From bug-request at octave dot org Tue Jan  4 19:54:26 2005
Subject: Re: Waiting for gnuplot to write file.
From: Peter JENSEN <peterjensen at agere dot com>
To: bug at octave dot org
Date: Tue, 4 Jan 2005 07:22:07 -0600

Hi,

I investigated this problem a little further.

I have not found a way to detect if a file
is already open for writing. C provides the
function flock to lock files, however gnuplot
does not lock the file.

So the only solution I can suggest right now
is to read the file and look for EOF. This is
unfortunately not very elegant. I hope somebody
can find a better solution.

I have enclosed the code snippet

86c86
< function my_legend (varargin)
---
 > function legend (varargin)
188,195d187
<
<   ## wait for file to be completed
<   attempt=0;
<   while( isempty(regexp("EOF",system(strcat("tail -1 ", tmpfilename)))) == 1 )  ;
<     if (++attempt > 20) error("gnuplot is not responding"); endif
<     usleep(1e5);
<   end
<

Peter


Peter JENSEN wrote:
> Hi,
> 
> On my system at work (with two processors) I see
> a problem with legend. The problem is intermittent
> however I managed to track it down, and I think I
> understand the underlying reason.
> 
> The chain of events are as follows.
> 
> Firstly legend.m asks gnuplot
> to save the original plotting command
> into a temp file.
> 
> This is done via :
> 
>   tmpfilename=tmpnam;
>   command=["save \"",tmpfilename,"\"\n"];
>   graw(command);
> 
> Then legend waits for file to appear
> and immediately after starts a
> shell command that processes the temp file.
> 
> The code looks like.
> 
>   while (isempty(stat(tmpfilename)))
>     if (++attempt > 20) error("gnuplot is not responding"); endif
>     usleep(1e5);
>   end
>   plot_cmd = split(system(shell_cmd),"\n");
> 
> The problem is that the file is empty at
> the time when the system call is made.
> 
> The problem is due to that the code only
> checks if the file exists. However at
> the time where the system call is made
> the file has actually not been written
> by gnuplot.
> 
> I verified this by modifying legend.m
> as follows :
> 
>   file_test = stat(tmpfilename) ;
>   while (isempty(file_test))
>     if (++attempt > 20) error("gnuplot is not responding"); endif
>     usleep(1e5);
>     file_test = stat(tmpfilename) ;
>   end
> 
>   file_test
> 
> When the code fails the size of the file is zero,
> and the code prints.
> 
> file_test =
> {
>   atime = 1104339140
>   blksize = 32768
>   blocks = 0
>   ctime = 1104339140
>   dev = 21
>   gid = 2100
>   ino = 12074898
>   modestr = -rw-rw-r--
>   mtime = 1104339140
>   nlink = 1
>   rdev = 0
>   size = 0
>   uid = 23737
> }
> 
> And when the code (occasionally works ) the file
> size is different from zero, and the code prints.
> 
> file_test =
> {
>   atime = 1104339203
>   blksize = 32768
>   blocks = 16
>   ctime = 1104339203
>   dev = 21
>   gid = 2100
>   ino = 12074898
>   modestr = -rw-rw-r--
>   mtime = 1104339203
>   nlink = 1
>   rdev = 0
>   size = 4316
>   uid = 23737
> }
> 
> On way to solve the problem is to look at the file
> written by gnuplot, and see if it is complete
> (it is not sufficient to look at the size of the
> file.....).
> 
> Gnuplot indicated that the file has been
> completed by writing "#    EOF" to the last line.
> We can therefore test this via a "tail -1 file_name".
> 
> We could also find out if the file is open for writing,
> as I assume that gnuplot will close the file when it has
> been fully written, however I don't know how to do this.
> 
> I have enclosed the code that I used for testing.
> 
> Peter Jensen
> 
> =======================================
> #!/opt/sbin/octave -q
> more("off") ;
> clear
> closeplot
> 
> level = [ 0 -1 -2 -3 -4 -5 -6 ] ;
> 
> plot_legend = {} ;
> 
> for i=1:length(level)
>   signal{i} = sin(0:0.01:10)*10**(level(i)/10) ;
>   plot_legend{i}  = strcat("L = ",num2str(level(i))) ;
> endfor
> 
> clearplot ;
> 
> ## Plot signals
> axis([ 1 length(signal{1}) ])
> title("Test") ;
> 
> plot(signal{1}) ;
> hold on ;
> 
> for k=2:length(level)
>   plot(signal{k});
> endfor
> 
> legend(plot_legend,-1) ;
> ==========================================
> 


-- 
Peter Kabell Jensen                     E-mail  : peterjensen at agere dot com
Agere Systems                           Tel     : +44 (0) 1344 86 5849
Microelectronics House                  Fax     : +44 (0) 1344 86 5990
Kings Ride, Ascot, SL5 8AD  England



-------------------------------------------------------------
Octave is freely available under the terms of the GNU GPL.

Octave's home on the web:  http://www.octave.org
How to fund new projects:  http://www.octave.org/funding.html
Subscription information:  http://www.octave.org/archive.html
-------------------------------------------------------------