Pointer Data

A pointer value is the address of a variable's storage. For example:

DECLARE K FIXED BINARY(15); 
DECLARE A(5) FIXED BINARY(15); 
DECLARE P POINTER;
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P = ADDR(A(K));

In this example, P is a pointer variable that is capable of holding the address of any variable, with the possible exception of unaligned bit strings (for more information, see your Open PL/I User's Guide). The assignment statement uses the ADDR built-in function to calculate the address of A (K) and assigns that address as the value of P.

A pointer is used with a template to access the storage to which it points. For example:

DECLARE X FIXED BINARY(15) BASED;
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P->X = 10;

In this example, X is a based variable (that is, a template) describing a fixed-point binary integer. P is the pointer variable from the example at the beginning of this section. The assignment assigns 10 to A (K).

Pointers are often used to locate the storage of dynamically allocated based variables as described in the section Based Variables in the chapter Storage Classes, but may provide a mechanism for accessing another variable's storage, as shown in the previous two examples.

The null pointer value is produced by the NULL built-in function. It is a unique value that does not address any variable and is used to indicate that a pointer variable does not currently address anything, as shown in the following example:

DECLARE CHAIN_HEAD POINTER;
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CHAIN_HEAD = NULL();

The internal representation of NULL is determined by the -setnull compiler option. For more information on compiler options, see your Open PL/I User's Guide.

If the variable whose storage is addressed by a pointer value is freed, the pointer value should no longer be used. Use of such a pointer to access the storage causes unpredictable results.

The ADDR built-in function must not be used to calculate the address of an unaligned bit data. Consequently, when such a restriction is imposed, pointer values never address unaligned bit-string data. However, all other data can always be addressed by a pointer, regardless of the addressing capabilities of the computer.

A pointer value cannot be represented by a constant. However, the NULL built-in function can be used in any context, including an INITIAL attribute, where a constant might be desired.