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<title>The GNU Fortran Compiler: Cray pointers</title>

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<a name="Cray-pointers"></a>
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<a name="Cray-pointers-1"></a>
<h4 class="subsection">6.1.14 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 &quot;pointer&quot; that holds a memory address, and a
&quot;pointee&quot; 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 ( &lt;pointer&gt; , &lt;pointee&gt; )
</pre></div>
<p>or,
</p><div class="smallexample">
<pre class="smallexample">        pointer ( &lt;pointer1&gt; , &lt;pointee1&gt; ), ( &lt;pointer2&gt; , &lt;pointee2&gt; ), ...
</pre></div>
<p>The pointer is an integer that is intended to hold a memory address.
The pointee may be an array or scalar.  A pointee can be an assumed
size array&mdash;that is, the last dimension may be left unspecified by
using a <code>*</code> in place of a value&mdash;but 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&rsquo;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>&amp;</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&rsquo;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 &ldquo;incorrectly&rdquo; 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>
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