Global Symbols

The GLOBALDEF attribute declares an external variable or an external file constant. You can optionally control the program section in which the data is allocated.

The format of the GLOBALDEF attribute is as follows:

GLOBALDEF [ (psect-name) ]

The GLOBALDEF attribute implies the EXTERNAL and STATIC attributes.

The following restrictions apply to the use of the GLOBALDEF attribute:

The GLOBALREF attribute indicates that the declared name is a global symbol defined in an external procedure.

The GLOBALREF attribute implies the EXTERNAL and STATIC attributes. The corresponding name must be declared in another procedure with the GLOBALDEF attribute or, if the external procedure is written in another programming language, its equivalent in that language.

The following restrictions apply to the use of the GLOBALREF attribute:

To create a global symbol definition in a PL/I program, you must declare it with the GLOBALDEF attribute in one, and only one, PL/I external procedure. The GLOBALDEF attribute implies the EXTERNAL attribute.

An external variable defined with the GLOBALDEF attribute can be accessed by external procedures that declare the name with the GLOBALREF attribute. For example, the procedure ABC contains the following lines:

ABC:  PROCEDURE;
   DECLARE UNIQUE_VALUE GLOBALDEF FIXED BINARY
       INITIAL (60);
   DECLARE XYZ EXTERNAL ENTRY (CHARACTER (*));
⋮
   CALL XYZ ('STRING');

The procedure XYZ contains the following lines:

XYZ:  PROCEDURE (STRING_VAL);
   DECLARE UNIQUE_VALUE GLOBALREF FIXED BINARY;
⋮

In these examples, the external variable UNIQUE_VALUE is declared with the GLOBALDEF attribute and initialized in the procedure ABC. The called external procedure XYZ declares this variable with the attribute GLOBALREF and the appropriate data type attributes.

Using the VAX MACRO programming language, it is possible to give a global external variable more than one name. However, in a PL/I procedure, you can use only one global symbol name for a particular variable. PL/I assumes that distinct global symbol names denote distinct storage locations; the storage associated with different names must not overlap. This rule applies only to global symbols that are declared without the VALUE attribute.

The READONLY attribute can be applied to any static variable whose value will not change during the program execution. For example, you can initialize fixed values with the PL/I attributes STATIC and INITIAL, and use the READONLY attribute as in this example:

DECLARE MSG_TEXT CHARACTER(80) STATIC READONLY
                  INITIAL ('Good morning');

This use of the READONLY attribute provides storage optimization and protects variables from inadvertent modification.

A variable declared with the VALUE attribute does not require an address reference in storage; instead, the compiler can refer to it by value during execution.

When you give a variable the VALUE attribute, you must specify either GLOBALDEF or GLOBALREF. If you specify GLOBALDEF, you must use the INITIAL attribute to define a value for the variable.

For example, the VALUE attribute can be specified in the declaration of an external global symbol, as follows:

DECLARE REQUEST_CODE GLOBALDEF VALUE FIXED BINARY
   STATIC INITIAL (10);

The variable REQUEST_CODE in this example can be accessed in any external procedure that declares it with the attribute GLOBALREF.

When the VALUE attribute is used with the GLOBALDEF or GLOBALREF attributes, the following rules apply:

A variable declared with the VALUE attribute can be specified as a value to initialize another variable; it must have the same data type as the variable that is being initialized. For example:

DECLARE TEMP GLOBALDEF FIXED VALUE INITIAL(10),
        ABC FIXED STATIC INIT(TEMP);

The declaration of ABC in this example gives ABC the value 10.