accessing a fifo file

[klintkrossa]

Hello

I am making a fake radar.

I have setup a a fifo.

Code: Select all

mkfifo -m a=rw aprs.fifo
nano aprs.fifo
rtl_fm -p 7.826 -f 144.390M -s 22050|multimon-ng -t raw -a AFSK1200 -f alpha -A /dev/stdin > aprs.fifo

I would like to access the aprs.fifo with a quick look to see if there is new info on the file.

There are other process that I would like to check, like gps.

[scotty101]

I would like to access the aprs.fifo with a quick look to see if there is new info on the file.

Do you want to access it from a bash script, python script, c code etc etc?
From bash the "tail" command can be used to read the last few lines of a file and the number of lines read is configurable. Read "man tail"

[blimpyway]

Code: Select all

$ mkfifo aprs.fifo
$ tail -f aprs.fifo &
$ echo blablabla > aprs.fifo
$ echo more data > aprs.fifo 

tail -f just keeps waiting for more bytes to be appended to a file and displays them as they come out of pipe. the "&" puts it in background.

[hippy]

I would like to access the aprs.fifo with a quick look to see if there is new info on the file.

This is what I've used, noting this is in the Python forum area -

Code: Select all

while True:
  with open("aprs.fifo", "r") as f:
    for s in f:
      s = s.strip()
      print(s)

There are other process that I would like to check, like gps.

Given that the above code appears to block, it is probably best to set up multiple threads for each FIFO you are reading.

[klintkrossa]

I would like to access the aprs.fifo with a quick look to see if there is new info on the file.

Do you want to access it from a bash script, python script, c code etc etc?
From bash the "tail" command can be used to read the last few lines of a file and the number of lines read is configurable. Read "man tail"

Thanks for the need to clarify.

I would like to access the program inpython3. when I do access the program with "with open() as" or "with popen() as" it stalls. and of coarse if it is accessed with "x = open()" it creates a mess in the file.

[hippy]

I would like to access the program inpython3. when I do access the program with "with open() as" or "with popen() as" it stalls. and of coarse if it is accessed with "x = open()" it creates a mess in the file.

See my example program above. Yes, the "with open()" does seem to stall - or the "for s in f:" does - when there is nothing in the FIFO, but that shouldn't be a problem.

Write some simple reader and writer code to get to understand how FIFO's work. Then apply that to what you are wanting to actually do.

[Paeryn]

You can check to see if there is any data to be read from a fifo with an ioctl(FIONREAD), you need to use something like an array.array to store the result in.

Code: Select all

import array
import time
import fcntl
import termios

bytesLeft = array.array('l', [0])
f = open('test.fifo', 'r')
while True:
  fcntl.ioctl(f, termios.FIONREAD, bytesLeft, 1)
  if bytesLeft[0] == 0:
    print('fifo is empty')
  else:
      print('length of data in buffer =', bytesLeft[0])
      for s in f:
      s = s.strip()
      print(s)
  time.sleep(1)