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127 lines
5.6 KiB
HTML
127 lines
5.6 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Volatiles - Using the GNU Compiler Collection (GCC)</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC)">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions">
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<link rel="prev" href="Inline.html#Inline" title="Inline">
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<link rel="next" href="Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C" title="Using Assembly Language with C">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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</head>
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<body>
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<div class="node">
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<a name="Volatiles"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C">Using Assembly Language with C</a>,
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Previous: <a rel="previous" accesskey="p" href="Inline.html#Inline">Inline</a>,
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Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a>
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<hr>
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</div>
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<h3 class="section">6.42 When is a Volatile Object Accessed?</h3>
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<p><a name="index-accessing-volatiles-3413"></a><a name="index-volatile-read-3414"></a><a name="index-volatile-write-3415"></a><a name="index-volatile-access-3416"></a>
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C has the concept of volatile objects. These are normally accessed by
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pointers and used for accessing hardware or inter-thread
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communication. The standard encourages compilers to refrain from
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optimizations concerning accesses to volatile objects, but leaves it
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implementation defined as to what constitutes a volatile access. The
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minimum requirement is that at a sequence point all previous accesses
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to volatile objects have stabilized and no subsequent accesses have
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occurred. Thus an implementation is free to reorder and combine
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volatile accesses that occur between sequence points, but cannot do
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so for accesses across a sequence point. The use of volatile does
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not allow you to violate the restriction on updating objects multiple
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times between two sequence points.
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<p>Accesses to non-volatile objects are not ordered with respect to
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volatile accesses. You cannot use a volatile object as a memory
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barrier to order a sequence of writes to non-volatile memory. For
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instance:
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<pre class="smallexample"> int *ptr = <var>something</var>;
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volatile int vobj;
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*ptr = <var>something</var>;
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vobj = 1;
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</pre>
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<p class="noindent">Unless <var>*ptr</var> and <var>vobj</var> can be aliased, it is not guaranteed
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that the write to <var>*ptr</var> occurs by the time the update
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of <var>vobj</var> happens. If you need this guarantee, you must use
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a stronger memory barrier such as:
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<pre class="smallexample"> int *ptr = <var>something</var>;
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volatile int vobj;
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*ptr = <var>something</var>;
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asm volatile ("" : : : "memory");
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vobj = 1;
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</pre>
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<p>A scalar volatile object is read when it is accessed in a void context:
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<pre class="smallexample"> volatile int *src = <var>somevalue</var>;
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*src;
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</pre>
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<p>Such expressions are rvalues, and GCC implements this as a
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read of the volatile object being pointed to.
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<p>Assignments are also expressions and have an rvalue. However when
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assigning to a scalar volatile, the volatile object is not reread,
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regardless of whether the assignment expression's rvalue is used or
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not. If the assignment's rvalue is used, the value is that assigned
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to the volatile object. For instance, there is no read of <var>vobj</var>
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in all the following cases:
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<pre class="smallexample"> int obj;
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volatile int vobj;
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vobj = <var>something</var>;
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obj = vobj = <var>something</var>;
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obj ? vobj = <var>onething</var> : vobj = <var>anotherthing</var>;
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obj = (<var>something</var>, vobj = <var>anotherthing</var>);
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</pre>
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<p>If you need to read the volatile object after an assignment has
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occurred, you must use a separate expression with an intervening
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sequence point.
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<p>As bit-fields are not individually addressable, volatile bit-fields may
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be implicitly read when written to, or when adjacent bit-fields are
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accessed. Bit-field operations may be optimized such that adjacent
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bit-fields are only partially accessed, if they straddle a storage unit
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boundary. For these reasons it is unwise to use volatile bit-fields to
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access hardware.
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</body></html>
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