1<html lang="en"> 2<head> 3<title>Volatiles - Using the GNU Compiler Collection (GCC)</title> 4<meta http-equiv="Content-Type" content="text/html"> 5<meta name="description" content="Using the GNU Compiler Collection (GCC)"> 6<meta name="generator" content="makeinfo 4.13"> 7<link title="Top" rel="start" href="index.html#Top"> 8<link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions"> 9<link rel="prev" href="Inline.html#Inline" title="Inline"> 10<link rel="next" href="Extended-Asm.html#Extended-Asm" title="Extended Asm"> 11<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 12<!-- 13Copyright (C) 1988-2013 Free Software Foundation, Inc. 14 15Permission is granted to copy, distribute and/or modify this document 16under the terms of the GNU Free Documentation License, Version 1.3 or 17any later version published by the Free Software Foundation; with the 18Invariant Sections being ``Funding Free Software'', the Front-Cover 19Texts being (a) (see below), and with the Back-Cover Texts being (b) 20(see below). 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These are normally accessed by 60pointers and used for accessing hardware or inter-thread 61communication. The standard encourages compilers to refrain from 62optimizations concerning accesses to volatile objects, but leaves it 63implementation defined as to what constitutes a volatile access. The 64minimum requirement is that at a sequence point all previous accesses 65to volatile objects have stabilized and no subsequent accesses have 66occurred. Thus an implementation is free to reorder and combine 67volatile accesses that occur between sequence points, but cannot do 68so for accesses across a sequence point. The use of volatile does 69not allow you to violate the restriction on updating objects multiple 70times between two sequence points. 71 72 <p>Accesses to non-volatile objects are not ordered with respect to 73volatile accesses. You cannot use a volatile object as a memory 74barrier to order a sequence of writes to non-volatile memory. For 75instance: 76 77<pre class="smallexample"> int *ptr = <var>something</var>; 78 volatile int vobj; 79 *ptr = <var>something</var>; 80 vobj = 1; 81</pre> 82 <p class="noindent">Unless <var>*ptr</var> and <var>vobj</var> can be aliased, it is not guaranteed 83that the write to <var>*ptr</var> occurs by the time the update 84of <var>vobj</var> happens. If you need this guarantee, you must use 85a stronger memory barrier such as: 86 87<pre class="smallexample"> int *ptr = <var>something</var>; 88 volatile int vobj; 89 *ptr = <var>something</var>; 90 asm volatile ("" : : : "memory"); 91 vobj = 1; 92</pre> 93 <p>A scalar volatile object is read when it is accessed in a void context: 94 95<pre class="smallexample"> volatile int *src = <var>somevalue</var>; 96 *src; 97</pre> 98 <p>Such expressions are rvalues, and GCC implements this as a 99read of the volatile object being pointed to. 100 101 <p>Assignments are also expressions and have an rvalue. However when 102assigning to a scalar volatile, the volatile object is not reread, 103regardless of whether the assignment expression's rvalue is used or 104not. If the assignment's rvalue is used, the value is that assigned 105to the volatile object. For instance, there is no read of <var>vobj</var> 106in all the following cases: 107 108<pre class="smallexample"> int obj; 109 volatile int vobj; 110 vobj = <var>something</var>; 111 obj = vobj = <var>something</var>; 112 obj ? vobj = <var>onething</var> : vobj = <var>anotherthing</var>; 113 obj = (<var>something</var>, vobj = <var>anotherthing</var>); 114</pre> 115 <p>If you need to read the volatile object after an assignment has 116occurred, you must use a separate expression with an intervening 117sequence point. 118 119 <p>As bit-fields are not individually addressable, volatile bit-fields may 120be implicitly read when written to, or when adjacent bit-fields are 121accessed. Bit-field operations may be optimized such that adjacent 122bit-fields are only partially accessed, if they straddle a storage unit 123boundary. For these reasons it is unwise to use volatile bit-fields to 124access hardware. 125 126 </body></html> 127 128