Calling another program in C++ and directing the program flow

[utab]

Dear all,

I am making a system call to the well known Gnuplot with

system("gnuplot ");

gnuplot opens if I only supply this command but I would like to pipe
that command line in my C++ program. For example I want to run a very
simple command like "plot sin(x)". So after running another program,
what is the way to pipe that with some commands from inside the c++
program.

Or should I ask this in Gnuplot forums?

Thx for the replies in advance.

[Markus Schoder]

utab wrote:

Dear all,

I am making a system call to the well known Gnuplot with

system("gnuplot ");

gnuplot opens if I only supply this command but I would like to pipe
that command line in my C++ program. For example I want to run a very
simple command like "plot sin(x)". So after running another program,
what is the way to pipe that with some commands from inside the c++
program.
You want popen(). Like in the following example:

#include <cerrno>
#include <cstdio>
#include <iostream>

using namespace std;

int
main()
{
FILE *pg = popen("/usr/bin/gnuplot", "w");
if(!pg)
{
cerr << "popen failed: " << strerror(errno) << '\n';
return 1;
}
fputs("plot sin(x)\n", pg);
fflush(pg);
sleep(3);
pclose(pg);
}

Or should I ask this in Gnuplot forums?

comp.os.linux.d evelopment.apps would probably be the right place (I
guess you use Linux).

[Phlip]

Markus Schoder wrote:

You want popen(). Like in the following example:

The OP doesn't need popen() (which I suspect is off-topic). Gnuplot has both
an interactive mode and a batch mode. The OP needs to write a batch as a
text file, and pass this to Gnuplot. Abusing Gnuplot via popen() would
solve the wrong problem, add might accidentally work, leaving the OP with a
lot of fragile complexity.

--
Phlip
http://www.greencheese.org/ZeekLand <-- NOT a blog!!!

[Markus Schoder]

Phlip wrote:

Markus Schoder wrote:

You want popen(). Like in the following example:

The OP doesn't need popen() (which I suspect is off-topic). Gnuplot has both
an interactive mode and a batch mode. The OP needs to write a batch as a
text file, and pass this to Gnuplot. Abusing Gnuplot via popen() would
solve the wrong problem, add might accidentally work, leaving the OP with a
lot of fragile complexity.

gnuplot reads commands from stdin providing them through popen seems to
work. If you want to make sure interactive mode does not do anything
unwanted use

popen("gnuplot /dev/fd/0", "w")

to put gnuplot in batch mode but I doubt there will be any noticeable
difference. Anyway popen or a similar mechanism is needed to prevent
gnuplot from exiting real quick (batch mode with a regular file suffers
from this problem as well).

[Phlip]

Markus Schoder wrote:

gnuplot reads commands from stdin providing them through popen seems to
work.

This is example why off-topic threads damage USENET.

Write a batch of gnuplot commands, including one command to output your plot
in a portable format, such as PNG. Then display this format, independent of
gnuplot, in a viewer such as a web browser.

I'm telling the OP to do it like that because I did it like that in my last
several projects that used Gnuplot, and I disregarded the popen() option
each time.

--
Phlip
http://www.greencheese.org/ZeekLand <-- NOT a blog!!!

[Markus Schoder]

Phlip wrote:

Markus Schoder wrote:

gnuplot reads commands from stdin providing them through popen seems to
work.
This is example why off-topic threads damage USENET.

OMG! I hope the fabric of the universe was not damaged as well...

You also managed to carefully omit the part were I show how you can
safely interact with gnuplot in batch mode using popen.

Also wasn't it you who proposed not long ago that platform neutral APIs
like POSIX should not be considered off-topic? Well, popen is POSIX.
Write a batch of gnuplot commands, including one command to output your plot
in a portable format, such as PNG. Then display this format, independent of
gnuplot, in a viewer such as a web browser.

I'm telling the OP to do it like that because I did it like that in my last
several projects that used Gnuplot, and I disregarded the popen() option
each time.
That may or may not be helpful to the OP. It certainly does not answer
his very specific question.
--
Phlip
http://www.greencheese.org/ZeekLand <-- NOT a blog!!!

You might want to check this link it currently points to a placeholder
page chock full of advertising.

[Phlip]

Markus Schoder wrote:

Also wasn't it you who proposed not long ago that platform neutral APIs
like POSIX should not be considered off-topic?
No.

The distinction is between high-level discussions of various programming
topics between regulars, and helping newbies. Help them with a reply that is
as detailed, accurate, and on-topic as you can, and direct them elsewhere.

As I pointed out many years ago, and someone reminded me not long ago,
off-topic answers cannot easily be reviewed by others.
Well, popen is POSIX.
Safetly interacting with gnuplot via popen is high-risk and fragile. The OP
needs a simple solution...
That may or may not be helpful to the OP. It certainly does not answer
his very specific question.
"Hi. How do I shoot my foot off with this machine gun?"

Here, use a plinker.
http://www.greencheese.org/ZeekLand <-- NOT a blog!!! You might want to check this link it currently points to a placeholder
page chock full of advertising.

Jeeze good free help is hard to find. My bro Peter Merel, who runs that
site, seems to have had his license stolen by a robot...

--
Phlip
http://www.greencheese.org/ZeekLand <-- NOT a blog!!!

[Noah Roberts]

Phlip wrote:

I'm telling the OP to do it like that because I did it like that in my last
several projects that used Gnuplot, and I disregarded the popen() option
each time.

Well, since _you_ do it that way and _you_ disregarded popen() then we
should all shut-up and let _you_ tell the OP how it should be done.

[Phlip]

Noah Roberts wrote:

Phlip wrote:

I'm telling the OP to do it like that because I did it like that in my
last
several projects that used Gnuplot, and I disregarded the popen() option
each time.

Well, since _you_ do it that way and _you_ disregarded popen() then we
should all shut-up and let _you_ tell the OP how it should be done.

Yes. Anyone else with Gnuplot experience, raise their hands.

BTW the Ruby library I suggested uses the equivalent of popen... ;-)

--
Phlip
http://www.greencheese.org/ZeekLand <-- NOT a blog!!!

[Diego Martins]

Markus Schoder wrote:

You want popen(). Like in the following example: [snip] fputs("plot sin(x)\n", pg);
fflush(pg);
sleep(3);
pclose(pg);
}

why are you using sleep(3) there?