strlen(), strcpy(), strncpy(), strcat(), strncat(), strcmp(), strncmp()函数的封装

size_t strlen(char* str)
{
	assert(str != NULL);
	int len = 0;
	while(*str != '\0')
	{
		str++;
		len++;
	}
	return len;
}

char* strcpy(char* dest, const char* src)
{
	assert(dest != NULL && src != NULL);
	char* tmp = dest;
	while(*src != '\0')
	{
		*tmp++ = *src++;
	}
    *tmp = '\0';
	return dest;
}

char* strncpy(char* dest, const char* src, size_t n)
{
	assert(dest != NULL && src != NULL);
	char* tmp = dest;
	int len = strlen(tmp);
	int m = n;
	while(n-- > 0 && *src != '\0')
	{
		*tmp++ = *src++;
	}
	if(m > len) //需要复制的字符数少于原字符串长度时不加'\0'
		*tmp = '\0';
	return dest;
}

char* strcat(char* dest, const char* src)
{
	assert(dest != NULL && src != NULL);
	char* tmp = dest;
	while(*tmp != '\0')	
	{
		tmp++;
	}	
	while(*src != '\0')
	{
		*tmp++ = *src++;
	}
    *tmp = '\0';
	return dest;
}

char* strncat(char* dest, const char* src, size_t n)
{
	assert(dest != NULL && src != NULL);
	char* tmp = dest;
	while(*tmp != '\0')
	{
		tmp++;
	}
	while(n-- > 0 && *src != '\0')
	{
		*tmp++ = *src++;
	}
	*tmp = '\0';
	return dest;	
}

int strcmp(const char* s1, const char* s2)
{
	assert(s1 != NULL && s2 != NULL);
	while(*s1 != '\0' && *s2 != '\0' && *s1 == *s2)
	{
		s1++;
		s2++;
	}
	if(*s1 < *s2)
		return -1;
	if(*s1 > *s2)
		return 1;
	return 0;
}

int strncmp(const char* s1, const char* s2, size_t n)
{
	assert(s1 != NULL && s2 != NULL);
	while(--n > 0 && *s1 != '\0' && *s2 != '\0' && *s1 == *s2)//使用前--不会导致最后多一位
	{
		s1++;
		s2++;
	}
	if(*s1 < *s2)
		return -1;
	if(*s1 > *s2)
		return 1;
	return 0;
}