C语言字符串库函数的实现

来源：互联网发布：魔兽争霸知乎编辑：程序博客网时间：2024/04/28 21:06

http://www.cnblogs.com/luxiaoxun/archive/2012/09/04/2670202.html

C语言字符串库函数的实现也是笔试题常考的题目，以下代码没有严格测试，只是简单的实现：

//字符串长度int strlen(const char *str)   {       assert(str != NULL);       int len = 0;       while (*str ++ != '\0')           ++ len;       return len;   }//字符串拷贝char *strcpy(char *to, const char *from){    assert((to != NULL) && (from != NULL));    char * result = to;    while( (*to++ = *from++) != '\0')        NULL;    return result;       }//strncpy()，如果from指向的字符个数少于count，则用'\0'补齐char *strncpy(char *to, const char *from, size_t count){    assert((to != NULL) && (from != NULL));    char * result = to;    while(count--)    {        if(*from != '\0')        {            *to++ = *from++;        }        else        {            *to++ = '\0';        }    }    return result;       }//memcpy(), 拷贝不重叠的内存块 void* memcpy(void* to, const void* from, size_t count){    assert((to != NULL) && (from != NULL));    void * result = to;    char * pto = (char *)to;    char * pfrom = (char *)from;    assert(pto < pfrom || pto > pfrom + count -1);    while(count--)    {       *pto++ = *pfrom++;    }    return result;}//memmove(), 拷贝重叠或者是不重叠的内存块 void* memmove(void* to, const void* from, size_t count){    assert((to != NULL) && (from != NULL));    void * result = to;    char * pto = (char *)to;    char * pfrom = (char *)from;    //to与from没有重叠    if(pto < pfrom || pto > pfrom + count -1)    {       while(count--)       {           *pto++ = *pfrom++;       }    }    //to与from有重叠，从后向前move    else    {       pto = pto + count -1;       pfrom = pfrom + count -1;       while(count--)       {          *pto-- = *pfrom--;       }    }    return result;}//memset():把指定内存区域的前count个字节设置成字符cvoid * memset(void* buffer, int c, size_t count){    assert(buffer != NULL);    char * p = (char *)buffer;    while(count--)        *p++ = (char)c;    return buffer;}//查找字符串s中首次出现字符c的位置   char *strchr(char *str, int c)   {       assert(str != NULL);       for (; *str != (char)c; ++ str)           if (*str == '\0')               return NULL;       return str;   }   //字符串比较int strcmp(const char *s, const char *t)   {       assert(s != NULL && t != NULL);       while (*s && *t && *s == *t)       {           ++ s;           ++ t;       }       return (*s - *t);   }   int strncmp(const char *s, const char *t, unsigned int count)   {       assert((s != NULL) && (t != NULL));       while (*s && *t && *s == *t && count --)       {           ++ s;           ++ t;       }       return (*s - *t);   }//字符串连接char *strcat(char *strDes, const char *strSrc)   {       assert((strDes != NULL) && (strSrc != NULL));       char *address = strDes;       while (*strDes != '\0')           ++ strDes;       while ((*strDes ++ = *strSrc ++) != '\0')           NULL;       return address;   }char *strncat(char *strDes, const char *strSrc, unsigned int count)   {       assert((strDes != NULL) && (strSrc != NULL));       char *address = strDes;       while (*strDes != '\0')           ++ strDes;       while (count -- && *strSrc != '\0' )           *strDes ++ = *strSrc ++;       *strDes = '/0';       return address;   }   //查找字符串第一次出现的位置char *strstr(const char *strSrc, const char *str)   {       assert(strSrc != NULL && str != NULL);       const char *s = strSrc;       const char *t = str;       for (; *strSrc != '\0'; ++ strSrc)       {           for (s = strSrc, t = str; *t != '\0' && *s == *t; ++s, ++t)               NULL;           if (*t == '\0')               return (char *) strSrc;       }       return NULL;   } //将字符串拷贝到新的位置 char *strdup_(char *strSrc){         if(strSrc!=NULL)         {             char *start=strSrc;             int len=0;             while(*strSrc++!='\0')                 len++;                       char *address=(char *)malloc(len+1);             assert(address != NULL);                    while((*address++=*start++)!='\0');              return address-(len+1);          }         return NULL;     }

标准库的实现：

char * __cdecl strcat (char * dst,const char * src)  {      char * cp = dst;      while( *cp )          cp++;                   /* find end of dst */      while( *cp++ = *src++ ) ;       /* Copy src to end of dst */      return( dst );                  /* return dst */  }    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 );  }    size_t __cdecl strlen (const char * str)  {      const char *eos = str;            while( *eos++ ) ;            return( (int)(eos - str - 1) );  }    char * __cdecl strncat (char * front,const char * back,size_t count)  {      char *start = front;            while (*front++)          ;      front--;            while (count--)          if (!(*front++ = *back++))              return(start);                    *front = '\0';          return(start);  }    int __cdecl strncmp (const char * first,const char * last,size_t count)  {      if (!count)          return(0);            while (--count && *first && *first == *last)      {          first++;          last++;      }            return( *(unsigned char *)first - *(unsigned char *)last );  }    /* Copy SRC to DEST.  */  char *  strcpy (char * dest,const char* src)   {      reg_char c;      char *__unbounded s = (char *__unbounded) CHECK_BOUNDS_LOW (src);      const ptrdiff_t off = CHECK_BOUNDS_LOW (dest) - s - 1;      size_t n;            do      {          c = *s++;          s[off] = c;      }      while (c != '\0');            n = s - src;      (void) CHECK_BOUNDS_HIGH (src + n);      (void) CHECK_BOUNDS_HIGH (dest + n);            return dest;  }    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);  }