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<a class="nav-entry-link" href="appendix-i-keywords.html">Appendix I: Keywords</a>
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<a class="nav-entry-link" href="appendix-ii-operators.html">Appendix II: Operators</a>
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<a class="nav-entry-link" href="other-online-resources.html">Other online resources</a>
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<a class="nav-entry-link" href="other-online-resources.html#v-contributing-guide"> V contributing guide</a>
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<a class="nav-entry-link" href="other-online-resources.html#awesome-v"> Awesome V</a>
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<a class="nav-entry-link" href="other-online-resources.html#the-v-language-discord"> The V language Discord</a>
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<article>
<div class="container">
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<main class="v-doc">
<h1 class="main">Concurrency</h1>
<h2 id="spawning-concurrent-tasks">Spawning Concurrent Tasks <a href="concurrency.html#spawning-concurrent-tasks" class="header-anchor" aria-hidden="true">#</a></h2>
<p>V's model of concurrency is going to be very similar to Go's.
For now, <code>spawn foo()</code> runs <code>foo()</code> concurrently in a different thread:</p>
<div class="language-v">import math
fn p(a f64, b f64) { // ordinary function without return value
c := math.sqrt(a * a + b * b)
println(c)
}
fn main() {
spawn p(3, 4)
// p will be run in parallel thread
// It can also be written as follows
// spawn fn (a f64, b f64) {
// c := math.sqrt(a * a + b * b)
// println(c)
// }(3, 4)
}
</div>
<blockquote class="tip">
<p>[!NOTE]
Threads rely on the machine's CPU (number of cores/threads).
Be aware that OS threads spawned with <code>spawn</code>
have limitations in regard to concurrency,
including resource overhead and scalability issues,
and might affect performance in cases of high thread count.</p>
</blockquote>
<p>There's also a <code>go</code> keyword. Right now <code>go foo()</code> will be automatically renamed via vfmt
to <code>spawn foo()</code>, and there will be a way to launch a coroutine with <code>go</code> (a lightweight
thread managed by the runtime).</p>
<p>Sometimes it is necessary to wait until a parallel thread has finished. This can
be done by assigning a <em>handle</em> to the started thread and calling the <code>wait()</code> method
to this handle later:</p>
<div class="language-v">import math
fn p(a f64, b f64) { // ordinary function without return value
c := math.sqrt(a * a + b * b)
println(c) // prints `5`
}
fn main() {
h := spawn p(3, 4)
// p() runs in parallel thread
h.wait()
// p() has definitely finished
}
</div>
<p>This approach can also be used to get a return value from a function that is run in a
parallel thread. There is no need to modify the function itself to be able to call it
concurrently.</p>
<div class="language-v">import math { sqrt }
fn get_hypot(a f64, b f64) f64 { // ordinary function returning a value
c := sqrt(a * a + b * b)
return c
}
fn main() {
g := spawn get_hypot(54.06, 2.08) // spawn thread and get handle to it
h1 := get_hypot(2.32, 16.74) // do some other calculation here
h2 := g.wait() // get result from spawned thread
println('Results: ${h1}, ${h2}') // prints `Results: 16.9, 54.1`
}
</div>
<p>If there is a large number of tasks, it might be easier to manage them
using an array of threads.</p>
<div class="language-v">import time
fn task(id int, duration int) {
println('task ${id} begin')
time.sleep(duration * time.millisecond)
println('task ${id} end')
}
fn main() {
mut threads := []thread{}
threads << spawn task(1, 500)
threads << spawn task(2, 900)
threads << spawn task(3, 100)
threads.wait()
println('done')
}
// Output:
// task 1 begin
// task 2 begin
// task 3 begin
// task 3 end
// task 1 end
// task 2 end
// done
</div>
<p>Additionally for threads that return the same type, calling <code>wait()</code>
on the thread array will return all computed values.</p>
<div class="language-v">fn expensive_computing(i int) int {
return i * i
}
fn main() {
mut threads := []thread int{}
for i in 1 .. 10 {
threads << spawn expensive_computing(i)
}
// Join all tasks
r := threads.wait()
println('All jobs finished: ${r}')
}
// Output: All jobs finished: [1, 4, 9, 16, 25, 36, 49, 64, 81]
</div>
<h2 id="channels">Channels <a href="concurrency.html#channels" class="header-anchor" aria-hidden="true">#</a></h2>
<p>Channels are the preferred way to communicate between threads. V's channels work basically like
those in Go. You can push objects into a channel on one end and pop objects from the other end.
Channels can be buffered or unbuffered and it is possible to <code>select</code> from multiple channels.</p>
<h3 id="syntax-and-usage">Syntax and Usage <a href="concurrency.html#syntax-and-usage" class="header-anchor" aria-hidden="true">#</a></h3>
<p>Channels have the type <code>chan objtype</code>. An optional buffer length can be specified as the <code>cap</code> field
in the declaration:</p>
<div class="language-v">ch := chan int{} // unbuffered - "synchronous"
ch2 := chan f64{cap: 100} // buffer length 100
</div>
<p>Channels do not have to be declared as <code>mut</code>. The buffer length is not part of the type but
a field of the individual channel object. Channels can be passed to threads like normal
variables:</p>
<div class="language-v">fn f(ch chan int) {
// ...
}
fn main() {
ch := chan int{}
spawn f(ch)
// ...
}
</div>
<p>Objects can be pushed to channels using the arrow operator. The same operator can be used to
pop objects from the other end:</p>
<div class="language-v">// make buffered channels so pushing does not block (if there is room in the buffer)
ch := chan int{cap: 1}
ch2 := chan f64{cap: 1}
n := 5
// push
ch <- n
ch2 <- 7.3
mut y := f64(0.0)
m := <-ch // pop creating new variable
y = <-ch2 // pop into existing variable
</div>
<p>A channel can be closed to indicate that no further objects can be pushed. Any attempt
to do so will then result in a runtime panic (with the exception of <code>select</code> and
<code>try_push()</code> - see below). Attempts to pop will return immediately if the
associated channel has been closed and the buffer is empty. This situation can be
handled using an <code>or {}</code> block (see <a href="type-declarations.html#handling-optionsresults">Handling options/results</a>).</p>
<div class="language-v">ch := chan int{}
ch2 := chan f64{}
// ...
ch.close()
// ...
m := <-ch or {
println('channel has been closed')
}
// propagate error
y := <-ch2 ?
</div>
<h3 id="channel-select">Channel Select <a href="concurrency.html#channel-select" class="header-anchor" aria-hidden="true">#</a></h3>
<p>The <code>select</code> command allows monitoring several channels at the same time
without noticeable CPU load. It consists of a list of possible transfers and associated branches
of statements - similar to the <a href="statements-&-expressions.html#match">match</a> command:</p>
<div class="language-v">import time
fn main() {
ch := chan f64{}