C標準庫string.h源碼八:memcmp/memset/memchr

int memcmp(const void *cs, const void *ct, size_t count)
{
    const unsigned char *su1, *su2;
    int res = 0;
    for (su1 = cs, su2 = ct; 0 < count; ++su1, ++su2, count--)
        if ((res = *su1 - *su2) != 0)
            break;
    return res;
}

void *memset(void *s, int c, size_t n)
{
	char * tmp = (char *)s;
	while(n-- > 0)
		*tmp++ = c;
	return s; 
}

/***
*char *memchr(buf, chr, cnt) - search memory for given character.
*
*Purpose:
*       Searches at buf for the given character, stopping when chr is
*       first found or cnt bytes have been searched through.
*
*Entry:
*       void *buf  - memory buffer to be searched
*       int chr    - character to search for
*       size_t cnt - max number of bytes to search
*
*Exit:
*       returns pointer to first occurence of chr in buf
*       returns NULL if chr not found in the first cnt bytes
*
*Exceptions:
*
*******************************************************************************/

void * memchr (const void * buf,int chr,size_t cnt)
{
        while ( cnt && (*(unsigned char *)buf != (unsigned char)chr) ) {
                buf = (unsigned char *)buf + 1;
                cnt--;
        }

        return(cnt ? (void *)buf : NULL);
}

 以上是memcmp,memset,memchr 函數的算法實現，如果相對算法優化，請參考前一章節memcpy函數的處理方式，這裏不展開討論了，另外提醒讀者一個訣竅，mem系列函數通常都帶3個參數，且第三個參數一般都指的是待操作的內存長度，有些人很容易弄混memset函數的第2以及第三個參數的意義。通過這個訣竅我們知道，memset第2個參數是指初始化初值，第三個參數指內存長度。至此，我們討論了string.h中所有的函數，並且學習了glibc以及crt的高效實現方式。