strcat : strcat « string.h « C Tutorial

Item	Value
Header file	string.h
Declaration	char strcat(char str1, const char *str2);
Function	concatenates a copy of str2 to str1 and terminates *str1 with a null.
Return	returns *str1.

You have to ensure that str1 is large enough to hold both its original contents and those of str2.

#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

24.6.strcat
	24.6.1.	strcat