sharedmutex3bobcat

Usage

Overview

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

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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>();

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~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.

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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

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

Examples

#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

Bobcat project files

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

Debian Bobcat project files:

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libbobcat6: debian package containing the shared library, changelog and copyright note;

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libbobcat-dev: debian package containing the static library, headers, manual pages, and developer info;

Bobcat

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