Sometimes a programmer wants an application
to read or write a record,
but does not want it to wait for I/O to complete.
For such an application, waiting is wasting time
when it could be doing other
processing.
Timeouts do not adequately address this problem. The
programmer wants this
application to start an I/O, continue processing immediately, and
check periodically to see if the I/O has finished.
MPE/iX provides a way to solve this problem with NOWAIT I/O.
This feature is requested by enabling the NOWAIT I/O option
(item #16) in HPFOPEN.
When using NOWAIT I/O, the process must make
at least two intrinsic calls to
perform the I/O, one to start it and one to finish it.
MPE/iX still handles the file in the same way.
But instead of waiting
for the I/O to complete, MPE/iX returns control to the application
so that the application can do
some useful processing.
NOWAIT I/O has been available to
users of standard files for a long time,
but to use it on standard files requires Privileged Mode.
On standard files the mechanics of NOWAIT I/O prevent
MPE/iX from protecting a process from corrupting its own stack.
However,
because Message files work differently, NOWAIT I/O on Message files
does not require Privileged Mode.
