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

[Steve C. <sc...@ja...>]

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?
>>
>>
>>
>>
>>
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>