trusty (3) sharedmemory.3bobcat.gz
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).