Coroutines are computer program components that generalize subroutines for non-preemptive multitasking, by allowing multiple entry points for suspending and resuming execution at certain locations. Coroutines are well-suited for implementing more familiar program components such as cooperative tasks, exceptions, event loops, iterators, infinite lists and pipes.
According to Donald Knuth, the term coroutine was coined by Melvin Conway in 1958, after he applied it to construction of an assembly program. The first published explanation of the coroutine appeared later, in 1963.
Subroutines are special cases of ... coroutines.
When subroutines are invoked, execution begins at the start, and once a subroutine exits, it is finished; an instance of a subroutine only returns once, and does not hold state between invocations. By contrast, coroutines can exit by calling other coroutines, which may later return to the point where they were invoked in the original coroutine; from the coroutine's point of view, it is not exiting but calling another coroutine. Thus, a coroutine instance holds state, and varies between invocations; there can be multiple instances of a given coroutine at once. The difference between calling another coroutine by means of "yielding" to it and simply calling another routine (which then, also, would return to the original point), is that the relationship between two coroutines which yield to each other is not that of caller-callee, but instead symmetric.
Any subroutine can be translated to a coroutine which does not call yield.
To implement a programming language with subroutines requires only a single stack that can be preallocated at the start of program execution. By contrast, coroutines, able to call on other coroutines as peers, are best implemented using continuations. Continuations may require allocation of additional stacks, and therefore are more commonly implemented in garbage-collected high-level languages. Coroutine creation can be done cheaply by preallocating stacks or caching previously allocated stacks.
Here is a simple example of how coroutines can be useful. Suppose you have a consumer-producer relationship where one routine creates items and adds them to a queue and another removes items from the queue and uses them. For reasons of efficiency, you want to add and remove several items at once. The code might look like this:
var q := new queue
coroutine produce loop while q is not full create some new items add the items to q yield to consume
coroutine consume loop while q is not empty remove some items from q use the items yield to produce
The queue is then completely filled or emptied before yielding control to the other coroutine using the yield command. The further coroutines calls are starting right after the yield, in the outer coroutine loop.
Although this example is often used to introduce multithreading, two threads are not needed for this: the yield statement can be implemented by a jump directly from one routine into the other.
Generators, also known as semicoroutines, are also a generalisation of subroutines, but are more limited than coroutines. Specifically, while both of these can yield multiple times, suspending their execution and allowing re-entry at multiple entry points, they differ in that coroutines can control where execution continues after they yield, while generators cannot, instead transferring control back to the generator's caller. That is, since generators are primarily used to simplify the writing of iterators, the
yield statement in a generator does not specify a coroutine to jump to, but rather passes a value back to a parent routine.
However, it is still possible to implement coroutines on top of a generator facility, with the aid of a top-level dispatcher routine (a trampoline, essentially) that passes control explicitly to child generators identified by tokens passed back from the generators:
var q := new queue
generator produce loop while q is not full create some new items add the items to q yield consume
generator consume loop while q is not empty remove some items from q use the items yield produce
subroutine dispatcher var d := new dictionary(generator -> iterator) d[produce] := start produce d[consume] := start consume var current := produce loop current := next d[current]
A number of implementations of coroutines for languages with generator support but no native coroutines (e.g. Python before 2.5) use this or a similar model.
Using coroutines for state machines or concurrency is similar to using mutual recursion with tail calls, as in both cases the control changes to a different one of a set of routines. However, coroutines are more flexible and generally more efficient. Since coroutines yield rather than return, and then resume execution rather than restarting from the beginning, they are able to hold state, both variables (as in a closure) and execution point, and yields are not limited to being in tail position; mutually recursive subroutines must either use shared variables or pass state as parameters. Further, each mutually recursive call of a subroutine requires a new stack frame (unless tail call elimination is implemented), while passing control between coroutines uses the existing contexts and can be implemented simply by a jump.
Coroutines are useful to implement the following:
Since continuations can be used to implement coroutines, programming languages that support them can also quite easily support coroutines.
Coroutines originated as an assembly language method, but are supported in some high-level programming languages. Early examples include Simula and Modula-2. More recent examples are Ruby, Lua, Julia, and Go.
As of 2003stack-based subroutine implementation.) An exception is the C++ library Boost.Context, part of boost libraries, which supports context swapping on ARM, MIPS, PowerPC, SPARC and X86 on POSIX, Mac OS X and Windows. Coroutines can be built upon Boost.Context., many of the most popular programming languages, including C and its derivatives, do not have direct support for coroutines within the language or their standard libraries. (This is, in large part, due to the limitations of
In situations where a coroutine would be the natural implementation of a mechanism, but is not available, the typical response is to use a closure - a subroutine with state variables (static variables, often boolean flags) to maintain an internal state between calls, and to transfer control to the correct point. Conditionals within the code result in the execution of different code paths on successive calls, based on the values of the state variables. Another typical response is to implement an explicit state machine in the form of a large and complex switch statement or via a goto statement, particularly a computed goto. Such implementations are considered difficult to understand and maintain, and a motivation for coroutine support.
Threads, and to a lesser extent fibers, are an alternative to coroutines in mainstream programming environments today. Threads provide facilities for managing the realtime cooperative interaction of simultaneously executing pieces of code. Threads are widely available in environments that support C (and are supported natively in many other modern languages), are familiar to many programmers, and are usually well-implemented, well-documented and well-supported. However, as they solve a large and difficult problem they include many powerful and complex facilities and have a correspondingly difficult learning curve. As such, when a coroutine is all that is needed, using a thread can be overkill.
One important difference between threads and coroutines is that threads are typically preemptively scheduled while coroutines are not. Because threads can be rescheduled at any instant and can execute concurrently, programs using threads must be careful about locking. In contrast, because coroutines can only be rescheduled at specific points in the program and do not execute concurrently, programs using coroutines can often avoid locking entirely. (This property is also cited as a benefit of event-driven or asynchronous programming.)
Since fibers are cooperatively scheduled, they provide an ideal base for implementing coroutines above. However, system support for fibers is often lacking compared to that for threads.
During the development of the .NET Framework 2.0, Microsoft extended the design of the Common Language Runtime (CLR) hosting APIs to handle fiber-based scheduling with an eye towards its use in fiber-mode for SQL server. Before release, support for the task switching hook ICLRTask::SwitchOut was removed due to time constraints. Consequently, the use of the fiber API to switch tasks is currently not a viable option in the .NET Framework.
There are several implementations for coroutines in Java. Despite the constraints imposed by Java's abstractions, the JVM does not preclude the possibility. There are four general methods used, but two break bytecode portability among standards-compliant JVMs.
Scala Coroutines rely on the
coroutine macro that transforms a normal block of code into a coroutine definition. Such a coroutine definition can be invoked with the
call operation, which instantiates a coroutine frame. A coroutine frame can be resumed with the
resume method, which resumes the execution of the coroutine's body, until reaching a
yieldval keyword, which suspends the coroutine frame. Scala Coroutines also expose a
snapshot method, which effectively duplicates the coroutine.
Several attempts have been made, with varying degrees of success, to implement coroutines in C with combinations of subroutines and macros. Simon Tatham's contribution, based on Duff's device, is a good example of the genre, and his own comments provide a good evaluation of the limitations of this approach. The use of such a device truly can improve the writability, readability and maintainability of a piece of code, but is likely to prove controversial. In Tatham's words: "Of course, this trick violates every coding standard in the book... [but] any coding standard which insists on syntactic clarity at the expense of algorithmic clarity should be rewritten. If your employer fires you for using this trick, tell them that repeatedly as the security staff drag you out of the building."
A more reliable approach to implementing coroutines in C is to give up on absolute portability and write processor-family-specific implementations, in assembly, of functions to save and restore a coroutine context. The setjmp and longjmp functions in the standard C library can be used to implement a form of coroutine. Unfortunately, as Harbison and Steele note, "the setjmp and longjmp functions are notoriously difficult to implement, and the programmer would do well to make minimal assumptions about them." What this means is if Harbison and Steele's many cautions and caveats are not carefully heeded, uses of setjmp and longjmp that appear to work in one environment may not work in another. Worse yet, faulty implementations of these routines are not rare. Indeed, setjmp/longjmp, because it only countenances a single stack, cannot be used to implement natural coroutines, as variables located on the stack will be overwritten as another coroutine uses the same stack space.
Thus for stack-based coroutines in C, functions are needed to create and jump between alternate stacks. A third function, which can usually be written in machine-specific C, is needed to create the context for a new coroutine. C libraries complying to POSIX or the Single Unix Specification (SUSv3) provide such routines as getcontext, setcontext, makecontext and swapcontext. The setcontext family of functions is thus considerably more powerful than setjmp/longjmp, but conforming implementations are as rare if not rarer. The main shortcoming of this approach is that the coroutine's stack is a fixed size and cannot be grown during execution. Thus, programs tend to allocate much more stack than they actually need to avoid the potential for stack overflow.
Due to the limits of standard libraries, some authors have written their own libraries for coroutines. Russ Cox's libtask library is a good example of this genre. It uses the context functions if they are provided by the native C library; otherwise it provides its own implementations for ARM, PowerPC, Sparc, and x86. Other notable implementations include libpcl, coro, lthread, libCoroutine, libconcurrency, libcoro, ribs2, and libdill.
Vala implements native support for coroutines. They are designed to be used with a Gtk Main Loop, but can be used alone if care is taken to ensure that the end callback will never have to be called before doing, at least, one yield.
Since, in most Smalltalk environments, the execution stack is a first-class citizen, coroutines can be implemented without additional library or VM support.
Since Scheme provides full support for continuations, implementing coroutines is nearly trivial, requiring only that a queue of continuations be maintained.
Since version 8.6, the Tool Command Language supports coroutines in the core language. 
Machine-dependent assembly languages often provide direct methods for coroutine execution. For example, in MACRO-11, the assembly language of the PDP-11 family of minicomputers, the "classic" coroutine switch is effected by the instruction "JSR PC,@(SP)+", which jumps to the address popped from the stack and pushes the current (i.e that of the next) instruction address onto the stack. On VAXen (in Macro-32) the comparable instruction is "JSB @(SP)+". Even on a Motorola 6809 there is the instruction "JSR [,S++]"; note the "++", as 2 bytes (of address) are popped from the stack. This instruction is much used in the (standard) 'monitor' Assist 09.
Simply calling back the routine whose address is on the top of the stack, does not, of course, exhaust the possibilities in assembly language(s)!
6. Symmetry is a complexity reducing concept (co-routines include sub-routines); seek it everywhere
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