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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