1、memcpy

void *memcpy(void *dst, const void *src, unsigned int len)
{
	void * ret = dst;
	while (len-- > 0) *((char *)dst)++ = *((char *)src)++;
	return ret;
}

2、memset

void * memset(void * s,char c,size_t count)
{
	char *xs = (char *) s;

	while (count--)
		*xs++ = c;

	return s;
}

3、strcpy

char * strcpy(char * dest,const char *src)
{
	char *tmp = dest;
	while ((*dest++ = *src++) != '\0')
		/* nothing */;
	return tmp;
}

4、strncpy

char * strncpy(char * dest,const char *src,size_t count)
{
	char *tmp = dest;

	while (count-- && (*dest++ = *src++) != '\0')
		/* nothing */;

	return tmp;
}

5、strcmp

int strcmp(const char * cs,const char * ct)
{
	register signed char __res;

	while (1) {
		if ((__res = *cs - *ct++) != 0 || !*cs++)
			break;
	}

	return __res;
}

6、strncmp

int strncmp(const char * cs,const char * ct,size_t count)
{
	register signed char __res = 0;

	while (count) {
		if ((__res = *cs - *ct++) != 0 || !*cs++)
			break;
		count--;
	}

	return __res;
}

7、strlen

size_t strlen(const char * s)
{
	const char *sc;

	for (sc = s; *sc != '\0'; ++sc)
		/* nothing */;
	return sc - s;
}