Why is _beginthreadex failing with ERROR_INVALID_ACCESS? - c++

Question

I have a bunch of QWebViews rendered onto a widget. There comes a point at which I start getting the error QThread::start: Failed to create thread (The access code is invalid.). Looking at the Qt source, it appears that _beginthreadex is returning a null handle and errno is ERROR_INVALID_ACCESS, but I have no idea why.
Here is the backtrace at the printing of the error:
0 qErrnoWarning qglobal.cpp 2451 0x69ccdd3c
1 QThread::start qthread_win.cpp 469 0x69cd5831
2 QThreadPoolPrivate::tryStart qthreadpool.cpp 203 0x69ccc3f5
3 QThreadPool::start qthreadpool.cpp 474 0x69cccdf4
4 QHostInfoLookupManager::work qhostinfo.cpp 633 0x6cb9b071
5 QHostInfoLookupManager::scheduleLookup qhostinfo.cpp 652 0x6cb9b143
6 QHostInfo::lookupHost qhostinfo.cpp 202 0x6cb9a220
7 qt_qhostinfo_lookup qhostinfo.cpp 722 0x6cb9b4b6
8 QAbstractSocket::connectToHostImplementation qabstractsocket.cpp 1427 0x6cbb17f5
9 QAbstractSocket::qt_static_metacall moc_qabstractsocket.cpp 166 0x6cbb4925
10 QMetaMethod::invoke qmetaobject.cpp 1664 0x69dc784f
11 QMetaObject::invokeMethod qmetaobject.cpp 1179 0x69dc6d6b
12 QMetaObject::invokeMethod qobjectdefs.h 418 0x6cd361dd
13 QAbstractSocket::connectToHost qabstractsocket.cpp 1342 0x6cbb13b3
14 QSslSocket::connectToHostImplementation qsslsocket.cpp 1744 0x6cbc7340
15 QSslSocket::qt_static_metacall moc_qsslsocket.cpp 91 0x6cbc93cf
16 QMetaMethod::invoke qmetaobject.cpp 1664 0x69dc784f
17 QMetaObject::invokeMethod qmetaobject.cpp 1179 0x69dc6d6b
18 QMetaObject::invokeMethod qobjectdefs.h 418 0x6cd361dd
19 QAbstractSocket::connectToHost qabstractsocket.cpp 1342 0x6cbb13b3
20 QSslSocket::connectToHostEncrypted qsslsocket.cpp 422 0x6cbc55e1
21 QHttpNetworkConnectionChannel::ensureConnection qhttpnetworkconnectionchannel.cpp 607 0x6cb6191f
22 QHttpNetworkConnectionPrivate::_q_startNextRequest qhttpnetworkconnection.cpp 862 0x6cb5e92c
23 QHttpNetworkConnectionPrivate::queueRequest qhttpnetworkconnection.cpp 501 0x6cb5c57d
24 QHttpNetworkConnection::sendRequest qhttpnetworkconnection.cpp 931 0x6cb5edf2
25 QHttpThreadDelegate::startRequest qhttpthreaddelegate.cpp 291 0x6cb8912a
26 QHttpThreadDelegate::qt_static_metacall moc_qhttpthreaddelegate_p.cpp 113 0x6cbd147c
27 QMetaCallEvent::placeMetaCall qobject.cpp 525 0x69dcf91c
28 QObject::event qobject.cpp 1195 0x69dd08db
29 QApplicationPrivate::notify_helper qapplication.cpp 4551 0x2582f44
30 QApplication::notify qapplication.cpp 3933 0x25808b7
31 QCoreApplication::notifyInternal qcoreapplication.cpp 915 0x69dc0dc6
32 QCoreApplication::sendEvent qcoreapplication.h 231 0x69e35185
33 QCoreApplicationPrivate::sendPostedEvents qcoreapplication.cpp 1539 0x69dc1d2a
34 qt_internal_proc qeventdispatcher_win.cpp 496 0x69de2590
35 USER32!OffsetRect C:\Windows\syswow64\user32.dll 0 0x74cc62fa
36 ?? 0 0x152404
37 ?? 0 0x401
38 ?? 0
The code at the call looks like:
d->handle = (Qt::HANDLE) _beginthreadex(NULL, d->stackSize, QThreadPrivate::start, //d->stackSize is 0
this, CREATE_SUSPENDED, &(d->id));
if (!d->handle) {
qErrnoWarning(errno, "QThread::start: Failed to create thread");
d->running = false;
d->finished = true;
return;
}
Why is this happening and how do I fix it?
EDIT: also of note, there are exactly 500 threads at the point in which this breaks.

Answer 1

There's a good chance you've run out of free address space in your process (for thread stacks) after creating 500 threads. On 32-bit Windows, processes only get 2GB of address space by default (the upper half of the address space being reserved for the kernel). 500 1MB thread stacks (the default size, Qt may go higher or lower) plus all the other allocations your process makes could easily be using that up.
See this Old New Thing article for more.
Possible fixes:
If you know your QThreads don't need very big stacks, you can call QThread::setStackSize() to set a smaller size before starting the thread.
Consider using a thread pool and/or just reducing the number of concurrent threads you start. It's unlikely that you have enough CPU cores to make 500+ threads productive.
Use the Windows /3GB switch and make your application LARGE ADDRESS AWARE to get 3GB of user-mode address space.
Go 64-bit (for 63 bits of user-mode address space).