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ID Category Severity Type Date Submitted Last Update
0000468 [1003.1(2008)/Issue 7] System Interfaces Editorial Clarification Requested 2011-06-29 20:34 2019-06-10 08:55
Reporter mscudder View Status public  
Assigned To ajosey
Priority normal Resolution Accepted As Marked  
Status Closed  
Name Mike Scudder
Organization MicroTools, Inc.
User Reference fgets() Description
Section fgets() Description
Page Number 852
Line Number 28283-28285
Interp Status Approved
Final Accepted Text Note: 0000871
Summary 0000468: fgets() description misleading in case of immediate EOF
Description The description says:
----
The fgets() function shall read bytes from stream into the array pointed to by s, until n-1 bytes are read, or a <newline> is read and transferred to s, or an end-of-file condition is encountered. The string is then terminated with a null byte.
----

This implies that the passed in array has a null byte placed in its first position if an end-of-file condition is immediately encountered.

However, the standard defers to the ISO C standard, which conflicts with this implication:
----
2. The fgets function reads at most one less than the number of characters specified by n from the stream pointed to by stream into the array pointed to by s. No additional characters are read after a new-line character (which is retained) or after end-of-file. A null character is written immediately after the last character read into the array.

Returns

3. The fgets function returns s if successful. If end-of-file is encountered and no characters have been read into the array, the contents of the array remain unchanged and a null pointer is returned.
----

since C99 explicitly requires that such a null character not be written into the array.
Desired Action Update the description to say:
----
he fgets() function shall read bytes from stream into the array pointed to by s, until n-1 bytes are read, or a <newline> is read and transferred to s, or an end-of-file condition is encountered. If any bytes are read, the string is then terminated with a null byte. However, if the end-of-file condition is encountered before any bytes are read, the contents of the array pointed to by s are unchanged.
----
Tags c99, tc2-2008
Attached Files

- Relationships
related to 0000316Closedajosey 1003.1(2004)/Issue 6 fprintf EOVERFLOW 

-  Notes
(0000870)
geoffclare (manager)
2011-06-30 09:22

 I find the reference to "the string" a little problematic; it would be
better to phrase that part like the C Standard. Also the new text
needs to use "shall" in some places.

Change:

    The fgets() function shall read bytes from stream into the array
    pointed to by s, until n-1 bytes are read, or a <newline> is read
    and transferred to s, or an end-of-file condition is encountered.
    The string is then terminated with a null byte.

to:

    The fgets() function shall read bytes from stream into the array
    pointed to by s until n-1 bytes are read, or a <newline> is read
    and transferred to s, or an end-of-file condition is encountered.
    A null byte shall be written immediately after the last byte read
    into the array. If the end-of-file condition is encountered
    before any bytes are read, the contents of the array pointed to by
    s shall not be changed.
(0000871)
msbrown (manager)
2011-06-30 15:38
edited on: 2011-06-30 15:41

 Interpretation response
------------------------
The standard states behavior for fgets requiring the null byte operation, and conforming implementations must conform to this. However, concerns have been raised about this which are being referred to the sponsor.

Rationale:
-------------
Conflicts with the C99 standard. All current implementations are thought to implement this as C99 requires.

Notes to the Editor (not part of this interpretation):
-------------------------------------------------------
Use the action recommended in Note: 0000870 .
(0000877)
mscudder (reporter)
2011-06-30 19:24

 Does it make sense to also include this sentence from the C99 spec?
----
If a read error occurs during the operation, the array contents are indeterminate and a null pointer is returned.
----
(0000881)
eblake (manager)
2011-07-01 18:17

 We do not need to go out of our way to document that an array might
be partially filled on error, thanks to 0000316, which added the
following sentence to the overriding documentation in XSH 2.3 Error
Numbers:
If an error condition is detected, the action requested may have
been partially performed, unless otherwise stated.


Therefore, the resolution in Note: 0000871 is still sufficient to
cover the conflict with C99, without having to incorporate any
changes from Note: 0000877.
(0000928)
ajosey (manager)
2011-08-10 15:22

 Interpretation proposed 10 August 2011 for final 30 day review
(0000960)
ajosey (manager)
2011-09-12 15:22

 Interpretation approved 12 Sept 2011

- Issue History
Date Modified Username Field Change
2011-06-29 20:34 mscudder New Issue
2011-06-29 20:34 mscudder Status New => Under Review
2011-06-29 20:34 mscudder Assigned To => ajosey
2011-06-29 20:34 mscudder Name => Mike Scudder
2011-06-29 20:34 mscudder Organization => MicroTools, Inc.
2011-06-29 20:34 mscudder User Reference => fgets() Description
2011-06-29 20:34 mscudder Section => fgets() Description
2011-06-29 20:34 mscudder Page Number => 852
2011-06-29 20:34 mscudder Line Number => ?
2011-06-30 09:22 geoffclare Note Added: 0000870
2011-06-30 15:07 nick Tag Attached: c99
2011-06-30 15:38 msbrown Line Number ? => 28283-28285
2011-06-30 15:38 msbrown Interp Status => Pending
2011-06-30 15:38 msbrown Final Accepted Text => Note: 0000870
2011-06-30 15:38 msbrown Note Added: 0000871
2011-06-30 15:38 msbrown Status Under Review => Interpretation Required
2011-06-30 15:38 msbrown Resolution Open => Accepted As Marked
2011-06-30 15:39 msbrown Final Accepted Text Note: 0000870 => Note: 0000871
2011-06-30 15:39 msbrown Tag Attached: tc2-2008
2011-06-30 15:41 msbrown Note Edited: 0000871
2011-06-30 19:24 mscudder Note Added: 0000877
2011-07-01 18:17 eblake Note Added: 0000881
2011-07-01 18:18 eblake Relationship added related to 0000316
2011-08-10 15:22 ajosey Interp Status Pending => Proposed
2011-08-10 15:22 ajosey Note Added: 0000928
2011-09-12 15:22 ajosey Interp Status Proposed => Approved
2011-09-12 15:22 ajosey Note Added: 0000960
2019-06-10 08:55 agadmin Status Interpretation Required => Closed

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