Re: [Gnuplot-info] strange behavior on calling gnuplot from cgi script written in python

[Thomas S. <t.s...@fz...>]

seems that your header:

print "Content-Type: image/png"     # This is an image
print                               # blank line, end of headers

and gnuplots output (png graphics) are going different
ways, because the png graphics is sent without header
(a png file starts with "\x89PNG").

have a look at the section "Object Headers" in
http://marc.merlins.org/htmlearn/misc/cgi-faq.txt


stevecoh1 wrote:
> 
> Here's the error:
> 
>  [error] [client ::1] malformed header from script. Bad header=\x89PNG:
> usage_graph.cgi
> 
> I don't understand this.  How could closing the gnuplot process create a
> malformed header yet leaving it open produces usable output?  Still, I
> guess that's a clue.  I just don't know what it means.
> 
> Here is the guts of the script, deliberately leaving out details:
> 
> #!/usr/bin/python
> 
> import cgi
> import MySQLdb
> import os
> 
> parms = cgi.FieldStorage()
> 
> print "Content-Type: image/png"     # This is an image
> print                               # blank line, end of headers
> 
> 
> # open database connection
> ...
> # create a cursor
> cursor = db.cursor()
> 
> # execute SQL query
> cursor.execute(query % (userid))
> 
> # get the resultset as a tuple
> result = cursor.fetchall()
> ...
> # open gnuplot process
> gp = os.popen("/usr/bin/gnuplot, 'w')
> 
> # initialize the plot with desired settings including definition of
> # "weekday" function.
> ...
> 
> # send plot command to gnuplot process
> gp.write("""plot '-' u 1:(weekday(timecolumn(1)) ? column(2) : 0/0) ti
> "Weekday" w impulses,  \
>      ''  u 1:(weekday(timecolumn(1)) ? column(2) : 0/0) ti "Weekend" w
> impulses\n""")
> 
> # send data to the plot command
> for record in result:
>     gp.write("%s %2.3f %d\n" % record)
> gp.write("e\n")
> 
> #bombs out if following is uncommented.
> #gp.close()
> 
> Again, I can't see anything here that would work if the gnuplot process
> was left open but fail if it is closed.
> 
> 
> Thomas Sefzick wrote:
> 
>> there should be information about what caused the error in
>> the error-log of the http-server.
>> 
>> 
>> stevecoh1 wrote:
>>> Am trying to use gnuplot as a subprocess to a CGI script written in
>>> python (similar to the perl script on p. 241 of "Gnuplot in Action").
>>>
>>> The python script itself follows the pattern of the python script on
>>> p.230-231, that is
>>>
>>> gp = os.popen( ' /usr/bin/gnuplot', 'w')
>>> ... plotting commands
>>> gp.close()
>>>
>>> The cgi script produces a 500 error.
>>>
>>> But, if I comment out the gp.close() command, it works perfectly, and
>>> produces the graph I want.  What is going on here?
>>>
>>>
>>>
>>>
>>>
>>> ------------------------------------------------------------------------------
>>> Let Crystal Reports handle the reporting - Free Crystal Reports 2008
>>> 30-Day 
>>> trial. Simplify your report design, integration and deployment - and
>>> focus
>>> on 
>>> what you do best, core application coding. Discover what's new with
>>> Crystal Reports now.  http://p.sf.net/sfu/bobj-july
>>> _______________________________________________
>>> Gnuplot-info mailing list
>>> Gnu...@li...
>>> https://lists.sourceforge.net/lists/listinfo/gnuplot-info
>>>
>>>
>> 
> 
> 
> ------------------------------------------------------------------------------
> Let Crystal Reports handle the reporting - Free Crystal Reports 2008
> 30-Day 
> trial. Simplify your report design, integration and deployment - and focus
> on 
> what you do best, core application coding. Discover what's new with
> Crystal Reports now.  http://p.sf.net/sfu/bobj-july
> _______________________________________________
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> 

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