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<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 similar to Go's.</p>
<p><code>go foo()</code> runs <code>foo()</code> concurrently in a lightweight thread managed by the V runtime.</p>
<p><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>
<p>A spawned call with a <code>mut param &T</code> parameter receives the caller's mutable pointer
storage, including when another function forwards that parameter. Keep that storage alive
until the thread finishes; <code>wait()</code> also makes its field changes and pointer replacement
available to the caller.</p>
<p>If a spawned thread's handle is discarded, including inside a discarded array or struct,
V detaches the thread. Keep its handle and call <code>wait()</code> when the result or completion matters.
The detached thread releases an owned return value after its function finishes.
If the return value is a thread handle, it joins that thread; a returned closure releases
its captured context.</p>
<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>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
}