C++ cgi using gnuplot to draw graph on the web

hpdp · 01-29-2020, 10:38 PM

>> when run from the command line?
>> What, specifically, happens in the browser? Nothing? An error? A blank page?

run from command line, ./mycode.cgi, the output

Quote:

Content-TYPE: image/svg+xml

<?xml version="1.0" encoding="utf-8" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN"
"http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg ....
</svg>

call from browser, http://.../mycode.cgi, it only displays
This XML file does not appear to have any style information associated with it. The document tree is shown below.
Blank below

run curl -isS http://.../mycode.cgi, output is

Quote:

HTTP/1.1 200 OK
Date: Thu, 30 Jan 2020 04:07:42 GMT
Server: Apache
Access-Control-Allow-Methods: POST, GET, HEAD, PUT, OPTIONS, PATCH, DELETE
Access-Control-Allow-Origin:
Access-Control-Allow-Credentials: true
Access-Control-Allow-Headers: X-Accept-Charset,X-Accept,Content-Type,X-Requested-With
Connection: close
Transfer-Encoding: chunked
Content-Type: image/svg+xml

The message from /web/logs/error is

Quote:

sh: gnuplot: command not found
[Wed Jan 29 23:12:23.799808 2020] [cgi:error] [pid 160685] AH01215: pclose returned error

I checked the PATH, echo $PATH, it has gnuplot included

Quote:

/opt/python-all/bin:/opt/perl/5.16.3/bin:/opt/fcron/bin:/usr/local/gnuplot/5.0.1/bin:/usr/local/eclipse/4.4.2/bin.....

I tried mycodeurl.cgi

Code:

#!/bin/bash
PATH=$PATH:/usr/local/gnuplot/5.0.1/bin/gnuplot
export PATH
./mycode.cgi

it gave the same message.

NevemTeve · 01-29-2020, 11:21 PM

> I checked the PATH, echo $PATH, it has gnuplot included

It might be set in you login-shell, but it isn't set in the context Apache runs your CGI. Here is how to debug your CGI: https://www.linuxquestions.org/quest...0/#post5707383

Also (as I've already told you) you don't put individual binaries into your PATH, but directories:

before:

Code:

#!/bin/bash
PATH=$PATH:/usr/local/gnuplot/5.0.1/bin/gnuplot
export PATH
./mycode.cgi

after (with some minor changes):

Code:

#!/bin/sh
PATH="$PATH:/usr/local/gnuplot/5.0.1/bin"
export PATH
exec "$(dirname -- "$0")/mycode.cgi"

Of course you don't actually need another file (mycodeurl.cgi), you can set the PATH within your mycode.cgi

hpdp · 01-30-2020, 01:33 PM

@NevemTeve
I checked the link about how to debug CGI, the good news is I can make it work by http://.../mycodeurl.cgi
mycodeurl.cgi

Code:

#!/bin/bash
echo "Content-Type: image/svg+xml"
echo
exec "$(dirname -- "$0")/mycode.cgi"

mycode.cgi

Code:

int main() {
        setenv("PATH", "$PATH:/usr/local/gnuplot/5.0.1/bin", true);
	Gnuplot gp;
        gp << "set term svg\n";	
	gp << "plot sin(x)\n";
}

what I don't understand is, if I put the line Content-Type: image/svg+xml in mycode.C, and call http://.../mycode.cgi directly, it didn't work

Code:

int main() {

	cout<<"Content-Type: image/svg+xml\n\n";
		
	setenv("PATH", "$PATH:/usr/local/gnuplot/5.0.1/bin", true);	
	Gnuplot gp;
        gp << "set term svg\n";	
	gp << "plot sin(x)\n";
}

I checked the /tmp/debug file, the output is

Quote:

<?xml version="1.0" encoding="utf-8" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN"
"http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg ...>
...
</svg>

Content-Type: image/svg+xml

The Content-Type: image/svg+xml was printed at the bottom, which caused the problem. Thanks a lot!

NevemTeve · 01-30-2020, 02:10 PM

You could try to force the data pushed out:

Code:

	cout<<"Content-Type: image/svg+xml\r\n\r\n"<<std::flush;

hpdp · 01-31-2020, 10:32 AM

Quote:

Originally Posted by NevemTeve

You could try to force the data pushed out:

Code:

	cout<<"Content-Type: image/svg+xml\r\n\r\n"<<std::flush;

That fixed the problem.
Thanks a million!!