# Asynchronous Specifier (ASYNCHRONOUS=)

The asynchronous specifier determines whether asynchronous I/O occurs for a data transfer statement. It takes the following form:

ASYNCHRONOUS=i-expr

i-expr | Is a scalar character constant expression that evaluates to 'YES' for asynchronous I/O or 'NO' for synchronous I/O. The value 'YES' should not appear unless the data transfer statement specifies a file unit number for io-unit. The default value is 'NO'. Trailing blanks in the expression are ignored. The values specified are without regard to case.

Asynchronous I/O is permitted only for external files opened with an OPEN statement that specifies ASYNCHRONOUS='YES'.

When an asynchronous I/O statement is executed, the pending I/O storage sequence for the data transfer operation is defined to be:

- The set of storage units specified by the I/O item list or by the NML= specifier

- The storage units specified by the SIZE= specifier

Parent topic: I/O Control List