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_6.02.02-x.dsc: detached signature;

o

bobcat_6.02.02-x.tar.gz: source archive;

o

bobcat_6.02.02-x_i386.changes: change log;

o

libbobcat1_6.02.02-x_*.deb: debian package containing the libraries;

o

libbobcat1-dev_6.02.02-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).