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comp.lang.ruby

Trouble with open/write/close cycle

Toby DiPasquale

3/24/2006 4:43:00 PM

Hi all,

I'm having trouble getting an open/write/close cycle to actually put the
correct data in the file. Here's some code that illustrates the problem:

<snip>
x = "012345678\n"
10.times do
len = 0
begin
len = File.stat( 'junk').size
rescue
len = 0
end
puts len.to_s
f = File.open 'crapper', 'w'
f.seek len
f.write x
f.close
end
</snip>

produces:

0
10
20
30
40
50
60
70
80
90

as output. However, the file 'junk' is filled with zeroes, except for
the last 10 bytes, which are what you'd expect them to be:

<snip>
puma:~> od -h junk
0000000 0000 0000 0000 0000 0000 0000 0000 0000
*
0000120 0000 0000 0000 0000 0000 3130 3332 3534
0000140 3736 0a38
0000144
puma:~>
</snip>

Anyone know what I'm doing wrong? I tried using sysseek and syswrite,
but I got the same results. Is Ruby's internal buffering screwing me up
here somehow? TIA.

--
Toby DiPasquale

--
Posted via http://www.ruby-....
6 Answers

Toby DiPasquale

3/24/2006 4:46:00 PM

0

From above:

- f = File.open 'crapper', 'w'
+ f = File.open 'junk', 'w'

Sorry.

--
Posted via http://www.ruby-....

Chris Hulan

3/24/2006 4:54:00 PM

0

> <snip>
> len = File.stat( 'junk').size
><snip>
> f = File.open 'crapper', 'w'

Are there really 2 files? or is this a typo?

The docs on IO::open says tha using the 'w' mode cause the file to be
truncated if it exists.
You need to look at 'a' mode

cheers

James Gray

3/24/2006 5:02:00 PM

0

On Mar 24, 2006, at 10:43 AM, Toby DiPasquale wrote:

> begin
> len = File.stat( 'junk').size
> rescue
> len = 0
> end
> puts len.to_s
> f = File.open 'crapper', 'w'
> f.seek len

The above procedure seems like a complex way to do:

File.open("junk", "a")...

Will that work for you?

James Edward Gray II

Toby DiPasquale

3/24/2006 5:04:00 PM

0

Mike Stok wrote:
> opening a file for writing truncates it.

ARRRGH! I'm a moron. Of course, you are right. Thanks for the mental
clue-by-four.

--
Toby DiPasquale

--
Posted via http://www.ruby-....

Toby DiPasquale

3/24/2006 5:04:00 PM

0

James Gray wrote:
> The above procedure seems like a complex way to do:
>
> File.open("junk", "a")...
>
> Will that work for you?

No, the above code was just a small example to illustrate the issue. The
real code does lots more before and after each write.

--
Toby DiPasquale


--
Posted via http://www.ruby-....

Chris Hulan

3/24/2006 5:08:00 PM

0


Toby DiPasquale wrote:
> From above:
>
> - f = File.open 'crapper', 'w'
> + f = File.open 'junk', 'w'
>
> Sorry.

Not a problem.
When I run your snippet i get:
>ruby -w fwrite-test.rb
0
11
22
33
44
55
66
77
88
99

It looks like seeking on an empty file fills it with null data. Not
sure if thats a Ruby thing or a feature of the underlying IO libs.

change the 'w' to an 'a' (an delete the current 'junk' file)
and the file produced looks like:
012345678
012345678
012345678
012345678
012345678
012345678
012345678
012345678
012345678
012345678


cheers