Re: Help wanted on some source codes

On 29 Nov 2005 18:34:15 -0800, "slebetman@xxxxxxxxx"
<slebetman@xxxxxxxxx> wrote in comp.lang.c:

> Old Wolf wrote:
> > Becker wrote:
> >
> > > 1.===========================================
> > > /* a.c */
> > > int x;
> > > int y;
> > >
> > > void main()
> > > {
> > > f();
> > > printf("%x %x\n", x, y);
> > > }
> > >
> > > /* b.c */
> > > double x;
> > >
> > > void f()
> > > {
> > > x = -0.0;
> > > }
> >
> > All of your programs have undefined behaviour because there
> > are two variables called 'x' with external linkage.
> >
>
> The programs are rubbish of course. But wouldn't the file scope of x
> mean that function f() is actually refering to the double x instead of
> the int x?

This is part of what I posted in a reply to the OP, but I'll repeat it
here for your benefit. The issue here is NOT file scope, but most
specifically IS external linkage:

The C standard says this in "6.9 External definitions" paragraph 5:

"An external definition is an external declaration that is also a
definition of a function (other than an inline definition) or an
object. If an identifier declared with external linkage is used in an
expression (other than as part of the operand of a sizeof operator
whose result is an integer constant), somewhere in the entire program
there shall be exactly one external definition for the identifier;
otherwise, there shall be no more than one."

Violating a 'shall' outside of a constraint section is just plain old
ordinary undefined behavior. You have left planet C behind and
entered the Twilight Zone, and there is no right or wrong.

--
Jack Klein
Home: http://JK-Technology.Com
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--
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.

Relevant Pages

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  • Re: Help wanted on some source codes
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