走进C标准库(7)——"string.h"中函数的实现memcmp,memcpy,memmove,memset

我的memcmp：

int memcmp(void *buf1, void *buf2, unsigned int count){
    int reval;
    while(count && !(reval = (*(unsigned char *)buf1) - (*(unsigned char *)buf2)))
    {
        buf1 = (unsigned char *)buf1 + 1;
        buf2 = (unsigned char *)buf2 + 1;
        --count;
    }
    return reval;
}

MS VC：

int __cdecl memcmp (
        const void * buf1,
        const void * buf2,
        size_t count
        )
{
        if (!count)
                return(0);

        while ( --count && *(char *)buf1 == *(char *)buf2 ) {
                buf1 = (char *)buf1 + 1;
                buf2 = (char *)buf2 + 1;
        }

        return( *((unsigned char *)buf1) - *((unsigned char *)buf2) );
}

应该使用const void *buf为宜，不改变该块内存的内容，最终使用unsigned char *进行运算，保证运算结果的符号正确。

我的memcpy：

void *memcpy(void *dest, const void *src, unsigned int count){
    void *reval = dest;
    while(count--){
         (*(unsigned char *)dest++) = (*(unsigned char *)src++);
    }
    return reval;
}

MSVC：

void * __cdecl memcpy (
        void * dst,
        const void * src,
        size_t count
        )
{
        void * ret = dst;

#if defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC)
        {
        extern void RtlMoveMemory( void *, const void *, size_t count );

        RtlMoveMemory( dst, src, count );
        }
#else  /* defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC) */
        /*
         * copy from lower addresses to higher addresses
         */
        while (count--) {
                *(char *)dst = *(char *)src;
                dst = (char *)dst + 1;
                src = (char *)src + 1;
        }
#endif  /* defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC) */

        return(ret);
}

我的memmove:

void *memmove(void *dest, const void *src, unsigned int count){
    void *reval = dest;
    int overlap = ((unsigned char *)src < (unsigned char *)dest && ((unsigned char *)src + count) > dest);
    while(count--){
        if(overlap)//src is in front of dest and overlap. copy direction is from endIndex to beginIndex
              (*((unsigned char *)dest + count)) = (*((unsigned char *)src + count));
        else
            (*(unsigned char *)dest++) = (*(unsigned char *)src++);
    }
    return reval;
}

MSVC:

void * __cdecl memmove (
        void * dst,
        const void * src,
        size_t count
        )
{
        void * ret = dst;

#if defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC)
        {
        extern void RtlMoveMemory( void *, const void *, size_t count );

        RtlMoveMemory( dst, src, count );
        }
#else  /* defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC) */
        if (dst <= src || (char *)dst >= ((char *)src + count)) {
                /*
                 * Non-Overlapping Buffers
                 * copy from lower addresses to higher addresses
                 */
                while (count--) {
                        *(char *)dst = *(char *)src;
                        dst = (char *)dst + 1;
                        src = (char *)src + 1;
                }
        }
        else {
                /*
                 * Overlapping Buffers
                 * copy from higher addresses to lower addresses
                 */
                dst = (char *)dst + count - 1;
                src = (char *)src + count - 1;

                while (count--) {
                        *(char *)dst = *(char *)src;
                        dst = (char *)dst - 1;
                        src = (char *)src - 1;
                }
        }
#endif  /* defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC) */

        return(ret);
}

关于memcpy和memmove的区别，memcpy不考虑内存区域重叠的情况而memmove保证内存区域重叠也能正常复制成功。

有时候我们的memcpy也可能在内存重叠的情况下正常使用，这取决于它的实现，不具有普遍性，C语言标准中未对其有这种要求。

参考资料：

《关于memcpy和memmove两函数的区别》

http://blog.csdn.net/caowei840701/article/details/8491836

《memcpy() vs memmove()》  

http://stackoverflow.com/questions/4415910/memcpy-vs-memmove

我的memset:

void *memset(void *buffer, int c, int count){
    void *reval = buffer;
    while(count--){
        (*(unsigned char *)buffer++) = (unsigned char)c;
    }
    return reval;
}

MSVC:

void * __cdecl memset (
        void *dst,
        int val,
        size_t count
        )
{
        void *start = dst;

#if defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC)
        {
        extern void RtlFillMemory( void *, size_t count, char );

        RtlFillMemory( dst, count, (char)val );
        }
#else  /* defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC) */
        while (count--) {
                *(char *)dst = (char)val;
                dst = (char *)dst + 1;
        }
#endif  /* defined (_M_MRX000) || defined (_M_ALPHA) || defined (_M_PPC) */

        return(start);
}

完