0000090: fscanf stds contradiction - Austin Group Issue Tracker

ID	Project	Category	View Status	Date Submitted	Last Update

0000090	1003.1(2008)/Issue 7	System Interfaces	public	2009-06-29 17:38	2013-04-16 13:06

Reporter	geoffclare	Assigned To	ajosey
Priority	normal	Severity	Objection	Type	Error
Status	Closed	Resolution	Accepted As Marked

Name	Geoff Clare
Organization	The Open Group
User Reference
Section	fscanf
Page Number	934
Line Number	31304
Interp Status	Approved
Final Accepted Text	0000090:0000135


Summary	0000090: fscanf stds contradiction
Description	_____________________________________________________________________________ OBJECTION Enhancement Request Number 20 gwc:xxxxxxxxxxxxx Defect in XSH fscanf (rdvk# 5) [gwc fscanf return] Fri, 16 Jan 2009 10:55:11 +0000 _____________________________________________________________________________ In austin-group-l 11809, Vincent Lefvre identified a conflict with the C Standard: ---- begin quote ---- I think there is a contradiction between POSIX.1-2008 and the ISO C standard concerning fscanf when an input failure occurs after the first conversion. The ISO C standard (at least N1124 and N1336) says: 7.19.6.2 The fscanf function 4 The fscanf function executes each directive of the format in turn. If a directive fails, as detailed below, the function returns. Failures are described as input failures (due to the occurrence of an encoding error or the unavailability of input characters), or matching failures (due to inappropriate input). 16 The fscanf function returns the value of the macro EOF if an input failure occurs before any conversion. Otherwise, the function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure. For instance, let us consider n = fscanf (stdin, "%d %d", &x, &y); where an input failure occurs after the first %d conversion (and before the second one). According to the C standard, n should be equal to 1 because * the input failure occurred after the first conversion, * 1 input item (exactly) has been assigned. However POSIX.1-2008 says (page 934): Upon successful completion, these functions shall return the number of successfully matched and assigned input items; this number can be zero in the event of an early matching failure. If the input ends before the first matching failure or conversion, EOF shall be returned. If any error occurs, EOF shall be returned, and errno shall be set to indicate the error. If a read error occurs, the error indicator for the stream shall be set. I wonder what POSIX really means by "If any error occurs, EOF shall be returned [...]". I suppose this means "input failure" in C (as opposed to "matching failure"). But, as shown above, the C standard does the difference between these two kinds of failure only when the failure occurs before the first conversion (indeed one gets EOF in case of an input failure, and 0 in case of a matching failure). ---- end quote ---- An interpretation should be issued stating that POSIX defers to the C Standard here, and conforming implementations must behave as described in the C Standard. The POSIX text should be corrected in TC1 to remove the conflict. During discussions it emerged that, by a strict reading, the phrase "before any conversion" is ambiguous, although the intention is clear (at least, it is to native English speakers). It was proposed to use the phrase "before the first successful conversion" instead, but this has a couple of problems: 1. If the error occurs before the first conversion, then that conversion never happens and cannot be thought of as "successful". 2. Even if the word "successful" is removed, it is still not clear whether the error has to occur before reading the first byte of input that would be used in the conversion, or if the requirement also applies after at least one such byte has been read but with more bytes needed to finish the conversion. I propose to use "without any successful conversions having been made" instead.
Desired Action	Change "If the input ends before the first matching failure or conversion, EOF shall be returned. If any error occurs, EOF shall be returned, [CX]and errno shall be set to indicate the error[/CX]. If a read error occurs, the error indicator for the stream shall be set." to "If the input ends without any successful conversions having been made, and without a matching failure having occurred, EOF shall be returned. If an error occurs without any successful conversions having been made, and without a matching failure having occurred, EOF shall be returned [CX]and errno shall be set to indicate the error[/CX]. If a read error occurs, the error indicator for the stream shall be set." Make the same change to the fwscanf() page (P988 L33162).
Tags	c99, tc1-2008

msbrown 2009-06-29 17:39 manager bugnote:0000135 Last edited: 2009-10-12 05:45	Interpretation response ------------------------ The required behaviour of fscanf for this standard is dependent on the requirements of the ISO C standard and conforming implementations must behave as described in the C Standard. Concerns have been raised about the fscanf function which are being referred to the sponsor. Rationale: ------------- This is a conflict with the C Standard. Notes to the Editor (not part of this interpretation): ------------------------------------------------------- Nick has progressed a proposal with WG14 which they appear to have accepted as an editorial. We agreed that since its been classified by WG14 as an editorial we can close this item. Change from "If the input ends before the first matching failure or conversion, EOF shall be returned. If any error occurs, EOF shall be returned, [CX]and errno shall be set to indicate the error[/CX]. If a read error occurs, the error indicator for the stream shall be set." to "If the input ends before the first conversion (if any) has completed, and without a matching failure having occurred, EOF shall be returned. If an error occurs before the first conversion (if any) has completed , and without a matching failure having occurred, EOF shall be returned [CX]and errno shall be set to indicate the error[/CX]. If a read error occurs, the error indicator for the stream shall be set." Add to Change History : A change to the second sentence in RETURN VALUE is made to align with expected wording changes in the next revision of the C standard. (recommended for TC1)

nick 2010-11-05 14:26 manager bugnote:0000605	Nick has progressed a proposal with WG14 which they appear to have accepted as an editorial: this change is included in the Committee Draft of the C1x standard now entering ballot.

Date Modified	Username	Field	Change
2009-06-29 17:38	msbrown	New Issue
2009-06-29 17:38	msbrown	Status	New => Under Review
2009-06-29 17:38	msbrown	Assigned To	=> ajosey
2009-06-29 17:38	msbrown	Name	=> Mark Brown
2009-06-29 17:38	msbrown	Organization	=> IBM
2009-06-29 17:38	msbrown	Section	=> fscanf
2009-06-29 17:38	msbrown	Page Number	=> 934
2009-06-29 17:38	msbrown	Line Number	=> 31304
2009-06-29 17:39	msbrown	Note Added: 0000135
2009-06-29 17:39	msbrown	Status	Under Review => Interpretation Required
2009-06-29 17:39	msbrown	Resolution	Open => Accepted As Marked
2009-06-29 17:40	msbrown	Final Accepted Text	=> 0000090:0000135
2009-07-01 17:56	~~Don Cragun~~	Name	Mark Brown => Geoff Clare
2009-07-01 17:56	~~Don Cragun~~	Organization	IBM => The Open Group
2009-07-01 17:56	~~Don Cragun~~	Reporter	msbrown => geoffclare
2009-08-06 16:33	nick	Tag Attached: c99
2009-08-11 16:37	~~Don Cragun~~	Interp Status	=> Pending
2009-09-17 15:41	nick	Interp Status	Pending => Proposed
2009-10-12 05:45	ajosey	Note Edited: 0000135
2009-10-12 05:45	ajosey	Interp Status	Proposed => Approved
2010-09-20 09:26	geoffclare	Tag Attached: tc1-2008
2010-11-05 14:26	nick	Note Added: 0000605
2013-04-16 13:06	ajosey	Status	Interpretation Required => Closed

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