plucky (3) sharedsegment.3bobcat.gz
NAME
FBB::SharedSegment - Shared Memory data structure
SYNOPSIS
#include <bobcat/sharedsegment>
Linking option: -lbobcat
DESCRIPTION
The class FBB::SharedSegment implements the shared memory data structure used by Bobcat’s shared memory
classes. Bobcat’s SharedMemory class accesses or defines a shared memory segment, controlling all its
read and write operations.
The requested amount of shared memory is always a lower bound to the maximum amount of shared memory that
eventually may become available. When defining a SharedSegment object not all of its potentially
available shared memory is immediately allocated. Shared memory will be allocated by the SharedSegment
object once needed (up to a calculated maximum).
As a fictitious example: assume 100 kB of memory is requested, then the SharedSegment object, maintains a
table of, e.g., 10 entries, each 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 segments.
The class SharedSegment therefore defines a gateway, controlling access to and allocating required shared
memory data segments. The mentioned table consists of nBlocks SharedBlock (sharedblock(3bobcat)) values,
offering mutexes and IDs of shared data segments. The mutexes control which process has access to a
particular block of shared data memory, and the IDs are either -1, meaning that their shared memory data
segments has as not yet been allocated, or they contain the IDs of defined shared memory data segments.
The class SharedSegment’s sole responsibility is to offer the framework as described. When used by a
FBB::SharedMemory object different processes may gain access to different parts of the shared memory data
without interfering each other’s read and write actions.
NAMESPACE
FBB
All constructors, members, operators and manipulators, mentioned in this man-page, are defined in the
namespace FBB.
INHERITS FROM
-
CONSTRUCTORS
No publicly accessible constructors have been defined for SharedSegment. A static member function create
(see below) is available, returning a pointer to a shared memory segment, in which a SharedSegment has
been defined.
OVERLOADED OPERATORS
o std::ostream &operator<<(std::ostream &out, SharedSegment const &sharedData):
The overloaded insertion operator inserts basic statistics of the shared memory data into the
ostream object. Information about the IDs of the shared segments, their sizes, the maximum number
of shared data segments and the number of bytes that can be read from the shared memory are
displayed.
o FBB::SharedBlock &operator[](size_t idx):
Table element idx of the table of FBB::SharedBlock block IDs is returned. The behavior of the
program is undefined if idx is or exceeds nBlocks().
Copy and move constructors (and assignment operators) are not available.
MEMBER FUNCTIONS
o size_t access() const:
Returns the access rights of the shared memory segment as a number which is usually interpreted as
an octal value, using the well-known (chmod(1)) way to define the access rights for owner, group
and others.
o void clear():
All the shared memory data blocks are unconditionally deleted and nReadable returns 0 (the shared
memory data blocks are not locked prior to deleting them). After calling clear all allocated
SharedSegment’s shared memory segments have ceased to exist and can no longer be used.
o void lock(size_t idx) const:
Access to shared data segment idx is locked. This member itself does not support recursive
locking.
o size_t nBlocks() const:
Returns the number of shared memory data blocks that can be used by the FBB::SharedSegment object.
o int newData(size_t idx):
Returns the ID of a newly created shared memory data segment. The ID is also stored in the table
of shared memory data segments that is maintained by the SharedSegment object.
An FBB::Exception is thrown if the shared memory data segment could not be allocated.
o std::streamsize nReadable() const:
Returns the number of characters (bytes) that can be read from the beginning of the shared memory.
o void nReadableLock() const:
When returning from this member function a lock has been obtained of SharedSegment’s mutex
controlling access the the object’s data member storing the number of characters that can be read
from the shared memory controlled by the SharedSegment object.
o void nReadableUnlock() const:
This member function releases the lock previously acquired by nReadableLock.
o size_t segmentSize() const:
Returns the size (in bytes) of the shared memory data blocks. The SharedSegment object can
accommodate at most segmentSize() * nBlocks() bytes.
o bool truncate(std::streamsize offset):
After calling nReadableLock, if offset is not exceeding the value returned by nReadable nReadable
is changed to offset and true is returned. Otherwise false is returned, and the value returned by
nReadable has not been changed. Before returning nReadableUnlock is called.
o void unlock(size_t idx) const:
Releases the lock on the shared memory data segment idx. If the current process does not own the
lock of shared memory data block idx nothing happens and the function immediately returns.
o void updateNreadable(std::streamsize offset):
The number of bytes that can be retrieved from the shared memory is updated to max(nReadable(),
offset).
STATIC MEMBER FUNCTIONS
o void *attach(int id):
Returns the address of shared memory segment id, mapped to the calling process’s memory area.
An FBB::Exception is thrown if the shared memory data segment could not be attached.
o SharedSegment *create(int *id, size_t nBlocks, size_t segmentSize, size_t access):
Returns a pointer to a newly created SharedSegment object, defined in the computer’s shared
memory.
The created shared memory’s ID is stored at *id. The remaining arguments define the potential
number of shared memory data blocks (nBlocks); the size of these data blocks (segmentSize); and
the shared memory’s access rights (access, using the well-known octal value representation as used
by (chmod(1)) to define access rights for the owner, the group and others).
To return the shared segment to the operating system deleteSegment should be used.
An FBB::Exception is thrown if the shared memory data segment could not be created.
o void deleteSegment(int id):
The shared memory segment having ID id is deleted. After calling deleteSegment shared memory
segment id doesn’t exist anymore. The id can be the shared memory ID of any segment for which the
current user has write permissions.
An FBB::Exception is thrown if shared memory data segment id could not be deleted.
o Type *detach(Type *sharedPtr, bool requireOK = true):
This member is defined as a member template. It expects a pointer to a shared memory segment,
previously mapped on the calling process’s memory space by attach, and detaches it from the
process’s memory space, returning 0.
By default, detaching the memory must succeed or an FBB::Exception is thrown. Throwing an
exception on failure can be prevented by passing false as the member’s second argument.
Note that detaching a memory segment does not destroy it. To return a shared segment to the
operating system deleteSegment should be used.
o size_t size(int id):
The size (in bytes) of shared memory data block having ID id is returned.
An FBB::Exception is thrown if the size of segment id could not be determined.
EXAMPLE
See the sharedstream(3bobcat) man page.
FILES
bobcat/sharedsegment - defines the class interface
SEE ALSO
bobcat(7), chmod(1), isharedstream(3bobcat), osharedstream(3bobcat), sharedblock(3bobcat), sharedcondition(3bobcat), sharedmemory(3bobcat), sharedmutex(3bobcat), sharedpos(3bobcat), sharedreadme(7bobcat), sharedstream(3bobcat), sharedbuf(3bobcat)
BUGS
None Reported.
BOBCAT PROJECT FILES
o https://fbb-git.gitlab.io/bobcat/: gitlab project page;
o bobcat_6.06.02-x.dsc: detached signature;
o bobcat_6.06.02-x.tar.gz: source archive;
o bobcat_6.06.02-x_i386.changes: change log;
o libbobcat1_6.06.02-x_*.deb: debian package containing the libraries;
o libbobcat1-dev_6.06.02-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).