strcpy (newstr, str1); strcpy (newstr + str1_length, str2);
concat (str1, str2)     char *str1, *str2;{  char *newstr;  int str1_length = strlen (str1);  newstr = malloc (str1_length + strlen (str2) + 1);  if (newstr == 0)    return 0;  strcpy (newstr, str1);  strcpy (newstr + str1_length, str2);  return newstr;}