memcpy()函数实现

void* my_memcpy_byte(void* dst, const void* src, size_t n) {
    // Copies n bytes from src to dst
     
    // Since we cannot dereference a void* ptr,
    // we first typecast it to a char* ptr
    // and then do the copying byte by byte,
    // since a char* ptr references a single byte
    char* char_dst = (char*) dst;
    char* char_src = (char*) src;
 
    for (int i=0; i<n; i++) {
        *char_dst++ = *char_src++;
    }
 
    return dst;
}

void *my_memcpy_byte(void *dst, const void *src, int n)
{
    if (dst == NULL || src == NULL || n <= 0)
        return NULL;

    char * pdst = (char *)dst;
    char * psrc = (char *)src;

    if (pdst > psrc && pdst < psrc + n)
    {
        pdst = pdst + n - 1;
        psrc = psrc + n - 1;
        while (n--)
            *pdst-- = *psrc--;
    }
    else
    {
        while (n--)
            *pdst++ = *psrc++;
    }
    return dst;
}　　　　
//*pdst-- = *psrc--;  查了下运算符优先级（*,--）优先级相同，从右向左结合，psrc--是先使用，后减减　　　　
//等价于*pdst = *psrc;psrc--;pdst--;