Sharing Variable between C and Python

Question

I'm using a named pipe FIFO to communicate between a C ( a loop generating numbers is C) and python (listening to the number generated by C and read them and process them). The code working well except for each generated number in C i need to close the fifo to be able to see it in Python, otherwise it won't be shown in python till a close command is called. I don't know if opening and closing FIFO is a good idea to be able to read them correctly in python code or not. I need to note that the C code generates number every 50 millisecond. This is why I doubt opening and closing is a good idea or not. Here are my codes in C and Python:

C as Server:

while (1){
            t=time_in_ms();
            if (t-t0>=50){
                    t0=t;
                    flag=1;
            }
            else{
                    flag=0;
            }
            if (flag==1){
                    flag=0;
                    printf("%lld %lld\n",count,t);
                    count+=1;
                    fprintf(f,"%lld\r",t);
                    fflush(f);
                    fclose(f);
                    f=fopen("fifo","w");

            }
    }

And Code in Python as Client:

with open(FIFO) as fifo:
print("FIFO opened")
while True:
    data = fifo.read()
    if len(data) == 0:
            count=count+1
    else:
            count=0
    if count>2000:
        print("Writer closed")
        break
    print data
    x=x+1

The problem is that on both ends of the fifo you're using buffered IO. You should use open/write on the C side and os.open/os.read on the python side! — Antti Haapala
– Antti Haapala, Commented Nov 12, 2017 at 16:25
May I know what you mean by use open/write on the C side and os.open/os.read on the python side? Could you please let me know how I should change the code to address your comments? — user2684657
– user2684657, Commented Nov 12, 2017 at 16:45
Regarding the C-code: man 2 open man 2 write Also have a look at: man 3 fflush When working with C, the man-pages in Linux is awesome and great resource — David Bern
– David Bern, Commented Nov 12, 2017 at 16:48
What I think @AnttiHaapala is saying; don't treat FIFO as STREAM, use the File descriptor instead. Like the write function does or dprintf. I should add, that I have never used dprintf before and just speculating on the behaviour — David Bern
– David Bern, Commented Nov 12, 2017 at 16:52

David Bern · Accepted Answer · 2017-11-12 17:11:47Z

0

Here is a small working example

Python side:

with open('./test_out.fifo', 'r') as fo:
    message = fo.readline()
    print(message)

On the "server" C-side

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int main()
{
    int fifo_fd;
    fifo_fd = open("./test_out.fifo", O_WRONLY);
    dprintf(fifo_fd, "testing123\n");

    while(1)
    {
        sleep(1);
    }
    return 0;
}

The endless looping in the end of the C-program is just to demonstrate that we do not need to close the file before the data is readable in the python program

I should also say that I haven't done C-code in a while.

answered Nov 12, 2017 at 17:11

David Bern

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8 Comments

user2684657 Over a year ago

Do you know if there is a way that in python side I can check the fifo if the data is ready catch it otherwise be able to do other thinks? It seems the right now I'm in clock mode that will wait to a new data to come. I'm asking because I need to process the data after getting it in python, I don't want to lose the process power of system being blocked by reading FIFO.

David Bern Over a year ago

let me think about this a moment. I'll get back to you here.

user2684657 Over a year ago

let me explain my case, my sensor in C gets data every 50 millisecond and puts in FIFO, in python side when the FIFO has a data, I read it and analyze it once analysis is finished I should go back and check if there is any new data in FIFO and read it and do it again. My only concern is that if I'm analyzing data takes more than 50msec, I won't lose any data from FIFO. And I don't know what is the length of the fifo.

David Bern Over a year ago

To answer the size. Try this command sysctl fs.pipe-max-size Regarding the other problem, there are solutions to this, im sure of it. Think of the problem for a while. Formulate a question and I will try my best to help you solve it. Why dont I help you here in the comments? Simple, perhaps someone else has the same problem, he/she wont find that answer here & if you think about the problem for a while, perhaps you find someone else that already had that exact question already. Feel free to give an url to your future question in this comment

user2684657 Over a year ago

Of course, I will think about it and let you know about my findings.

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