Experimental Physics and
Industrial Control System

Till Straumann via Tech-talk <tech-talk at aps.anl.gov> · Fri, 5 Mar 2021 15:50:51 +0100

/* Demonstrate a problem with pthread_join() from initialization
 * code under matlab2020b
 *
 * For a more realistic example you can compile this file into
 * a library (mimicks an external library):
 *
 *   g++ -DBUILD_AS_LIBRARY -shared -fPIC mexJoin.cc -o libXXX.so
 * 
 * and a separate mex-file:
 *
 *   mex -cxx -DBUILD_AS_MEXFILE mexJoin.cc -L. -lXXX
 *
 * Alternatively, the code can be compiled into a single mexfile:
 *
 *   mex -cxx mexJoin.cc
 */

/* Use stdio for printing messages to ensure matlab is not interfering.
 * Must use matlab in CLI mode, however, in order to see the messages:
 *
 *   matlab -nodisplay -nosplash -nojvm
 *
 * (or watch the terminal window from where the matlab GUI was started)
 */

/* Author: Till Straumann <till.straumann at psi.ch>, 2021 */

#include <string.h>
#include <errno.h>
#include <pthread.h>
#include <stdlib.h>
#include <stdio.h>

#if ! defined(BUILD_AS_LIBRARY) && ! defined(BUILD_AS_MEXFILE)
#define BUILD_AS_LIBRARY
#define BUILD_AS_MEXFILE
#endif

extern "C" {
	int join_a_thread(int skip);
};

#ifdef BUILD_AS_LIBRARY
static void * some_thread(void *arg)
{
	fprintf(stderr, "Thread terminated\n");
	return 0;
}

extern "C" {

int join_a_thread(int skip)
{
pthread_t id;
	switch ( skip ) {
		case 1:
			fprintf(stderr, "Skipping thread creation and joining during initialization phase\n\n");
		return -1;

		case 2:
			fprintf(stderr, "Skipping thread creation and joining during finalization phase\n\n");
		return -1;

		case -1:
			fprintf(stderr, "Attempting thread creation and joining during initialization phase\n");
		break;

		case -2:
			fprintf(stderr, "Attempting thread creation and joining during finalization phase\n");
		break;

		default:
			fprintf(stderr, "Attempting thread creation and joining from mexFunction\n");
		break;
	}

	if ( pthread_create( &id, 0, some_thread, 0 ) ) {
		fprintf(stderr, "Unable to create thread\n\n");
		return -1;
	}
	fprintf(stderr, "Created a thread\n");
	// 2020b under RHEL7 (compiled with g++ 9.3.0)
	// deadlocks in 'pthread_join' -- somewhere in
	// MW's glibc-2.17_shim.so. The same happens
	// when code which attempts 'pthread_join' from
	// a global initializer is loaded with dlopen().
	//
	// NOTE: 'join_a_thread()' works fine when executed
	//       from the mex-function itself; the deadlock
	//       occurs only when this is attempted during
	//       library initialization!
	fprintf(stderr, "Attempting to join the thread\n");
	if ( pthread_join( id, 0 ) ) {
		fprintf(stderr, "Unable to join thread\n\n");
		return -1;
	}
	fprintf(stderr, "Successfully joined!\n\n");
	return 0;
}

}

// Set the environment variable 'SKIP_JOIN_DURING_INIT' (prior to starting
// matlab) to verify that joining works when executed from the mexFunction
// itself.

class Initializer {
public:
	Initializer()
	{
		join_a_thread( ( (!! getenv("SKIP_JOIN_DURING_INIT")) ? 1 : -1) );
	}

	~Initializer()
	{
		join_a_thread( ( (!! getenv("SKIP_JOIN_DURING_EXIT")) ? 2 : -2) );
	}
};

static Initializer v;

#endif

#ifdef BUILD_AS_MEXFILE
#include <mex.hpp>
#include <mexAdapter.hpp>

using namespace matlab::data;
using matlab::mex::ArgumentList;

class MexFunction : public matlab::mex::Function {
public:
	void operator()(matlab::mex::ArgumentList o, matlab::mex::ArgumentList i)
	{
		//mexLock(); -- could prevent unloading
		join_a_thread(0);
	}
};
#endif

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System