Re: How to use __DYNAMIC_REENT__ correctly in multithreaded applications?

["J. Johnston" <jjohnstn at redhat dot com>]

A monster patch has been checked in.  I found a problem in stdio64/fseeko64.c
using struct stat when calling _fstat64_r. I could not switch to use stat64
as Cygwin has not defined it externally.  For now, stdio64/fseeko64.c will use _fstat_r
instead of _fstat64_r.

I have rebuilt on linux and cygwin.  Give it a try.

There is still some reentrancy fixups that could be done but they will
not have any impact on a __DYNAMIC_REENT__ or non-threaded build which
work correctly by virtue of using _REENT.

-- Jeff J.

Thomas Pfaff wrote:
> I can not do it this way because it must be fixed for cygwin and not on 
> an application level.
>
> As far as i can see the only problem with __DYNAMIC_REENT__ is this 
> stdio related stuff, so i would suggest to fix it temporarly in newlib.
>
> My proposal is:
>
> Create a new macro _GLOBAL_REENT which returns _impure_ptr.
> Change all functions in stdio to use _GLOBAL_REENT instead of _REENT 
> until there is a better solution.
>
> Thomas
>
> J. Johnston wrote:
>
> > It's a design problem that isn't the fault of __DYNAMIC_REENT__ and
> > is going to require a little bit of thought.  One way you might bypass 
> > this problem is
> > to create an fopen stub for your application and have it call _fopen_r 
> > with the default global reentrancy struct.
> >
> > -- Jeff J.
> >
> > Thomas Pfaff wrote:
> >
> > > Hello,
> > >
> > > some time ago i have added a __DYNAMIC_REENT__ to the cygwin part of
> > > config.h. The main purpose was to get thread safe errnos (without this
> > > define errno is stored process global in _impure_ptr).
> > >
> > > Now the reent structure for every thread is thread local and will be
> > > destroyed when the thread has terminated. A __getreent function was
> > > implemented to get the thread local reent.
> > >
> > > This leads to following problem:
> > >
> > > Each FILE structure contains a pointer to the reent structure of the
> > > thread that initially opened the file. If the thread terminates than the
> > >   reent pointer will point to an invalid address.
> > >
> > > Here is a simple testcase to illustrate this probleme:
> > >
> > > #include <stdio.h>
> > > #include <pthread.h>
> > >
> > > static FILE *fp;
> > >
> > > static void *threadfunc (void *parm)
> > > {
> > >   fp = fopen ("/tmp/testreent", "w");
> > >   return NULL;
> > > }
> > >
> > > int main(void)
> > > {
> > >   pthread_t thread;
> > >
> > >   pthread_create (&thread, NULL, threadfunc, NULL);
> > >   pthread_join (thread, NULL);
> > >
> > >   fprintf (fp, "test");
> > >   fclose (fp);
> > >
> > >   return 0;
> > > }
> > >
> > > A thread opens a file and terminates immediately. The mainthread will
> > > try to write to the opened file and segfaults in the CHECK_INIT macro in
> > > fwrite since the data part of the FILE pointer is no longer valid.
> > >
> > > I am a bit clueless now:
> > > How was DYNAMIC_REENT stuff meant to work  ?
> > >
> > > IMHO the only way to get this working is to remove the reent part in the
> > > FILE structure, but there might be a better solution.
> > >
> > > TIA,
> > > Thomas


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