• 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 */
    View Code

    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?

    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

     
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  • 原文地址:https://www.cnblogs.com/youxin/p/3270974.html
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