strcpy()、memcpy()、memmove()、memset() 、strcmp()、strlen()、strcat()的实现

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  1 strcpy(), 字符串拷贝.
  2 char *strcpy(char *strDest, const char *strSrc)
  3 {
  4     assert((strDest!=NULL) && (strSrc !=NULL));
  5     char *address = strDest;     
  6     while( (*strDest++ = * strSrc++) != '\0') 
  7        NULL ; 
  8     return address ;       
  9 }
 10 
 11 memcpy, 拷贝不重叠的内存块 
 12 void *memcpy(void* pvTo, void* pvFrom, size_t size) //byte是java里的变量类型
 13 {
 14 assert(pvTo != NULL && pvFrom != NULL);
 15 void* pbTo = (byte*)pvTo;
 16 void* pbFrom = (byte*)pvFrom;
 17 /* 内存块重叠吗？如果重叠，就使用memmove */
 18 assert(pbTo>=pbFrom+size || pbFrom>=pbTo+size);
 19 while(size-->0)
 20     *pbTo++ == *pbFrom++;
 21 return pvTo;
 22 }
 23 
 24 void *MemCopy(void *dest,const void *src,size_t count)
 25 {
 26     char *pDest=static_cast<char *>(dest);
 27     const char *pSrc=static_cast<const char *>(src);
 28     if( pDest>pSrc && pDest<pSrc+count )
 29     {
 30         for(size_t i=count-1; i<=0; ++i)
 31         {
 32             pDest[i]=pSrc[i];
 33         }
 34     }
 35     else
 36     {
 37         for(size_t i=0; i<count; ++i)
 38         {
 39              pDest[i]=pSrc[i];
 40         }
 41     }
 42     return pDest;
 43 }
 44 
 45 void *Memmove(void *Dst, const void*Src,size_t count)
 46 {
 47 assert(Dst && Src);
 48 void* pDst = Dst;
 49 if (Dst<Src && (char*)Dst > (char*)Src + count)
 50 {
 51 while(count--)
 52 {
 53    *(char*)Dst = *(char*)Src;
 54    Dst = (char*)Dst + 1;
 55    Src = (char*)Src + 1;
 56 }
 57 }
 58 else
 59 {
 60    Dst = (char*)Dst + count - 1;
 61    Src = (char*)Src + count - 1;
 62    while(count--)
 63    {
 64       *(char*)Dst = *(char*)Src;
 65       Dst = (char*)Dst -1 ;
 66       Src = (char*)Src -1 ;
 67    }
 68 }
 69 return pDst;
 70 }
 71 
 72 
 73 void* memmove(void *dest, const void *src,size_t n) 
 74 { 
 75     if (n == 0) return 0; 
 76     if (dest == NULL) return 0; 
 77     if (src == NULL)    return 0; 
 78     char *psrc = (char*)src; 
 79     char *pdest = (char*)dest; 
 80     if((dest <= psrc) || (pdest >= psrc + n)) /*检查是否有重叠问题 */ 
 81         { 
 82          for(int i=0; i < n; i++) /*正向拷贝*/ 
 83           { 
 84            *pdest = *psrc; 
 85            psrc++; 
 86            pdest++; 
 87           } 
 88         } 
 89         else /*反向拷贝*/ 
 90         { 
 91           psrc += n; 
 92           pdest += n; 
 93           for(int i=0;i<n;i++) 
 94            { 
 95             psrc--; 
 96             pdest--; 
 97             *pdest = *psrc; 
 98            } 
 99         } 
100    return dest;
101 }
102 
103 memset:把buffer所指内存区域的前count个字节设置成字符c
104 
105 void * Memset(void* buffer, int c, int count)
106 {
107 char* pvTo=(char*)buffer;
108 assert(buffer != NULL);
109 while(count-->0)
110 *pvTo++=(char)c;
111 return buffer;
112 }
113  

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 1 Char* strcat ( char * dst , const char * src )
 2 
 3 {
 4 
 5 char * cp = dst;
 6 
 7 while( *cp )
 8 
 9 cp++; /* find end of dst */
10 
11 while( *cp++ = *src++ ) ; /* Copy src to end of dst */
12 
13 return( dst ); /* return dst */
14 
15 }

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1 int strlen1(char s[])
2 {
3      int n;
4      n = 0;
5      while (s[n++] != '\0')
6           ;
7      return n;
8 }

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 1 int strcmp(const char *dest, const char *source)  
 2 {  
 3    assert((NULL != dest) && (NULL != source));  
 4    while (*dest && *source && (*dest == *source))  
 5            {  
 6                     dest ++;  
 7                    source ++;  
 8            }  
 9    return *dest - *source;  
10 /*如果dest > source,则返回值大于0，如果dest = source,则返回值等于0，如果dest  < source ,则返回值小于0。*/  
11 }