FBB::OSharedStream - std::ostream operations on shared memory

Author

       Frank B. Brokken (f.b.brokken@rug.nl).

libbobcat-dev_6.07.01                               2005-2025                        FBB::OSharedStream(3bobcat)

Bobcat

       Bobcat is an acronym of `Brokken’s Own Base Classes And Templates’.

Bobcat Project Files

       o      https://fbb-git.gitlab.io/bobcat/: gitlab project page;

       Debian Bobcat project files:

       o      libbobcat6: debian package containing the shared library, changelog and copyright note;

       o      libbobcat-dev:  debian package containing the static library, headers, manual pages, and developer
              info;

Bugs

       Note that by default exceptions thrown from within a std::stream object are caught by the stream  object,
       setting its ios::failbit flag. To allow exceptions to leave a stream object, its exceptions member can be
       called, e.g., using:

           myStream.exceptions(ios::failbit | ios::badbit | ios::eofbit);

Constructors

       o      OSharedStream():
              The default constructor defines a stub a OSharedStream object that cannot immediately be  used  to
              access shared memory. To use it, its member open must first be called.

       o      OSharedStream(size_tmaxSize,SizeUnitsizeUnit,std::ios::openmodeopenMode=std::ios::out,size_taccess=0600):
              This constructor creates an std::ostream that interfaces to  a  shared  memory  segment  having  a
              capacity of at least maxSize*sizeUnit bytes. By default, the shared memory segment is opened for
              writing.  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,  using
              octal  digits. If construction succeeds the shared memory is ready for use. If construction fails,
              an FBB::Exception is thrown.

       o      OSharedBuf(intid,std::ios::openmodeopenMode=std::ios::out|std::ios::in):
              This constructor creates an std::istream that 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.

              Copy and move constructors (and assignment operators) are not available.

Copyright

       This is free software, distributed under the terms of the GNU General Public License (GPL).

Description

       This class offers the features of the std::ostream class, operating on shared memory.

Example

       See the sharedstream(3bobcat) man page.

Files

bobcat/osharedstream - defines the class interface

Inherits From

FBB::OSharedBuf (private inheritance),
       std::ostream,
       FBB::SharedEnum__ (cf. sharedmemory(3bobcat) for a description of this last base class).

Member Functions

All members of std::istream and std::ostream and the enum values kB,MB, and GB, defined by
FBB::OSharedEnum__ are available.

o FBB::SharedConditionattachSharedCondition(std::ios::off_typeoffset,std::ios::seekdirorigin):
Returns an FBB::SharedCondition(3) object, interfacing to a shared condition variable located at
offset offset (relative to origin) in the SharedMemory object to which the OSharedStream object
interfaces. This member does not alter the value returned by the stream’s tellp member.

An FBB::Exception is thrown if the FBB::SharedCondition object could not be constructed.

o FBB::SharedConditioncreateSharedCondition():
Returns an FBB::SharedCondition(3) object, interfacing to a newly created shared condition
variable which is created at the current offset of the SharedMemory object to which the
OSharedStream object interfaces (or at the first offset of the next physical shared memory data
block, cf. sharedcondition(3bobcat)). Creating a SharedCondition object does not alter the value
returned by the stream’s tellp member.

An FBB::Exception is thrown if the FBB::SharedCondition object could not be constructed.

o intid()const:
The ID of the shared memory segment is returned.

o voidkill():
Without locking the shared memory first, all shared memory is returned to the operating system.
The FBB::OSharedStream object is unusable after returning from kill. Other processes that were
using the shared memory can continue to do so.

o voidmemInfo(std::ostream&out,charconst*end="\n"):
Information about the OSharedMemory object is inserted 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. Following the information about the shaed memory, end is inserted into out.

o voidopen(size_tmaxSize,SizeUnitsizeUnit,std::ios::openmodeopenMode=std::ios::out,size_taccess=0600):
This member creates a shared memory segment having a capacity of at least maxSize*sizeUnit
bytes, and connects the shared memory segment to the FBB::OSharedStream. A matching close member
does not exist and is not required. By default, the shared memory segment is opened for reading.
Different from the open modes used for file streams, creating a shared memory stream with open
modes ios::in is OK. In this case the shared memory segment is created and once information has
been written to the shared memory by another process it can be read. 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, using octal digits. If opening succeeds the
shared memory is ready for use. If opening fails, an FBB::Exception is thrown.

o voidopen(intid,std::ios::openmodeopenMode=std::ios::in|std::ios::out):
This member connects the FBB::OSharedStream object to a shared memory segment having ID id. A
matching close member does not exist and is not required. If opening succeeds the shared memory is
ready for use. If opening fails (e.g., no shared memory segment having ID id exists), an
FBB::Exception is thrown.

o voidremove():
The shared memory is first locked. Next, all shared memory is returned to the operating system.
The FBB::OSharedStream object is unusable after returning from remove. Other processes that were
using the shared memory can continue to do so.

o booltruncate(std::streamsizeoffset):
If offset is not exceeding the value returned by seekp(0,std::ios::end) this latter value is
changed to offset and true is returned. Otherwise false is returned, and the value returned by
seekp is not altered. If the value returned by tellp exceeded offset, tellp’s return value is
reduced to offset as well.

Name

       FBB::OSharedStream - std::ostream operations on shared memory

Namespace

FBB
       All  constructors,  members,  operators  and manipulators, mentioned in this man-page, are defined in the
       namespace FBB.

Sizeunit Enumeration

       The enumSizeUnit defines the following symbolic constants:

       o      kB, representing 1024 (2**10) bytes of memory;

       o      MB, representing 1048576 (2**20 bytes of memory;

       o      GB, representing 1073741824 (2**30) bytes of memory )

Synopsis

#include<bobcat/osharedstream>
       Linking option: -lpthread-lbobcat