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global-variables.html
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<a class="link" href="appendix-i-keywords.html">Appendix I: Keywords</a>
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<a class="link" href="appendix-ii-operators.html">Appendix II: Operators</a>
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<main>
<h1 class="main">Global Variables</h1>
<p>By default V does not allow global variables. However, in low level applications they have their
place so their usage can be enabled with the compiler flag <code>-enable-globals</code>.
Declarations of global variables must be surrounded with a <code>__global ( ... )</code>
specification – as in the example <a href="atomics.html">above</a>.</p>
<p>An initializer for global variables must be explicitly converted to the
desired target type. If no initializer is given a default initialization is done.
Some objects like semaphores and mutexes require an explicit initialization <em>in place</em>, i.e.
not with a value returned from a function call but with a method call by reference.
A separate <code>init()</code> function can be used for this purpose – it will be called before <code>main()</code>:</p>
<div class="language-v">import sync
__global (
sem sync.Semaphore // needs initialization in `init()`
mtx sync.RwMutex // needs initialization in `init()`
f1 = f64(34.0625) // explicily initialized
shmap shared map[string]f64 // initialized as empty `shared` map
f2 f64 // initialized to `0.0`
)
fn init() {
sem.init(0)
mtx.init()
}
</div>
<p>Be aware that in multi threaded applications the access to global variables is subject
to race conditions. There are several approaches to deal with these:</p>
<ul>
<li>use <code>shared</code> types for the variable declarations and use <code>lock</code> blocks for access.
This is most appropriate for larger objects like structs, arrays or maps.</li>
<li>handle primitive data types as "atomics" using special C-functions (see <a href="atomics.html">above</a>).</li>
<li>use explicit synchronization primitives like mutexes to control access. The compiler
cannot really help in this case, so you have to know what you are doing.</li>
<li>don't care – this approach is possible but makes only sense if the exact values
of global variables do not really matter. An example can be found in the <code>rand</code> module
where global variables are used to generate (non cryptographic) pseudo random numbers.
In this case data races lead to random numbers in different threads becoming somewhat
correlated, which is acceptable considering the performance penalty that using
synchronization primitives would represent.</li>
</ul>
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<time datetime="2024-06-04T15:42:48.277Z">Last updated:
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