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

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

you're right, there are no 'separate ways' (i first thought the
'Content-Type' lines would go somewhere else than the gnuplot
output, but that's not the case).
both, the print commands and gnuplots output go through stdout - but,
i just ran your little script and realized that the 'Content-Type' lines are
at the end.
you can reproduce this by running your script in a terminal and
piping the output into a file.

you need a 'flush' after 'print'ing the two lines:

#!/usr/bin/python

import os
import sys
import math

print "Content-Type: image/png"     # This is an image
print                               # blank line, end of headers
sys.stdout.flush();

gp = os.popen("/usr/bin/gnuplot", 'w')
gp.write("set term png;set o;")
gp.write("plot '-' u 1:2 w linesp\n")

xs = [a/10.0 for a in range(0,101,1)]
for x in xs:
    y = x * math.sin(x)
    gp.write( "%f %f\n" % (x, y))
gp.write("e\n")
gp.close()


stevecoh1 wrote:
> 
> OK, I feel like an idiot but I guess I'm really not understanding you
> even after reading the document you referred me to.  The specific words
> I'm not understanding are "going separate ways".
> 
> To that end I've gone back to basics and rewritten the example on p. 241
> in python.  I believe my translation is more or less exact:
> 
> ================================
> #!/usr/bin/python
> 
> import os
> import math
> 
> print "Content-Type: image/png"     # This is an image
> print                               # blank line, end of headers
> 
> gp = os.popen("/usr/bin/gnuplot", 'w')
> gp.write("set term png;set o;")
> gp.write("plot '-' u 1:2 w linesp\n")
> 
> xs = [a/10.0 for a in range(0,101,1)]
> for x in xs:
>     y = x * math.sin(x)
>     gp.write( "%f %f\n" % (x, y))
> gp.write("e\n")
> gp.close()
> 
> 
> 
> =================================
> 
> This fails with the following message
> [error] [client ::1] malformed header from script. Bad header=\x89PNG:
> 
> whether or not the gp.close() line is commented out.
> 
> What am I missing?
> 
> 
> 
> 
> 
> Thomas Sefzick wrote:
>> 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
>>>
> 
> ...
> 
> 
> 
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