Guardian Procedure Calls Reference Manual

Interprocess Communication Considerations
• Replying to messages
Each message read in a call to READUPDATE[X|XL], including system messages, must be
replied to in a corresponding call to the REPLY[X|XL] procedure.
• Queuing several messages before replying
Several interprocess messages can be read and queued by the application process before a
reply must be made. The maximum number of messages that the application process expects
to read before a corresponding reply is made must be specified in the receive-depth
parameter to the FILE_OPEN_ or OPEN procedure.
• If $RECEIVE is opened with receive-depth = 0, only READ[X]s can be performed, and
READUPDATE[X|XL] and REPLY[X|XL] fail with error 2 ("operation not allowed on this type
of file").
• Message tags when replying to queued messages
If more than one message is to be queued by the application process (that is, receive-depth
> 1), a message tag that is associated with each incoming message must be obtained in a
call to the FILE_GETRECEIVEINFOL_ (or FILE_GETRECEIVEINFO_ or LASTRECEIVE or
RECEIVEINFO) procedure following each call to READUPDATE[X|XL]. To direct a reply back
to the originator of the message, the message tag associated with the incoming message is
passed to the system in a parameter to the REPLY[X|XL] procedure. If messages are not to be
queued, it is not necessary to call FILE_GETRECEIVEINFOL_ or FILE_GETRECEIVEINFO_.
Considerations for READUPDATEX and READUPDATEXL
• The buffer and count transferred may be in the user stack or in an extended data segment.
The buffer and count transferred cannot be in the user code space.
• If the buffer or count transferred is in a selectable extended data segment, the segment must
be in use at the time of the call. Flat segments allocated by a process are always accessible
to the process.
• The size of the transfer is subject to current restrictions for the type of file.
• If the file is opened for nowait I/O, and the buffer is in an extended data segment, you cannot
deallocate or reduce the size of the extended data segment before the I/O completes with a
call to AWAITIOX or AWAITIOXL or is canceled by a call to CANCEL or CANCELREQ or
CANCELREQL.
• If the file is opened for nowait I/O, you must not modify the buffer before the I/O completes
with a call to AWAITIOX or AWAITIOXL. This also applies to other processes that may be
sharing the segment. It is the application's responsibility to ensure this.
• If the file is opened for nowait I/O, and the I/O has been initiated with these routines, the
I/O must be completed with a call to AWAITIOX or AWAITIOXL (not AWAITIO).
• If the file is opened for nowait I/O, a selectable extended data segment containing the buffer
need not be in use at the time of the call to AWAITIOX or AWAITIOXL.
• Nowait I/O initiated with these routines may be canceled with a call to CANCEL or
CANCELREQ or CANCELREQL. The I/O is canceled if the file is closed before the I/O
completes or AWAITIOX or AWAITIOXL is called with a positive time limit and specific file
number and the request times out.
• A file opened by FILE_OPEN_ uses direct I/O transfers by default except on NSAA systems;
you can use SETMODE function 72 to force the system to use an intermediate buffer in the
READUPDATE[X|XL] Procedures 1219