A non-reentrant function in an API being used in a multi-threaded program

Question

I've using the QT API in C++, but I imagine answers can come effectively from people without any prior experience with QT.

QT has a function in its XML-handling class, called setContent(), which is specified as non-reentrant. When called, setContent() reads an XML file into memory, and returns it as a data structure.

As I understand it, a non-reentrant function is one that is not safe to call simultaneously from multiple threads even if the function is called to operate on different files/objects.

So based on this, my understanding is that I would not be able to have more than one thread that opens XML files using this function unless somehow both of these threads are protected against accessing the setContent() function at the same time.

Is this correct? If so, seems like a really poor way to write an API as this doesn't seem like a function at all that intuitively would raise multi-threading problems. In addition, no mutex is provided at all by the API.

So in order to use this function in my multi-threaded program, where more than one thread will be opening different XML files, what's the best way to handle access to the setContent() function? Should I create an extern mutex in a header file on its own that is included by every file that will access XML?

Answer 1

Looks like it's all about static QDomImplementation::InvalidDataPolicy invalidDataPolicy. It's the only static data that QDom*** classes use.

setContent and a bunch of global functions use its value when parsing, and if another thread changes it in the middle, obviously something may happen.

I suppose if your program never calls setInvalidDataPolicy(), you're safe to parse XML from different threads.

Answer 2

So based on this, my understanding is that I would not be able to have more than one thread that opens XML files using this function unless somehow both of these threads are protected against accessing the setContent() function at the same time.

I think you're correct.

So in order to use this function in my multi-threaded program, where more than one thread will be opening different XML files, what's the best way to handle access to the setContent() function? Should I create an extern mutex in a header file on its own that is included by every file that will access XML?

Again, I tend to agree with you regarding the mutex. (By the way, Qt provides the QMutex) But I'm not sure what you mean by an extern mutex in a header file, so I'll just make sure to instantiate exactly one mutex, and dispatch a pointer to this mutex to all the threads that require it.