boost/libs/coroutine/doc/intro.qbk

[/
          Copyright Oliver Kowalke 2009.
 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:intro Introduction]

[heading Definition]

In computer science routines are defined as a sequence of operations. The
execution of routines forms a parent-child relationship and the child terminates
always before the parent. Coroutines (the term was introduced by Melvin
Conway [footnote Conway, Melvin E.. "Design of a Separable Transition-Diagram Compiler".
Commun. ACM, Volume 6 Issue 7, July 1963, Article No. 7]),
are a generalization of routines (Donald Knuth [footnote Knuth, Donald Ervin (1997).
"Fundamental Algorithms. The Art of Computer Programming 1", (3rd ed.)].
The principal difference between coroutines and routines
is that a coroutine enables explicit suspend and resume of its progress via
additional operations by preserving execution state and thus provides an
[*enhanced control flow] (maintaining the execution context).


[heading How it works]

Functions foo() and bar() are supposed to alternate their execution (leave and
enter function body).

[$../../../../libs/coroutine/doc/images/foo_bar.png [align center]]

If coroutines were called exactly like routines, the stack would grow with
every call and would never be popped. A jump into the middle of a coroutine
would not be possible, because the return address would be on top of
stack entries.

The solution is that each coroutine has its own stack and control-block
(__fcontext__ from __boost_context__).
Before the coroutine gets suspended, the non-volatile registers (including stack
and instruction/program pointer) of the currently active coroutine are stored in
the coroutine's control-block.
The registers of the newly activated coroutine must be restored from its
associated control-block before it is resumed.

The context switch requires no system privileges and provides cooperative
multitasking convenient to C++. Coroutines provide quasi parallelism.
When a program is supposed to do several things at the same time, coroutines
help to do this much more simply and elegantly than with only a single flow of
control.
The advantages can be seen particularly clearly with the use of a recursive
function, such as traversal of binary trees (see example 'same fringe').


[heading characteristics]
Characteristics [footnote Moura, Ana Lucia De and Ierusalimschy, Roberto.
"Revisiting coroutines". ACM Trans. Program. Lang. Syst., Volume 31 Issue 2,
February 2009, Article No. 6] of a coroutine are:

* values of local data persist between successive calls (context switches)
* execution is suspended as control leaves coroutine and is resumed at certain time later
* symmetric or asymmetric control-transfer mechanism; see below
* first-class object (can be passed as argument, returned by procedures,
   stored in a data structure to be used later or freely manipulated by
   the developer)
* stackful or stackless

Coroutines are useful in simulation, artificial intelligence, concurrent
programming, text processing and data manipulation, supporting
the implementation of components such as cooperative tasks (fibers), iterators,
generators, infinite lists, pipes etc.


[heading execution-transfer mechanism]
Two categories of coroutines exist: symmetric and asymmetric coroutines.

An asymmetric coroutine knows its invoker, using a special operation to
implicitly yield control specifically to its invoker. By contrast, all symmetric
coroutines are equivalent; one symmetric coroutine may pass control to any
other symmetric coroutine. Because of this, a symmetric coroutine ['must]
specify the coroutine to which it intends to yield control.

[$../../../../libs/coroutine/doc/images/foo_bar_seq.png [align center]]

Both concepts are equivalent and a fully-general coroutine library can provide
either symmetric or asymmetric coroutines. For convenience, Boost.Coroutine
provides both.


[heading stackfulness]
In contrast to a stackless coroutine a stackful coroutine can be suspended
from within a nested stackframe. Execution resumes at exactly the same point
in the code where it was suspended before.
With a stackless coroutine, only the top-level routine may be suspended. Any
routine called by that top-level routine may not itself suspend. This prohibits
providing suspend/resume operations in routines within a general-purpose library.

[heading first-class continuation]
A first-class continuation can be passed as an argument, returned by a
function and stored in a data structure to be used later.
In some implementations (for instance C# ['yield]) the continuation can
not be directly accessed or directly manipulated.

Without stackfulness and first-class semantics, some useful execution control
flows cannot be supported (for instance cooperative multitasking or
checkpointing).

[endsect]