[ruby-core:108167] [Ruby master Bug#18676] Misleading Documentation/Bug: Incorrect File.position on a file opened with "a" mode
From:
"jeremyevans0 (Jeremy Evans)" <noreply@...>
Date:
2022-04-04 01:13:40 UTC
List:
ruby-core #108167
Issue #18676 has been updated by jeremyevans0 (Jeremy Evans).
apatniv (Vivek Ak) wrote in #note-2:
> Hi Jeremy. Thanks you for your response. Should we update the documentation to indicate the same so that it help anyone who encounters the same issue?
I don't think doing so is a good idea. This is not Ruby-specific behavior, this is operating system behavior. If you run an operating system that sets the default file position for files opened in append mode to the end of the file, then that's the position that Ruby would report.
----------------------------------------
Bug #18676: Misleading Documentation/Bug: Incorrect File.position on a file opened with "a" mode
https://bugs.ruby-lang.org/issues/18676#change-97141
* Author: apatniv (Vivek Ak)
* Status: Rejected
* Priority: Normal
* ruby -v: ruby 2.7.0p0 (2019-12-25 revision 647ee6f091) [x86_64-linux-gnu]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
Documentation says that whenever a file is opened in "a" mode, the stream is positioned at the end of stream. However, #tell method returns 0 instead of the size of the file. (https://ruby-doc.org/core-3.1.1/IO.html#class-IO-label-Position)
Issue happens with latest version of ruby 3.1.1 as well.
How to reproduce:
``` ruby
cat code_to_reproduce.txt
fp = File.open("sample_data.txt", "a")
puts "File size=#{fp.size} position=#{fp.tell}"
```
Output: See the **position value**
``` shell
->ruby -v
ruby 2.7.0p0 (2019-12-25 revision 647ee6f091) [x86_64-linux-gnu]
->wc sample_data.txt
1 2 12 sample_data.txt
->cat sample_data.txt
hello world
->ruby code_to_reproduce.txt
File size=12 position=0
->cat code_to_reproduce.txt
fp = File.open("sample_data.txt", "a")
puts "File size=#{fp.size} position=#{fp.tell}"
```
Further debugging with **`irb`**
``` ruby
rb(main):009:0> fp = File.open("sample_data.txt", "a")
irb(main):010:0> fp.tell
=> 0
irb(main):011:0> fp.size
=> 12
irb(main):012:0> fp.fileno
=> 7
irb(main):013:0> Process.pid
=> 22675
```
Examining the offset in /proc filesytem
``` shell
->cat /proc/22675/fdinfo/7
pos: 0
flags: 02102001
mnt_id: 30
```
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