Re: Reentrancy
Krat <kursattheking@gmail.com> wrote:
"Igor Tandetnik" <itandetnik@mvps.org> wrote in message
news:uYHFlaSuJHA.1240@TK2MSFTNGP02.phx.gbl...
Krat <kursattheking@gmail.com> wrote:
The wikipedia
article(http://en.wikipedia.org/wiki/Reentrant_(subroutine)) that
defines "reentrant" gives 6 rules to write reentrant code. According
to the definition a function like this is reentrant :
char reentrant_func (char * ptr)
{
return *(ptr + 2);
}
CALL-2
char * buf = 0;
int main ()
{
buf = (char * ) malloc (256);
strcpy (buf, "Reentrant");
char c = reentrant_func (buf);
}
In call-2 we supplied a pointer to a global data so the function
call may not be reentrant.
A function can be reentrant or not. It's meaningless to say that a
particular function call is reentrant or not - you can't make the
same call twice.
if a function that manipulates a global variable is non-reentrant
then can we be sure that a function which indirectly refers to a
global variable via a pointer is reentrant?
Being reentrant doesn't mean being resistant to abuse. Let's put it this
way: there are functions that can be reentrant if used correctly, and
there are functions that can't be made reentrant no matter how nice the
caller is to them. "Reentrant when called correctly" is then abbreviated
simply to "reentrant": an assumption is made that the caller is
reasonable.
The article explains that
if a function that uses a global variable is non-reentrant because
value of the varible can be changed by another thread. The global
variable referred by the pointer can be changed by another thread
too. So I think the definition in the article is short, it should add
another condition that states "a function that takes a reference to a
global resource is non-reentrant".
Such a requirement won't be particularly helpful. How are you, as a
library author, going to satisfy it? By restricting yourself to
functions that don't take any parameters at all?
According to this new condition,
if a caller passes a pointer to a local variable then the function
will be reentrant but otherwise the function will be non-reentrant.
The function is either reentrant or non-reentrant. It can't be reentrant
one minute and non-reentrant the next. Otherwise, the term "reentrant"
would be useless for any practical purposes. Being reentrant is a
property of the function, not a property of the program using said
function.
In any case, in what sense is the situation you show non-reentrant?
Does the function not successfully return the third character of the
passed-in buffer? Will it not continue to do so even if called
concurrently?
It will work in either way but it it refers to a global variable then
the variable is subject to be manipulated by another thread.
And that renders the function non-reentrant because ...?
--
With best wishes,
Igor Tandetnik
With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
overhead. -- RFC 1925