strncmp.c

C语言库函数的原型,有用的拿去

/***
*strncmp.c - compare first n characters of two strings
*
*   Copyright (c) Microsoft Corporation. All rights reserved.
*
*Purpose:
*   defines strncmp() - compare first n characters of two strings
*   for lexical order.
*
*******************************************************************************/

#include <cruntime.h>
#include <string.h>

/***
*int strncmp(first, last, count) - compare first count chars of strings
*
*Purpose:
*   Compares two strings for lexical order.  The comparison stops
*   after: (1) a difference between the strings is found, (2) the end
*   of the strings is reached, or (3) count characters have been
*   compared.
*
*Entry:
*   char *first, *last - strings to compare
*   unsigned count - maximum number of characters to compare
*
*Exit:
*   returns <0 if first < last
*   returns  0 if first == last
*   returns >0 if first > last
*
*Exceptions:
*
*******************************************************************************/

int __cdecl strncmp (
    const char * first,
    const char * last,
    size_t count
)
{
    size_t n = 0;

    if (!count)
        return(0);

    if( count >= 4 )
    {
        /* unroll by four */
        for (; n < count-4; n += 4)
        {
            first += 4;
            last += 4;

            if (*(first - 4) == 0 || *(first - 4) != *(last - 4))
            {
                return (*(unsigned char *)(first - 4) - *(unsigned char *)(last - 4));
            }

            if (*(first - 3) == 0 || *(first - 3) != *(last - 3))
            {
                return (*(unsigned char *)(first - 3) - *(unsigned char *)(last - 3));
            }

            if (*(first - 2) == 0 || *(first - 2) != *(last - 2))
            {
                return (*(unsigned char *)(first - 2) - *(unsigned char *)(last - 2));
            }

            if (*(first - 1) == 0 || *(first - 1) != *(last - 1))
            {
                return (*(unsigned char *)(first - 1) - *(unsigned char *)(last - 1));
            }
        }
    }

    /* residual loop */
    for (; n < count; ++n)
    {
        if (*first == 0 || *first != *last)
        {
            return (*(unsigned char *)first - *(unsigned char *)last);
        }
        ++first;
        ++last;
    }

    return 0;
}