Re: Cannot run Qt5 applications.

[Jon TURNEY <jon.turney@dronecode.org.uk>]

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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

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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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