Non-Reentrant Class Use In Multithreaded Program
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These are relatively hard questions to answer, because it will depend on the way the data's organized. I'd make an effort to elaborate the best I can, nonetheless.
For 1:
Race conditions are an inevitable consequence of memory access, so if a race condition occurs depends heavily on the memory layout of the object/memory block in question. Say we are talking about a regular reentrant object, then it is safe in theory to call two methods simultaneously (i.e. from different threads) provided the mentioned two methods work on a different data member of that class. For example, if you have the very simple:
struct MyStruct { int a, b; };Then you can completely safely read or modify
afrom one thread andbfrom from another as long as you don't touch the other member. The same considerations apply to plain C arrays of data, e.g.:int * myarray = new int[200];Then you can read or modify simultaneously from different threads different parts of the array; meaning you can write to the first element while reading the second from different threads safely. If you can guarantee there's no overlap between the memory accesses made from the different threads, then there's no race condition, and you're completely fine. This is exploited, for example, if you have the consumer-producer problem and there's an example in Qt's documentation of how it can be achieved with semaphores. To make your regular objects work like this, however, is often too finicky and error prone, so you'd usually opt for a serial access (i.e. through a mutex). Also see answer to question #2 for related information.
Now, if you use a container (for example
std::vectoror especiallyQVectordue to implicit sharing) it's a bit more complicated. If you're working withstd::vectoryou're fine obtaining a read or write iterator directly, because there's no data sharing behind the scenes. Then the only problem to solve is like with regular arrays - to ensure there are no concurrent memory accesses of the different elements. If you useQVector(or aQList,QHash, etc.) on the other hand then obtaining the write iterator is by itself a write operation, because the data may be shared and data copy might occur at that point. After the possible detach has happened then the same considerations as forstd::vectorapply.For 2:
If you want to permit multiple readers and writers to access a specific data block, then a more elaborate scheme for locking is needed. As you correctly observed mutexes are exclusive, hence the name - mutual exclusive lock. They also guard a single resource, for example the whole array. If you need to have a threading primitive that provides guarding of multiple resources, then you need a semaphore (like in the consumer-producer example sourced above). On the other hand if you're satisfied with exclusive access only for write-induced races, which is exactly your question - allowing a single write at one time, but simultaneous reads, you could use Qt's QReadWriteLock which provides a locking scheme for that specific case (internal implementation isn't that important here, so I'll not venture into it).Here I also would like to note that if you can prepopulate the data, then it could be a very viable approach to avoid needless locking. Because after you have the data in a container (provided it holds reentrant objects and the classes respect the method's
constmodifier, meaning noconst_castormutable), you could use const-only access to that container and data in as many threads as you need. I quite often do that in my work with hashes.@kshegunov
First, thanks for your comprehensive explanation.For #1, you're making it way more complex than I had in mind. Say I have one thread writer & one thread reader, and my data is, say, a list/array of integers (forget vectors), no structures or whatever. I wish to use
QMutex. All I wanted to verify is: (a) writer can insert/delete/update list/array; (b) reader wants just to read element #10; (c) confirm that the implementation must beQMutexfor whole object/list/array, so that writer cannot change elements while reader trying to read one element.For #2,
QReadWriteLockis just the ticket instead ofQMutex, thank you. -
@kshegunov
First, thanks for your comprehensive explanation.For #1, you're making it way more complex than I had in mind. Say I have one thread writer & one thread reader, and my data is, say, a list/array of integers (forget vectors), no structures or whatever. I wish to use
QMutex. All I wanted to verify is: (a) writer can insert/delete/update list/array; (b) reader wants just to read element #10; (c) confirm that the implementation must beQMutexfor whole object/list/array, so that writer cannot change elements while reader trying to read one element.For #2,
QReadWriteLockis just the ticket instead ofQMutex, thank you.@JNBarchan said in Non-Reentrant Class Use In Multithreaded Program:
confirm that the implementation must be QMutex for whole object/list/array, so that writer cannot change elements while reader trying to read one element.
Yes, I confirm. While the mutex is locked the whole object/list/array is owned by the locking thread. All other threads requesting access are waiting and one of them (in an unspecified order) will acquire the mutex when the owning thread unlocks it.
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These are relatively hard questions to answer, because it will depend on the way the data's organized. I'd make an effort to elaborate the best I can, nonetheless.
For 1:
Race conditions are an inevitable consequence of memory access, so if a race condition occurs depends heavily on the memory layout of the object/memory block in question. Say we are talking about a regular reentrant object, then it is safe in theory to call two methods simultaneously (i.e. from different threads) provided the mentioned two methods work on a different data member of that class. For example, if you have the very simple:
struct MyStruct { int a, b; };Then you can completely safely read or modify
afrom one thread andbfrom from another as long as you don't touch the other member. The same considerations apply to plain C arrays of data, e.g.:int * myarray = new int[200];Then you can read or modify simultaneously from different threads different parts of the array; meaning you can write to the first element while reading the second from different threads safely. If you can guarantee there's no overlap between the memory accesses made from the different threads, then there's no race condition, and you're completely fine. This is exploited, for example, if you have the consumer-producer problem and there's an example in Qt's documentation of how it can be achieved with semaphores. To make your regular objects work like this, however, is often too finicky and error prone, so you'd usually opt for a serial access (i.e. through a mutex). Also see answer to question #2 for related information.
Now, if you use a container (for example
std::vectoror especiallyQVectordue to implicit sharing) it's a bit more complicated. If you're working withstd::vectoryou're fine obtaining a read or write iterator directly, because there's no data sharing behind the scenes. Then the only problem to solve is like with regular arrays - to ensure there are no concurrent memory accesses of the different elements. If you useQVector(or aQList,QHash, etc.) on the other hand then obtaining the write iterator is by itself a write operation, because the data may be shared and data copy might occur at that point. After the possible detach has happened then the same considerations as forstd::vectorapply.For 2:
If you want to permit multiple readers and writers to access a specific data block, then a more elaborate scheme for locking is needed. As you correctly observed mutexes are exclusive, hence the name - mutual exclusive lock. They also guard a single resource, for example the whole array. If you need to have a threading primitive that provides guarding of multiple resources, then you need a semaphore (like in the consumer-producer example sourced above). On the other hand if you're satisfied with exclusive access only for write-induced races, which is exactly your question - allowing a single write at one time, but simultaneous reads, you could use Qt's QReadWriteLock which provides a locking scheme for that specific case (internal implementation isn't that important here, so I'll not venture into it).Here I also would like to note that if you can prepopulate the data, then it could be a very viable approach to avoid needless locking. Because after you have the data in a container (provided it holds reentrant objects and the classes respect the method's
constmodifier, meaning noconst_castormutable), you could use const-only access to that container and data in as many threads as you need. I quite often do that in my work with hashes.@kshegunov said in Non-Reentrant Class Use In Multithreaded Program:
and the classes respect the method's const modifier, meaning no const_cast or mutable
Just a small note, the above is not exhaustive. For example:
class BreakingConst{ int* a; BreakingConst(int b=0) : a(new int(b)){} ~BreakingConst(){delete a;} int getIncreasingA() const{ return (*a)++;} };has no
const_castormutablebut still breaks if used as described"La mort n'est rien, mais vivre vaincu et sans gloire, c'est mourir tous les jours"
~Napoleon BonaparteOn a crusade to banish setIndexWidget() from the holy land of Qt
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@kshegunov said in Non-Reentrant Class Use In Multithreaded Program:
and the classes respect the method's const modifier, meaning no const_cast or mutable
Just a small note, the above is not exhaustive. For example:
class BreakingConst{ int* a; BreakingConst(int b=0) : a(new int(b)){} ~BreakingConst(){delete a;} int getIncreasingA() const{ return (*a)++;} };has no
const_castormutablebut still breaks if used as describedIt's probably somewhat of a fringe use case, but yes, you are right. The
constmodifier doesn't implicitly propagate to the indirectly referenced data.ais of typeint * constin that case, which allows you to actually dereference the pointer and modify the underlying data. The same behavior is true for a heap-allocated array too, however with auto-storage:class BreakingConst { int a[1]; BreakingConst(int b=0) { a[0] = b; } ~BreakingConst() { delete a; } int getIncreasingA() const { return a[0]++; } };The compiler knows what's going on and will prevent it.
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Last question for this thread - if you go here to the first answer Kuba is complaining about iterators. Any merit to this claim? Anything to worry about? When he says internal pointers that doesn' t mean a pointer to a qt class does it? Only an iterator pointer right?
He says:Sidebar: If you use iterators or internal pointers to data on non-const instances, you must forcibly detach() the object before constructing the iterators/pointers. The problem with iterators is that they become invalidated when an object's data is detached, and detaching can happen in any thread where the instance is non-const - so at least one thread will end up with invalid iterators. I won't talk any more of this, the takeaway is that implicitly shared data types are tricky to implement and use safely. With C++11, there's no need for implicit sharing anymore: they were a workaround for the lack of move semantics in C++98. -
Last question for this thread - if you go here to the first answer Kuba is complaining about iterators. Any merit to this claim? Anything to worry about? When he says internal pointers that doesn' t mean a pointer to a qt class does it? Only an iterator pointer right?
He says:Sidebar: If you use iterators or internal pointers to data on non-const instances, you must forcibly detach() the object before constructing the iterators/pointers. The problem with iterators is that they become invalidated when an object's data is detached, and detaching can happen in any thread where the instance is non-const - so at least one thread will end up with invalid iterators. I won't talk any more of this, the takeaway is that implicitly shared data types are tricky to implement and use safely. With C++11, there's no need for implicit sharing anymore: they were a workaround for the lack of move semantics in C++98.@Crag_Hack said in Non-Reentrant Class Use In Multithreaded Program:
Any merit to this claim?
Yes, some. See this thread on the mailing list.
Anything to worry about?
Don't call write operations, including non-const iterator retrieval, on an object from different threads!
When he says internal pointers that doesn' t mean a pointer to a qt class does it?
No, he means pointers to data that are kept internally by Qt. You don't have access to them, but you need to take into a account they exist. That's also the reason why in the docs it's said to not keep iterators to a container that may get modified (i.e. the container getting out of scope).
Only an iterator pointer right?
The iterator is a pointer to the data or an object keeping a pointer to the data. So in a sense you can say the iterator is a pointer (and it is for contiguous data classes like
QVector).With C++11, there's no need for implicit sharing anymore: they were a workaround for the lack of move semantics in C++98.
This is incorrect though. Qt's signal-slot mechanism could not work effectively without implicit sharing. Move semantics is not a panacea, and certainly solves no problems when an object has to be queued through Qt's event loop.
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@kshegunov said in Non-Reentrant Class Use In Multithreaded Program:
Anything to worry about?
Don't call write operations, including non-const iterator retrieval, on an object from different threads!
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I have two quick followup questions:
- There's no possibility of two different non-reentrant classes sharing the same global (program-wise) resources, e. g. static variables? So say QStorageInfo uses the same globals/statics as QPair, you use QStorageInfo in one thread then QPair in another - they're not gonna fight each other and end up with a race condition and everything is safe right? (arbitrary choice of classes)
- I'm not confident enough in my understanding of multithreading to feel comfortable with my code right now - so I'll just state it explicity. I am calling job.sortFileList(); in my worker thread. The job object is a custom class for backup jobs. I send it to the worker thread using signals and slots. The sortFileList() function follows as well as the comparison function. fileList is QList<std::pair<QString, QString> > fileList;
void BackupJob::sortFileList() { qSort(fileList.begin(), fileList.end(), fileListCompare); } bool fileListCompare(const std::pair<QString, QString> &leftFileStringPair, const std::pair<QString, QString> &rightFileStringPair) {...}fileListCompare() is a function declared in the BackupJob.h file but not a member of the BackupJob class (global right?). Is this call to qSort safe? I'm guessing yes since the worker thread uses its own instance of the class (the class contains no non-reentrant objects, functions, or anything) and the iterators are constant (they're constant right?).
*edit - when I hover above the iterators in Qt Creator they appear as const_iterators not sure why though I didn't declare fileList as const
Thanks for the help! -
I have two quick followup questions:
- There's no possibility of two different non-reentrant classes sharing the same global (program-wise) resources, e. g. static variables? So say QStorageInfo uses the same globals/statics as QPair, you use QStorageInfo in one thread then QPair in another - they're not gonna fight each other and end up with a race condition and everything is safe right? (arbitrary choice of classes)
- I'm not confident enough in my understanding of multithreading to feel comfortable with my code right now - so I'll just state it explicity. I am calling job.sortFileList(); in my worker thread. The job object is a custom class for backup jobs. I send it to the worker thread using signals and slots. The sortFileList() function follows as well as the comparison function. fileList is QList<std::pair<QString, QString> > fileList;
void BackupJob::sortFileList() { qSort(fileList.begin(), fileList.end(), fileListCompare); } bool fileListCompare(const std::pair<QString, QString> &leftFileStringPair, const std::pair<QString, QString> &rightFileStringPair) {...}fileListCompare() is a function declared in the BackupJob.h file but not a member of the BackupJob class (global right?). Is this call to qSort safe? I'm guessing yes since the worker thread uses its own instance of the class (the class contains no non-reentrant objects, functions, or anything) and the iterators are constant (they're constant right?).
*edit - when I hover above the iterators in Qt Creator they appear as const_iterators not sure why though I didn't declare fileList as const
Thanks for the help!@Crag_Hack said in Non-Reentrant Class Use In Multithreaded Program:
There's no possibility of two different non-reentrant classes sharing the same global (program-wise) resources, e. g. static variables? So say QStorageInfo uses the same globals/statics as QPair, you use QStorageInfo in one thread then QPair in another - they're not gonna fight each other and end up with a race condition and everything is safe right? (arbitrary choice of classes)
Actually this is exactly what makes the classes non-reentrant - having a shared global/static variable. Objects of those classes are safe to use from different threads only if they're explicitly thread-safe, e.g.
QSemaphore/QMutexin contrast toQWidget, which is neither reentrant nor thread-safe.Is this call to qSort safe?
As long as 2 conditions are met:
- the
fileListComparedoesn't use global data (i.e. static/global variables) - it's reentrant - or if it does, then all accesses to the globals is serialized between different threads (with a mutex) - it's thread-safe. fileListisn't modified whileqSortis executed. This would be true ifBackupJobis reentrant, or more specifically - the thread operates on it's own instance of thefileListdata member.
when I hover above the iterators in Qt Creator they appear as const_iterators not sure why though I didn't declare fileList as const
It's a deficiency of the IDE you use.
qSortrequires non-const iterators. As a side note, you should usestd::sortinstead of the legacyqSort. -
Thanks kshegunov! :)
Actually this is exactly what makes the classes non-reentrant - having a shared global/static variable.
I understand that but I wonder whether two different classes ever share the same global/static variable. Then you might be in trouble if you used one in one thread and the other in another thread, even though each object is isolated to its own thread there still is an issue since they share those globals/statics.1. the fileListCompare doesn't use global data (i.e. static/global variables) - it's reentrant - or if it does, then all accesses to the globals is serialized between different threads (with a mutex) - it's thread-safe.
It only uses the referance parameters passed to the function.2. fileList isn't modified while qSort is executed. This would be true if BackupJob is reentrant, or more specifically - the thread operates on it's own instance of the fileList data member.
The thread operates on its own instance of a list of backupjob objects or an individual backupjob object which is passed to the thread through signals/slots. fileList is a member variable of the backupJob objects and the backupJob objects only use reentrant variables/objects. So safe right?As a side note, you should use std::sort instead of the legacy qSort.
I wasn't even aware qSort had been obsoleted. The first google result says nothing of such matters since it's Qt 4.8. For std::sort we just pass it the same QList::begin() QList::constBegin() QList::end() QList::constEnd() right?And how do I make those neat bars on the left for styling replied to comments?
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Thanks kshegunov! :)
Actually this is exactly what makes the classes non-reentrant - having a shared global/static variable.
I understand that but I wonder whether two different classes ever share the same global/static variable. Then you might be in trouble if you used one in one thread and the other in another thread, even though each object is isolated to its own thread there still is an issue since they share those globals/statics.1. the fileListCompare doesn't use global data (i.e. static/global variables) - it's reentrant - or if it does, then all accesses to the globals is serialized between different threads (with a mutex) - it's thread-safe.
It only uses the referance parameters passed to the function.2. fileList isn't modified while qSort is executed. This would be true if BackupJob is reentrant, or more specifically - the thread operates on it's own instance of the fileList data member.
The thread operates on its own instance of a list of backupjob objects or an individual backupjob object which is passed to the thread through signals/slots. fileList is a member variable of the backupJob objects and the backupJob objects only use reentrant variables/objects. So safe right?As a side note, you should use std::sort instead of the legacy qSort.
I wasn't even aware qSort had been obsoleted. The first google result says nothing of such matters since it's Qt 4.8. For std::sort we just pass it the same QList::begin() QList::constBegin() QList::end() QList::constEnd() right?And how do I make those neat bars on the left for styling replied to comments?
@Crag_Hack said in Non-Reentrant Class Use In Multithreaded Program:
Actually this is exactly what makes the classes non-reentrant - having a shared global/static variable.
I understand that but I wonder whether two different classes ever share the same global/static variable. Then you might be in trouble if you used one in one thread and the other in another thread, even though each object is isolated to its own thread there still is an issue since they share those globals/statics.Yes, this does happen.
Such classes must be used from the GUI thread only (e.g. QWidget, QPixmap must only be used in the same thread that created a QApplication)
2. fileList isn't modified while qSort is executed. This would be true if BackupJob is reentrant, or more specifically - the thread operates on it's own instance of the fileList data member.
The thread operates on its own instance of a list of backupjob objects or an individual backupjob object which is passed to the thread through signals/slots. fileList is a member variable of the backupJob objects and the backupJob objects only use reentrant variables/objects. So safe right?Sounds good.
As a side note, you should use std::sort instead of the legacy qSort.
I wasn't even aware qSort had been obsoleted. The first google result says nothing of such matters since it's Qt 4.8.See http://doc.qt.io/qt-5/qtalgorithms-obsolete.html
Qt 4.8 is obsolete too. Support ended in December 2015.
For std::sort we just pass it the same QList::begin() QList::constBegin() QList::end() QList::constEnd() right?
Yes.
And how do I make those neat bars on the left for styling replied to comments?
Click the "reply"/"quote" link at the bottom-right of each post.
Paragraphs that start with
>are treated as quotes. -
@Crag_Hack said in Non-Reentrant Class Use In Multithreaded Program:
Actually this is exactly what makes the classes non-reentrant - having a shared global/static variable.
I understand that but I wonder whether two different classes ever share the same global/static variable. Then you might be in trouble if you used one in one thread and the other in another thread, even though each object is isolated to its own thread there still is an issue since they share those globals/statics.Yes, this does happen.
Such classes must be used from the GUI thread only (e.g. QWidget, QPixmap must only be used in the same thread that created a QApplication)
2. fileList isn't modified while qSort is executed. This would be true if BackupJob is reentrant, or more specifically - the thread operates on it's own instance of the fileList data member.
The thread operates on its own instance of a list of backupjob objects or an individual backupjob object which is passed to the thread through signals/slots. fileList is a member variable of the backupJob objects and the backupJob objects only use reentrant variables/objects. So safe right?Sounds good.
As a side note, you should use std::sort instead of the legacy qSort.
I wasn't even aware qSort had been obsoleted. The first google result says nothing of such matters since it's Qt 4.8.See http://doc.qt.io/qt-5/qtalgorithms-obsolete.html
Qt 4.8 is obsolete too. Support ended in December 2015.
For std::sort we just pass it the same QList::begin() QList::constBegin() QList::end() QList::constEnd() right?
Yes.
And how do I make those neat bars on the left for styling replied to comments?
Click the "reply"/"quote" link at the bottom-right of each post.
Paragraphs that start with
>are treated as quotes.Thanks JKSH
Yes, this does happen.
Such classes must be used from the GUI thread only (e.g. QWidget, QPixmap must only be used in the same thread that created a QApplication)
I forgot to clarify in the previous post not just two objects isolated to their own threads but two objects of two different classes... But you knew that right? Anything else to know? Are these classes the exception, perhaps only graphical classes?
- the fileListCompare doesn't use global data (i.e. static/global variables) - it's reentrant - or if it does, then all accesses to the globals is serialized between different threads (with a mutex) - it's thread-safe.
It only uses the referance parameters passed to the function.
And of course this sounds good as well right...
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Thanks JKSH
Yes, this does happen.
Such classes must be used from the GUI thread only (e.g. QWidget, QPixmap must only be used in the same thread that created a QApplication)
I forgot to clarify in the previous post not just two objects isolated to their own threads but two objects of two different classes... But you knew that right? Anything else to know? Are these classes the exception, perhaps only graphical classes?
- the fileListCompare doesn't use global data (i.e. static/global variables) - it's reentrant - or if it does, then all accesses to the globals is serialized between different threads (with a mutex) - it's thread-safe.
It only uses the referance parameters passed to the function.
And of course this sounds good as well right...
@Crag_Hack said in Non-Reentrant Class Use In Multithreaded Program:
I forgot to clarify in the previous post not just two objects isolated to their own threads but two objects of two different classes... But you knew that right?
Yes, I knew that :)
The widget example still stands: Suppose you create a very simple program that consists of only 1
QApplicationobject and 1QWidgetobject. These two objects are not allowed to be in different threads.Anything else to know? Are these classes the exception, perhaps only graphical classes?
None that I can think of. I'm quite sure it's only the graphical classes with QApplication/QGuiApplication.
- the fileListCompare doesn't use global data (i.e. static/global variables) - it's reentrant - or if it does, then all accesses to the globals is serialized between different threads (with a mutex) - it's thread-safe.
It only uses the referance parameters passed to the function.
And of course this sounds good as well right...
You said:
fileListCompare()only uses references parameters. I presume you were confirming thatfileListCompare()doesn't use global/static variables.As @kshegunov said: If
fileListCompare()doesn't use global/static variables, thenfileListCompare()is a reentrant function. -
A couple more questions creeped into my domain... thankfully they're quick :) is qDebug() reentrant or should it not be used from multiple threads? All STL classes are reentrant right except for <atomic> and std::atomic guys right?
@Crag_Hack said in Non-Reentrant Class Use In Multithreaded Program:
A couple more questions creeped into my domain... thankfully they're quick :) is qDebug() reentrant or should it not be used from multiple threads?
qDebug()itself is thread-safe. It can be called simultaneously from multiple threads, without endangering your application.However,
qDebug()sends data tostderrby default, which is unbuffered. That means: If you send a long stream of "A"s from one thread and a long stream of "B"s from another thread, you might see interleaved text when viewing stderr:"AAAAAAAAAABBBBBBBBBBAAAAAAAAAABBBBBBBBBB"
To avoid this, call
qInstallMessageHandler()to sendqDebug()output to a buffered stream (likestdoutor a file).All STL classes are reentrant right except for <atomic> and std::atomic guys right?
I'd say so
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Another related question I conjured up - how come some Qt classes aren't reeentrant that would be quite more useful if they were? I.E. QPair, QStorageInfo. My program makes use of drive storage info in both the main GUI thread and the worker thread (I was forced to use the win32 api in the worker thread since QStorageInfo wasn't reentrant) and I could easily foresee QPair being used in such a way as well.
Thanks ! That might finalize this thread permanently :) -
Another related question I conjured up - how come some Qt classes aren't reeentrant that would be quite more useful if they were? I.E. QPair, QStorageInfo. My program makes use of drive storage info in both the main GUI thread and the worker thread (I was forced to use the win32 api in the worker thread since QStorageInfo wasn't reentrant) and I could easily foresee QPair being used in such a way as well.
Thanks ! That might finalize this thread permanently :)QPairis reentrant, if the docs don't state it, then it's a "bug" in the documentation, please file it as such.As for
QStorageInfo- I don't know if it's reentrant or not, I would have to check the source to find out for sure, but usually the class not being reentrant is due to the underlying API architecture (e.g. the widget classes). -
Thanks. Perhaps they are both bugs - I would think a useful class that would have any possibility of being used in multiple threads like QStorageInfo would most definitely be reentrant. Is there another factor at play here besides utility?
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Thanks. Perhaps they are both bugs - I would think a useful class that would have any possibility of being used in multiple threads like QStorageInfo would most definitely be reentrant. Is there another factor at play here besides utility?
@Crag_Hack said in Non-Reentrant Class Use In Multithreaded Program:
utility
What do you mean by "utility"? Whether it's reentrant or thread-safe (or both or neither) just depends on the specific implementation. As Qt's (mostly) a platform agnostic toolkit, implementations for some platforms may be reentrant/thread-safe, while implementations for other platforms may be not, in that case the worst-case is documented - i.e. non reentrant/not thread-safe.
