Detect stop process Python

Question

I have a C program. I want it to run and stop at specific points, and let a python script do some things while it is stopped.

What I did was to put this line at the stopping points in the C program:

kill(getpid(), SIGSTOP);

If my python code has this line:

subprocess.call("./prog");

It is stuck forever, because the C program is never terminated.

If I use this:

subprocess.Popen("./prog");

It resumes before the code section was done.

How can I check if the subprocess is stopped? Or is there another solution? It is very important for me to keep the C program running.

Thank you, Marina

Answer 1

I don't know what's your purpose, if you want to check a subprocess' status, use Popen.poll() , if you want to wait for it to stop, use Popen.wait() .

Or you need to check its status periodically? If so, try threading.Timer() .

Answer 2

To synchronize the execution of your Python script with the child C process, you could use communication eg, write a byte to stdout putchar('\\0') in C program in the place where you want it to stop and wait while Python does something and try to read back a char getchar() to continue execution and in reverse in the Python script: read a byte, start doing something, and write byte to signal C program to continue:

from subprocess import Popen, PIPE

# start child process, do not block Python
p = Popen("./prog", stdin=PIPE, stdout=PIPE, bufsize=0)
while p.poll() is None: # while C program is running
    # block here until C program writes a byte and flushes its stdout buffer
    if not p.stdout.read(1): # EOF
        break
    # .. do something while C program waits
    p.stdin.write(b'\0') # signal C program to continue

To unbuffer stdout in C, call at the very beginning of the C program:

setvbuf(stdout, NULL,_IONBF,0);

Otherwise Python program won't see any output until you call fflush() explicitly or the corresponding buffer overflows.