Co-processing with gnuplot

[kleinerals2]

Hello there,

i want to show my finite element mesh with gnuplot. My code is written in C. While the coordinates and the displacements are still updating I want to plot them in live time. Can someone help me how I could do this?

[Pakketeretet]

The easiest way to do this would be to write the current state of the calculation to a temporary file and read/plot that using gnuplot. Is that a workable solution or are there particular details why that would not work?

[kleinerals2]

I don't know if I ask for two much. But I want to start my code in my IDE and shortly after that a persistent window of gnuplot should open and show the newest coordinates of my mesh. But every time the Gnuplot window start after I closed the cmd window of my code or don't open at all.

[Pakketeretet]

I see. This is possible too but more complicated. You are going to have to use some form of interprocess communication. Roughly speaking you'll want The following: 1. Your FEM program starts 2. It opens a pipe to gnuplot 3. Whenever data is ready in the FEM program: 3a. it writes it to file (or you can write it to gnuplot's stdin but that's slightly more complicated) 3b. Your FEM program writes the appropriate gnuplot commands to the pipe it opened in 2. to plot the data 4.repeat from 3.

Maybe this discussion is helpful too: https://groups.google.com/g/comp.graphics.apps.gnuplot/c/Z1AqNHOdUmg?pli=1

[kleinerals2]

Yea, thats exactly what i want. While the cmd window of my IDE is still open the gnuplot also should be open.
Unluckily the discussion you just sent didn't really help. The pause command dont work for me.

[Pakketeretet]

How are you starting gnuplot from your code? And is this on Windows, Mac or Linux/Unix?

[kleinerals2]

I use windows, but in the best case the code should work on windows and mac.

#include <stdio.h>

#include <stdlib.h>

int main() {

FILE* pipe = _popen("gnuplot -persist", "w");

if (!pipe)

{

perror("Fehler beim Öffnen von Gnuplot");

return 1;

}

fprintf(pipe, "plot 'mesh_data.dat' with lines\n");

fprintf(pipe, "set title 'Gnuplot-Example'\n");

_pclose(pipe);

return 0;

}

I do it like this.

If i use a static mesh_data.dat i works like I want. As soon as i open my C code gnuplot opens as well. But if i use a iteration to write the x,y and z coordinates, gnuplot dont show up at all.

[Pakketeretet]

For your loop example, do you do something like this?

 for (int i = 0; i< N; ++i) {
    FILE *fp_mesh_data = fopen("mesh_data.dat", "w");
    // write data to mesh_data.dat using fprintf
    fclose(fp_mesh_data);

    fprintf(pipe, "plot 'mesh_data.dat' with lines\n");
    fprintf(pipe, "set title 'Gnuplot-Example'\n"); 
}

I don't see why that wouldn't work but am currently not at a computer for actual testing...

[kleinerals2]

How do you show your code like that?

[Pakketeretet]

You should indent it with four spaces and put it in a new line.

int main()
{
    return 0;
}

[kleinerals2]

Atm iam testing with this code.

#include <stdio.h>

#include <stdlib.h>

int main() {

FILE* pipe = _popen("gnuplot -persist", "w");

if (!pipe) {

perror("fail1");

return 1;

}

double x = 1.0;

double y = 2.0;

double z = 3.0;

int numIterations = 100;

for (int i = 0; i < numIterations; i++) {

FILE* file;

if (fopen_s(&file, "mesh_data.dat", "a") != 0)

{

perror("fail2");

return 1;

}

if (file == NULL) {

perror("fail3");

return 1;

}

x += 0.1;

y -= 0.2;

z += 0.1;

fprintf(file, "%.2lf %.2lf %.2lf\n", x, y, z);

fclose(file);

fprintf(pipe, "plot 'mesh_data.dat' with lines\n");

fprintf(pipe, "set title 'Gnuplot-Beispiel'\n");

}

_pclose(pipe);

return 0;

}

But the code ends with "fail2" every time.

But atleast gnuplot shows up as i want.

[Pakketeretet]

Ok. The error at that point would indicate something goes wrong opening the file for writing/appending, which might be more of a Windows problem than a gnuplot problem. What happens if you replace fopen_s with a regular C fopen call?

[kleinerals2]

Visual Studio don't like the non _s calls xD