字符串操作函数<string.h>相关函数strcpy,strcat,等源码。

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 Filesvisual studioVCincludecstring

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 > 1), 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) == 0 ? _Dst : 0;
    }
}
#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==0) ? 0 : 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) ? 0 : 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Vcinclude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 Filesvisual studioVCcrtsrc 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++) != ‘/0’ )   // 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!='') && (*q!=''))
    {
        if(*p==*q)
        {
            p++;
            q++;
        }
        else if(*p<*q)
        {
            return -1;
        }
        else
        {
            return 1;
        }
    }
    return 0;
}

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 = 0 ;

        while( ! (ret = *(unsigned char *)src - *(unsigned char *)dst) && *dst)
                ++src, ++dst;

        if ( ret < 0 )
                ret = -1 ;
        else if ( ret > 0 )
                ret = 1 ;

        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 ",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''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 1;

　　}

　　else if(*(unsiged char *)str1 < *(unsiged char *)str2)

　　{

　　　　return -1;

　　}

　　else

　　{
　　　　return 0;
　　}

}

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 = 0;
       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=0;
    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 - 1 );
}

 用递归求：

int strlen4(const char *str)
{
    if(*str=='')
        return 0;
    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（''），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[] = "abcdef";

strncpy(des,source,5);

此时，des区域是这样的：a,b,c,,,逗号,i,空格,a,m,!

在测试时不要用prinf("%s"),因为遇到就结束了， 

for(int i=0;i<sizeof(d)/sizeof(d[0]);i++)
        printf("%c",d[i]);

注意：,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=0;
    while( *src && (*p++=*src++) && (i<num))
        i++;
    if(i<num)
    {
        for(int j=1;j<=num-i;j++)
        {
            *p++='';
        }
    }
    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++=''写成*p='';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++ = '';

        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)-1;
    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