SizeInWords, _In_ wchar_t _Val, _In_ size_t _MaxCount);
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_2(errno_t, _wcsnset_s, _Deref_prepost_z_ wchar_t, _Dst, wchar_t, _Val, _In_ size_t, _MaxCount)
__DEFINE_CPP_OVERLOAD_STANDARD_NFUNC_0_2_EX(wchar_t *, __RETURN_POLICY_DST, _CRTIMP, _wcsnset, _wcsnset_s, _Deref_inout_z_cap_c_(_Size) wchar_t, _Inout_z_cap_(_MaxCount), wchar_t, _Str, wchar_t, _Val, _In_ size_t, _MaxCount)
_CRTIMP wchar_t * __cdecl _wcsrev(_Inout_z_ wchar_t * _Str);
_Check_return_wat_ _CRTIMP_ALTERNATIVE errno_t __cdecl _wcsset_s(_Inout_z_cap_(_SizeInWords) wchar_t * _Dst, _In_ size_t _SizeInWords, _In_ wchar_t _Value);
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_1(errno_t, _wcsset_s, _Deref_prepost_z_ wchar_t, _Str, wchar_t, _Val)
__DEFINE_CPP_OVERLOAD_STANDARD_FUNC_0_1_EX(wchar_t *, __RETURN_POLICY_DST, _CRTIMP, _wcsset, _wcsset_s, _Deref_inout_z_cap_c_(_Size) wchar_t, _Inout_z_, wchar_t, _Str, wchar_t, _Val) _Check_return_wat_ _CRTIMP errno_t __cdecl _wcslwr_s(_Inout_z_cap_(_SizeInWords) wchar_t * _Str, _In_ size_t _SizeInWords);
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_0(errno_t, _wcslwr_s, _Deref_prepost_z_ wchar_t, _String)
__DEFINE_CPP_OVERLOAD_STANDARD_FUNC_0_0(wchar_t *, __RETURN_POLICY_DST, _CRTIMP, _wcslwr, _Inout_z_, wchar_t, _String)
_Check_return_wat_ _CRTIMP errno_t __cdecl _wcslwr_s_l(_Inout_z_cap_(_SizeInWords) wchar_t * _Str, _In_ size_t _SizeInWords, _In_opt_ _locale_t _Locale);
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_1(errno_t, _wcslwr_s_l, _Deref_prepost_z_ wchar_t, _String, _In_opt_ _locale_t, _Locale)
__DEFINE_CPP_OVERLOAD_STANDARD_FUNC_0_1_EX(wchar_t *, __RETURN_POLICY_DST, _CRTIMP, _wcslwr_l, _wcslwr_s_l, _Deref_inout_z_cap_c_(_Size) wchar_t, _Inout_z_, wchar_t, _String, _In_opt_ _locale_t, _Locale)
_Check_return_wat_ _CRTIMP errno_t __cdecl _wcsupr_s(_Inout_z_cap_(_Size) wchar_t * _Str, _In_ size_t _Size);
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_0(errno_t, _wcsupr_s, _Deref_prepost_z_ wchar_t, _String)
__DEFINE_CPP_OVERLOAD_STANDARD_FUNC_0_0(wchar_t *, __RETURN_POLICY_DST, _CRTIMP, _wcsupr, _Inout_z_, wchar_t, _String)
_Check_return_wat_ _CRTIMP errno_t __cdecl _wcsupr_s_l(_Inout_z_cap_(_Size) wchar_t * _Str, _In_ size_t _Size, _In_opt_ _locale_t _Locale);
__DEFINE_CPP_OVERLOAD_SECURE_FUNC_0_1(errno_t, _wcsupr_s_l, _Deref_prepost_z_ wchar_t, _String, _In_opt_ _locale_t, _Locale)
__DEFINE_CPP_OVERLOAD_STANDARD_FUNC_0_1_EX(wchar_t *, __RETURN_POLICY_DST, _CRTIMP, _wcsupr_l, _wcsupr_s_l, _Deref_inout_z_cap_c_(_Size) wchar_t, _Inout_z_, wchar_t, _String, _In_opt_ _locale_t, _Locale)
_Check_return_opt_ _CRTIMP size_t __cdecl wcsxfrm(_Out_opt_cap_(_MaxCount) _Post_maybez_ wchar_t * _Dst, _In_z_ const wchar_t * _Src, _In_ size_t _MaxCount);
_Check_return_opt_ _CRTIMP size_t __cdecl _wcsxfrm_l(_Out_opt_cap_(_MaxCount) _Post_maybez_ wchar_t * _Dst, _In_z_ const wchar_t *_Src, _In_ size_t _MaxCount, _In_opt_ _locale_t _Locale);
_Check_return_ _CRTIMP int __cdecl wcscoll(_In_z_ const wchar_t * _Str1, _In_z_ const wchar_t * _Str2);
_Check_return_ _CRTIMP int __cdecl _wcscoll_l(_In_z_ const wchar_t * _Str1, _In_z_ const wchar_t * _Str2, _In_opt_ _locale_t _Locale);
_Check_return_ _CRTIMP int __cdecl _wcsicoll(_In_z_ const wchar_t * _Str1, _In_z_ const wchar_t * _Str2);
_Check_return_ _CRTIMP int __cdecl _wcsicoll_l(_In_z_ const wchar_t * _Str1, _In_z_ const wchar_t *_Str2, _In_opt_ _locale_t _Locale);
_Check_return_ _CRTIMP int __cdecl _wcsncoll(_In_z_ const wchar_t * _Str1, _In_z_ const wchar_t * _Str2, _In_ size_t _MaxCount);
_Check_return_ _CRTIMP int __cdecl _wcsncoll_l(_In_z_ const wchar_t * _Str1, _In_z_ const wchar_t * _Str2, _In_ size_t _MaxCount, _In_opt_ _locale_t _Locale);
_Check_return_ _CRTIMP int __cdecl _wcsnicoll(_In_z_ const wchar_t * _Str1, _In_z_ const wchar_t * _Str2, _In_ size_t _MaxCount);
_Check_return_ _CRTIMP int __cdecl _wcsnicoll_l(_In_z_ const wchar_t * _Str1, _In_z_ const wchar_t * _Str2, _In_ size_t _MaxCount, _In_opt_ _locale_t _Locale); #ifdef __cplusplus
#ifndef _CPP_WIDE_INLINES_DEFINED
#define _CPP_WIDE_INLINES_DEFINED
extern "C++" {
_Check_return_ inline wchar_t * __CRTDECL wcschr(_In_z_ wchar_t *_Str, wchar_t _Ch)
{return ((wchar_t *)wcschr((const wchar_t *)_Str, _Ch)); }
_Check_return_ inline wchar_t * __CRTDECL wcspbrk(_In_z_ wchar_t *_Str, _In_z_ const wchar_t *_Control)
{return ((wchar_t *)wcspbrk((const wchar_t *)_Str, _Control)); }
_Check_return_ inline wchar_t * __CRTDECL wcsrchr(_In_z_ wchar_t *_Str, _In_ wchar_t _Ch)
{return ((wchar_t *)wcsrchr((const wchar_t *)_Str, _Ch)); }
_Check_return_ inline wchar_t * __CRTDECL wcsstr(_In_z_ wchar_t *_Str, _In_z_ const wchar_t *_SubStr)
{return ((wchar_t *)wcsstr((const wchar_t *)_Str, _SubStr)); }
}
#endif
#endif #if !__STDC__ #if defined(_DEBUG) && defined(_CRTDBG_MAP_ALLOC)
#pragma push_macro("wcsdup")
#undef wcsdup
#endif _Check_return_ _CRT_NONSTDC_DEPRECATE(_wcsdup) _CRTIMP wchar_t * __cdecl wcsdup(_In_z_ const wchar_t * _Str); #if defined(_DEBUG) && defined(_CRTDBG_MAP_ALLOC)
#pragma pop_macro("wcsdup")
#endif /* old names */
#define wcswcs wcsstr /* prototypes for oldnames.lib functions */
_Check_return_ _CRT_NONSTDC_DEPRECATE(_wcsicmp) _CRTIMP int __cdecl wcsicmp(_In_z_ const wchar_t * _Str1, _In_z_ const wchar_t * _Str2);
_Check_return_ _CRT_NONSTDC_DEPRECATE(_wcsnicmp) _CRTIMP int __cdecl wcsnicmp(_In_z_ const wchar_t * _Str1, _In_z_ const wchar_t * _Str2, _In_ size_t _MaxCount);
_CRT_NONSTDC_DEPRECATE(_wcsnset) _CRTIMP wchar_t * __cdecl wcsnset(_Inout_z_cap_(_MaxCount) wchar_t * _Str, _In_ wchar_t _Val, _In_ size_t _MaxCount);
_CRT_NONSTDC_DEPRECATE(_wcsrev) _CRTIMP wchar_t * __cdecl wcsrev(_Inout_z_ wchar_t * _Str);
_CRT_NONSTDC_DEPRECATE(_wcsset) _CRTIMP wchar_t * __cdecl wcsset(_Inout_z_ wchar_t * _Str, wchar_t _Val);
_CRT_NONSTDC_DEPRECATE(_wcslwr) _CRTIMP wchar_t * __cdecl wcslwr(_Inout_z_ wchar_t * _Str);
_CRT_NONSTDC_DEPRECATE(_wcsupr) _CRTIMP wchar_t * __cdecl wcsupr(_Inout_z_ wchar_t * _Str);
_Check_return_ _CRT_NONSTDC_DEPRECATE(_wcsicoll) _CRTIMP int __cdecl wcsicoll(_In_z_ const wchar_t * _Str1, _In_z_ const wchar_t * _Str2); #endif /* !__STDC__ */ #define _WSTRING_DEFINED
#endif #ifdef __cplusplus
}
#endif #endif /* _INC_STRING */
在Vc\include下面倒是有一个string的cpp文件，但是没有string操作函数的代码，那么strcpy,strcat源码到底在哪里？
在D:\Program Files\visual studio\VC\crt\src 目录下面。
这个目录下面包含所有C语言函数的源代码。
大多数函数的名字都是以名字命名的.c文件里，但是strcpy并没有strcpy.c文件，它的源码在strcat.c里面：
strcat.c内容如下：
/***
*strcat.c - contains strcat() and strcpy() *
*Purpose:
* Strcpy() copies one string onto another.
*
* Strcat() concatenates (appends) a copy of the source string to the
* end of the destination string, returning the destination string.
*
*******************************************************************************/ #include <cruntime.h>
#include <string.h> #ifndef _MBSCAT
#pragma function(strcat,strcpy)
#endif /* _MBSCAT */ /***
*char *strcat(dst, src) - concatenate (append) one string to another
*
*Purpose:
* Concatenates src onto the end of dest. Assumes enough
* space in dest.
*
*Entry:
* char *dst - string to which "src" is to be appended
* const char *src - string to be appended to the end of "dst"
*
*Exit:
* The address of "dst"
*
*Exceptions:
*
*******************************************************************************/ char * __cdecl strcat (
char * dst,
const char * src
)
{
char * cp = dst; while( *cp )
cp++; /* find end of dst */
/*不能写成while(*cp++)/ 因为这样的话为*cp为0时还会+1，导致cp执向了\0的后一个。 while( *cp++ = *src++ ) ; /* Copy src to end of dst */ return( dst ); /* return dst */ } /***
*char *strcpy(dst, src) - copy one string over another
*
*Purpose:
* Copies the string src into the spot specified by
* dest; assumes enough room.
*
*Entry:
* char * dst - string over which "src" is to be copied
* const char * src - string to be copied over "dst"
*
*Exit:
* The address of "dst"
*
*Exceptions:
*******************************************************************************/ char * __cdecl strcpy(char * dst, const char * src)
{
char * cp = dst; while( *cp++ = *src++ )
; /* Copy src over dst */ return( dst );
}
strcpy另一种实现策略：
林锐《高质量C++/C编程指南》
#include <assert.h>
char *strcpy(char *strDest, const char *strSrc)
{ assert((strDest!=NULL) && (strSrc !=NULL)); // 2分 char *address = strDest; // 2分 while( (*strDest++ = * strSrc++) != ‘/’ ) // 2分 ; return address ; // 2分 }
附：
strcpy能把strSrc的内容复制到strDest，为什么还要char * 类型的返回值？
答：为了实现链式表达式。 // 2分
例如 int length = strlen( strcpy( strDest, “hello world”) );
个人觉得
(*strDest++ = * strSrc++) != ‘/0’ 不能没有必要判断是否=0,因为但char内容为字符串结束符时ascii为0，dst=src; dst=0;
while 判断dst是否等于0，等于0就退出了。 strcmp怎么写，最开始我这么写的：
int strcmp(char *src,char *dst)
{
char *p=src,*q=dst;
while( (*p!='\0') && (*q!='\0'))
{
if(*p==*q)
{
p++;
q++;
}
else if(*p<*q)
{
return -;
}
else
{
return ;
}
}
return ;
}
表面上看没有问题，但是有很大的问题，但2个长度不等时，退出循环还是输出了0,。如de和def比较还是0.
可以把return改下，判断*P和*q是否后面还有。显然太麻烦了。
函数源码：
/***
*strcmp - compare two strings, returning less than, equal to, or greater than
*
*Purpose:
* STRCMP compares two strings and returns an integer
* to indicate whether the first is less than the second, the two are
* equal, or whether the first is greater than the second.
*
* Comparison is done byte by byte on an UNSIGNED basis, which is to
* say that Null (0) is less than any other character (1-255).
*
*Entry:
* const char * src - string for left-hand side of comparison
* const char * dst - string for right-hand side of comparison
*
*Exit:
* returns -1 if src < dst
* returns 0 if src == dst
* returns +1 if src > dst
*
*Exceptions:
*
*******************************************************************************/ int __cdecl strcmp (
const char * src,
const char * dst
)
{
int ret = ; while( ! (ret = *(unsigned char *)src - *(unsigned char *)dst) && *dst)
++src, ++dst; if ( ret < )
ret = - ;
else if ( ret > )
ret = ; return( ret );
}
刚开始对
while( ! (ret = *(unsigned char *)src - *(unsigned char *)dst) && *dst)
++src, ++dst;
不是很理解。
如果是de和def比较，*src没有为0，还是在继续比较吗？
刚开始我们认为是在继续比较，其实没有比较了。因为。
! ret
ret=0才继续比较。
ret=0-'f'=负数。
!负数 为假。 0为假，非0为真。
!0为真。
这里取非很巧妙。没有 用
! (ret = *(unsigned char *)src - *(unsigned char *)dst) && *src && *dst)这种。
strcmp算法的可以有多种，不过我觉的可以把这么多算法分为两种：一种是利用减法运算判断结果，另一种是利用比较运算==得出结果。上面的就是用减法。判断。
为什么要把src转成：
unsigned char *
这个函数要注意的几点：
（1）使用*（unsigned char *）str1而不是用*str1。这是因为传入的参数为有符号数，
有符号字符值的范围是-128-127，无符号字符值的范围是0-255，而字符串的ASCII没有负值，若不转化为无符号数，在减法实现时出现错误。
例如：str的值为1，str2的值为255。
作为无符号数计算时ret=-254，结果为负值，正确。
作为有符号数计算时ret=-2，结果为正值，错误。
ascii char范围为0-127,为什么上面说的255.怎么才能打印255的字符。
在cmd下，按alt+255 回车，就可以打印ÿ。这个不是ascii字符。
我们把这个字符粘贴到vs2010文件中，会提示当前编码文件为ascii格式，不能存储当前的字符，是否转换为unicode，转换为unicode就可以存储这个字符了。
char *c="dÿ";
printf("%s\n",c);
运行控制台会输出：
d?
？说明第二个字符不可打印。
）
（2）while循环中（ret=*（unsigned char *）str1 - *（unsigned char *）str2）&& *str1，最后的str1也可以换成str2，因为前面已经做了相减，无论哪个先为'\0'都会退出。
（3）这个函数没有判断参数为NULL时的情况，所以当传入NULL时程序会崩溃。
可以看到我们传递的参数有const，这会为面试加分。
用比较实现
int strcmp(const char *str1,const char * str2)
{
while((*str1)&&(*str1 == *str2))
{
str1++;
str2++;
}
if(*(unsiged char *)str1 > *(unsiged char *)str2)
{
return ;
}
else if(*(unsiged char *)str1 < *(unsiged char *)str2)
{
return -;
}
else
{
return ;
}
}
注意：把while循环简写成while((*str1)&&(*str1++ == *str2++))
当str1为abcd, str2为abfd时，由于判断到第三个字符时while推出，而str指针又加了1，str都指向第四个字符输出结果为0，显然 这是错误的。
参考了：http://www.cnblogs.com/kaifublog/archive/2012/09/23/2699017.html
strlen：
int strlen(const char *str)
{
int len = ;
assert(str != NULL);
while(*str++)
{
len++;
}
return len;
}
我刚开始写的是：
int strlen2(const char *src)
{
const char *p=src;
int i=;
while(*p++)
i++;
return i;
}
用了一个临时的p来存储src，有没有必要，是没有必要的，调用时，形参只是copy一份实参的值，所以，src只是copy了一份
原理src的值。
源码：
size_t __cdecl strlen ( const char * str)
{
const char *eos = str; while( *eos++ ) ; return( eos - str - );
}
用递归求：
int strlen4(const char *str)
{
if(*str=='\0')
return ;
else
return strlen4(++str)+1;
}
有点非常值得注意，最开始我写的是strlen4(str++),程序崩溃了，为什么？
每次函数内部调用时，str都指向h,strlen4(str)执行，str根本没动，导致了无穷递归，实在是一个很大的错误。
strncpy：
原型：char * strncpy(char *dest, char *src,size_tnum);
功能：（c/c++）复制src中的内容（字符，数字、汉字....）到dest，复制多少由num的值决定，返回指向dest的指针。如果遇到null字符（'\0'），且还没有到num个字符时，就用（num - n）（n是遇到null字符前已经有的非null字符个数）个null字符附加到destination。注意：并不是添加到destination的最后，而是紧跟着由source中复制而来的字符后面。下面举例说明[1]：
char des[] = "Hello,i am!";
char source[] = "abc\0def";
strncpy(des,source,5);
此时，des区域是这样的：a,b,c,\0,\0,逗号,i,空格,a,m,!
在测试时不要用prinf("%s"),因为遇到\0就结束了，
for(int i=;i<sizeof(d)/sizeof(d[]);i++)
printf("%c",d[i]);
注意：\0,\0并不是添加在!的后面。
头文件：#include "string.h"
说明：
如果n > dest串长度，dest栈空间溢出产生崩溃异常。
否则：
1）src串长度<=dest串长度,(这里的串长度包含串尾NULL字符)
如果n<src串长度，src的前n个字符复制到dest中。但是由于没有NULL字符，所以直接访问dest串会发生栈溢出的异常情况。
如果n = src串长度，与strcpy一致。
如果n >src串长度，src串存放于desk字串的[0,src串长度]，dest串的(src串长度, dest串长度]处存放NULL。
2）src串长度>dest串长度
如果n =dest串长度，则dest串没有NULL字符，会导致输出会有乱码。如果不考虑src串复制完整性，可以将dest最后一字符置为NULL。
综上，一般情况下，使用strncpy时，建议将n置为dest串长度（除非你将多个src串都复制到dest数组，并且从dest尾部反向操作)，复制完毕后，为保险起见，将dest串最后一字符置NULL，避免发生在第2)种情况下的输出乱码问题。当然喽，无论是strcpy还是strncpy，保证src串长度<dest串长度才是最重要的。
我的实现：
char *strncpy2(char *dest,char *src,int num)
{
char *p=dest;
int i=;
while( *src && (*p++=*src++) && (i<num))
i++;
if(i<num)
{
for(int j=;j<=num-i;j++)
{
*p++='\0';
}
}
return dest;
}
写这几行代码时经常发生各种各样的错误，如在for(j=1;j<num-i;j++)内，j写成了i，导致与前面的i重复。
*p++='\0'写成*p='\0';导致最后一个num字符没有赋0.
源码：
/***
*strncpy.c - copy at most n characters of string
*
* Copyright (c) Microsoft Corporation. All rights reserved.
*
*Purpose:
* defines strncpy() - copy at most n characters of string
*
*******************************************************************************/ #include <cruntime.h>
#include <string.h> /***
*char *strncpy(dest, source, count) - copy at most n characters
*
*Purpose:
* Copies count characters from the source string to the
* destination. If count is less than the length of source,
* NO NULL CHARACTER is put onto the end of the copied string.
* If count is greater than the length of sources, dest is padded
* with null characters to length count.
*
*
*Entry:
* char *dest - pointer to destination
* char *source - source string for copy
* unsigned count - max number of characters to copy
*
*Exit:
* returns dest
*
*Exceptions:
*
*******************************************************************************/ char * __cdecl strncpy (
char * dest,
const char * source,
size_t count
)
{
char *start = dest; while (count && (*dest++ = *source++)) /* copy string */
count--; if (count) /* pad out with zeroes */
while (--count)
*dest++ = '\0'; return(start);
}
为什么我写的代码比官方写的代码要长，因为我用了一个变量i 来计算，看有没有达到num，官方则自己用num--来判断。在while
循环里用count来代替i<num.这样不加少了一个变量i，而且很紧凑。在后面的填充null时，也没用用j来技术，还是count字减。
官方的源码写的漂亮。
strrev:
自己写的:
char * strrev2(char *src)
{
char *p,*q;
p=src;
q=src+strlen(src)-;
while(p<q)
{
char tmp=*p;*p=*q;*q=tmp; //注意是char tmp,我已经习惯写成int tmp
p++;
q--;
}
return src;
}
官方源码：
char * __cdecl _strrev (
char * string
)
{
char *start = string;
char *left = string;
char ch; while (*string++) /* find end of string */
;
string -= 2; while (left < string)
{
ch = *left;
*left++ = *string;
*string-- = ch;
} return(start);
}
更多参考：
http://www.cnblogs.com/JCSU/articles/1305401.html
http://www.cnblogs.com/winnxm/archive/2011/05/17/2049266.html
http://www.cppblog.com/dawnbreak/archive/2009/08/24/94262.html