Interesting C question

From: Cini, Richard <RCini_at_congressfinancial.com>
Date: Wed Feb 6 08:34:04 2002

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.

        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.

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

        Any thoughts??

Rich
Received on Wed Feb 06 2002 - 08:34:04 GMT

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