C/string.h/strcat
strcat: concatenates a copy of *str2 to *str1 and terminates *str1 with a null
//Declaration: char *strcat(char *str1, const char *str2);
//Return: returns *str1.
#include <stdio.h>
#include <string.h>
int main(void)
{
char s1[80], s2[80];
printf("s1:");
gets(s1);
printf("s2:");
gets(s2);
strcat(s2, s1);
printf(s2);
return 0;
}
/*
s1:s
s2:2
2s*/