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Revert "iOS: Replace variable length array in event dispatcher with QVarLengthArray"

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This reverts commit be93c06758260b1e0738ab17a2ecc4ee8f90e87e.

The assumption was that using a QVarLengthArray was equivalent to
the existing C-style VLA, as we initialized the QVarLengthArray
with defaultStackSize as the Prealloc template argument.

However we failed to take into account that we adjust the stack
size via Stack::computeSize(), by adding space for our memory
guard, so we would effectively always end up with the user-main
stack on the heap.

This caused problems for Qt Quick's V4 engine, because it checks
for C++ stack overflow by comparing the current stack pointer to
the base and limit given by pthread_get_stackaddr_np and RLIMIT_STACK,
resulting in "RangeError: Maximum call stack size exceeded." errors
and failure to load the QML.

We can't tweak what pthread_get_stackaddr_np reports for an already
started thread, so in practice we can not put the user-main stack
on the heap, so we revert back to the use of C-style variable length
arrays, which should be fine for this specific use-case.

Change-Id: I05a4bd32697abee71aa9e70f9f5a011cc338f915
Pick-to: 6.9 6.8
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
This commit is contained in:
Tor Arne Vestbø 2025-04-30 15:12:46 +00:00
parent 981757189c
commit ccce91461a
1 changed files with 11 additions and 4 deletions
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15
src/plugins/platforms/ios/qioseventdispatcher.mm
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@ -188,16 +188,23 @@ extern "C" int qt_main_wrapper(int argc, char *argv[])
s_isQtApplication = true;
@autoreleasepool {
constexpr size_t defaultStackSize = 512 * kBytesPerKiloByte; // Same as secondary threads
size_t defaultStackSize = 512 * kBytesPerKiloByte; // Same as secondary threads
uint requestedStackSize = qMax(0, infoPlistValue(@"QtRunLoopIntegrationStackSize", defaultStackSize));
if (infoPlistValue(@"QtRunLoopIntegrationDisableSeparateStack", false))
requestedStackSize = 0;
QVarLengthArray<uchar, defaultStackSize> reservedStack(Stack::computeSize(requestedStackSize));
if (reservedStack.size() > 0) {
userMainStack.adopt(reservedStack.data(), reservedStack.size());
QT_WARNING_PUSH
QT_WARNING_DISABLE_CLANG("-Wunknown-warning-option")
QT_WARNING_DISABLE_CLANG("-Wvla-cxx-extension")
// The user-main stack _must_ live on the stack, so that the stack pointer
// during user-main is within pthread_get_stackaddr_np/pthread_get_stacksize_np.
char reservedStack[Stack::computeSize(requestedStackSize)];
QT_WARNING_POP
if (sizeof(reservedStack) > 0) {
userMainStack.adopt(reservedStack, sizeof(reservedStack));
if (infoPlistValue(@"QtRunLoopIntegrationDebugStackUsage", false)) {
debugStackUsage = true;