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| author | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
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| committer | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
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diff --git a/share/doc/gfortran/Cray-pointers.html b/share/doc/gfortran/Cray-pointers.html new file mode 100644 index 0000000..34e1ecb --- /dev/null +++ b/share/doc/gfortran/Cray-pointers.html @@ -0,0 +1,216 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1999-2023 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>The GNU Fortran Compiler: Cray pointers</title> + +<meta name="description" content="The GNU Fortran Compiler: Cray pointers"> +<meta name="keywords" content="The GNU Fortran Compiler: Cray pointers"> +<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="Option-Index.html#Option-Index" rel="index" title="Option Index"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="Extensions-implemented-in-GNU-Fortran.html#Extensions-implemented-in-GNU-Fortran" rel="up" title="Extensions implemented in GNU Fortran"> +<link href="CONVERT-specifier.html#CONVERT-specifier" rel="next" title="CONVERT specifier"> +<link href="Character-conversion.html#Character-conversion" rel="previous" title="Character conversion"> +<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" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Cray-pointers"></a> +<div class="header"> +<p> +Next: <a href="CONVERT-specifier.html#CONVERT-specifier" accesskey="n" rel="next">CONVERT specifier</a>, Previous: <a href="Character-conversion.html#Character-conversion" accesskey="p" rel="previous">Character conversion</a>, Up: <a href="Extensions-implemented-in-GNU-Fortran.html#Extensions-implemented-in-GNU-Fortran" accesskey="u" rel="up">Extensions implemented in GNU Fortran</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="Cray-pointers-1"></a> +<h4 class="subsection">5.1.16 Cray pointers</h4> +<a name="index-pointer_002c-Cray"></a> + +<p>Cray pointers are part of a non-standard extension that provides a +C-like pointer in Fortran. This is accomplished through a pair of +variables: an integer "pointer" that holds a memory address, and a +"pointee" that is used to dereference the pointer. +</p> +<p>Pointer/pointee pairs are declared in statements of the form: +</p><div class="smallexample"> +<pre class="smallexample"> pointer ( <pointer> , <pointee> ) +</pre></div> +<p>or, +</p><div class="smallexample"> +<pre class="smallexample"> pointer ( <pointer1> , <pointee1> ), ( <pointer2> , <pointee2> ), ... +</pre></div> +<p>The pointer is an integer that is intended to hold a memory address. +The pointee may be an array or scalar. +If an assumed-size array is permitted within the scoping unit, a +pointee can be an assumed-size array. +That is, the last dimension may be left unspecified by using a <code>*</code> +in place of a value. A pointee cannot be an assumed shape array. +No space is allocated for the pointee. +</p> +<p>The pointee may have its type declared before or after the pointer +statement, and its array specification (if any) may be declared +before, during, or after the pointer statement. The pointer may be +declared as an integer prior to the pointer statement. However, some +machines have default integer sizes that are different than the size +of a pointer, and so the following code is not portable: +</p><div class="smallexample"> +<pre class="smallexample"> integer ipt + pointer (ipt, iarr) +</pre></div> +<p>If a pointer is declared with a kind that is too small, the compiler +will issue a warning; the resulting binary will probably not work +correctly, because the memory addresses stored in the pointers may be +truncated. It is safer to omit the first line of the above example; +if explicit declaration of ipt’s type is omitted, then the compiler +will ensure that ipt is an integer variable large enough to hold a +pointer. +</p> +<p>Pointer arithmetic is valid with Cray pointers, but it is not the same +as C pointer arithmetic. Cray pointers are just ordinary integers, so +the user is responsible for determining how many bytes to add to a +pointer in order to increment it. Consider the following example: +</p><div class="smallexample"> +<pre class="smallexample"> real target(10) + real pointee(10) + pointer (ipt, pointee) + ipt = loc (target) + ipt = ipt + 1 +</pre></div> +<p>The last statement does not set <code>ipt</code> to the address of +<code>target(1)</code>, as it would in C pointer arithmetic. Adding <code>1</code> +to <code>ipt</code> just adds one byte to the address stored in <code>ipt</code>. +</p> +<p>Any expression involving the pointee will be translated to use the +value stored in the pointer as the base address. +</p> +<p>To get the address of elements, this extension provides an intrinsic +function <code>LOC()</code>. The <code>LOC()</code> function is equivalent to the +<code>&</code> operator in C, except the address is cast to an integer type: +</p><div class="smallexample"> +<pre class="smallexample"> real ar(10) + pointer(ipt, arpte(10)) + real arpte + ipt = loc(ar) ! Makes arpte is an alias for ar + arpte(1) = 1.0 ! Sets ar(1) to 1.0 +</pre></div> +<p>The pointer can also be set by a call to the <code>MALLOC</code> intrinsic +(see <a href="MALLOC.html#MALLOC">MALLOC</a>). +</p> +<p>Cray pointees often are used to alias an existing variable. For +example: +</p><div class="smallexample"> +<pre class="smallexample"> integer target(10) + integer iarr(10) + pointer (ipt, iarr) + ipt = loc(target) +</pre></div> +<p>As long as <code>ipt</code> remains unchanged, <code>iarr</code> is now an alias for +<code>target</code>. The optimizer, however, will not detect this aliasing, so +it is unsafe to use <code>iarr</code> and <code>target</code> simultaneously. Using +a pointee in any way that violates the Fortran aliasing rules or +assumptions is illegal. It is the user’s responsibility to avoid doing +this; the compiler works under the assumption that no such aliasing +occurs. +</p> +<p>Cray pointers will work correctly when there is no aliasing (i.e., when +they are used to access a dynamically allocated block of memory), and +also in any routine where a pointee is used, but any variable with which +it shares storage is not used. Code that violates these rules may not +run as the user intends. This is not a bug in the optimizer; any code +that violates the aliasing rules is illegal. (Note that this is not +unique to GNU Fortran; any Fortran compiler that supports Cray pointers +will “incorrectly” optimize code with illegal aliasing.) +</p> +<p>There are a number of restrictions on the attributes that can be applied +to Cray pointers and pointees. Pointees may not have the +<code>ALLOCATABLE</code>, <code>INTENT</code>, <code>OPTIONAL</code>, <code>DUMMY</code>, +<code>TARGET</code>, <code>INTRINSIC</code>, or <code>POINTER</code> attributes. Pointers +may not have the <code>DIMENSION</code>, <code>POINTER</code>, <code>TARGET</code>, +<code>ALLOCATABLE</code>, <code>EXTERNAL</code>, or <code>INTRINSIC</code> attributes, nor +may they be function results. Pointees may not occur in more than one +pointer statement. A pointee cannot be a pointer. Pointees cannot occur +in equivalence, common, or data statements. +</p> +<p>A Cray pointer may also point to a function or a subroutine. For +example, the following excerpt is valid: +</p><div class="smallexample"> +<pre class="smallexample"> implicit none + external sub + pointer (subptr,subpte) + external subpte + subptr = loc(sub) + call subpte() + [...] + subroutine sub + [...] + end subroutine sub +</pre></div> + +<p>A pointer may be modified during the course of a program, and this +will change the location to which the pointee refers. However, when +pointees are passed as arguments, they are treated as ordinary +variables in the invoked function. Subsequent changes to the pointer +will not change the base address of the array that was passed. +</p> +<hr> +<div class="header"> +<p> +Next: <a href="CONVERT-specifier.html#CONVERT-specifier" accesskey="n" rel="next">CONVERT specifier</a>, Previous: <a href="Character-conversion.html#Character-conversion" accesskey="p" rel="previous">Character conversion</a>, Up: <a href="Extensions-implemented-in-GNU-Fortran.html#Extensions-implemented-in-GNU-Fortran" accesskey="u" rel="up">Extensions implemented in GNU Fortran</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |