Interesting C question

From: Bryan Pope <bpope_at_wordstock.com>
Date: Wed Feb 6 09:11:40 2002

And thusly Cini, Richard spake:
>
> Hello, all:
>
> I have discovered an interesting anomaly while finishing up the 2.2
> version of the Altair32 Emulator (to be released this weekend, by the way).
> Maybe it's my lack of understanding of C or the service pack I applied to
> MSVC :-)
>
> Between version 2.0 and 2.1, BASIC 3.2 was broken -- it was hanging
> in a loop in the console input code waiting for the right status bits to be
> set by my port emulator code. While I was setting the right bits, the
> internal emulator variable wasn't reflecting the assignment.
>
> In the sio00h routine in the sio.c module, I declare a local-scope
> variable of type int that's not initialized...
>
> int sio00h (params...){
>
> int nSioStatus ;
>
> ... stuff
> }
>
> When in the MSVC debugger, the undeclared variable has the value
> 0xcccccccc. OK.
>
> In the body of the routine, the variable can be set to three values
> using #defines from the header. For some reason, the variable is not being
> set to the #define value. As you trace execution, the variable to be
> returned by the routine remains 0xcccccccc after assignment:
>
> nSioStatus = SIO_WDB ; //#defines to 0x80
>
> So, I initialized the variable to 0 and for good measure, returned
> the int as:
>
> return (nSioStatus & 0xff) ;
>
> In one of the top calling routines, it gets truncated to a BYTE-size
> (so to speak) anyway.

If you only want a byte sized data type, why not use a char type for the
variable and return value.

Have you tried using fprintf (stderr, ...); to print out the value of
nSioStatus right after it is initialized?

Another thing that comes to mind: Do you use any blocks of malloc'd memory?
Maybe you are going outside the allocated range?


>
> So I figured that maybe there was a scoping problem but the
> nSioStatus variable is not used in any other module, nor is it declared
> extern or module global. The #define appears in multiple headers but are all
> defined to be the same value.

You could also try fprintf'ing the #define value in the sio00h function.


>
> Now, BASIC 3.2 it works but I can't really figure out why.
>
> Any thoughts??
>

This is all I can come up without having more code to look at...

Cheers,

Bryan
Received on Wed Feb 06 2002 - 09:11:40 GMT