Someone

#include <stdio.h>
#define MAXLEN 20
 
void strcat2(char *ps, char *pt);
 
int main()
{
  char t[MAXLEN];
  char s[MAXLEN];
  int i = 0, c;

  while ((c = getchar()) != EOF) {
    t[i++] = c;
    /* i is now incremented, so t[i] points to the wrong place. */
    if (t[i-1] == '\n') {
      strcat2(s, t);
      printf("%s", s);
      i = 0;
    }
  }
  return 0;
}

int s_index = 0;

void strcat2(char *ps,  char *pt)
{
  ps += s_index;
  while (s_index < MAXLEN && (*ps++ = *pt++) != '\n')
    s_index++;

  /* The '\0' character (0), signifies the end of a string. */
  *ps = '\0';
}