Clearing StringIO

[James Edward Gray II]

I'm working with StringIO to write some test code and it is behaving
strangely. Check out this example:

irb(main):001:0> require "stringio"
=> true
irb(main):002:0> fake_file = StringIO.new
=> #<StringIO:0x3119cc>
irb(main):003:0> fake_file.puts "This is a test."
=> nil
irb(main):004:0> fake_file.string
=> "This is a test.\n"
irb(main):005:0> fake_file.truncate(0)
=> 0
irb(main):006:0> fake_file.string
=> ""
irb(main):007:0> fake_file.puts " <= Junk!"
=> nil
irb(main):008:0> fake_file.string
=> "\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000 <=
Junk!\n"

Is that a bug? Yuck.

I hand my "fake_file" off to some code to play with, but keep a
reference for my testing purposes. At times, I need to clear the
accumulated output, but as you can see truncate() is giving me fits.

Can anyone suggest how I might clear the StringIO object, without
replacing it?

Thanks.

James Edward Gray II

 

[Saynatkari]

I'm working with StringIO to write some test code and it is behaving
strangely. Check out this example:

irb(main):001:0> require "stringio"
=> true
irb(main):002:0> fake_file = StringIO.new
=> #<StringIO:0x3119cc>
irb(main):003:0> fake_file.puts "This is a test."
=> nil
irb(main):004:0> fake_file.string
=> "This is a test.\n"
irb(main):005:0> fake_file.truncate(0)
=> 0
irb(main):006:0> fake_file.string
=> ""
irb(main):007:0> fake_file.puts " <= Junk!"
=> nil
irb(main):008:0> fake_file.string
=> "\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000 <=
Junk!\n"

Is that a bug? Yuck.

I hand my "fake_file" off to some code to play with, but keep a
reference for my testing purposes. At times, I need to clear the
accumulated output, but as you can see truncate() is giving me fits.

Can anyone suggest how I might clear the StringIO object, without
replacing it?

I think #truncate simply memset()s the buffer but it does not
change the position of the file pointer, so you would probably
need to #seek 0, as well.

Thanks.

James Edward Gray II

E

 

[Ara.T.Howard]

I'm working with StringIO to write some test code and it is behaving
strangely. Check out this example:

irb(main):001:0> require "stringio"
=> true
irb(main):002:0> fake_file = StringIO.new
=> #<StringIO:0x3119cc>
irb(main):003:0> fake_file.puts "This is a test."
=> nil
irb(main):004:0> fake_file.string
=> "This is a test.\n"
irb(main):005:0> fake_file.truncate(0)
=> 0

this is consistent with file behaviour:

[ahoward@localhost ~]$ env PAGER=cat man truncate | grep pointer
The file pointer is not changed.

try:

irb(main):005:0> fake_file.truncate(fake_file.rewind)

hth.

-a
--
===============================================================================
| email :: ara [dot] t [dot] howard [at] noaa [dot] gov
| phone :: 303.497.6469
| although gold dust is precious, when it gets in your eyes, it obstructs
| your vision. --hsi-tang
===============================================================================

 

[B. K. Oxley (binkley)]

I think #truncate simply memset()s the buffer but it does not
change the position of the file pointer, so you would probably
need to #seek 0, as well.

Very odd. From the man page:

http://www.opengroup.org/onlinepubs/009695399/functions/truncate.html

truncate - truncate a file to a specified length

The truncate() function shall cause the regular file named by path to
have a size which shall be equal to length bytes.

If the file previously was larger than length, the extra data is
discarded. If the file was previously shorter than length, its size is
increased, and the extended area appears as if it were zero-filled.

And on Linux:

http://linux.com.hk/PenguinWeb/manpage.jsp?name=truncate&section=2


Cheers,
--binkley

 

[James Edward Gray II]

I think #truncate simply memset()s the buffer but it does not
change the position of the file pointer, so you would probably
need to #seek 0, as well.

Thanks for the tip.

James Edward Gray II

 

[B. K. Oxley (binkley)]

[ahoward@localhost ~]$ env PAGER=cat man truncate | grep pointer
The file pointer is not changed.

This is highly system-dependent (although I am unable to tell if the
behavior has standardized over time):

http://www.faqs.org/faqs/unix-faq/faq/part3/section-3.html

<<
For systems which support the ftruncate function, there are
three known behaviours:

BSD 4.2 - Ultrix, SGI, LynxOS
- truncation doesn't grow file
- truncation doesn't move file pointer


BSD 4.3 - SunOS, Solaris, OSF/1, HP/UX, Amiga
- truncation can grow file
- truncation doesn't move file pointer

Cray - UniCOS 7, UniCOS 8
- truncation doesn't grow file
- truncation changes file pointer

Cheers,
--binkley