Cannot run Qt5 applications.

Cannot run Qt5 applications.

[David Stacey]

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I'm having difficulty running any Qt5 application. These are the 
commands I'm issuing:

     XWin -multiwindow &
     export DISPLAY=:0.0
     xclock &

and I see the clock, so X is up and running. Then:

     /usr/lib/qt5/examples/gui/analogclock/analogclock
     QXcbConnection: XCB error: 145 (Unknown), sequence: 162, resource 
id: 0, major c
     ode: 140 (Unknown), minor code: 20
     Bad system call (core dumped)

and no clock.

I've tried compiling a really trivial Qt5 example (attached):

     g++ -I/usr/include/qt5 -I/usr/include/qt5/QtWidgets 
-I/usr/include/qt5/QtGui \
     -o qt5-demo qt5-demo.cpp -lQt5Widgets -lQt5Core

And then running ./qt5-demo gives:

     QObject::connect: signal not found in QPushButton
     QObject::connect: signal not found in QWidget
     QObject::connect: signal not found in QWidget
     QXcbConnection: XCB error: 145 (Unknown), sequence: 162, resource 
id: 0, major
     code: 140 (Unknown), minor code: 20
     Bad system call (core dumped)

The demo runs fine in Fedora 21. It also runs in Cygwin when compiled 
with Qt4:

     g++ -I/usr/include/qt4 -I/usr/include/qt4/QtGui -o qt5-demo 
qt5-demo.cpp \
     -lQtGui -lQtCore

And now running the (incorrectly-named!) ./qt5-demo launches the 
application.

Please could you advise what I'm doing wrong.

Many thanks in advance,

Dave.


[-- Attachment #2: qt5-demo.cpp --]
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#include <QApplication>
#include <QPushButton>

int main(int argc, char** argv)
{
  QApplication app(argc, argv);
  QPushButton *button = new QPushButton("Hello!");
  QObject::connect(button, SIGNAL(clicked()), &app, SLOT(quit()));
  button->show();
  return app.exec();
}

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Re: Cannot run Qt5 applications.

[Jon TURNEY]

On 04/02/2015 23:20, David Stacey wrote:
> I'm having difficulty running any Qt5 application. These are the
> commands I'm issuing:
>
>      XWin -multiwindow &
>      export DISPLAY=:0.0
>      xclock &
>
> and I see the clock, so X is up and running. Then:
>
>      /usr/lib/qt5/examples/gui/analogclock/analogclock
>      QXcbConnection: XCB error: 145 (Unknown), sequence: 162, resource
> id: 0, major c
>      ode: 140 (Unknown), minor code: 20
>      Bad system call (core dumped)

Possibly you need to install and start cygserver (See [1])

If so, this is because Qt5 is assuming shared memory is available, which 
could possibly be handled in a better way...

[1]  http://x.cygwin.com/docs/ug/using-shared-memory.html

-- 
Jon TURNEY
Volunteer Cygwin/X X Server maintainer

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Re: Cannot run Qt5 applications.

[David Stacey]

On 05/02/15 01:40, Jon TURNEY wrote:
> On 04/02/2015 23:20, David Stacey wrote:
>> I'm having difficulty running any Qt5 application. These are the
>> commands I'm issuing:
>>
>>      XWin -multiwindow &
>>      export DISPLAY=:0.0
>>      xclock &
>>
>> and I see the clock, so X is up and running. Then:
>>
>>      /usr/lib/qt5/examples/gui/analogclock/analogclock
>>      QXcbConnection: XCB error: 145 (Unknown), sequence: 162, resource
>> id: 0, major c
>>      ode: 140 (Unknown), minor code: 20
>>      Bad system call (core dumped)
>
> Possibly you need to install and start cygserver (See [1])
>
> If so, this is because Qt5 is assuming shared memory is available, 
> which could possibly be handled in a better way...

Yes, that did the trick - thank you. I would never have figured that our 
from the error message!

Dave.


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Re: Cannot run Qt5 applications.

[Jon TURNEY]

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On 05/02/2015 01:40, Jon TURNEY wrote:
> On 04/02/2015 23:20, David Stacey wrote:
>> I'm having difficulty running any Qt5 application. These are the
>> commands I'm issuing:
>>
>>      XWin -multiwindow &
>>      export DISPLAY=:0.0
>>      xclock &
>>
>> and I see the clock, so X is up and running. Then:
>>
>>      /usr/lib/qt5/examples/gui/analogclock/analogclock
>>      QXcbConnection: XCB error: 145 (Unknown), sequence: 162, resource
>> id: 0, major c
>>      ode: 140 (Unknown), minor code: 20
>>      Bad system call (core dumped)
>
> Possibly you need to install and start cygserver (See [1])
>
> If so, this is because Qt5 is assuming shared memory is available, which
> could possibly be handled in a better way...
>
> [1]  http://x.cygwin.com/docs/ug/using-shared-memory.html

Yaakov,

This looks like a portability problem in Qt5, where it only handles 
shmget() failing with a return value of -1, not with SIGSYS, to fallback 
to using an image in unshared memory.

Patch attached.

-- 
Jon TURNEY
Volunteer Cygwin/X X Server maintainer

[-- Attachment #2: qt5-shm.patch --]
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--- origsrc/qtbase-opensource-src-5.3.2/src/plugins/platforms/xcb/qxcbbackingstore.cpp	2014-09-11 11:48:06.000000000 +0100
+++ src/qtbase-opensource-src-5.3.2/src/plugins/platforms/xcb/qxcbbackingstore.cpp	2015-02-13 17:30:11.410525500 +0000
@@ -75,6 +75,7 @@ public:
 
 private:
     void destroy();
+    static bool isShmSupported();
 
     xcb_shm_segment_info_t m_shm_info;
 
@@ -88,6 +89,44 @@ private:
     QRegion m_dirty;
 };
 
+static bool shmNotSupported = false;
+
+static void
+SigSysHandler(int signo)
+{
+    shmNotSupported = true;
+}
+
+bool
+QXcbShmImage::isShmSupported()
+{
+    static bool checked = false;
+    if (!checked)
+      {
+        void (*oldHandler)(int);
+        int shmid = -1;
+
+        /* If no SHM support in the kernel, the bad syscall will generate SIGSYS */
+        oldHandler = signal(SIGSYS, SigSysHandler);
+
+        shmNotSupported = false;
+        shmid = shmget(IPC_PRIVATE, 4096, IPC_CREAT);
+        if (shmid != -1)
+          {
+            /* Successful allocation - clean up */
+            shmctl(shmid, IPC_RMID, NULL);
+          }
+        else
+          {
+            /* Allocation failed */
+            shmNotSupported = true;
+          }
+        signal(SIGSYS, oldHandler);
+        checked = true;
+      }
+    return (!shmNotSupported);
+}
+
 QXcbShmImage::QXcbShmImage(QXcbScreen *screen, const QSize &size, uint depth, QImage::Format format)
     : QXcbObject(screen->connection())
     , m_gc(0)
@@ -116,7 +155,9 @@ QXcbShmImage::QXcbShmImage(QXcbScreen *s
     if (!segmentSize)
         return;
 
-    int id = shmget(IPC_PRIVATE, segmentSize, IPC_CREAT | 0600);
+    int id = -1;
+    if (isShmSupported())
+      id = shmget(IPC_PRIVATE, segmentSize, IPC_CREAT | 0600);
     if (id == -1)
         qWarning("QXcbShmImage: shmget() failed (%d) for size %d (%dx%d)",
                  errno, segmentSize, size.width(), size.height());
@@ -130,7 +171,7 @@ QXcbShmImage::QXcbShmImage(QXcbScreen *s
     xcb_generic_error_t *error = NULL;
     if (shm_present)
         error = xcb_request_check(xcb_connection(), xcb_shm_attach_checked(xcb_connection(), m_shm_info.shmseg, m_shm_info.shmid, false));
-    if (!shm_present || error) {
+    if (!shm_present || error || (id == -1)) {
         free(error);
 
         shmdt(m_shm_info.shmaddr);

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Re: Cannot run Qt5 applications.

[Yaakov Selkowitz]

On Fri, 2015-02-13 at 18:32 +0000, Jon TURNEY wrote:
> On 05/02/2015 01:40, Jon TURNEY wrote:
> > On 04/02/2015 23:20, David Stacey wrote:
> >> I'm having difficulty running any Qt5 application. These are the
> >> commands I'm issuing:
> >> [snip]
> >> and I see the clock, so X is up and running. Then:
> >> [snip]
> >
> > Possibly you need to install and start cygserver (See [1])
> >
> > If so, this is because Qt5 is assuming shared memory is available, which
> > could possibly be handled in a better way...
> 
> This looks like a portability problem in Qt5, where it only handles 
> shmget() failing with a return value of -1, not with SIGSYS, to fallback 
> to using an image in unshared memory.
> 
> Patch attached.

Or is it a problem with our shmget()?

http://pubs.opengroup.org/onlinepubs/9699919799/functions/shmget.html
http://man7.org/linux/man-pages/man2/shmget.2.html

Perhaps we should be just returning -1 with an errno (ENOSYS?) instead
of raise(SIGSYS)?

--
Yaakov



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Re: Cannot run Qt5 applications.

[Corinna Vinschen]

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On Mar  2 17:34, Yaakov Selkowitz wrote:
> On Fri, 2015-02-13 at 18:32 +0000, Jon TURNEY wrote:
> > On 05/02/2015 01:40, Jon TURNEY wrote:
> > > On 04/02/2015 23:20, David Stacey wrote:
> > >> I'm having difficulty running any Qt5 application. These are the
> > >> commands I'm issuing:
> > >> [snip]
> > >> and I see the clock, so X is up and running. Then:
> > >> [snip]
> > >
> > > Possibly you need to install and start cygserver (See [1])
> > >
> > > If so, this is because Qt5 is assuming shared memory is available, which
> > > could possibly be handled in a better way...
> > 
> > This looks like a portability problem in Qt5, where it only handles 
> > shmget() failing with a return value of -1, not with SIGSYS, to fallback 
> > to using an image in unshared memory.
> > 
> > Patch attached.
> 
> Or is it a problem with our shmget()?
> 
> http://pubs.opengroup.org/onlinepubs/9699919799/functions/shmget.html
> http://man7.org/linux/man-pages/man2/shmget.2.html
> 
> Perhaps we should be just returning -1 with an errno (ENOSYS?) instead
> of raise(SIGSYS)?

Or you just add signal(SIGSYS, SIG_IGN) prior to calling shmget.
SIGSYS is raised when calling a system call which isn't available.
That's perfectly valid.

Of course, it would be nice if Qt5 used POSIX shared memory objects
iunstead, but that's asked too much, probably.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
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Red Hat

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Re: Cannot run Qt5 applications.

[Jon TURNEY]

On 03/03/2015 09:04, Corinna Vinschen wrote:
> On Mar  2 17:34, Yaakov Selkowitz wrote:
>> On Fri, 2015-02-13 at 18:32 +0000, Jon TURNEY wrote:
>>> On 05/02/2015 01:40, Jon TURNEY wrote:
>>>> On 04/02/2015 23:20, David Stacey wrote:
>>>>> I'm having difficulty running any Qt5 application. These are the
>>>>> commands I'm issuing:
>>>>> [snip]
>>>>> and I see the clock, so X is up and running. Then:
>>>>> [snip]
>>>>
>>>> Possibly you need to install and start cygserver (See [1])
>>>>
>>>> If so, this is because Qt5 is assuming shared memory is available, which
>>>> could possibly be handled in a better way...
>>>
>>> This looks like a portability problem in Qt5, where it only handles
>>> shmget() failing with a return value of -1, not with SIGSYS, to fallback
>>> to using an image in unshared memory.
>>>
>>> Patch attached.
>>
>> Or is it a problem with our shmget()?
>>
>> http://pubs.opengroup.org/onlinepubs/9699919799/functions/shmget.html
>> http://man7.org/linux/man-pages/man2/shmget.2.html
>>
>> Perhaps we should be just returning -1 with an errno (ENOSYS?) instead
>> of raise(SIGSYS)?

I think it was a BSD-ism to raise SIGSYS if shared memory is not 
available due to policy.

Historically, there must have been some OS which did this, because there 
is code to detect this situation in the X server [1]

[1] http://cgit.freedesktop.org/xorg/xserver/tree/Xext/shm.c#n167

Looking at [2],[3] it seems perhaps this was added in Cygwin because 
some FreeBSD test code was used?

[2] https://cygwin.com/ml/cygwin-cvs/2003-q4/msg00130.html
[3] https://cygwin.com/ml/cygwin-cvs/2003-q4/msg00128.html

I'd kind of assumed that modern BSDs behave in the same way, but that 
doesn't actually seem likely.

I have absolutely no problem with changing this to return ENOSPC (there 
is a limit of 0 SHM identifiers, and we have reached it), or whatever is 
appropriate in this state.

> Or you just add signal(SIGSYS, SIG_IGN) prior to calling shmget.
> SIGSYS is raised when calling a system call which isn't available.
> That's perfectly valid.

This is true.

I guess it's not how Linux behaves, though, so I think changing it ought 
to be considered to minimize porting effort.

I'll also note that checking once at startup, as the X server does, is 
not enough.  If the cygserver is stopped, it will die with an unexpected 
SIGSYS when a client tried to use shared memory.

> Of course, it would be nice if Qt5 used POSIX shared memory objects
> instead, but that's asked too much, probably.

Unfortunately, it has to use whatever the X server's MIT-SHM extension 
uses...

-- 
Jon TURNEY
Volunteer Cygwin/X X Server maintainer

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Re: Cannot run Qt5 applications.

[Corinna Vinschen]

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On Mar  3 14:48, Jon TURNEY wrote:
> On 03/03/2015 09:04, Corinna Vinschen wrote:
> >Or you just add signal(SIGSYS, SIG_IGN) prior to calling shmget.
> >SIGSYS is raised when calling a system call which isn't available.
> >That's perfectly valid.
> 
> This is true.
> 
> I guess it's not how Linux behaves, though, so I think changing it ought to
> be considered to minimize porting effort.

Done.


HTH,
Corinna

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