| Re: Bug in IDL's FILE_INFO function [message #55604] |
Fri, 31 August 2007 05:44  |
Dave Wuertz
Messages: 6
Registered: August 2007
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Junior Member |
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I suspect FSTAT is executing FLUSH whereas FILE_INFO must not be. I
doubt FSTAT is closing and re-opening, unless that is the only way to
perform the flush.
I know the Linux OS has a buffering mechanism, but my experience has
been that it is very smart and efficient. For example, I've witnessed
Linux holding some recently written data in a buffer until *either* the
buffer gets filled *or* another application needs to read from the file
the data still being held in the buffer. It's like Linux is smart
enough to know that it doesn't *really* need to physically write small
amounts of information (a configurable OS parameter) unless it really
*has* to.
There is no mention of flushing or buffering in the documentation for
FILE_INFO and FSTAT and both are supposed to return the current size of
an open file. The fact is they behave differently (at least on my IDL
version and platform). Interestingly, the doc for FLUSH, however, states
that *IDL* "uses buffered output for reasons of efficiency". I'm
wondering if IDL has it's own buffering mechanism on top of Linux's,
though I cannot imagine why it would need it.
My only real purpose in this post is to point out that those functions
behave differently. This behavior should be either documented or the
code modified to to give the same result.
-Dave Wuertz
Jean H. said the following on 8/30/2007 4:10 PM:
> FSTAT returns the info of an open file, while FILE_INFO returns the
> info of a file, opened or not.
>
> When you write to a file, it probably goes through some buffers (not
> sure of this / how)... for example, if one writes to a text file and
> tries to read this file in another program before it is closed, then
> you would see nothing in the file... though as soon as it has been
> closed by IDL, you can access it.
>
> So it doesn't look surprising that the FILE_INFO returns the previous
> size... the question is, does FSTAT close and re-open the file for
> you? ... it appear so as after a call to it, you get the correct size!
>
> Jean
>
> Dave Wuertz wrote:
>> Folks,
>>
>> I believe there's a bug in the FILE_INFO function. I am running IDL
>> v6.4 on Linux.
>>
>> I'm writing a program that does a lot of file updates and it's
>> necessary for me to get the current file size after an update to an
>> open file. I decided to use FILE_INFO rather than FSTAT because I
>> also must first check to see if the file exists. FILE_INFO can tell
>> you if the file exists as well it's size in bytes. It's also newer
>> than FSTAT, so I thought I'd just use FILE_INFO exclusively in my
>> program.
>>
>> Well, things just weren't making sense, and I boiled it down to
>> this. If I append a new record to a file and immediately check the
>> file size with FILE_INFO it gives me the wrong size. It returns the
>> size BEFORE the record was added. However, FSTAT will give the
>> correct new size. And, once FSTAT has been called, then FILE_INFO
>> knows about the new size. It's like FSTAT issues a FLUSH, because
>> the only way FILE_INFO gives the correct size is if FLUSH (or FSTAT)
>> is called first. This is fine, however there is no mention in the
>> documentation that FLUSH must be called first.
>>
>> Below is some code to illustrate the problem:
>>
>>
>> pro file_info_vs_fstat
>>
>> fname = 'test.txt'
>> openw, lun, /get_lun, fname
>> nrec = 3
>> for i = 0, nrec-1 do begin
>>
>> print, 'Before writing record file_info.size, fstat.size:', $
>> (file_info( fname )).size, (fstat( lun )).size,
>> format='(a,1x,2i6)'
>>
>> printf, lun, 'This is record number ', i
>>
>> print, 'After writing record file_info.size, fstat.size:', $
>> (file_info( fname )).size, (fstat( lun )).size,
>> format='(a,1x,2i6)'
>>
>> print, ' ' ; print blank line for readability
>>
>> endfor
>> free_lun, lun
>>
>> return
>> end
>> ;;;;;;;;;;;;;;;; Run above procedure
>> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
>> IDL> file_info_vs_fstat Before writing record file_info.size,
>> fstat.size: 0 0
>> After writing record file_info.size, fstat.size: 0 31
>>
>> Before writing record file_info.size, fstat.size: 31 31
>> After writing record file_info.size, fstat.size: 31 62
>>
>> Before writing record file_info.size, fstat.size: 62 62
>> After writing record file_info.size, fstat.size: 62 93
>>
>> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ;;;;;;;;;;;;;;;;;;;;;;
>>
>>
>> Now, if you replace the "After" print statement with the following
>> one that simply
>> reverses the order the two functions are called, you then get the
>> correct result from
>> the FILE_INFO function:
>>
>> print, 'After writing record fstat.size, file_info.size:', $
>> (fstat( lun )).size, (file_info( fname )).size,
>> format='(a,1x,2i6)'
>>
>> IDL> file_info_vs_fstat Before writing record file_info.size,
>> fstat.size: 0 0
>> After writing record fstat.size, file_info.size: 31 31
>>
>> Before writing record file_info.size, fstat.size: 31 31
>> After writing record fstat.size, file_info.size: 62 62
>>
>> Before writing record file_info.size, fstat.size: 62 62
>> After writing record fstat.size, file_info.size: 93 93
>>
>> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ;;;;;;;;;;;;;;;;;;;;;;
>>
>>
>> Ciao,
>>
>> -Dave Wuertz
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