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.Last update: 18 March,1997
1003.1b-93 #12
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Interpretation Number: XXXX
Topic: messages
Relevant Sections: 2.2.2.65, 2.2.2.66
Interpretation Request:
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From: Andrew Josey (on behalf of the X/Open Base Working group)
Part 1. Interpretation Request for:
IEEE Std 1003.1b-1993 as contained in:
ISO/IEC 9945-1:1996(E), ANSI/IEEE Std. 1003.1, 1996 Edition.
Subsection 2.2.2.65, Page 20, Line 298.
Question:
The sentence "A message consists of a fixed-size message buffer." seems
contradictory to the description of a message presented in subsections 15.2.4
and 15.2.5 (Interfaces to send and receive a message).
When a message is sent, the size of the message buffer is not fixed,
but is user specified for each message sent. When a message is received,
the size of the buffer into which the message is to be fetched is
effectively fixed by the mq_msgsize attribute of the message queue
(although the user may specify a larger buffer, with no
apparent benefit), but each message so received is of variable size, with the
size returned by mq_receive().
Therefore, can it be that a message "consists" of a fixed-size message buffer?
Suggested Correction:
Change the sentence to "The size of a message is variable but limited to a
fixed maximum size established at the time the associated message queue is
created.
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Part 2. Interpretation Request for:
IEEE Std 1003.1b-1993 as contained in:
ISO/IEC 9945-1:1996(E), ANSI/IEEE Std. 1003.1, 1996 Edition.
Subsection 2.2.2.66, Page 20, Line 301-302.
Question:
The sentence "Messages may be removed in the order in which they were added or
in priority order." implies (because of the "or") that it is the user's option
as to how messages are removed, independent of how they were queued. This is
contradictory to the way messages are described as queued in section 15.2.4 and
received in section 15.2.5. Both of these subsections are clear that messages
are queued FIFO within priority, and may be removed only in the order found in
the queue (i.e. oldest of the highest priority messages first). There is no
option to remove messages in the order they were added if they were queued with
different priorities.
Therefore, is the sentence quoted above technically correct?
Suggested Correction:
Change the sentence to "When all messages in a message queue are of the same
priority, messages are removed from the queue in the order in which they were
added to the queue. When messages in a message queue are of different
priorities, messages are removed in priority order, first-in-first-out (FIFO)
within the same priority.
Interpretation response
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The standard is very clear in section 15.2.4 lines 255-260 on the
precise ordering of the messages. The definition allows this particular
behavior and notes that the precise characteristics are determined by
the interface and characteristics of the queue. It is the view of the
interpretations committee that no further clarification is needed.
Rationale
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The definition is clear enough without going into the details of the
precise order that is the current interface. Since additional
interfaces may be defined in the future, it is not viewed as advisable
to unnecessarily constrain the definition.
Forwarded to Interpretations group: Dec 9 1996
Finalised: 26th Feb 1997