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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>
<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
}
</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 &lt;&lt; spawn task(1, 500)
    threads &lt;&lt; spawn task(2, 900)
    threads &lt;&lt; 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 &lt;&lt; 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. They allow threads to exchange data
safely without requiring explicit locking. V's channels are similar to those in Go, enabling you
to push objects into a channel on one end and pop objects from the other.
Channels can be buffered or unbuffered, and you can use the <code>select</code> statement to monitor multiple
channels simultaneously.</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 are declared with the type <code>chan objtype</code>.
You can optionally specify a buffer length using the <code>cap</code> field:</p>
<div class="language-v">ch := chan int{} // unbuffered - &quot;synchronous&quot;
ch2 := chan f64{cap: 100} // buffered with a capacity of 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">import time

fn worker(ch chan int) {
    for i in 0 .. 5 {
        ch &lt;- i // push values into the channel
    }
}

fn clock(ch chan int) {
    for i in 0 .. 5 {
        time.sleep(1 * time.second)
        println('Clock tick')
        ch &lt;- (i + 1000) // push a value into the channel
    }
    ch.close() // close the channel when done
}

fn main() {
    ch := chan int{cap: 5}
    spawn worker(ch)
    spawn clock(ch)
    for {
        value := &lt;-ch or { // receive/pop values from the channel
            println('Channel closed')
            break
        }
        println('Received: ${value}')
    }
}
</div>
<h3 id="buffered-channels">Buffered Channels <a href="concurrency.html#buffered-channels" class="header-anchor" aria-hidden="true">#</a></h3>
<p>Buffered channels allow you to push multiple items without blocking,
as long as the buffer is not full:</p>
<div class="language-v">ch := chan string{cap: 2}
ch &lt;- 'hello'
ch &lt;- 'world'
// ch &lt;- '!' // This would block because the buffer is full

println(&lt;-ch) // &quot;hello&quot;
println(&lt;-ch) // &quot;world&quot;
</div>
<h3 id="closing-channels">Closing Channels <a href="concurrency.html#closing-channels" class="header-anchor" aria-hidden="true">#</a></h3>
<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 := &lt;-ch or {
    println('channel has been closed')
}

// propagate error
y := &lt;-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{}
    ch2 := chan f64{}
    ch3 := chan f64{}
    mut b := 0.0
    c := 1.0
    // ... setup spawn threads that will send on ch/ch2
    spawn fn (the_channel chan f64) {
        time.sleep(5 * time.millisecond)
        the_channel &lt;- 1.0
    }(ch)
    spawn fn (the_channel chan f64) {
        time.sleep(1 * time.millisecond)
        the_channel &lt;- 1.0
    }(ch2)
    spawn fn (the_channel chan f64) {
        _ := &lt;-the_channel
    }(ch3)

    select {
        a := &lt;-ch {
            // do something with `a`
            eprintln('&gt; a: ${a}')
        }
        b = &lt;-ch2 {
            // do something with predeclared variable `b`
            eprintln('&gt; b: ${b}')
        }
        ch3 &lt;- c {
            // do something if `c` was sent
            time.sleep(5 * time.millisecond)
            eprintln('&gt; c: ${c} was send on channel ch3')
        }
        500 * time.millisecond {
            // do something if no channel has become ready within 0.5s
            eprintln('&gt; more than 0.5s passed without a channel being ready')
        }
    }
    eprintln('&gt; done')
}
</div>
<p>The timeout branch is optional. If it is absent <code>select</code> waits for an unlimited amount of time.
It is also possible to proceed immediately if no channel is ready in the moment <code>select</code> is called
by adding an <code>else { ... }</code> branch. <code>else</code> and <code>&lt;timeout&gt;</code> are mutually exclusive.</p>
<p>The <code>select</code> command can be used as an <em>expression</em> of type <code>bool</code>
that becomes <code>false</code> if all channels are closed:</p>
<div class="language-v">if select {
    ch &lt;- a {
        // ...
    }
} {
    // channel was open
} else {
    // channel is closed
}
</div>
<h3 id="special-channel-features">Special Channel Features <a href="concurrency.html#special-channel-features" class="header-anchor" aria-hidden="true">#</a></h3>
<p>For special purposes there are some builtin fields and methods:</p>
<div class="language-v">ch := chan int{cap: 2}
println(ch.try_push(42)) // `.success` if pushed, `.not_ready` if full, `.closed` if closed
println(ch.len) // Number of items in the buffer
println(ch.cap) // Buffer capacity
println(ch.closed) // Whether the channel is closed
</div>
<div class="language-v">struct Abc {
    x int
}

a := 2.13
ch := chan f64{}
res := ch.try_push(a) // try to perform `ch &lt;- a`
println(res)
l := ch.len // number of elements in queue
c := ch.cap // maximum queue length
is_closed := ch.closed // bool flag - has `ch` been closed
println(l)
println(c)
mut b := Abc{}
ch2 := chan Abc{}
res2 := ch2.try_pop(mut b) // try to perform `b = &lt;-ch2`
</div>
<p>The <code>try_push/pop()</code> methods will return immediately with one of the results
<code>.success</code>, <code>.not_ready</code> or <code>.closed</code> - dependent on whether the object has been transferred or
the reason why not.
Usage of these methods and fields in production is not recommended -
algorithms based on them are often subject to race conditions. Especially <code>.len</code> and
<code>.closed</code> should not be used to make decisions.
Use <code>or</code> branches, error propagation or <code>select</code> instead (see <a href="concurrency.html#syntax-and-usage">Syntax and Usage</a>
and <a href="concurrency.html#channel-select">Channel Select</a> above).</p>
<h2 id="shared-objects">Shared Objects <a href="concurrency.html#shared-objects" class="header-anchor" aria-hidden="true">#</a></h2>
<p>Data can be exchanged between a thread and the calling thread via a shared variable.
Such variables should be created as <code>shared</code> and passed to the thread as such, too.
The underlying <code>struct</code> contains a hidden <em>mutex</em> that allows locking concurrent access
using <code>rlock</code> for read-only and <code>lock</code> for read/write access.</p>
<p>Note: Shared variables must be structs, arrays or maps.</p>
<h3 id="example-of-shared-objects">Example of Shared Objects <a href="concurrency.html#example-of-shared-objects" class="header-anchor" aria-hidden="true">#</a></h3>
<div class="language-v">struct Counter {
mut:
    value int
}

fn (shared counter Counter) increment() {
    lock counter {
        counter.value += 1
        println('Incremented to: ${counter.value}')
    }
}

fn main() {
    shared counter := Counter{}

    spawn counter.increment()
    spawn counter.increment()

    rlock counter {
        println('Final value: ${counter.value}')
    }
}
</div>
<h2 id="difference-between-channels-and-shared-objects">Difference Between Channels and Shared Objects <a href="concurrency.html#difference-between-channels-and-shared-objects" class="header-anchor" aria-hidden="true">#</a></h2>
<p><strong>Purpose</strong>:</p>
<ul>
<li>Channels: Used for message passing between threads, ensuring safe communication.</li>
<li>Shared objects: Used for direct data sharing and modification between threads.</li>
</ul>
<p><strong>Synchronization</strong>:</p>
<ul>
<li>Channels: Implicit (via channel operations)</li>
<li>Shared objects:  Explicit (via <code>rlock</code>/<code>lock</code> blocks)</li>
</ul>

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