关于四个strlen，strcpy，strcat，strcmp函数的编写方法

#include <assert.h>

size_t str_len(const char* str)
{
	assert(NULL != str);
	const char* tmp = str;
	while(*tmp) tmp++;
	return tmp - str;
}

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

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

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