How do you print to console in a Multi-Threaded MEX Function?

I'm writing a simple producer consumer MEX function which uses the Boost library. I have manged to get the following program to work without any issues.

#include "mex.h"

#include <boost/thread/thread.hpp>

#include <boost/lockfree/spsc_queue.hpp>

#include <iostream>

#include <boost/atomic.hpp>



int producer_count = 0;

boost::atomic_int consumer_count (0);

boost::lockfree::spsc_queue<int, boost::lockfree::capacity<1024> > spsc_queue;

const int iterations = 10000000;



void producer()

{

    for (int i = 0; i != iterations; ++i) {

        int value = ++producer_count;

        while (!spsc_queue.push(value));

    }

}



boost::atomic<bool> done (false);



void consumer()

{

    int value;

    while (!done) {

        while (spsc_queue.pop(value))

            ++consumer_count;

    }



    while (spsc_queue.pop(value))

        ++consumer_count;

}



void mexFunction(int nlhs, mxArray *plhs, int nrhs, const mxArray *prhs)

{

    if (!spsc_queue.is_lock_free())

    {

      mexPrintf("boost::lockfree::queue is not lockfreen");

      mexEvalString("drawnow;");

    }

    else

    {

      mexPrintf("boost::lockfree::queue is lockfreen");

      mexEvalString("drawnow;");

    }



    boost::thread producer_thread(producer);

    boost::thread consumer_thread(consumer);



    producer_thread.join();

    done = true;

    consumer_thread.join();



    cout << "produced " << producer_count << " objects." << endl;

    cout << "consumed " << consumer_count << " objects." << endl;

}

The big problem is I try to include a mexPrintf() into the either the producer or consumer method MATLAB just crashes. After doing some investigation I found this post which explained this happens because of race conditions. Does anyone know how I can fix this issue? I read what the answer said about Mutex, but I do not understand how I would implement such a functionality.

edited Jan 2 at 18:41

Cris Luengo

22.3k52253

asked Jan 2 at 17:49

Crystal Pritzker

168210

add a comment |

I'm writing a simple producer consumer MEX function which uses the Boost library. I have manged to get the following program to work without any issues.

#include "mex.h"

#include <boost/thread/thread.hpp>

#include <boost/lockfree/spsc_queue.hpp>

#include <iostream>

#include <boost/atomic.hpp>



int producer_count = 0;

boost::atomic_int consumer_count (0);

boost::lockfree::spsc_queue<int, boost::lockfree::capacity<1024> > spsc_queue;

const int iterations = 10000000;



void producer()

{

    for (int i = 0; i != iterations; ++i) {

        int value = ++producer_count;

        while (!spsc_queue.push(value));

    }

}



boost::atomic<bool> done (false);



void consumer()

{

    int value;

    while (!done) {

        while (spsc_queue.pop(value))

            ++consumer_count;

    }



    while (spsc_queue.pop(value))

        ++consumer_count;

}



void mexFunction(int nlhs, mxArray *plhs, int nrhs, const mxArray *prhs)

{

    if (!spsc_queue.is_lock_free())

    {

      mexPrintf("boost::lockfree::queue is not lockfreen");

      mexEvalString("drawnow;");

    }

    else

    {

      mexPrintf("boost::lockfree::queue is lockfreen");

      mexEvalString("drawnow;");

    }



    boost::thread producer_thread(producer);

    boost::thread consumer_thread(consumer);



    producer_thread.join();

    done = true;

    consumer_thread.join();



    cout << "produced " << producer_count << " objects." << endl;

    cout << "consumed " << consumer_count << " objects." << endl;

}

edited Jan 2 at 18:41

Cris Luengo

22.3k52253

asked Jan 2 at 17:49

Crystal Pritzker

168210

add a comment |

I'm writing a simple producer consumer MEX function which uses the Boost library. I have manged to get the following program to work without any issues.

#include "mex.h"

#include <boost/thread/thread.hpp>

#include <boost/lockfree/spsc_queue.hpp>

#include <iostream>

#include <boost/atomic.hpp>



int producer_count = 0;

boost::atomic_int consumer_count (0);

boost::lockfree::spsc_queue<int, boost::lockfree::capacity<1024> > spsc_queue;

const int iterations = 10000000;



void producer()

{

    for (int i = 0; i != iterations; ++i) {

        int value = ++producer_count;

        while (!spsc_queue.push(value));

    }

}



boost::atomic<bool> done (false);



void consumer()

{

    int value;

    while (!done) {

        while (spsc_queue.pop(value))

            ++consumer_count;

    }



    while (spsc_queue.pop(value))

        ++consumer_count;

}



void mexFunction(int nlhs, mxArray *plhs, int nrhs, const mxArray *prhs)

{

    if (!spsc_queue.is_lock_free())

    {

      mexPrintf("boost::lockfree::queue is not lockfreen");

      mexEvalString("drawnow;");

    }

    else

    {

      mexPrintf("boost::lockfree::queue is lockfreen");

      mexEvalString("drawnow;");

    }



    boost::thread producer_thread(producer);

    boost::thread consumer_thread(consumer);



    producer_thread.join();

    done = true;

    consumer_thread.join();



    cout << "produced " << producer_count << " objects." << endl;

    cout << "consumed " << consumer_count << " objects." << endl;

}

edited Jan 2 at 18:41

Cris Luengo

22.3k52253

asked Jan 2 at 17:49

Crystal Pritzker

168210

I'm writing a simple producer consumer MEX function which uses the Boost library. I have manged to get the following program to work without any issues.

#include "mex.h"

#include <boost/thread/thread.hpp>

#include <boost/lockfree/spsc_queue.hpp>

#include <iostream>

#include <boost/atomic.hpp>



int producer_count = 0;

boost::atomic_int consumer_count (0);

boost::lockfree::spsc_queue<int, boost::lockfree::capacity<1024> > spsc_queue;

const int iterations = 10000000;



void producer()

{

    for (int i = 0; i != iterations; ++i) {

        int value = ++producer_count;

        while (!spsc_queue.push(value));

    }

}



boost::atomic<bool> done (false);



void consumer()

{

    int value;

    while (!done) {

        while (spsc_queue.pop(value))

            ++consumer_count;

    }



    while (spsc_queue.pop(value))

        ++consumer_count;

}



void mexFunction(int nlhs, mxArray *plhs, int nrhs, const mxArray *prhs)

{

    if (!spsc_queue.is_lock_free())

    {

      mexPrintf("boost::lockfree::queue is not lockfreen");

      mexEvalString("drawnow;");

    }

    else

    {

      mexPrintf("boost::lockfree::queue is lockfreen");

      mexEvalString("drawnow;");

    }



    boost::thread producer_thread(producer);

    boost::thread consumer_thread(consumer);



    producer_thread.join();

    done = true;

    consumer_thread.join();



    cout << "produced " << producer_count << " objects." << endl;

    cout << "consumed " << consumer_count << " objects." << endl;

}

c++ multithreading matlab boost mex

edited Jan 2 at 18:41

Cris Luengo

22.3k52253

asked Jan 2 at 17:49

Crystal Pritzker

168210

edited Jan 2 at 18:41

Cris Luengo

22.3k52253

asked Jan 2 at 17:49

Crystal Pritzker

168210

edited Jan 2 at 18:41

Cris Luengo

22.3k52253

edited Jan 2 at 18:41

Cris Luengo

22.3k52253

edited Jan 2 at 18:41

Cris Luengo

22.3k52253

asked Jan 2 at 17:49

Crystal Pritzker

168210

asked Jan 2 at 17:49

Crystal Pritzker

168210

asked Jan 2 at 17:49

Crystal Pritzker

168210

add a comment |

1 Answer
1

active

oldest

votes

You cannot call mexPrintf from any thread except the main thread. A mutex will not solve your problem.

From the MATLAB documentation:

MEX API Is Not Thread Safe

Do not call a single session of MATLAB® on separate threads from a MEX file. The MEX and Matrix Library APIs are not multi-threaded.

You can create threads from a C MEX file; however, accessing MATLAB from those threads is not supported. Do not call any MEX API functions from the spawned threads, including printf, which is defined as mexPrintf in the mex.h header file.

If you really need to produce output from these threads, consider implementing a simple messaging system where the threads post a message with the text to be output, and the main thread, instead of waiting with producer_thread.join();, sits in a loop looking for messages to print, and prints them with mexPrintf.

The code below is not tested. It's not even been compiled. Consider it pseudo-code. I think this is a reasonable attempt at a solution but there might be much better approaches. Continue at your own risk. :)

boost::lockfree::queue<std::string> message_queue;



void producer() {

    //...

    message_queue.push("A string to print!");

    //...

}



void mexFunction( /*...*/ ) {

    // ...

    boost::thread producer_thread(producer);

    boost::thread consumer_thread(consumer);

    while(producer_thread.joinable()) {

        join_for(boost::chrono::milliseconds(50));

        std::string s;

        while (message_queue.pop(s)) {

            mexPrintf("%sn", s.c_str());

        }

    }

    producer_thread.join();

    done = true;

    consumer_thread.join();

    // ...

}

edited Jan 2 at 18:40

answered Jan 2 at 18:02

Cris Luengo

22.3k52253

Thanks for explaining why it's not possible. Could you expand on you explanation of how I could implement a messaging system? Some example code would really be appreciated.

– Crystal Pritzker
Jan 2 at 18:20

@CrystalPritzker: Sorry, I have little experience with that, I'm afraid I'd recommend something that's wrong. I imagine you'd do, in main, a while(joinable()) loop, with a join_for(boost::chrono::milliseconds(50)) wait inside. Then you'd have a linked list of messages, which you'd print and remove from the list within this loop. The worker threads would append to the list. The list would be protected by a std::mutex for multi-threaded access (or use boost::lockfree::queue instead, which has the mutex built in). But this is my low-experience guess at a solution.

– Cris Luengo
Jan 2 at 18:34

@CrystalPritzker: I've added a bit of code, for how I thought about this solution. I'm not sure even if it would compile... :)

– Cris Luengo
Jan 2 at 18:42

I appreciate the help. Thank you very much!

– Crystal Pritzker
Jan 2 at 18:45

add a comment |

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1 Answer
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You cannot call mexPrintf from any thread except the main thread. A mutex will not solve your problem.

From the MATLAB documentation:

MEX API Is Not Thread Safe

Do not call a single session of MATLAB® on separate threads from a MEX file. The MEX and Matrix Library APIs are not multi-threaded.

You can create threads from a C MEX file; however, accessing MATLAB from those threads is not supported. Do not call any MEX API functions from the spawned threads, including printf, which is defined as mexPrintf in the mex.h header file.

boost::lockfree::queue<std::string> message_queue;



void producer() {

    //...

    message_queue.push("A string to print!");

    //...

}



void mexFunction( /*...*/ ) {

    // ...

    boost::thread producer_thread(producer);

    boost::thread consumer_thread(consumer);

    while(producer_thread.joinable()) {

        join_for(boost::chrono::milliseconds(50));

        std::string s;

        while (message_queue.pop(s)) {

            mexPrintf("%sn", s.c_str());

        }

    }

    producer_thread.join();

    done = true;

    consumer_thread.join();

    // ...

}

edited Jan 2 at 18:40

answered Jan 2 at 18:02

Cris Luengo

22.3k52253

Thanks for explaining why it's not possible. Could you expand on you explanation of how I could implement a messaging system? Some example code would really be appreciated.

– Crystal Pritzker
Jan 2 at 18:20

@CrystalPritzker: Sorry, I have little experience with that, I'm afraid I'd recommend something that's wrong. I imagine you'd do, in main, a while(joinable()) loop, with a join_for(boost::chrono::milliseconds(50)) wait inside. Then you'd have a linked list of messages, which you'd print and remove from the list within this loop. The worker threads would append to the list. The list would be protected by a std::mutex for multi-threaded access (or use boost::lockfree::queue instead, which has the mutex built in). But this is my low-experience guess at a solution.

– Cris Luengo
Jan 2 at 18:34

@CrystalPritzker: I've added a bit of code, for how I thought about this solution. I'm not sure even if it would compile... :)

– Cris Luengo
Jan 2 at 18:42

I appreciate the help. Thank you very much!

– Crystal Pritzker
Jan 2 at 18:45

add a comment |

You cannot call mexPrintf from any thread except the main thread. A mutex will not solve your problem.

From the MATLAB documentation:

MEX API Is Not Thread Safe

Do not call a single session of MATLAB® on separate threads from a MEX file. The MEX and Matrix Library APIs are not multi-threaded.

You can create threads from a C MEX file; however, accessing MATLAB from those threads is not supported. Do not call any MEX API functions from the spawned threads, including printf, which is defined as mexPrintf in the mex.h header file.

boost::lockfree::queue<std::string> message_queue;



void producer() {

    //...

    message_queue.push("A string to print!");

    //...

}



void mexFunction( /*...*/ ) {

    // ...

    boost::thread producer_thread(producer);

    boost::thread consumer_thread(consumer);

    while(producer_thread.joinable()) {

        join_for(boost::chrono::milliseconds(50));

        std::string s;

        while (message_queue.pop(s)) {

            mexPrintf("%sn", s.c_str());

        }

    }

    producer_thread.join();

    done = true;

    consumer_thread.join();

    // ...

}

edited Jan 2 at 18:40

answered Jan 2 at 18:02

Cris Luengo

22.3k52253

Thanks for explaining why it's not possible. Could you expand on you explanation of how I could implement a messaging system? Some example code would really be appreciated.

– Crystal Pritzker
Jan 2 at 18:20

@CrystalPritzker: Sorry, I have little experience with that, I'm afraid I'd recommend something that's wrong. I imagine you'd do, in main, a while(joinable()) loop, with a join_for(boost::chrono::milliseconds(50)) wait inside. Then you'd have a linked list of messages, which you'd print and remove from the list within this loop. The worker threads would append to the list. The list would be protected by a std::mutex for multi-threaded access (or use boost::lockfree::queue instead, which has the mutex built in). But this is my low-experience guess at a solution.

– Cris Luengo
Jan 2 at 18:34

@CrystalPritzker: I've added a bit of code, for how I thought about this solution. I'm not sure even if it would compile... :)

– Cris Luengo
Jan 2 at 18:42

I appreciate the help. Thank you very much!

– Crystal Pritzker
Jan 2 at 18:45

add a comment |

You cannot call mexPrintf from any thread except the main thread. A mutex will not solve your problem.

From the MATLAB documentation:

MEX API Is Not Thread Safe

Do not call a single session of MATLAB® on separate threads from a MEX file. The MEX and Matrix Library APIs are not multi-threaded.

You can create threads from a C MEX file; however, accessing MATLAB from those threads is not supported. Do not call any MEX API functions from the spawned threads, including printf, which is defined as mexPrintf in the mex.h header file.

boost::lockfree::queue<std::string> message_queue;



void producer() {

    //...

    message_queue.push("A string to print!");

    //...

}



void mexFunction( /*...*/ ) {

    // ...

    boost::thread producer_thread(producer);

    boost::thread consumer_thread(consumer);

    while(producer_thread.joinable()) {

        join_for(boost::chrono::milliseconds(50));

        std::string s;

        while (message_queue.pop(s)) {

            mexPrintf("%sn", s.c_str());

        }

    }

    producer_thread.join();

    done = true;

    consumer_thread.join();

    // ...

}

edited Jan 2 at 18:40

answered Jan 2 at 18:02

Cris Luengo

22.3k52253

You cannot call mexPrintf from any thread except the main thread. A mutex will not solve your problem.

From the MATLAB documentation:

MEX API Is Not Thread Safe

Do not call a single session of MATLAB® on separate threads from a MEX file. The MEX and Matrix Library APIs are not multi-threaded.

You can create threads from a C MEX file; however, accessing MATLAB from those threads is not supported. Do not call any MEX API functions from the spawned threads, including printf, which is defined as mexPrintf in the mex.h header file.

boost::lockfree::queue<std::string> message_queue;



void producer() {

    //...

    message_queue.push("A string to print!");

    //...

}



void mexFunction( /*...*/ ) {

    // ...

    boost::thread producer_thread(producer);

    boost::thread consumer_thread(consumer);

    while(producer_thread.joinable()) {

        join_for(boost::chrono::milliseconds(50));

        std::string s;

        while (message_queue.pop(s)) {

            mexPrintf("%sn", s.c_str());

        }

    }

    producer_thread.join();

    done = true;

    consumer_thread.join();

    // ...

}

edited Jan 2 at 18:40

answered Jan 2 at 18:02

Cris Luengo

22.3k52253

edited Jan 2 at 18:40

answered Jan 2 at 18:02

Cris Luengo

22.3k52253

answered Jan 2 at 18:02

Cris Luengo

22.3k52253

answered Jan 2 at 18:02

Cris Luengo

22.3k52253

Thanks for explaining why it's not possible. Could you expand on you explanation of how I could implement a messaging system? Some example code would really be appreciated.

– Crystal Pritzker
Jan 2 at 18:20

@CrystalPritzker: Sorry, I have little experience with that, I'm afraid I'd recommend something that's wrong. I imagine you'd do, in main, a while(joinable()) loop, with a join_for(boost::chrono::milliseconds(50)) wait inside. Then you'd have a linked list of messages, which you'd print and remove from the list within this loop. The worker threads would append to the list. The list would be protected by a std::mutex for multi-threaded access (or use boost::lockfree::queue instead, which has the mutex built in). But this is my low-experience guess at a solution.

– Cris Luengo
Jan 2 at 18:34

@CrystalPritzker: I've added a bit of code, for how I thought about this solution. I'm not sure even if it would compile... :)

– Cris Luengo
Jan 2 at 18:42

I appreciate the help. Thank you very much!

– Crystal Pritzker
Jan 2 at 18:45

add a comment |

Thanks for explaining why it's not possible. Could you expand on you explanation of how I could implement a messaging system? Some example code would really be appreciated.

– Crystal Pritzker
Jan 2 at 18:20

@CrystalPritzker: Sorry, I have little experience with that, I'm afraid I'd recommend something that's wrong. I imagine you'd do, in main, a while(joinable()) loop, with a join_for(boost::chrono::milliseconds(50)) wait inside. Then you'd have a linked list of messages, which you'd print and remove from the list within this loop. The worker threads would append to the list. The list would be protected by a std::mutex for multi-threaded access (or use boost::lockfree::queue instead, which has the mutex built in). But this is my low-experience guess at a solution.

– Cris Luengo
Jan 2 at 18:34

@CrystalPritzker: I've added a bit of code, for how I thought about this solution. I'm not sure even if it would compile... :)

– Cris Luengo
Jan 2 at 18:42

I appreciate the help. Thank you very much!

– Crystal Pritzker
Jan 2 at 18:45

Thanks for explaining why it's not possible. Could you expand on you explanation of how I could implement a messaging system? Some example code would really be appreciated.

– Crystal Pritzker
Jan 2 at 18:20

@CrystalPritzker: Sorry, I have little experience with that, I'm afraid I'd recommend something that's wrong. I imagine you'd do, in main, a while(joinable()) loop, with a join_for(boost::chrono::milliseconds(50)) wait inside. Then you'd have a linked list of messages, which you'd print and remove from the list within this loop. The worker threads would append to the list. The list would be protected by a std::mutex for multi-threaded access (or use boost::lockfree::queue instead, which has the mutex built in). But this is my low-experience guess at a solution.

– Cris Luengo
Jan 2 at 18:34

@CrystalPritzker: I've added a bit of code, for how I thought about this solution. I'm not sure even if it would compile... :)

– Cris Luengo
Jan 2 at 18:42

I appreciate the help. Thank you very much!

– Crystal Pritzker
Jan 2 at 18:45

add a comment |

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