Bug #17673: sysseek(0) and '1A' in header return "EOFError (end of file reached)"

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 3.0.0p0 (2020-12-25 revision 95aff21468)
• Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN

[stiuna (Juan Gregorio)]

The first byte of my file starts with the value '1A' .

Then:

descriptor = IO.sysopen(filePath)
file2Read = IO.new(descriptor)
file2Read.sysseek(0)
p file2Read.sysread(2).unpack('H*')
#=> "EOFError (end of file reached)"

But when I change the first byte of my file to 'AA' for example.
Then:

#=> "aa45"

Is it a bug or am I missing something? I thought sysseek only changed the cursor and nothing else.

This error only occurs with byte '1A' I did the test with the rest of the bytes [00..FF] and there is no problem except with '1A' .

[stiuna (Juan Gregorio)]

• Description updated (diff)

[stiuna (Juan Gregorio)]

• Description updated (diff)

[jeremyevans0 (Jeremy Evans)]

• Status changed from Open to Feedback

I tried your example and was unable to reproduce your problem:

filePath = 'test.txt'
File.binwrite(filePath, "\x1A\x45")
descriptor = IO.sysopen(filePath)
file2Read = IO.new(descriptor)
file2Read.sysseek(0)
p file2Read.sysread(2).unpack('H*')
# => ["1a45"]

Can you try on a different machine to check if this is a local problem?

[stiuna (Juan Gregorio)]

Can you try on a different machine to check if this is a local problem?

Unfortunately I don't have another computer to test the bug with, what version of ruby do you use?

[stiuna (Juan Gregorio)]

Aaah! I don't know if it matters but I use Windows, did you run it on Linux all of a sudden?

[jeremyevans0 (Jeremy Evans)]

• Status changed from Feedback to Closed

I tried on Windows and I was able to reproduce the issue:

C:\Users\jeremye>c:\Ruby30-x64\bin\ruby

filePath = 'test.txt'
File.binwrite(filePath, "\x1A\x45")
descriptor = IO.sysopen(filePath)
file2Read = IO.new(descriptor)
file2Read.sysseek(0)
p file2Read.sysread(2).unpack('H*')
__END__
-:7:in `sysread': end of file reached (EOFError)
        from -:7:in `<main>'

From some brief research, this is expected behavior on Windows for text files. You need to put the file in binary mode (IO#binmode):

:\Users\jeremye>c:\Ruby30-x64\bin\ruby
filePath = 'test.txt'
File.binwrite(filePath, "\x1A\x45")
descriptor = IO.sysopen(filePath)
file2Read = IO.new(descriptor)
file2Read.binmode
file2Read.sysseek(0)
p file2Read.sysread(2).unpack('H*')
__END__
["1a45"]

[stiuna (Juan Gregorio)]

Right, that solved it, the Ruby documentation should specify that. Ok, thanks.

[nobu (Nobuyoshi Nakada)]

This is not specific to Ruby, but the very (in)famous text mode spec of Windows (and other dosish systems).