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diff --git a/share/doc/gcc/Volatiles.html b/share/doc/gcc/Volatiles.html new file mode 100644 index 0000000..452ba3e --- /dev/null +++ b/share/doc/gcc/Volatiles.html @@ -0,0 +1,170 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- This file documents the use of the GNU compilers. + +Copyright (C) 1988-2023 Free Software Foundation, Inc. + +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.3 or +any later version published by the Free Software Foundation; with the +Invariant Sections being "Funding Free Software", the Front-Cover +Texts being (a) (see below), and with the Back-Cover Texts being (b) +(see below). A copy of the license is included in the section entitled +"GNU Free Documentation License". + +(a) The FSF's Front-Cover Text is: + +A GNU Manual + +(b) The FSF's Back-Cover Text is: + +You have freedom to copy and modify this GNU Manual, like GNU + software. Copies published by the Free Software Foundation raise + funds for GNU development. --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>Using the GNU Compiler Collection (GCC): Volatiles</title> + +<meta name="description" content="Using the GNU Compiler Collection (GCC): Volatiles"> +<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Volatiles"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Indices.html#Indices" rel="index" title="Indices"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="C-Extensions.html#C-Extensions" rel="up" title="C Extensions"> +<link href="Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C" rel="next" title="Using Assembly Language with C"> +<link href="Inline.html#Inline" rel="previous" title="Inline"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en_US" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Volatiles"></a> +<div class="header"> +<p> +Next: <a href="Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C" accesskey="n" rel="next">Using Assembly Language with C</a>, Previous: <a href="Inline.html#Inline" accesskey="p" rel="previous">Inline</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="When-is-a-Volatile-Object-Accessed_003f"></a> +<h3 class="section">6.46 When is a Volatile Object Accessed?</h3> +<a name="index-accessing-volatiles"></a> +<a name="index-volatile-read"></a> +<a name="index-volatile-write"></a> +<a name="index-volatile-access"></a> + +<p>C has the concept of volatile objects. These are normally accessed by +pointers and used for accessing hardware or inter-thread +communication. The standard encourages compilers to refrain from +optimizations concerning accesses to volatile objects, but leaves it +implementation defined as to what constitutes a volatile access. The +minimum requirement is that at a sequence point all previous accesses +to volatile objects have stabilized and no subsequent accesses have +occurred. Thus an implementation is free to reorder and combine +volatile accesses that occur between sequence points, but cannot do +so for accesses across a sequence point. The use of volatile does +not allow you to violate the restriction on updating objects multiple +times between two sequence points. +</p> +<p>Accesses to non-volatile objects are not ordered with respect to +volatile accesses. You cannot use a volatile object as a memory +barrier to order a sequence of writes to non-volatile memory. For +instance: +</p> +<div class="smallexample"> +<pre class="smallexample">int *ptr = <var>something</var>; +volatile int vobj; +*ptr = <var>something</var>; +vobj = 1; +</pre></div> + +<p>Unless <var>*ptr</var> and <var>vobj</var> can be aliased, it is not guaranteed +that the write to <var>*ptr</var> occurs by the time the update +of <var>vobj</var> happens. If you need this guarantee, you must use +a stronger memory barrier such as: +</p> +<div class="smallexample"> +<pre class="smallexample">int *ptr = <var>something</var>; +volatile int vobj; +*ptr = <var>something</var>; +asm volatile ("" : : : "memory"); +vobj = 1; +</pre></div> + +<p>A scalar volatile object is read when it is accessed in a void context: +</p> +<div class="smallexample"> +<pre class="smallexample">volatile int *src = <var>somevalue</var>; +*src; +</pre></div> + +<p>Such expressions are rvalues, and GCC implements this as a +read of the volatile object being pointed to. +</p> +<p>Assignments are also expressions and have an rvalue. However when +assigning to a scalar volatile, the volatile object is not reread, +regardless of whether the assignment expression’s rvalue is used or +not. If the assignment’s rvalue is used, the value is that assigned +to the volatile object. For instance, there is no read of <var>vobj</var> +in all the following cases: +</p> +<div class="smallexample"> +<pre class="smallexample">int obj; +volatile int vobj; +vobj = <var>something</var>; +obj = vobj = <var>something</var>; +obj ? vobj = <var>onething</var> : vobj = <var>anotherthing</var>; +obj = (<var>something</var>, vobj = <var>anotherthing</var>); +</pre></div> + +<p>If you need to read the volatile object after an assignment has +occurred, you must use a separate expression with an intervening +sequence point. +</p> +<p>As bit-fields are not individually addressable, volatile bit-fields may +be implicitly read when written to, or when adjacent bit-fields are +accessed. Bit-field operations may be optimized such that adjacent +bit-fields are only partially accessed, if they straddle a storage unit +boundary. For these reasons it is unwise to use volatile bit-fields to +access hardware. +</p> +<hr> +<div class="header"> +<p> +Next: <a href="Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C" accesskey="n" rel="next">Using Assembly Language with C</a>, Previous: <a href="Inline.html#Inline" accesskey="p" rel="previous">Inline</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |