# FBB::MmapBuf(3bobcat)

## std::streambuf using mmap(2)
(libbobcat-dev_6.14.00)

## 2005-2026

## NAME

FBB::MmapBuf - a std::streambuf using mmap(2) for file I/O

## SYNOPSIS

#include <bobcat/mmapbuf>

Linking option: -lbobcat

## DESCRIPTION

The mmap(2) functions map files in the virtual address space of the calling process. Using mmap in C++ is doable, but complex. The MmapBuf std::streambuf class embeds the features of mmap(2), allowing the use of C++'s streams instead of using the raw mmap calls. the FBB::Mmapbase class defines a std::streambuf which can be used by istream, ostream, and iostream objects.

An std::exception is thrown, and an error message is written to cerr if the details about the specified filename (cf. section CONSTRUCORS cannot be determined, or when mmap or munmap return errors.

## NAMESPACE

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

## INHERITS FROM

std::streambuf

## CONSTRUCTORS

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

- MmapBuf(std::string const &fname, std::ios::openmode openMode, char const *bufSize = 0, mode_t mode = 0644):
  The constructor initializes the MmapBuf object for a file named fname. The openmode can be any combination of the std::ios::openmode values. The required values are usually provided when constructing ImapStream, OmapStream, or IOmapStream objects (cf. section SEE ALSO below).

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

  By default bfMmapBuf

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::stringstream are available, as FBB::MmapBuf inherits from that class.

- size_t bufSize() const:
  returns the used t(mmap) buffer size;

- size_t fileSize() const:
  returns the size of the used file. The size is updated to a larger size when writing beyond the current file size. Once MmapBuf objects cease to exist the used file's size is modified to the current fileSize value. The size of a file that can only be read is never altered.

- 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/mmapbuf/demo.

## FILES

bobcat/mmapbuf - defines the class interface

## SEE ALSO

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

## BUGS

None Reported.

## BOBCAT PROJECT FILES

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

Debian Bobcat project files:

- libbobcat6: debian package containing the shared library, changelog and copyright note;
- libbobcat-dev: debian package containing the static library, headers, manual pages, and developer info;

## 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).