buffers.qbk

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[/
 / Copyright (c) 2003-2008 Christopher M. Kohlhoff (chris at kohlhoff dot com)
 /
 / Distributed under the Boost Software License, Version 1.0. (See accompanying
 / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 /]

[section:buffers Buffers]

Fundamentally, I/O involves the transfer of data to and from contiguous regions
of memory, called buffers. These buffers can be simply expressed as a tuple
consisting of a pointer and a size in bytes. However, to allow the development
of efficient network applications, Boost.Asio includes support for scatter-gather
operations. These operations involve one or more buffers:

* A scatter-read receives data into multiple buffers.
* A gather-write transmits multiple buffers.

Therefore we require an abstraction to represent a collection of buffers. The
approach used in Boost.Asio is to define a type (actually two types) to
represent a single buffer. These can be stored in a container, which may be
passed to the scatter-gather operations.

In addition to specifying buffers as a pointer and size in bytes, Boost.Asio makes a
distinction between modifiable memory (called mutable) and non-modifiable
memory (where the latter is created from the storage for a const-qualified
variable). These two types could therefore be defined as follows:

  typedef std::pair<void*, std::size_t> mutable_buffer;
  typedef std::pair<const void*, std::size_t> const_buffer;

Here, a mutable_buffer would be convertible to a const_buffer, but conversion
in the opposite direction is not valid.

However, Boost.Asio does not use the above definitions as-is, but instead defines two
classes: `mutable_buffer` and `const_buffer`. The goal of these is to provide
an opaque representation of contiguous memory, where:

* Types behave as std::pair would in conversions. That is, a `mutable_buffer` is
  convertible to a `const_buffer`, but the opposite conversion is disallowed.

* There is protection against buffer overruns. Given a buffer instance, a user
  can only create another buffer representing the same range of memory or a
  sub-range of it. To provide further safety, the library also includes
  mechanisms for automatically determining the size of a buffer from an array,
  `boost::array` or `std::vector` of POD elements, or from a `std::string`.

* Type safety violations must be explicitly requested using the `buffer_cast`
  function. In general an application should never need to do this, but it is
  required by the library implementation to pass the raw memory to the
  underlying operating system functions.

Finally, multiple buffers can be passed to scatter-gather operations (such as
[link boost_asio.reference.read read()] or [link boost_asio.reference.write write()]) by
putting the buffer objects into a container. The `MutableBufferSequence` and
`ConstBufferSequence` concepts have been defined so that containers such as
`std::vector`, `std::list`, `std::vector` or `boost::array` can be used.

[heading Streambuf for Integration with Iostreams]

The class `boost::asio::basic_streambuf` is derived from `std::basic_streambuf` to
associate the input sequence and output sequence with one or more objects of
some character array type, whose elements store arbitrary values. These
character array objects are internal to the streambuf object, but direct access
to the array elements is provided to permit them to be used with I/O
operations, such as the send or receive operations of a socket:

* The input sequence of the streambuf is accessible via the [link
  boost_asio.reference.basic_streambuf.data data()] member function. The return type
  of this function meets the `ConstBufferSequence` requirements.

* The output sequence of the streambuf is accessible via the [link
  boost_asio.reference.basic_streambuf.data prepare()] member function. The return
  type of this function meets the `MutableBufferSequence` requirements.

* Data is transferred from the front of the output sequence to the back of the
  input sequence by calling the [link boost_asio.reference.basic_streambuf.commit
  commit()] member function.

* Data is removed from the front of the input sequence by calling the [link
  boost_asio.reference.basic_streambuf.consume consume()] member function.

The streambuf constructor accepts a `size_t` argument specifying the maximum of
the sum of the sizes of the input sequence and output sequence. Any operation
that would, if successful, grow the internal data beyond this limit will throw
a `std::length_error` exception.

[heading Bytewise Traversal of Buffer Sequences]

The `buffers_iterator<>` class template allows buffer sequences (i.e. types
meeting `MutableBufferSequence` or `ConstBufferSequence` requirements) to be
traversed as though they were a contiguous sequence of bytes. Helper functions
called buffers_begin() and buffers_end() are also provided, where the
buffers_iterator<> template parameter is automatically deduced.

As an example, to read a single line from a socket and into a `std::string`,
you may write:

  boost::asio::streambuf sb;
  ...
  std::size_t n = boost::asio::read_until(sock, sb, '\n');
  boost::asio::streambuf::const_buffers_type bufs = sb.data();
  std::string line(
      boost::asio::buffers_begin(bufs),
      boost::asio::buffers_begin(bufs) + n);

[heading Buffer Debugging]

Some standard library implementations, such as the one that ships with
Microsoft Visual C++ 8.0 and later, provide a feature called iterator
debugging. What this means is that the validity of iterators is checked at
runtime. If a program tries to use an iterator that has been invalidated, an
assertion will be triggered. For example:

  std::vector<int> v(1)
  std::vector<int>::iterator i = v.begin();
  v.clear(); // invalidates iterators
  *i = 0; // assertion!

Boost.Asio takes advantage of this feature to add buffer debugging. Consider the
following code:

  void dont_do_this()
  {
   std::string msg = "Hello, world!";
   boost::asio::async_write(sock, boost::asio::buffer(msg), my_handler);
  }

When you call an asynchronous read or write you need to ensure that the buffers
for the operation are valid until the completion handler is called. In the
above example, the buffer is the `std::string` variable `msg`. This variable is
on the stack, and so it goes out of scope before the asynchronous operation
completes. If you're lucky then the application will crash, but random failures
are more likely.

When buffer debugging is enabled, Boost.Asio stores an iterator into the string until
the asynchronous operation completes, and then dereferences it to check its
validity. In the above example you would observe an assertion failure just
before Boost.Asio tries to call the completion handler.

This feature is automatically made available for Microsoft Visual Studio 8.0 or
later and for GCC when `_GLIBCXX_DEBUG` is defined. There is a performance cost
to this checking, so buffer debugging is only enabled in debug builds. For
other compilers it may be enabled by defining `BOOST_ASIO_ENABLE_BUFFER_DEBUGGING`.
It can also be explicitly disabled by defining `BOOST_ASIO_DISABLE_BUFFER_DEBUGGING`.

[heading See Also]

[link boost_asio.reference.buffer buffer],
[link boost_asio.reference.buffers_begin buffers_begin],
[link boost_asio.reference.buffers_end buffers_end],
[link boost_asio.reference.buffers_iterator buffers_iterator],
[link boost_asio.reference.const_buffer const_buffer],
[link boost_asio.reference.const_buffers_1 const_buffers_1],
[link boost_asio.reference.mutable_buffer mutable_buffer],
[link boost_asio.reference.mutable_buffers_1 mutable_buffers_1],
[link boost_asio.reference.streambuf streambuf],
[link boost_asio.reference.ConstBufferSequence ConstBufferSequence],
[link boost_asio.reference.MutableBufferSequence MutableBufferSequence],
[link boost_asio.examples.buffers buffers example].

[endsect]