Re: reentrantcy?
On Sat, 19 Nov 2011 23:22:34 -0600
Paul Elliott <pelliott@blackpatchpanel.com> wrote:
> On Saturday, November 19, 2011 07:27:14 PM you wrote:
> > On Sat, Nov 19, 2011 at 07:03:19PM -0600, Paul Elliott wrote:
> > > Is there any requirement that a shared library be reentrant, if the
> > > upstream wrote it that way?
> > >
> > > Am I correct in my assumption, that although non-reentrancy presents a
> > > problem for multi threaded programs, it is not a problem for
> > > multi-programming, that is two programs using the same shared library at
> > > the same time, because the linker will assure that the separate programs
> > > will have separate copies of any writable global or static data?
> >
>
> A
> > It's not the linker that does it, it's the kernel. Separate programs
The linker sets flags that the kernel used to enforce, or not that requirement.
> >
> B
> > If all linkings are shared, it is my understanding that the global state
> > of the multiple-linked library would be shared by all references.
this is true. both linkings are with in the same process space. program A includes
a library B and one C. B is linked also with C. if A calls C function F() that sets
a global variable then B will see that value. If there is "state" to the value the
state that A experiences may be inconsistent.
>
> Statement A and B above seem to contradict each other.
>
> If global state means global variables, my experiments show that B is not true
> but A is true.
the B case has both in the same process. its not directly related to threading. If there
were 2 copies of B running (B and B') then they wouldn't share any data that was not explicitly
marked as share-able.
>
> The state of a global variable in one program does not effect the state of the
> same global variable in another program that links the same shared library.
> Both programs have seperate copies of static or global writable varriables of
> the shared library.
> Unless the programmer takes special action to force only one copy.
>
> Best Wishes.
>
Hope that helps
Joe
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