Provided by: libbobcat-dev_4.08.02-2build1_amd64 
NAME
SharedREADME - Description of Bobcat’s shared memory classes
DESCRIPTION
o SharedLock(3bobcat)
The class SharedLock(3bobcat) is a wrapper around a pthread_mutex. SharedLock has
member functions lock and unlock, with lock throwing an exception when it fails to
obtain the lock (i.e., calling pthread_mutex’s lock member fails, otherwise it
waits until pthread_mutex’s lock succeeds).
SharedLock is a convenience class for other shared memory classes. By itself
SharedLock is unrelated to shared memory.
o SharedBlock(3bobcat)
The class SharedBlock(3bobcat) defines a small object consisting of a SharedMutex
and a (shared segment) id. If offers functionality to set and retrieve the id, and
to lock and unlock the SharedMutex. SharedBlock objects are used by
SharedSegment(3bobcat) objects to keep track of which segments actually contain the
shared memory data. It’s a bookkeeping helper class.
o SharedSegment(3bobcat)
The class SharedSegment (cf. figure images/sharedsegment1.jpg) interfaces to actual
shared memory. Its main member function is the static member create, returning a
pointer to a newly allocated SharedMemory object.
The member create receives as its input parameters the number of blocks, the size
of the segments allocated by SharedSegment, and the segment’s access mode (as used
by, e.g., chmod(1)). It returns a pointer to the allocated SharedSegment, and its
ID (set using a pointer to an int, passed as its first parameter).
Although the SharedSegment class defines an array of a single SharedBlock(3bobcat)
object, in fact it will contain d_nBlocks SharedBlock objects. The member create
first allocates a block of raw memory of the required total size. Next it
initializes the first part of this raw, shared memory, using the SharedSegment
constructor and placement new. Next, the remaining amount of raw shared memory is
initialized with d_nBlocks - 1 SharedBlock objects, again using placement new (cf.
figure images/sharedsegment2.jpg).
o SharedPos(3bobcat)
The class SharedPos(3bobcat) monitors position information in the shared memory
segments managed by SharedSegment objects. SharedPos only has a default
constructor, but offers a reset member receiving at construction time a pointer to
a SharedSegment object.
o SharedMemory(3bobcat)
The class SharedMemory(3bobcat) offers the standard interface to shared memory,
defining members like read, get, write and put. It interfaces to a SharedSegment
object, and uses a SharedPos object to keep track of the SharedMemory’s offsets.
A SharedMemory object itself resides in a program’s working memory, and not in the
computer’s shared memory area. At any time the SharedMemory object only loads at
most one actual block of the shared memory block managed by SharedSegment. The
current position in the shared memory managed by SharedSegment is monitored by the
SharedMemory’s SharedBlock d_pos member.
Following read/write operations the offset is updated accordingly. There’s only one
offset, which is used by [IO]?SharedStream objects when requesting or updating
offsets. a SharedMemory object offers safeguards against inappropriate use. E.g.,
if no memory has been allocated yet it may thow exceptions. Also, offsets can never
exceed the SharedMemory’s maximum possible offset.
o SharedMutex(3bobcat)
The class SharedMutex implements a non-recursive mutex that can be stored in shared
memory. It merely offers lock and unlock members. Within one thread lcok should
never be called repeatedly unless unlock is called first. The unlock member may
safely repeatedly be called; once the mutex has been unlocked, repeated requests to
unlock the mutex are simply ignored. When a SharedMutex is destroyed it calls
unlock. Use SharedMemory’s install member to define a SharedMutex object in shared
memory, and use ptr->~SharedMutex() (with SharedMutex *ptr pointing at the
SharedMutex object) to destroy it again.
o SharedCondition(3bobcat)
The class SharedCondition implements a condition variable and an associated mutex,
both created in shared memory controlled by a SharedMemory object. It offers all
the functionality of std::condition_variable. A SharedCondition object itself is
defined in a thread’s working memory, but interfaces to a `Condition’ object
containing a condition variable and a mutex which are installed in shared memory.
The size of this `Condition’ object is returned by the static member
SharedCondition::size. SharedCondition members ensure that the thread has access to
the `Condition’ object, and that the shared memory’s offset has not changed when
returning from the SharedCondition’s members.
o SharedStreambuf(3bobcat)
The class SharedStreambuf implements a std::streambuf specialization interfacing to
a SharedMemory object. In addition to a default constructor which is an empty stub
the class offers constructors which immediately interface to a SharedMemory object,
as well as a member to (re)associate a SharedStreambuf object with a SharedMemory
object.
o SharedStream, ISharedStream, OSharedStream(3bobcat)
These stream classes offer stream-facilities operating on shared memory maintained
by a SharedMemory object. The SharedStream class uses the SharedMemory’s seek
member for both seekp and seekg, and uses SharedMemory’s offset member for both
tellp and tellg. All shared stream objects, including ISharedStream, offer a member
to initialize a SharedCondition
An overview of currently defined shared memory segments is shown by the command
ipcs -ma. To remove a defined shared memory segment the command ipcrm -m <id> can
be used, where <id> is the shared memory segment’s ID.
EXAMPLE
See the sharedstream(3bobcat) man page.
FILES
Images referred to in this man-page are located in the source distribution in
bobcat/documents/images, or they can be found in the standard location of documentation
files in your distribution (e.g., they are located in /usr/share/doc/libbobcat4/images).
SEE ALSO
bobcat(7), ipcs(1), ipcrm(1), isharedstream(3bobcat), osharedstream(3bobcat),
sharedblock(3bobcat), sharedcondition(3bobcat), sharedmemory(3bobcat),
sharedmutex(3bobcat), sharedpos(3bobcat), sharedsegment(3bobcat), sharedstream(3bobcat),
sharedstreambuf(3bobcat)
BUGS
None Reported.
DISTRIBUTION FILES
o bobcat_4.08.02-x.dsc: detached signature;
o bobcat_4.08.02-x.tar.gz: source archive;
o bobcat_4.08.02-x_i386.changes: change log;
o libbobcat1_4.08.02-x_*.deb: debian package holding the libraries;
o libbobcat1-dev_4.08.02-x_*.deb: debian package holding the libraries, headers and
manual pages;
o http://sourceforge.net/projects/bobcat: public archive location;
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).