IEEE Standards AssociationCopyright © 2006 IEEE. All rights reserved.
Topic: interaction of file descriptors and I/O streams
Relevant Sections: 2.5.1
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7 Defect Report concerning (number and title of International Standard
or DIS final text, if applicable):
The System Interfaces Volume of IEEE Std 1003.1-2001
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8 Qualifier (e.g. error, omission, clarification required):
1: Error
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9 References in document (e.g. page, clause, figure, and/or table
numbers):
Page: 36 Line: 1472 Section: 2.5.1
XSH Enhancement Request Number 4
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10 Nature of defect (complete, concise explanation of the perceived
problem):
Defect code : 1. Error
This could be filed as an error in this section or an ommision in
fflush().
The problem is that in section 2.5.1 of XSH, which deals with the
interaction of file descriptors and I/O streams, it is written:
If the stream is open with a mode that allows reading and the
underlying open file description refers to a device that is capable of
seeking, the application shall either perform an fflush( ), or the
stream shall be closed.
The man page for fflush() explicitly does not mention what happens if
fflush() is applied to an input stream or an update stream with the last
operation being input. This wording comes straight from ISO C.
The problem is that ISO C does not have to deal with file descriptors.
POSIX does. In the early POSIX versions the text in section 2.5.1 was
added to make clear how the interaction works. But the fflush()
documentation was never updated.
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11 Solution proposed by the submitter (optional):
There are two possible options:
- the original intend was to define fflush() on input streams and it
was just forgotten to update the fflush() documentation. In this
case the page should be updated with the following in line 11607
(CX shaded):
If /stream/ points to an input stream or an update stream in which
the most recent operation was not input, fflush() shall cause
all buffered input to be discarded and on seekable files the file
offset for the file descriptor be reset to the first unprocessed
byte.
- if fflush() is not meant or cannot be changed some other mechanism
for synchronizing input streams must be found. As far as I can see
no existing function fits the bill so I cannot make any concrete
proposal.
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Interpretation response
------------------------
The standards states the requirements for fflush() and interaction
between file descriptors in section 2.5.1 . However, concerns have been raised
about this which are being referred to the sponsor.
Rationale
-------------
The standard developers noted that action is a future revision
should be to restore the wording to that of the
original 1003.1-1988 standard in fflush() and fclose().
Notes to the Editor (not part of this interpretation):
-----------------------------------------------------------
For a future revision:
Add to fflush() as a new para at the end of DESCRIPTION:
[CX shading on]
For a stream open for reading, if the file is not already at EOF, and the
file is one capable of seeking, the file offset of the underlying open file
description shall be adjusted so that the next operation on the open file
description deals with the byte after the last one read from, or written to,
the stream being flushed.
[CX shading off]
Add to fclose() as a new third paragraph in DESCRIPTION
[CX shading on]
If the file is not already at EOF, and the file is one capable of
seeking, the file offset of the underlying open file description
shall be adjusted so that the next operation on the open
file description deals with the byte after the last one read from
or written to the stream being closed.
[CX shading off]
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