oracular (3) sharedmutex.3bobcat.gz

NAME

       FBB::SharedMutex - Mutex for shared memory

SYNOPSIS

       #include <bobcat/sharedmutex>
       Linking option: -lpthread, -lbobcat

DESCRIPTION

       Shared   memory  may  be  used  by  multiple  processes.  To  synchronize  access  to  shared  memory  an
       FBB::SharedMutex may be defined inside a shared memory segment. SharedMutex  objects  allows  clients  to
       lock   a  shared  memory  segment  before  reading  or  writing  its  content.  E.g.,  the  Bobcat  class
       FBB::SharedSegment defines a SharedMutex in its shared memory segment.

       The SharedMutex class uses the facilities offered by the PThread library  to  implement  (non  recursive)
       shared  memory  locking. To force unlocking a (possibly) locked shared memory segment, its destructor can
       be called.

       SharedMutex mutexes are non-recursive, resulting in deadlocks if their lock member is called  twice  from
       the  same thread of execution without an intermediate call to unlock the mutex. If this causes concern, a
       variable can be defined indicating whether the lock has already been obtained.

NAMESPACE

       FBB
       All constructors, members, operators and manipulators, mentioned in this man-page,  are  defined  in  the
       namespace FBB.

INHERITS FROM

       -

CONSTRUCTORS, DESTRUCTOR

       o      SharedMutex():
              The default constructor initializes an FBB::SharedMutex object to a shared memory mutex (using the
              PTHREAD_PROCESS_SHARED attribute). As an FBB::SharedMutex object will normally be defined inside a
              shared  memory segment the object’s memory is already available. In this case placement new should
              be used to call the constructor. E.g., if a shared memory  segment  is  attached  to  the  current
              process  at  d_shared,  and  an FBB::SharedMutex should be defined at d_shared’s address, then the
              FBB::SharedMutex object can be initialized like this:

                  new (d_shared) FBB::SharedMutex;

              Caveat: when using placement new to initialize a SharedMutex make sure that the  SharedMutex  fits
              inside  a block (i.e., shared.blockOffset() + sizeof(SharedMemory) < shared.dataSegmentSize()). If
              not,  use  seek  to  switch  to  an  offset  where  this  equality  holds  true,  or  simply   use
              SharedMemory::create like this:

                  FBB::SharedMemory::create<FBB::SharedMutex>();

       o      ~SharedMutex():
              The  class’s  destructor releases all of the current process’s nested shared memory segment locks.
              To destroy an FBB::SharedMutex object that has been constructed using the placement  new  operator
              use

              d_sharedMutex->~SharedMutex();

              (assuming  SharedMutex  *d_sharedMutex  points  to the location where placement new has previously
              initialized the FBB::SharedMutex object.)

              Copy and move constructors (and assignment operators) are not available.

MEMBER FUNCTIONS

       o      void lock() const:
              When returning from this member, the current process has locked the shared  memory  segment.  Note
              that SharedMutex objects are non-recursive.

       o      void unlock() const:
              The  object’s lock of the shared memory segment is released. This member can also be called if the
              SharedMutex’s lock has not been obtained.

PROTECTED MEMBER FUNCTION

       o      pthread_mutex_t *mutexPtr():
              A pointer is returned to the pthread_mutex_t object used by the SharedMutex object;

EXAMPLE

       #include <sys/types.h>
       #include <signal.h>

       #include <iostream>
       #include <string>
       #include <chrono>
       #include <bobcat/fork>
       #include <bobcat/semaphore>
       #include <bobcat/sharedsegment>
       #include <bobcat/sharedmutex>

       using namespace std;
       using namespace FBB;

       class Wait: public Fork
       {
           SharedSegment *d_shared;
           int d_id;
           SharedMutex *d_mutex;

           public:
               Wait();
               ~Wait();
               void childProcess() override;
               void parentProcess() override;
       };

       Wait::Wait()
       :
           d_shared(SharedSegment::create(&d_id, 1, 100, 0700)),
           d_mutex(new (d_shared) SharedMutex)
       {
           cout << "shared memory ID = " << d_id << ’\n’;
       }

       Wait::~Wait()
       {
           d_mutex->~SharedMutex();
           SharedSegment::deleteSegment(d_id);
           cout << "deleted the shared memory\n";
       }

       void Wait::childProcess()
       {
           Semaphore waiter{0};

           while (true)
           {
               waiter.wait_for(chrono::seconds(2));
               d_mutex->lock();
               cout << "child hello\n";
               d_mutex->unlock();
           }
       }

       void Wait::parentProcess()
       {
           string line;
           do
           {
               cout << "press enter to allow the parent to locck\n";
               cin.ignore(100, ’\n’);
               d_mutex->lock();
               cout << "parent has the lock, press enter to continue "
                       "(to end: some input)\n";
               getline(cin, line);
               d_mutex->unlock();
           }
           while (line.empty());

           kill(pid(), SIGTERM);
       }

       int main()
       {
           Wait waiter;
           waiter.fork();
       }

FILES

       bobcat/sharedmutex - defines the class interface

SEE ALSO

       bobcat(7) isharedstream(3bobcat), osharedstream(3bobcat), sharedblock(3bobcat), sharedcondition(3bobcat),
       sharedmemory(3bobcat),  (e.g.,)  pthread_mutex_init(3posix),  sharedpos(3bobcat),  sharedreadme(7bobcat),
       sharedsegment(3bobcat), sharedstream(3bobcat), sharedbuf(3bobcat)

BUGS

       None Reported.

BOBCAT PROJECT FILES

       o      https://fbb-git.gitlab.io/bobcat/: gitlab project page;

       o      bobcat_6.06.01-x.dsc: detached signature;

       o      bobcat_6.06.01-x.tar.gz: source archive;

       o      bobcat_6.06.01-x_i386.changes: change log;

       o      libbobcat1_6.06.01-x_*.deb: debian package containing the libraries;

       o      libbobcat1-dev_6.06.01-x_*.deb: debian package containing the libraries, headers and manual pages;

BOBCAT

       Bobcat is an acronym of `Brokken’s Own Base Classes And Templates’.

COPYRIGHT

       This is free software, distributed under the terms of the GNU General Public License (GPL).

AUTHOR

       Frank B. Brokken (f.b.brokken@rug.nl).