strcpy not acting as expected.

Projects Programming
2 
  aOneString = (char*) malloc((sizeof(username)+1));
  strcpy(aOneString, username);
  strcat(aOneString, ":");  
  strcat(aOneString, realm);  
  strcat(aOneString, ":");  
  strcat(aOneString, password);

You allocate exactly enough space to hold username, then you write username, a colon, realm, another colon, and password to that space. How do you expect all that to fit?