几种string库函数的实现

1. strcpy

char *strcpy(char *strDst, char *strSrc)
{
	assert(strDst != NULL  && strSrc != NULL);
	char *tmpDst = strDst;
	while((*tmpDst++ = *strSrc++) != '\0')
		;
	return strDst;
}

2. strcat

char *strcat(char *strDst, char *strSrc)
{
	assert(strDst != NULL && strSrc != NUULL);
	char *tmpDst = strDst;
	while(*tmpDst)	//不可使用while（*tmpDst++）
	{
		tmpDst++;
	}
	while((*tmpDst++ = *strSrc++) != '\0')
		;
	return strDst;
}

3. strcmp

int strcmp(const char *str1, const char *str2)
{
	assert(str1 != NULL && str2 != NULL);
	int ret;
	while(!(ret = *(unsigned char*)str1 - *(unsigned char*)str2) && *str1 && *str2)
	{
		str1++;
		str2++;
	}
	
	if (ret < 0)
		ret = -1;
	else if (ret == 0)
		ret = 0;
	else
		ret = 1;
	return ret;
	
}

4. strlen

int strlen(const char *str)
{
	const char *tmpStr;
	for (tmpStr = str; *tmpStr++;)
		;
	return (tmpStr - str -1);
}