Prevent subprocess PIPE from blocking

Question

I want to utilize subprocess Popen to call strace on Linux. I also want to catch every line of output strace gives, in realtime if possible.

I came up with the following code for that, but for some reason I can't get it working. I'll only get the output AFTER I terminate the program.

from threading import Thread
from queue import Queue, Empty

pid = 1

def enqueue_output(out, queue):
    for line in iter(out.readline, b''):
        queue.put(line)
    out.close()

p = Popen(["strace", "-p", pid], stdout=subprocess.PIPE, bufsize=1)
q = Queue()
t = Thread(target=enqueue_output, args=(p.stdout, q))
t.daemon = True # thread dies with the program
t.start()

try:
    line = q.get_nowait()
    print("Got it! "+line)
except Empty:
    pass

Wouldn't you be better with installing signals, as in https://stackoverflow.com/questions/132058/showing-the-stack-trace-from-a-running-python-application ? — Demi-Lune, Feb 09 '19 at 17:53
Hi! I don't want to trace a Python programm, so there is no use for installing signals, isn't it? — Druckermann, Feb 09 '19 at 20:39
What is the intended purpose of the Queue? You could directly read from the subprocess output. — VPfB, Feb 09 '19 at 20:42
The Queue was meant to read the output from a separate thread as a kind of buffer. I tried it without the queue too, but that didn't work either. — Druckermann, Feb 09 '19 at 20:47

score 1 · Accepted Answer · answered Feb 10 '19 at 08:42

Here is a short working example:

Please note that:

strace writes to stderr (unless -o filename is given)
all arguments must be strings (or bytes), i.e. pid must be given as "1"
line buffering works only with universal newlines
you must be root to trace process 1

import subprocess

PID = 1 

p = subprocess.Popen(
    ["strace", "-p", str(PID)],
    stdin=subprocess.DEVNULL, stderr=subprocess.PIPE,
    universal_newlines=True, bufsize=1)
for line in p.stderr:
    line = line.rstrip()
    print(line)

Works like a charm! Thank you very much! – Druckermann Feb 10 '19 at 13:58 — Druckermann, Feb 10 '19 at 13:58

Prevent subprocess PIPE from blocking

1 Answers1