Developing parallel applications with Open CASCADE. Part 4

( continued...)

Open CASCADE threads
Open CASCADE provides thread abstraction in the form of OSD_Thread class that encapsulates OS-specific threads. It accepts a variable of the OSD_ThreadFunction type which is essentially a pointer to a function defined as follows:

typedef Standard_Address (*OSD_ThreadFunction) (Standard_Address data);

Here is a simple example of using Open CASCADE threads:

static Standard_Mutex aMutex;

Standard_Address Test_ThreadFunction (Standard_Address /*theData*/)
Standard_Mutex::Sentry aSentry (aMutex);
std::cout << "Running in worker thread id:" << OSD_Thread::Current() << std::endl;

int main (int argc, char *argv[])
const int MAX_THREAD = 5;
OSD_Thread aThreadArr[MAX_THREAD];

std::cout << "Running in master thread id: " << OSD_Thread::Current() << std::endl;

OSD_ThreadFunction aFunc = Test_ThreadFunction;
int i;
for (i = 0; i < MAX_THREAD; i++) {
aThreadArr[i].SetFunction (aFunc);
for (i = 0; i < MAX_THREAD; i++) {
if (!aThreadArr[i].Run (NULL))
std::cerr << "Error: Cannot start thread " << i << std::endl;

for (i = 0; i < MAX_THREAD; i++) {
Standard_Address aRes;
if (!aThreadArr[i].Wait (aRes))
std::cerr << "Error: Cannot get result of thread " << i << std::endl;
return 0;

The code of a function accepted by OSD_Thread::SetFunction() will be executed in a separate thread. Running the above example you should see different id's for master and worker threads.

OSD_Thread::Run() instance accepts a parameter of type void* which can be a pointer to some data object or is a type cast from some compatible type (e.g. integer). The same convention applies to an output parameter of OSD_Thread::Wait().

Of course, you are not obliged to use OSD_Threads and may prefer to use other implementations (system threads, Qt, Intel Threading Building Blocks, Open MP, etc). I have become a big fan of Intel TBB and am using it in CAD Exchanger.

Synchronization objects
Currently Open CASCADE provides the only synchronization object called Standard_Mutex which corresponds to Critical Section on Windows. You should get some knowledge on this subject to effectively use synchronization in your code.

For instance, to minimize contention you should protect the minimum required section of the code with a synchronization object. Using Standard_Mutex::Sentry you may create an additional nested scope for that:

void MyClass::MyMethodThatCanRunConcurrently()
//thread-safe code
//code requiring serialization
Standard_Mutex::Sentry aSentry (myMutex);

//thread-safe code again

The most convenient and efficient use of Standard_Mutex is through Standard_Mutex::Sentry which implements a scoped object which acquire and releases locks on a mutex. The Sentry object locks the mutex during its construction and unlocks it during destruction. This guarantees that the mutex will be unlocked even in the case of an exception raised during execution. Otherwise this could create a deadlock – other threads continue to wait for the locked mutex while there is no one who could unlock it.

It must be noted that Standard_Mutex is currently implemented inefficiently and therefore can hardly be recommended for use as is. Its method Standard_Mutex::Lock() is uses repetitive calls to TryEnterCriticalSection() and sleep(). This may result in wasting processor resources instead of yielding them to another task. It seems that developers attempted to implement a spin lock which can be useful for very short critical sections (where it is more efficient to spin than to immediately go into wait mode). However this must have been done differently – using critical section with spin count. I had to patched Standard_Mutex to eliminate this TryEnter...() behavior.

During my earlier experiments I have implemented other objects – wait condition and semaphore – but as they did not make part of Open CASCADE, there is no point describing them here.

(to be continued...)


  1. Are there some realy reasons to use the OpenCascade threads? It#s look like simple wrapper of the system's threads. May be, there are some default thread#s OCC-functions or something else.

  2. Indeed, Open CASCADE classes are just a convenience. They can be helpful if you are not well familiar with direct thread API or need cross-platform implementation without any other 3rd party libraries.