FBB::SharedMutex(3bobcat) Shared Memory Mutex FBB::SharedMutex(3bobcat)

FBB::SharedMutex - Mutex for shared memory

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

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.

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


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:

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

(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.

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

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

#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;
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() {
cout << "deleted the shared memory\n"; } void Wait::childProcess() {
Semaphore waiter{0};
while (true)
cout << "child hello\n";
} } void Wait::parentProcess() {
string line;
cout << "press enter to allow the parent to locck\n";
cin.ignore(100, ’\n’);
cout << "parent has the lock, press enter to continue "
"(to end: some input)\n";
getline(cin, line);
while (line.empty());
kill(pid(), SIGTERM); } int main() {
Wait waiter;
waiter.fork(); }

bobcat/sharedmutex - defines the class interface

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)

None Reported.

https://fbb-git.gitlab.io/bobcat/: gitlab project page;
bobcat_6.02.02-x.dsc: detached signature;
bobcat_6.02.02-x.tar.gz: source archive;
bobcat_6.02.02-x_i386.changes: change log;
libbobcat1_6.02.02-x_*.deb: debian package containing the libraries;
libbobcat1-dev_6.02.02-x_*.deb: debian package containing the libraries, headers and manual pages;

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

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

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

2005-2022 libbobcat-dev_6.02.02