How to use __DYNAMIC_REENT__ correctly in multithreaded applications?

Fri Aug 22 19:40:00 GMT 2003

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
>>>>>>>>
--
Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple
Problem reports: http://cygwin.com/problems.html
Documentation: http://cygwin.com/docs.html
FAQ: http://cygwin.com/faq/