C/string.h/strncat
strncat: concatenates not more than count characters of *str2 to *str1 and terminates str1 with a null
//Declaration: char *strncat(char *str1, const char *str2, size_t count);
//Return: returns *str1.
#include <stdio.h>
#include <string.h>
int main(void)
{
char s1[80], s2[80];
unsigned int len;
gets(s1);
gets(s2);
len = 79-strlen(s2);
strncat(s2, s1, len);
printf(s2);
return 0;
}
/*
2
1
12*/