Why doesn't rrdtool generate any PNG output in my Perl CGI program?

Question

I'm trying to output an image from RRD Tool using Perl. I've posted the relevant part of the CGI script below:

sub graph
{
my $rrd_path = $co->param('rrd_path');
my $RRD_DIR = "../data/";

#generate a PNG from the RRD
my $png_filename = "-"; # a '-' as the filename send the PNG to stdout
my $rrd = "$RRD_DIR/$rrd_path";

my $png = `rrdtool graph $png_filename -a PNG -r -l 0 --base 1024 --start -151200 -- vertical-label 'bits per second' --width 500 --height 200 DEF:bytesInPerSec=$rrd:bytesInPerSec:AVERAGE DEF:bytesOutPerSec=$rrd:bytesOutPerSec:AVERAGE CDEF:sbytesInPerSec=bytesInPerSec,8,* CDEF:sbytesOutPerSec=bytesOutPerSec,8,* AREA:sbytesInPerSec#00cf00:AvgIn LINE1:sbytesOutPerSec#002a97:AvgOut VRULE:1246428000#ff0000:`;

#print the image header 
use bytes;
print $co->header(-type=>"image/png",-Content_length=>length($png));

binmode STDOUT;
print $png;
}#end graph

This works fine on the command line (perl graph.cgi > test.png) - commenting out the header, of course, as well as on my Ubuntu 10.04 development machine. However, when I move to the Centos 5 production server, it doesn't, and the browser receives a content-length of 0:

Ubuntu 10.04/Apache:

Request URL:http://noc-student.nmsu.edu/grasshopper/web/graph.cgi
Request Method:GET
Status Code:200 OK
Request Headers
Accept:application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5
Cache-Control:max-age=0
User-Agent:Mozilla/5.0 (X11; U; Linux i686; en-US) AppleWebKit/534.7 (KHTML, like Gecko)     Chrome/7.0.517.36 Safari/534.7
Response Headers
Connection:Keep-Alive
Content-Type:image/png
Content-length:12319
Date:Fri, 08 Oct 2010 21:40:05 GMT
Keep-Alive:timeout=15, max=97
Server:Apache/2.2.14 (Ubuntu)

And from the Centos 5/Apache Server:

Request URL:http://grasshopper-new.nmsu.edu/grasshopper/branches/michael_dev/web/graph.cgi
Request Method:GET
Status Code:200 OK
Request Headers
Accept:application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5
Cache-Control:max-age=0
User-Agent:Mozilla/5.0 (X11; U; Linux i686; en-US) AppleWebKit/534.7 (KHTML, like Gecko) Chrome/7.0.517.36 Safari/534.7
Response Headers
Connection:close
Content-Type:image/png
Content-length:0
Date:Fri, 08 Oct 2010 21:40:32 GMT
Server:Apache/2.2.3 (CentOS)

The use bytes and manual setting of the content length are in there to try to fix the problem, but it's the same without them. Same with setting binmode on STDOUT. The script works fine from the command line on both machines.

Answer 1

See my How can I troubleshoot my Perl CGI program. Typically, the difference between running your program on the command line and from the web server is a matter of difference environments. In this case I'd expect that either rddtool is not in the path or the webserver user can't run it.

The backticks only capture standard output. There is probably some standard error output in the web server error log.

Answer 2

Are you sure your web user has access to your data? Try having the CGI writing the png to the filesystem, so you can make sure it's generated properly. If it is, the problem is in the transmission (headers, encodings, etc). If not, it's unrelated to the web server, and probably related to permissions.