Trouble with open/write/close cycle

[Toby DiPasquale]

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]

From above:

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

Sorry.

 

[ChrisH]

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:pen says tha using the 'w' mode cause the file to be
truncated if it exists.
You need to look at 'a' mode

cheers

 

[James Edward Gray II]

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]

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]

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.

 

[ChrisH]

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