From: Thomas Demmer <demmer AT LSTM DOT Ruhr-UNI-Bochum DOT De>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: A bug in djgpp?
Date: Wed, 15 Apr 1998 19:35:58 +0200
Organization: Lehrstuhl fuer Stroemungsmechanik
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Tom Wilson wrote:
> 
> Hi People :I
> 
> I'm a newbie C programmer using djgpp, and while mucking about i found
> something weird and i dont really understand it :)
> 
> ok if i compile this peice of code..
> 
> #include <stdio.h>
> int main()
> {
>         short b, a;
>         printf("Choose a value for a: ");
>         scanf("%i", &a);
>         printf("Choose a value for b: ");
>         scanf("%i", &b);
>         printf("A is equal to %i\n", a);
>         printf("B is equal to %i\n", b);
>         return 0;
> }
> 
> i get the expected results... The program prints the value of A and B that
> you type in.
> The problem i have is.. if i declare the variables a and b in the oposite
> order.. ie,
> 
> #include <stdio.h>
> int main()
> {
>         short a, b;
[...]
 
> It prints a out as being the value 0, when it shoudn't be.
> 
> I've tried using both peices of code in Visual C++ and they both worked, so
> why does DJGPP stuff this up? anybody know? :I
Well, type
info libc scanf
and thoroghly read the format specifiers. And, recompile with -Wall
-pedantic.
In a nutshell: 
%i is basically an int specifier, so scanf will affect 4 bytes. You
declare
them being shorts, two bytes. The first version reads a, then screws up
b.
Then, you read in b, screwing up two bytes in your stack. It is just
plain luck that
this version works at all. 

Why the second version shows the described behavior is left as an
exercise.

The bottom line: Before suspecting bugs in library code, recompile your 
code with -Wall -pedantic, or better, -Wall -pedantic-errors
and check the warnings. 

Oh yes, this works in Visual C++ because probably there is
sizeof(int)==sizeof(short)
-- 
Ciao
Tom

*************************************************************
* Thomas Demmer                                             *
* Lehrstuhl fuer Stroemungsmechanik                         *
* Ruhr-Uni-Bochum                                           *
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