Edit report at https://bugs.php.net/bug.php?id=76995&edit=1
ID: 76995
Updated by: requinix@php.net
Reported by: miloslav dot hula at gmail dot com
Summary: fseek() breaks reading in read/append (a+) mode
-Status: Not a bug
+Status: Open
-Type: Bug
+Type: Documentation Problem
Package: Streams related
PHP Version: 7.3.0RC2
Block user comment: N
Private report: N
New Comment:
And seconds after I hit Submit, I realize that documenting append mode's fseek quirks would be
a good idea.
Previous Comments:
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[2018-10-10 16:07:12] requinix@php.net
Unfortunately this really is one of those "undefined behavior" situations, and PHP
isn't the one responsible for it.
Look at the fopen(3) docs on Linux: https://linux.die.net/man/3/fopen
> Reads and writes may be intermixed on read/write streams in any order. Note
> that ANSI C requires that a file positioning function intervene between output
> and input, unless an input operation encounters end-of-file. (If this condition
> is not met, then a read is allowed to return the result of writes other than
> the most recent.) Therefore it is good practice (and indeed sometimes necessary
> under Linux) to put an fseek(3) or fgetpos(3) operation between write and read
> operations on such a stream. This operation may be an apparent no-op (as in
> fseek(..., 0L, SEEK_CUR) called for its synchronizing side effect.
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[2018-10-10 15:11:26] miloslav dot hula at gmail dot com
Or: "In this mode, fseek() only affects the reading position". Sound like positive claim.
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[2018-10-10 15:07:02] miloslav dot hula at gmail dot com
At least, documentation is misleading. "a+ mode is for reading/append". How can one read
from file, when cannot seek?
Could be clearer with: "only appended data can be read, seeking not supported"
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[2018-10-10 15:03:30] spam2 at rhsoft dot net
but what is the bug?
> seeking in a+ mode is bad idea
> and the result of calling fseek() will be undefined
undefined is what it is - undfined - therw is no right/wrong for undefined
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[2018-10-10 14:58:08] miloslav dot hula at gmail dot com
Description:
------------
URL in a test script shows, how fseek() in 'a+' mode breaks contents reading.
Documentation of fopen() says, that fseek() is OK, but documentation of fseek() says, that seeking
in a+ mode is bad idea.
In documentation of fopen():
a+ - Open for reading and writing; place the file pointer at the end of the file. If the file does
not exist, attempt to create it. In this mode, fseek() only affects the reading position, writes are
always appended.
In documentation of fseek():
If you have opened the file in append (a or a+) mode, any data you write to the file will always be
appended, regardless of the file position, and the result of calling fseek() will be undefined.
Test script:
---------------
https://3v4l.org/gTXsR
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Edit this bug report at https://bugs.php.net/bug.php?id=76995&edit=1