C语言字符串库函数的实现

1.strlen（字符串的长度）

size_t Strlen(const char* str)
{
    assert(str);
    for (size_t i = 0;; ++i)
    {
        if (str[i] == ‘\0‘)
            return i;
    }
}

size_t StrlenByRecursion(const char* str)
{
    return *str == ‘\0‘ ? 0 : StrlenByRecursion(str + 1) + 1;
}

size_t __cdecl strlen (const char * str)  //函数库里面实现的代码{
    const char *eos = str;  

    while( *eos++ ) ;  

    return( (int)(eos - str - 1) );
}  

2.strcpy（字符串拷贝）

char* Strcpy1(char* dst, const char* src)
{
    assert(dst && src);
    int i = 0;
    for (; src[i] != ‘\0‘; ++i)
    {
        dst[i] = src[i];
    }
    dst[i] = ‘\0‘;
    return dst;
}

char* Strcpy2(char* dst, const char* src)
{
    assert(dst && src);
    char* tmp = dst;
    while ((*tmp++ = *src++) != ‘\0‘);
    *tmp = ‘\0‘;
    return dst;
}

char *  strcpy (char * dest,const char* src)     //函数库中的实现{      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;  }

3.strncpy

char* Strncpy(char* dst,const char* src,int lenth)
{
    assert(dst && src);
    char* tmp = dst;
    while (lenth--)
    {
        if ((*tmp++ = *src) != ‘\0‘)
            src++;
    }
    *tmp = ‘\0‘;
    return dst;
}

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);
} 

4.strcat（字符串连接）

char* Strcat(char* dst, const char* src)
{
    assert(dst && src);
    char* tmp = dst;
    while (*tmp)
        tmp++;
    while ((*tmp++ = *src++) != ‘\0‘);
    return dst;
}

char * __cdecl strcat (char * dst,const char * src)     //库中的实现方法
{
    char * cp = dst;  

    while( *cp )
        cp++;                   /* find end of dst */  

    while( *cp++ = *src++ ) ;       /* Copy src to end of dst */  

    return( dst );                  /* return dst */
} 

5.strncat

char* Strncat(char* dst,const char* src,size_t lenth)
{
    assert(dst && src);
    char* tmp = dst;
    while (*tmp)
        ++tmp;
    while (lenth-- && (*tmp++ = *src++));
    *tmp = ‘\0‘;
    return dst;
}

char * __cdecl strncat (char * front,const char * back,size_t count)     //库中实现的方法
{
    char *start = front;  

    while (*front++)
        ;
    front--;  

    while (count--)
        if (!(*front++ = *back++))
            return(start);  

        *front = ‘\0‘;
        return(start);
} 

6.strcmp

int Strcmp(const char* str1, const char* str2)
{
    assert(str1 && str2);
    while (*str1 == *str2 && *str2)
    {
        str1++;
        str2++;
    }
    return *（unsigned char*）str1 - *（unsigend char*）str2;
}

int __cdecl strcmp (const char * src,const char * dst)      //库中的实现方式
{
    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 );
}  

7.strncmp

int Strncmp(const char* str1, const char* str2,size_t lenth)
{
    assert(str1 && str2);
    while (lenth-- && *str2)
    {
        if (*str1 != *str2)
            return *(unsigned char*)str1 - *(unsigned char*)str2;
        str1++;
        str2++;
    }
    return 0;
}

int __cdecl strncmp (const char * first,const char * last,size_t count)     //库里实现的方式
{
    if (!count)
        return(0);  

    while (--count && *first && *first == *last)
    {
        first++;
        last++;
    }  

    return( *(unsigned char *)first - *(unsigned char *)last );
}