# FBB::SharedMutex(3bobcat)

## Shared Memory Mutex
(libbobcat-dev_5.11.00)

## 2005-2022

## 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

- 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>();

- ~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

- 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.

## PROTECTED MEMBER FUNCTION

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

## EXAMPLE

See the sharedstream(3bobcat) man page.

## 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

- https://fbb-git.gitlab.io/bobcat/: gitlab project page;
- bobcat_5.11.00-x.dsc: detached signature;
- bobcat_5.11.00-x.tar.gz: source archive;
- bobcat_5.11.00-x_i386.changes: change log;
- libbobcat1_5.11.00-x_*.deb: debian package containing the libraries;
- libbobcat1-dev_5.11.00-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).