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215 lines
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<!-- Copyright (C) 1999-2016 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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(see below). A copy of the license is included in the section entitled
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>The GNU Fortran Compiler: Cray pointers</title>
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<meta name="description" content="The GNU Fortran Compiler: Cray pointers">
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<meta name="keywords" content="The GNU Fortran Compiler: Cray pointers">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Extensions-implemented-in-GNU-Fortran.html#Extensions-implemented-in-GNU-Fortran" rel="up" title="Extensions implemented in GNU Fortran">
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<link href="CONVERT-specifier.html#CONVERT-specifier" rel="next" title="CONVERT specifier">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Cray-pointers"></a>
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<div class="header">
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<p>
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Next: <a href="CONVERT-specifier.html#CONVERT-specifier" accesskey="n" rel="next">CONVERT specifier</a>, Previous: <a href="Hollerith-constants-support.html#Hollerith-constants-support" accesskey="p" rel="prev">Hollerith constants support</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>
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</div>
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<hr>
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<a name="Cray-pointers-1"></a>
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<h4 class="subsection">6.1.14 Cray pointers</h4>
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<a name="index-pointer_002c-Cray"></a>
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<p>Cray pointers are part of a non-standard extension that provides a
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C-like pointer in Fortran. This is accomplished through a pair of
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variables: an integer "pointer" that holds a memory address, and a
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"pointee" that is used to dereference the pointer.
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</p>
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<p>Pointer/pointee pairs are declared in statements of the form:
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</p><div class="smallexample">
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<pre class="smallexample"> pointer ( <pointer> , <pointee> )
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</pre></div>
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<p>or,
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</p><div class="smallexample">
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<pre class="smallexample"> pointer ( <pointer1> , <pointee1> ), ( <pointer2> , <pointee2> ), ...
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</pre></div>
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<p>The pointer is an integer that is intended to hold a memory address.
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The pointee may be an array or scalar. A pointee can be an assumed
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size array—that is, the last dimension may be left unspecified by
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using a <code>*</code> in place of a value—but a pointee cannot be an
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assumed shape array. No space is allocated for the pointee.
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</p>
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<p>The pointee may have its type declared before or after the pointer
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statement, and its array specification (if any) may be declared
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before, during, or after the pointer statement. The pointer may be
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declared as an integer prior to the pointer statement. However, some
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machines have default integer sizes that are different than the size
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of a pointer, and so the following code is not portable:
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</p><div class="smallexample">
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<pre class="smallexample"> integer ipt
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pointer (ipt, iarr)
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</pre></div>
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<p>If a pointer is declared with a kind that is too small, the compiler
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will issue a warning; the resulting binary will probably not work
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correctly, because the memory addresses stored in the pointers may be
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truncated. It is safer to omit the first line of the above example;
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if explicit declaration of ipt’s type is omitted, then the compiler
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will ensure that ipt is an integer variable large enough to hold a
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pointer.
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</p>
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<p>Pointer arithmetic is valid with Cray pointers, but it is not the same
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as C pointer arithmetic. Cray pointers are just ordinary integers, so
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the user is responsible for determining how many bytes to add to a
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pointer in order to increment it. Consider the following example:
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</p><div class="smallexample">
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<pre class="smallexample"> real target(10)
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real pointee(10)
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pointer (ipt, pointee)
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ipt = loc (target)
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ipt = ipt + 1
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</pre></div>
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<p>The last statement does not set <code>ipt</code> to the address of
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<code>target(1)</code>, as it would in C pointer arithmetic. Adding <code>1</code>
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to <code>ipt</code> just adds one byte to the address stored in <code>ipt</code>.
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</p>
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<p>Any expression involving the pointee will be translated to use the
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value stored in the pointer as the base address.
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</p>
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<p>To get the address of elements, this extension provides an intrinsic
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function <code>LOC()</code>. The <code>LOC()</code> function is equivalent to the
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<code>&</code> operator in C, except the address is cast to an integer type:
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</p><div class="smallexample">
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<pre class="smallexample"> real ar(10)
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pointer(ipt, arpte(10))
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real arpte
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ipt = loc(ar) ! Makes arpte is an alias for ar
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arpte(1) = 1.0 ! Sets ar(1) to 1.0
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</pre></div>
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<p>The pointer can also be set by a call to the <code>MALLOC</code> intrinsic
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(see <a href="MALLOC.html#MALLOC">MALLOC</a>).
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</p>
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<p>Cray pointees often are used to alias an existing variable. For
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example:
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</p><div class="smallexample">
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<pre class="smallexample"> integer target(10)
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integer iarr(10)
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pointer (ipt, iarr)
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ipt = loc(target)
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</pre></div>
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<p>As long as <code>ipt</code> remains unchanged, <code>iarr</code> is now an alias for
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<code>target</code>. The optimizer, however, will not detect this aliasing, so
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it is unsafe to use <code>iarr</code> and <code>target</code> simultaneously. Using
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a pointee in any way that violates the Fortran aliasing rules or
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assumptions is illegal. It is the user’s responsibility to avoid doing
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this; the compiler works under the assumption that no such aliasing
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occurs.
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</p>
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<p>Cray pointers will work correctly when there is no aliasing (i.e., when
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they are used to access a dynamically allocated block of memory), and
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also in any routine where a pointee is used, but any variable with which
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it shares storage is not used. Code that violates these rules may not
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run as the user intends. This is not a bug in the optimizer; any code
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that violates the aliasing rules is illegal. (Note that this is not
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unique to GNU Fortran; any Fortran compiler that supports Cray pointers
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will “incorrectly” optimize code with illegal aliasing.)
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</p>
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<p>There are a number of restrictions on the attributes that can be applied
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to Cray pointers and pointees. Pointees may not have the
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<code>ALLOCATABLE</code>, <code>INTENT</code>, <code>OPTIONAL</code>, <code>DUMMY</code>,
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<code>TARGET</code>, <code>INTRINSIC</code>, or <code>POINTER</code> attributes. Pointers
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may not have the <code>DIMENSION</code>, <code>POINTER</code>, <code>TARGET</code>,
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<code>ALLOCATABLE</code>, <code>EXTERNAL</code>, or <code>INTRINSIC</code> attributes, nor
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may they be function results. Pointees may not occur in more than one
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pointer statement. A pointee cannot be a pointer. Pointees cannot occur
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in equivalence, common, or data statements.
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</p>
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<p>A Cray pointer may also point to a function or a subroutine. For
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example, the following excerpt is valid:
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</p><div class="smallexample">
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<pre class="smallexample"> implicit none
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external sub
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pointer (subptr,subpte)
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external subpte
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subptr = loc(sub)
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call subpte()
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[...]
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subroutine sub
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[...]
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end subroutine sub
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</pre></div>
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<p>A pointer may be modified during the course of a program, and this
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will change the location to which the pointee refers. However, when
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pointees are passed as arguments, they are treated as ordinary
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variables in the invoked function. Subsequent changes to the pointer
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will not change the base address of the array that was passed.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="CONVERT-specifier.html#CONVERT-specifier" accesskey="n" rel="next">CONVERT specifier</a>, Previous: <a href="Hollerith-constants-support.html#Hollerith-constants-support" accesskey="p" rel="prev">Hollerith constants support</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>
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