Re: typedefs and structs

[linux-os (Dick Johnson)]

On Thu, 10 Nov 2005, Nikita Danilov wrote:

> J.A. Magallon writes:
>
> [...]
>
> > >
> > > However, if the code is as follows:
> > > 	void foo (void) {
> > > 		int myvar = 0;
> > > 		printf("%d\n", myvar);
> > > 		bar(&myvar);
> > > 		printf("%d\n", myvar);
> > > 	}
> > > If bar is declared in _another_ file as
> > > 	void bar (const int * var);
> > > then I think the compiler can validly cache the value of 'myvar' for the
> > > second printf without re-reading it. Correct/incorrect?
> > >
> >
> > Nope. You can't trust bar() not doing something like
> >
> > bar(const int* local_var)
> > {
> >    ... use local_var as ro...
> >    extern int myvar;
> >    myvar = 7;
> > }
> >
> > For the compiler to do that, you must tag bar() with attribute(pure).
>
> extern declaration in your version of bar() cannot refer to the
> automatic variable myvar in foo().

Also, just because some called function may use casts to write
to a variable behind a void pointer doesn't mean that the function's
code must respect the potential of such buggy code.

The compiler must properly assume that the local variable, 'myvar'
will never be modified through the void pointer, even though some
jerk's buggy code may actually do that. Infortunately most protection
is page-based so there isn't any way for an OS to seg-fault somebody
who writes through void pointers.

The only hint that somebody did the wrong thing is that the wrong
value got printed. Because 'C' allows the use of casts, which
allows anything to be cast to anything else (may have to cheat by
first casting to void), the 'C' language not only allows you to
shoot yourself in the foot, but also hands you all the tools
necessary to do that, including loading and cocking the trigger.

Code inspection is sometimes necessary to avoid such problems.
In particular one must be on the lookout for 'excessive' casts.


Cheers,
Dick Johnson
Penguin : Linux version 2.6.13.4 on an i686 machine (5589.55 BogoMips).
Warning : 98.36% of all statistics are fiction.
.

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