# 5.19 VOLATILE Attribute and Statement

The VOLATILE attribute specifies that the value of an object is entirely unpredictable, based on information local to the current program unit. It prevents objects from being optimized during compilation.

The VOLATILE attribute can be specified in a type declaration statement or a VOLATILE statement, and takes one of the following forms:

Type Declaration Statement:

  type, [att-ls,] VOLATILE [, att-ls] :: object [, object]...

Statement:

  VOLATILE object [, object]...
  type
  Is a data type specifier.
  att-ls
  Is an optional list of attribute specifiers.
  object
  Is the name of an object, or the name of a common block enclosed in slashes.

Rules and Behavior

A variable or COMMON block must be declared VOLATILE if it can be read or written in a way that is not visible to the compiler. For example:

- If an operating system feature is used to place a variable in shared memory (so that it can be accessed by other programs), the variable must be declared VOLATILE.
- If a variable is accessed or modified by a routine called by the operating system when an asynchronous event occurs, the variable must be declared VOLATILE.

If an array is declared VOLATILE, each element in the array becomes volatile. If a common block is declared VOLATILE, each variable in the common block becomes volatile.

If an object of derived type is declared VOLATILE, its components become volatile.

If a pointer is declared VOLATILE, the pointer itself becomes volatile.

A VOLATILE statement cannot specify the following:

- A procedure
- A function result
- A namelist group

Examples

The following example shows a type declaration statement specifying the VOLATILE attribute:

    INTEGER, VOLATILE :: D, E

The following example shows a VOLATILE statement:

    PROGRAM TEST
    LOGICAL(1) IPI(4)
    INTEGER(4) A, B, C, D, E, ILOOK
    INTEGER(4) P1, P2, P3, P4
    COMMON /BLK1/A, B, C

    VOLATILE /BLK1/, D, E
    EQUIVALENCE(ILOOK, IPI)
    EQUIVALENCE(A, P1)
    EQUIVALENCE(P1, P4)

The named common block, BLK1, and the variables D and E are volatile. Variables P1 and P4 become volatile because of the direct equivalence of P1 and the indirect equivalence of P4.

For More Information:

- On type declaration statements, see Section 5.1.
- On compatible attributes, see Table 5-1.
- On optimizations performed by the compiler, see your user manual or programmer's guide.

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