PACK Directive

General Compiler Directive: Specifies the memory starting addresses of derived-type items (and record structure items).

Syntax

cDEC$ PACK[: [{1 | 2 | 4 | 8}] ]

c
Is one of the following: C (or c), !, or *. (See Syntax Rules for Compiler Directives.)

Description

Items of derived types, unions, and structures are aligned in memory on the smaller of two sizes: the size of the type of the item, or the current alignment setting. The current alignment setting can be 1, 2, 4, or 8 bytes. The default initial setting is 8 bytes (unless compiler option vms or align recnbytes is specified). By reducing the alignment setting, you can pack variables closer together in memory.

The PACK directive lets you control the packing of derived-type or record structure items inside your program by overriding the current memory alignment setting.

For example, if PACK:1 is specified, all variables begin at the next available byte, whether odd or even. Although this slightly increases access time, no memory space is wasted. If PACK:4 is specified, INTEGER(1), LOGICAL(1), and all character variables begin at the next available byte, whether odd or even. INTEGER(2) and LOGICAL(2) begin on the next even byte; all other variables begin on 4-byte boundaries.

If the PACK directive is specified without a number, packing reverts to the compiler option setting (if any), or the default setting of 8.

The directive can appear anywhere in a program before the derived-type definition or record structure definition. It cannot appear inside a derived-type or record structure definition.

See Also

TYPE, STRUCTURE...END STRUCTURE, UNION...END UNION, General Compiler Directives, Building Applications: Compiler Directives Related to Options, align recnbytes compiler option, vms compiler option

Example

 ! Use 4-byte packing for this derived type
 ! Note PACK is used outside of the derived type definition

 !DEC$ PACK:4
 TYPE pair
    INTEGER a, b
 END TYPE
 ! revert to default or compiler option
 !DEC$ PACK