Doc #76995 [Com]: fseek() breaks reading in read/append (a+) mode
Edit report at https://bugs.php.net/bug.php?id=76995&edit=1
ID: 76995
Comment by: riz dot ali0001 at gmail dot com
Reported by: miloslav dot hula at gmail dot com
Summary: fseek() breaks reading in read/append (a+) mode
Status: Open
Type: Documentation Problem
Package: Streams related
PHP Version: 7.3.0RC2
Block user comment: N
Private report: N
New Comment:
The issue highlighted in the test script relates to the use of fseek() in 'a+' mode, which
appears to disrupt content reading. This seems to stem from conflicting information in the
documentation. While the fopen() documentation indicates that fseek() can be used in 'a+'
mode to adjust the reading position (with writes always being appended), the fseek() documentation
contradicts this by stating that using fseek() in append mode ('a' or 'a+') can
lead to undefined behavior. This discrepancy suggests a need for clarification in the documentation
to ensure proper usage and avoid potential
issues in file handling
Previous Comments:
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[2023-12-10 04:21:24] sincerelyjulie0 at gmail dot com
position the file pointer for reading.
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[2023-05-13 12:02:14] cynthiamarshal0010 at gmail dot com
To address the doc bug where fseek() breaks reading in read/append (a+) mode, it's important to
understand the behavior of fseek() in that mode and how to correctly position the file pointer for
reading. Here's an explanation and solution to update the documentation:
Explanation:
In the a+ mode, when you use fseek() with a positive offset, it is expected to move the file pointer
to that offset from the beginning of the file. However, in certain implementations, fseek() may not
work as expected in a+ mode and instead positions the file pointer relative to the current end of
the file. This behavior can lead to unexpected results when trying to read from the file.
Solution:
To correctly position the file pointer for reading in a+ mode, you should use fseek() with a
negative offset relative to the end of the file. Here's an example of how to update the
documentation to reflect this:
vbnet
When using fseek() in read/append (a+) mode, it's important to note
that its behavior may vary depending on the implementation. In some cases, fseek() may
not correctly position the file pointer for reading. To ensure proper positioning, follow the
recommended approach below:
To move the file pointer to a specific position from the end of the file, use a negative offset with
fseek(). For example, to move the file pointer to the last 100 bytes of the file:
fseek(file, -100, SEEK_END);
This ensures that the file pointer is correctly positioned for subsequent read operations.
Please note that this workaround may not be necessary in all implementations of the C standard
library. It's recommended to test and verify the behavior of fseek() in your
specific environment to ensure compatibility and consistent results.
By providing this updated information, users will be informed about the potential issue with fseek()
in a+ mode and the recommended approach to correctly position the file pointer for reading.
(https://dgmeportal.live/)github.com
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[2018-10-10 16:08:23] requinix@php.net
And seconds after I hit Submit, I realize that documenting append mode's fseek quirks would be
a good idea.
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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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