# FBB::OmmapStream(3bobcat)

## std::istream using FBB::MmapBuf as streambuf
(libbobcat-dev_6.06.02)

## 2005-2024

## NAME

FBB::OmmapStream - Input Stream using mmap(2) through FBB::MmapBuf

## SYNOPSIS

#include <bobcat/ommapstream>

Linking option: -lbobcat

## DESCRIPTION

FBB::OmmapStream objects are used to write information to files which are made available in the virtual address space of the calling process. Using the virtual address space is handled by the class's privately inherited FBB::MmapBuf (std::streambuf class.

By mapping files in the process's virtual address space the time required for processing such files is usually dramatically reduced.

An std::exception is thrown, and an error message is written to cerr if the FBB::MmapBuf base class cannot determine the size of the specified file, or when the (un)mapping cannot be performed.

## NAMESPACE

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

## INHERITS FROM

std::ostream, privately from FBB::MmapBuf.

## CONSTRUCTORS

- OmmapStream():
  The default constructor merely constructs an empty OmmapStream object. To change it to a usable object use move assignment;

- OmmapStream(std::string const &fname, char const *bufSize = 0, std::ios::openmode openMode = std::ios::out, mode_t mode = 0644):
  The constructor initializes the OmmapStream object for a file named fname.

  The openMode parameter specifies how fname is used. The standard ios::in, ios::out, ios::trunc, ios::app, and ios::ate modes are supported.

  By default bfOmmapStream) uses a mapping buffer size of 10 times the standard page size, (cf. sysconf(3), and the member pageSize below). The size of the mapping buffer can also be specified using the bufSize parameter. To specify it use a value followed by K, M, or G, representing, resp. 1024, 1024 * 1024, and 1024 * 1024 * 1024 bytes. The final buffer size is at least equal to the standard page size. When a larger value is specified the used buffer size is set to specified / pageSize * pageSize.

  The mode parameter specifies the user/group/other access mode which is used when the file is created by MmapBuf. Its default value specifies read/write access by the user, and read access by others. )

  The move constructor and move assignment operator are available.
  The copy constructor and copy assignment operator are not available.

  ## MEMBER FUNCTIONS

  All members of std::istream are available, as FBB::OmmapStream inherits from that class.
  - size_t bufSize() const:
    returns the used t(mmap) buffer size;

  - size_t fileSize() const:
    returns the current size of the used file. The size is updated to a larger size when writing beyond the current file size. Once the OmmapStream object ceases to exist the used file's size is modified to the current fileSize value.

  - size_t pageSize() const:
    returns the smallest page size used by t(mmap).

  ## EXAMPLE

  An example is provided in bobcat's source archive and gitlab repository at bobcat/ommapstream/demo.

  ## FILES

  bobcat/ommapstream - defines the class interface

  ## SEE ALSO

  bobcat(7), chmod(2), immapstream(3bobcat), iommapstream(3bobcat), mmap(2), mmapbuf(3bobcat) sysconf(3)

  ## BUGS

  None Reported.

  ## BOBCAT PROJECT FILES

  - https://fbb-git.gitlab.io/bobcat/: gitlab project page;
  - bobcat_6.06.02-x.dsc: detached signature;
  - bobcat_6.06.02-x.tar.gz: source archive;
  - bobcat_6.06.02-x_i386.changes: change log;
  - libbobcat1_6.06.02-x_*.deb: debian package containing the libraries;
  - 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).