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_6.03.02-x.dsc: detached signature;

       o      bobcat_6.03.02-x.tar.gz: source archive;

       o      bobcat_6.03.02-x_i386.changes: change log;

       o      libbobcat1_6.03.02-x_*.deb: debian package containing the libraries;

       o      libbobcat1-dev_6.03.02-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).