qt.qpa.xcb: xcb_shm_attach() failed

[Rafael Sadowski]

I don't know if this has worked in the past, but when debugging some Qt
applications I saw the following debug messages:

$ gwenview
qt.qpa.xcb: Has MIT-SHM     : true
qt.qpa.xcb: Has MIT-SHM FD  : true
qt.qpa.xcb: xcb_shm_attach() failed
qt.qpa.xcb: failed to create System V shared memory segment (remote X11 
connection?), disabling SHM
qt.qpa.xcb: Using XInput version 2.2
qt.qpa.screen: Output DP-1 is not connected
qt.qpa.screen: Output HDMI-1 is not connected

The code that fails is here. xcb_shm_attach_checked and/or
xcb_request_check failed with all my Qt applications.

pobj/qtbase-5.15.6/qtbase-everywhere-src-5.15.6/src/plugins/platforms/xcb/qxcbbackingstore.cpp

bool QXcbBackingStoreImage::createSystemVShmSegment(xcb_connection_t *c, size_t 
segmentSize,
                                                    xcb_shm_segment_info_t 
*shmInfo)
{
    const int id = shmget(IPC_PRIVATE, segmentSize, IPC_CREAT | 0x1C0);
    if (id == -1) {
        qCWarning(lcQpaXcb, "shmget() failed (%d: %s) for size %zu", errno, 
strerror(errno), segmentSize);
        return false;
    }

    void *addr = shmat(id, nullptr, 0);
    if (addr == (void *)-1) {
        qCWarning(lcQpaXcb, "shmat() failed (%d: %s) for id %d", errno, 
strerror(errno), id);
        return false;
    }

    if (shmctl(id, IPC_RMID, nullptr) == -1)
        qCWarning(lcQpaXcb, "Error while marking the shared memory segment to 
be destroyed");

    const auto seg = xcb_generate_id(c);
    auto cookie = xcb_shm_attach_checked(c, seg, id, false);
    auto *error = xcb_request_check(c, cookie);
    if (error) {
        qCWarning(lcQpaXcb(), "xcb_shm_attach() failed");
        free(error);
        if (shmdt(addr) == -1)
            qCWarning(lcQpaXcb, "shmdt() failed (%d: %s) for %p", errno, 
strerror(errno), addr);
        return false;


If you want to test it run any Qt application with
QT_LOGGING_RULES="qt.qpa.xcb.debug=true" exported.

Should SHM with XCB generally work on OpenBSD?

Rafael