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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 "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. 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—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’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> |
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