[^^mac]  [Main TECH page]  
Evolving Pains

For this next trick i wish to show that a program
that had been used for YEARS had a hidden flaw.

It was simple function that Input a string from the
console. In order to make it "foolproof" a number
of checks were put into it. And so it went.

Then it started crashing - but only with a certain
new proogram. 

The catch was that you would call the function:

    Input_string (Junk3);

And it would return what the user had typed in
via the return variable Junk3 (char *) and nicely
terminated by a \0.

Except of course you have to allocte Junk3 first; 
eg,

       Junk3 = (char *) malloc (STRING_SIZE);
      *Junk3 = '\0';

What happens if Junk3 isn't allocated? As it turns
out, in most cases, it uses the NEXT allocated space.
And that (in most cases was Junk4) - which was almost
never used anyway - so who cared? (or more sinisterly,
who knew?  Murphy natch).

So, in a new program the string wasn't allocated 
and it was a new variable (eg, Working_buffer).

Of course, a new addition 

    void Input_string (char *Xs)
    {
       if (Xs == NULL)
       { 
          printf ("*** ERR: Un-allocated string passed to Input_string\n");
          exit;
       }

Of course some comilers don't initialise strings to NULL.

And on it goes!