Provided by: libbobcat-dev_3.19.01-1ubuntu1_amd64 
NAME
FBB::SharedMemory - Shared Memory memory structure
SYNOPSIS
#include <bobcat/sharedmemory>
Linking option: -lbobcat
DESCRIPTION
The class FBB::SharedMemory implements a usable interface to a shared memory segment made
available by FBB::SharedSegment and monitored by FBB::SharedPos. It is the main building
block for FBB::SharedStreambuf, defining the `device’ to which FBB::SharedStreambuf
interfaces. All shared memory related I/O should be performed through an FBB::SharedMemory
object, which is a true object, not itself residing in shared memory. An FBB::SharedMemory
object defines, connects to and manages access to shared memory, encapsulating all raw
shared memory operations. In addition to the class FBB::SharedMemory the header file
bobcat/sharedmemory also defines a struct SharedEnum__ defining an enum SizeUnit.
The total amount of requested shared memory is always a lower bound to the actual amount
of shared memory that eventually may become available. As a fictitious example: assume 100
kB of memory is requested, then a table of, e.g., 10 entries is maintained, each entry
controlling the access to a shared memory block of, 10 kB. These 10 kB blocks aren’t
immediately allocated, but become available once the program reads from or writes to
addresses located in these data blocks. Whenever a data block is created it is initialized
to 0-bytes.
Caveat: when constructing a shared memory segment make sure the segment’s ID is stored at
a retrievable location. This allows other processes to access the shared segment. The
shared segment ID is also required to delete a shared memory segment. If the shared
segment ID is lost, the memory occupied by the shared memory segment remains inaccessible,
and will reduce the amount of usable memory until the computer is rebooted. The member
function id returns the ID of the shared memory currently monitored by an
FBB::SharedMemory object.
NAMESPACE
FBB
All constructors, members, operators and manipulators, mentioned in this man-page, are
defined in the namespace FBB.
INHERITS FROM
FBB::SharedEnum__
The struct SharedEnum__ is a wrapper struct around enum SizeUnit, which is available
through inheritance in several FBB::Shared* classes, and offers symbolic constants
defining standard memory sizes. The enum SizeUnit defines the following symbolic
constants:
o kB:
, representing 2 bytes of memory;
o MB:
, representing 2 bytes of memory;
o GB:
, representing 2 bytes of memory
CONSTRUCTORS, DESTRUCTOR
o SharedMemory():
The default constructor does not yet access a shared memory segment, but can be
used to define a stub FBB::SharedMemory object, to be used at some later point
during the execution of a program.
o SharedMemory(size_t maxSize, SizeUnit sizeUnit, size_t access = 0600):
This constructor creates a shared memory segment having a capacity of at least
maxSize * sizeUnit bytes. The shared memory’s access rights are defined by the
access parameter, using the well-known (chmod(1)) way to define the access rights
for the owner, the group and others. If construction succeeds the shared memory is
ready for use. If construction fails, an FBB::Exception is thrown.
o SharedMemory(int id):
This constructor connects to a shared memory segment having ID id. If construction
succeeds the shared memory is ready for use. If construction fails (e.g., no shared
memory segment having ID id exists), an FBB::Exception is thrown.
o ~SharedMemory():
The destructor detaches any attached shared memory segments from the
FBB::SharedMemory object. If the shared memory segment is currently locked by the
FBB::SharedMemory object, the lock is removed.
copy and move constructors are not available.
OVERLOADED OPERATORS
o std::ostream &operator<<(std::ostream &out, SharedMemory const &sharedMemory):
The overloaded insertion operator inserts information about the SharedMemory object
into the provide ostream object. The IDs of the shared segments, their sizes, the
maximum number of shared memory segments, the number of bytes that can be read from
the shared memory, and its actual storage capacity, etc., are displayed.
o SharedMemory &operator=(SharedMemory &&rhs):
The overloaded move assignment operator is available. It is used to (re)define the
shared memory segment an FBB::SharedMemory object is interfacing with.
The overloaded copy assignement operator is not available.
MEMBER FUNCTIONS
o void clear():
First, the shared memory is locked. Next, all shared data segment are deleted, and
the shared memory’s own data are reset to indicate it is completely empty.
Following this the shared memory segment is unlocked again. Returning from clear
the shared memory The FBB::SharedMemory object is effectively re-initialized, with
offset and nReadable returning 0.
o int get():
First the FBB::SharedMemory object calls lock to lock the shared memory segment.
Next the character at offset is retrieved and offset is incremented. Then unlock is
called, and the retrieved character is returned. If offset is at least equal to
nReadable, EOF is immediately returned.
o int id() const:
The ID of the shared memory segment is returned.
o void kill():
Without locking the shared memory all shared memory controlled by the
FBB::SharedMemory object is deleted. The FBB::SharedMemory object is unusable after
returning from kill.
o std::streamsize maxOffset() const:
The maximum possible offset that can be used with the shared memory segment is
returned. The members offset and nReadable never exceed the value returned by
maxOffset.
o std::streamsize nReadable() const:
The number of characters (bytes) that can be read from the beginning of the shared
memory is returned.
o std::streamsize offset() const:
The offset within the shared memory segment is returned.
o char *ptr():
Returns 0 if offset() == maxOffset(). Otherwise it returns a pointer to the
character at index offset within the shared memory segment.
o int put(int ch):
After locking the appropriate shared data segment, ch is written at position
offset, incrementing offset thereafter. If ch == EOF, EOF is immediately returned.
o int read(char *data, std::streamsize len):
While locking the appropriate shared data segment(s) at most len bytes are read
from the shared memory, starting at offset. The bytes read from shared memory are
stored at data. The number of bytes actually written is returned. This member
returns -1 if initially offset was at least equal to nReadable.
o void remove():
The shared memory is locked, after which all shared memory controlled by the
FBB::SharedMemory object is deleted. The FBB::SharedMemory object is unusable after
returning from remove.
o std::ios::pos_type seek(std::ios::off_type offset, std::ios::seekdir way =
std::ios::beg):
Moves the offset position relative to way. The value -1 is returned when seeking
before offset 0 or beyond maxOffset.
o std::streamsize showmanyc() const:
The number of characters that can be read from the current shared segment data
block is returned. This member interrogates the number of readable characters in
the shared memory segment. This number may change while this member is being
executed. In order to receive a stable return value, calling functions should have
obtained a lock on the shared memory segment before calling this member.
o void swap(SharedMemory &other):
The current and other FBB::SharedMemory objects are swapped.
o bool try_lock() const:
When returning true the current process has obtained the lock, and the object’s
lock count is set to 1. Otherwise, false is returned (which includes the case where
the process already has obtained the lock).
o void unlock() const:
If the object’s lock count is zero, the function immediately returns. Otherwise,
the object’s lock count is decremented. Once the lock count has decremented to zero
the lock of the shared memory segment is released.
o int write(char const *data, std::streamsize len):
The FBB::SharedMemory object calls lock to lock the shared memory, and writes at
most len bytes into the shared memory, starting at offset. Next, unlock is called.
The number of bytes actually written is returned. The member function returns -1 if
initially offset is equal to maxOffset.
EXAMPLE
See the sharedstream(3bobcat) man page.
FILES
bobcat/sharedmemory - defines the class interface
SEE ALSO
bobcat(7), chmod(1), isharedstream(3bobcat), osharedstream(3bobcat), sharedblock(3bobcat),
sharedmutex(3bobcat), sharedpos(3bobcat), sharedsegment(3bobcat), sharedstream(3bobcat),
sharedstreambuf(3bobcat)
BUGS
None Reported.
DISTRIBUTION FILES
o bobcat_3.19.01-x.dsc: detached signature;
o bobcat_3.19.01-x.tar.gz: source archive;
o bobcat_3.19.01-x_i386.changes: change log;
o libbobcat1_3.19.01-x_*.deb: debian package holding the libraries;
o libbobcat1-dev_3.19.01-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).