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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>System.Timeout</title><link href="ocean.css" rel="stylesheet" type="text/css" title="Ocean" /><script src="haddock-util.js" type="text/javascript"></script><script type="text/javascript">//<![CDATA[
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</script></head><body><div id="package-header"><ul class="links" id="page-menu"><li><a href="index.html">Contents</a></li><li><a href="doc-index.html">Index</a></li></ul><p class="caption">base-4.5.1.0: Basic libraries</p></div><div id="content"><div id="module-header"><table class="info"><tr><th>Portability</th><td>non-portable</td></tr><tr><th>Stability</th><td>experimental</td></tr><tr><th>Maintainer</th><td>libraries@haskell.org</td></tr><tr><th>Safe Haskell</th><td>Trustworthy</td></tr></table><p class="caption">System.Timeout</p></div><div id="description"><p class="caption">Description</p><div class="doc"><p>Attach a timeout event to arbitrary <code><a href="System-IO.html#t:IO">IO</a></code> computations.
</p></div></div><div id="synopsis"><p id="control.syn" class="caption expander" onclick="toggleSection('syn')">Synopsis</p><ul id="section.syn" class="hide" onclick="toggleSection('syn')"><li class="src short"><a href="#v:timeout">timeout</a> ::  <a href="Data-Int.html#t:Int">Int</a> -&gt; <a href="System-IO.html#t:IO">IO</a> a -&gt; <a href="System-IO.html#t:IO">IO</a> (<a href="Data-Maybe.html#t:Maybe">Maybe</a> a)</li></ul></div><div id="interface"><h1>Documentation</h1><div class="top"><p class="src"><a name="v:timeout" class="def">timeout</a> ::  <a href="Data-Int.html#t:Int">Int</a> -&gt; <a href="System-IO.html#t:IO">IO</a> a -&gt; <a href="System-IO.html#t:IO">IO</a> (<a href="Data-Maybe.html#t:Maybe">Maybe</a> a)</p><div class="doc"><p>Wrap an <code><a href="System-IO.html#t:IO">IO</a></code> computation to time out and return <code>Nothing</code> in case no result
 is available within <code>n</code> microseconds (<code>1/10^6</code> seconds). In case a result
 is available before the timeout expires, <code>Just a</code> is returned. A negative
 timeout interval means &quot;wait indefinitely&quot;. When specifying long timeouts,
 be careful not to exceed <code>maxBound :: Int</code>.
</p><p>The design of this combinator was guided by the objective that <code>timeout n f</code>
 should behave exactly the same as <code>f</code> as long as <code>f</code> doesn't time out. This
 means that <code>f</code> has the same <code><a href="Control-Concurrent.html#v:myThreadId">myThreadId</a></code> it would have without the timeout
 wrapper. Any exceptions <code>f</code> might throw cancel the timeout and propagate
 further up. It also possible for <code>f</code> to receive exceptions thrown to it by
 another thread.
</p><p>A tricky implementation detail is the question of how to abort an <code>IO</code>
 computation. This combinator relies on asynchronous exceptions internally.
 The technique works very well for computations executing inside of the
 Haskell runtime system, but it doesn't work at all for non-Haskell code.
 Foreign function calls, for example, cannot be timed out with this
 combinator simply because an arbitrary C function cannot receive
 asynchronous exceptions. When <code>timeout</code> is used to wrap an FFI call that
 blocks, no timeout event can be delivered until the FFI call returns, which
 pretty much negates the purpose of the combinator. In practice, however,
 this limitation is less severe than it may sound. Standard I/O functions
 like <code><a href="System-IO.html#v:hGetBuf">hGetBuf</a></code>, <code><a href="System-IO.html#v:hPutBuf">hPutBuf</a></code>, Network.Socket.accept, or
 <code><a href="System-IO.html#v:hWaitForInput">hWaitForInput</a></code> appear to be blocking, but they really don't
 because the runtime system uses scheduling mechanisms like <code>select(2)</code> to
 perform asynchronous I/O, so it is possible to interrupt standard socket
 I/O or file I/O using this combinator.
</p></div></div></div></div><div id="footer"><p>Produced by <a href="http://www.haskell.org/haddock/">Haddock</a> version 2.11.0</p></div></body></html>