摘自 linux 源代码


想知道怎么实现的看看，其实都很简单，没有想象的那么难
1.常用字符串操作
/**
* strcpy - Copy a %NUL terminated string
* @dest: Where to copy the string to
* @src: Where to copy the string from
*/
char * strcpy(char * dest,const char *src)
{
    char *tmp = dest;

    while ((*dest++ = *src++) != '\0')
        /* nothing */;
    return tmp;
}




/**
* strncpy - Copy a length-limited, %NUL-terminated string
* @dest: Where to copy the string to
* @src: Where to copy the string from
* @count: The maximum number of bytes to copy
*
* The result is not %NUL-terminated if the source exceeds
* @count bytes.
*/
char * strncpy(char * dest,const char *src,size_t count)
{
    char *tmp = dest;

    while (count) {
        if ((*tmp = *src) != 0) src++;
        tmp++;
        count--;
    }
    return dest;
}





/**
* strcat - Append one %NUL-terminated string to another
* @dest: The string to be appended to
* @src: The string to append to it
*/
char * strcat(char * dest, const char * src)
{
    char *tmp = dest;

    while (*dest)
        dest++;
    while ((*dest++ = *src++) != '\0')
        ;

    return tmp;
}

/**
* strncat - Append a length-limited, %NUL-terminated string to another
* @dest: The string to be appended to
* @src: The string to append to it
* @count: The maximum numbers of bytes to copy
*
* Note that in contrast to strncpy, strncat ensures the result is
* terminated.
*/
char * strncat(char *dest, const char *src, size_t count)
{
    char *tmp = dest;

    if (count) {
        while (*dest)
            dest++;
        while ((*dest++ = *src++) != 0) {
            if (--count == 0) {
                *dest = '\0';
                break;
            }
        }
    }

    return tmp;
}
    
/**
* strcmp - Compare two strings
* @cs: One string
* @ct: Another string
*/
int strcmp(const char * cs,const char * ct)
{
    register signed char __res;

    while (1) {
        if ((__res = *cs - *ct++) != 0 || !*cs++)
            break;
    }

    return __res;
}


/**
* strncmp - Compare two length-limited strings
* @cs: One string
* @ct: Another string
* @count: The maximum number of bytes to compare
*/
int strncmp(const char * cs,const char * ct,size_t count)
{
    register signed char __res = 0;

    while (count) {
        if ((__res = *cs - *ct++) != 0 || !*cs++)
            break;
        count--;
    }

    return __res;
}




/**
* strlen - Find the length of a string
* @s: The string to be sized
*/
size_t strlen(const char * s)
{
    const char *sc;

    for (sc = s; *sc != '\0'; ++sc)
        /* nothing */;
    return sc - s;
}