Issue #5530 has been updated by zzak (Zachary Scott).

Status changed from Closed to Assigned
Assignee changed from drbrain (Eric Hodel) to kosaki (Motohiro KOSAKI)
% Done changed from 100 to 90


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Bug #5530: SEEK_SET malfunctions when used with 'append' File.open mode
https://bugs.ruby-lang.org/issues/5530#change-33167

Author: jduck (Joshua J. Drake)
Status: Assigned
Priority: Normal
Assignee: kosaki (Motohiro KOSAKI)
Category: lib
Target version: 2.0.0
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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