Issue #5530 has been updated by Motohiro KOSAKI.


The code is correct. Ruby's append mode is derived from C and C's append mode mean "ignore file position".
Which documentation bring you confusing? 


http://pubs.opengroup.org/onlinepubs/009695399/functions/fopen.html

    Opening a file with append mode (a as the first character in the mode argument) shall cause all 
    subsequent writes to the file to be forced to the then current end-of-file, regardless of intervening 
    calls to fseek().
----------------------------------------
Bug #5530: SEEK_SET malfunctions when used with 'append' File.open mode
http://redmine.ruby-lang.org/issues/5530

Author: Joshua J. Drake
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 
ruby -v: all


The following code demonstrates the issue. As documented, IO#seek says "SEEK_SET" will move to a position relative to the start of the file. Using 'ab', it doesn't actually seek where it should. It's possible the documentation is just wrong here, either the code or documentation should change though.

<code lang=ruby>
#!/usr/bin/env ruby

modes = [ 'ab', 'wb', 'w+b', 'r+b' ]

filename = '/tmp/test.txt'
desired = "AABBBBAA"

modes.each { |mode|
        File.unlink(filename) rescue nil
        File.open(filename, "wb") { |fd|
                fd.write('AAAAAAAA')
        }

        File.open(filename, mode) { |fd|
                fd.seek(2, IO::SEEK_SET)
                fd.write("BBBB")
                fd.close
        }

        data = ''
        File.open(filename, "rb") { |fd|
                data = fd.read(fd.stat.size)
        }

        puts "%3s #{data.inspect}" % mode
}

File.unlink(filename)
puts ""
</code>

Output:
<pre>

fear:0:ruby$ rvm all ruby -v file_mode_test.rb
ruby 1.8.6 (2010-09-02 patchlevel 420) [x86_64-linux]
 ab "AAAAAAAABBBB"
 wb "\000\000BBBB"
w+b "\000\000BBBB"
r+b "AABBBBAA"

ruby 1.8.7 (2011-02-18 patchlevel 334) [x86_64-linux]
 ab "AAAAAAAABBBB"
 wb "\000\000BBBB"
w+b "\000\000BBBB"
r+b "AABBBBAA"

ruby 1.9.1p378 (2010-01-10 revision 26273) [x86_64-linux]
 ab "AAAAAAAABBBB"
 wb "\x00\x00BBBB"
w+b "\x00\x00BBBB"
r+b "AABBBBAA"

ruby 1.9.2p290 (2011-07-09 revision 32553) [x86_64-linux]
 ab "AAAAAAAABBBB"
 wb "\x00\x00BBBB"
w+b "\x00\x00BBBB"
r+b "AABBBBAA"

ruby 1.8.7 (2011-06-30 patchlevel 352) [x86_64-linux]
 ab "AAAAAAAABBBB"
 wb "\000\000BBBB"
w+b "\000\000BBBB"
r+b "AABBBBAA"

ruby 1.9.2p180 (2011-02-18 revision 30909) [x86_64-linux]
 ab "AAAAAAAABBBB"
 wb "\x00\x00BBBB"
w+b "\x00\x00BBBB"
r+b "AABBBBAA"

ruby 1.9.1p431 (2011-02-18 revision 30908) [x86_64-linux]
Error loading gem paths on load path in gem_prelude
can't modify frozen string
<internal:gem_prelude>:69:in `force_encoding'
<internal:gem_prelude>:69:in `set_home'
<internal:gem_prelude>:38:in `dir'
<internal:gem_prelude>:76:in `set_paths'
<internal:gem_prelude>:47:in `path'
<internal:gem_prelude>:286:in `push_all_highest_version_gems_on_load_path'
<internal:gem_prelude>:355:in `<compiled>'
 ab "AAAAAAAABBBB"
 wb "\x00\x00BBBB"
w+b "\x00\x00BBBB"
r+b "AABBBBAA"

ruby 1.9.3dev (2011-07-31 revision 32789) [x86_64-linux]
 ab "AAAAAAAABBBB"
 wb "\x00\x00BBBB"
w+b "\x00\x00BBBB"
r+b "AABBBBAA"

fear:0:ruby$
</pre>

The other modes behave as expected.

See also - http://dev.metasploit.com/redmine/issues/3199


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