从原字符串的第m个开始的全部字符复制成为另一个字符串

#include<stdio.h>
#include<string.h>

int main ()
{
	void copystr(char *,char *,int);
	int m;
	char str1[20],str2[20];
	printf("input string:");
	gets(str1);
	printf("which character that begin to copy?");
	scanf("%d",&m);
	if(strlen(str1) < m)
	{
		printf("input error!");
	}
	else
	{
		copystr(str1,str2,m);
		printf("result:%s
",str2);
	}
	return 0;
}

void copystr(char *p1,char *p2,int m)
{
	int n;
	n = 0;
	while(n < m - 1)
	{
		n++;
		p1++;
	}
	while(*p1 != "")
	{
		*p2 = *p1;
		p1++;
		p2++;
	}
	*p2 = "";
}