strcpy memcpy strcat strcmp 函数的实现

strcpy函数的实现

• 不考虑内存重叠

//这种实现如果遇到内存重叠就会出错，如mystrcpy(Src+1,Src);'\0'由于被覆盖导致出现死循环
char* strcpy(char* dst, const char* src)
{
    assert(dst);
    assert(src);
    char* ret = dst;
    while((*dst++ = *src++) != '\0');
    return ret;
}

• 考虑内存重叠

char* strcpy(char* dst, const char* src)
{
    assert((dst != NULL) && (src != NULL));
    char* ret = dst;
    int size = strlen(src) + 1;
    if(dst > src && dst < src + len)
    {
        dst = dst + size - 1;
        src = src + size - 1;
        while(size--)
        {
            *dst-- = *src--;
        }
    }
    else
    {
        while(size--)
        {
            *dst++ = *src++;
        }
    }
    return ret;
}

memcpy函数的实现

void* memcpy(void* dst, void* src, size_t size)
{
    if(dst == NULL || src == NULL)
    {
        return NULL;
    }
    void* res = dst;
    char* pdst = (char*)dst;
    char* psrc = (char*)src;
    
    if(pdst > Psrc && pdst < psrc + size)//有重叠
    {
        pdst += size - 1;
        psrc += size - 1;
        while(size--)
            *pdst-- = *psrc--; 
    }else//无重叠
    {
        while(size--)
        {
            *pdst++ = *psrc++;
        }
    }
    return res;
}

strcat函数的实现

char* strcat(char* dst, const char* src)
{
    char* res = dst;
    while(*dst != '/0')
        dst++;
    while((*dst++ = *src++) != '/0');
    return res;
}

strcmp函数实现

int strcmp(const char* str1, const char* str2)
{
    while(*str1 == *str2 && *str1 != '\0')
    {
        str1++;
        str2++;
    }
    return *str1 - *str2;
}