字符串函数和内存函数

一、strlen

strlen

int my_strlen1(const char* s)
{
    
	int cnt = 0;
	while (*s)
	{
    
		s++;
		cnt++;
	}

	return cnt;
}

int my_strlen2(const char* s)
{
    
	if (!*s) return 0;

	return my_strlen2(s + 1) + 1;
}

int my_strlen3(const char* s)
{
    
	const char* p = s;
	while (*p) p++;

	return (int)(p - s);

二、strcpy

strcpy

char* my_strcpy(char* dest, const char* src)
{
    
	char* ret = dest;
	assert(*src);
	while (*dest++ = *src++);
	return ret;
}

三、strcat

strcat

char* my_strcat(char* dest, const char* src)
{
    
	assert(*dest);
	assert(*src);
	char* ret = dest;
	while (*dest)
		dest++;
	while (*dest++ = *src++);
	return ret;
}

四、strstr

strstr

char* my_strstr(const char* str1, const char* str2)
{
    
	if (!*str2) return (char*)str1;

	char* cp = (char*)str2;
	char* s1, *s2;

	while (*cp)
	{
    
		s1 = (char*)cp;
		s2 = (char*)str2;

		if (*str2) return (char*)str1;

		while (*s1 & *s2 && !(*s1 - *s2))
			s1++, s2++;

		if (!*s2) return cp;

		cp++;
	}

	return nullptr;
}

五、strcmp

strcmp

int my_strcmp(const char* src, const char* dst)
{
    
	assert(*src);
	assert(*dst);
	int ret = 0;
	while (!(ret = (unsigned char*)src - (unsigned char*)dst) && *dst)
		dst++, src++;

	if (ret < 0) return -1;
	else if (ret) return 1;
	return 0;
}

六、memcpy

memcpy

void* my_memcpy(void* dst, const void* src, size_t count)
{
    
	assert(dst);
	assert(src);

	void* ret = dst;
	while (count--)
	{
    
		*(char*)dst = *(char*)src;
		dst = (char*) dst + 1;
		src = (char*)src + 1;
	}

	return ret;
}

七、memmove

memmove

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

	if (dst <= src || (char*)dst >= ((char*)src + count)) {
    

		while (count--) {
    
			*(char*)dst = *(char*)src;
			dst = (char*)dst + 1;
			src = (char*)src + 1;
		}
	}
	else {
    
		dst = (char*)dst + count - 1;
		src = (char*)src + count - 1;
		while (count--) {
    
			*(char*)dst = *(char*)src;
			dst = (char*)dst - 1;
			src = (char*)src - 1;
		}
	}
	return ret;
}