#include <stdio.h>
char *mystrncpy(char *p1, char *p2, int n);
int main(void)
{
char str1[81];
char str2[81];
int n;
do
{
puts("input string1:");
gets(str1);
puts("input string2:");
gets(str2);
puts("input the number of copying char:");
scanf("%d",&n);
getchar();
puts("After copying:");
puts(mystrncpy(str1, str2, n));
puts("input any char except q to go on.");
gets(str1);
}
while(*str1 != 'q');
puts("Quit.");
return 0;
}
char *mystrncpy(char *p1, char *p2, int n)
{
char *p=p1;
while(*p1++ != '\0') continue;
*--p1 = *p2;
n--;
while(n>0 && *p2 != '\0')
{
*++p1 = *++p2;
n--;
}
return p;
}

因为新拼接的字符没有加结束符。在末尾修改为：

while(n>0 && *p2 != '\0')
{
*++p1 = *++p2;
n--;
}
*++p1='\0';
return p;
}

添加上一个结束符就可以