> On 29 Nov 2021, at 22:31, Jen Kris <jenk...@tutanota.com> wrote:
>
> Thanks to you and Cameron for your replies. The C side has an epoll_ctl set,
> but no event loop to handle it yet. I'm putting that in now with a pipe
> write in Python-- as Cameron pointed out that is the likely source of
> blocking on C. The pipes are opened as rdwr in Python because that's
> nonblocking by default. The child will become more complex, but not in a way
> that affects polling. And thanks for the tip about the c-string termination.
>
flags is a bit mask. You say its BLOCKing by not setting os.O_NONBLOCK.
You should not use O_RDWR when you only need O_RDONLY access or only O_WRONLY
access.
You may find
man 2 open
useful to understand in detail what is behind os.open().
Barry
>
>
> Nov 29, 2021, 14:12 by ba...@barrys-emacs.org:
>
> On 29 Nov 2021, at 20:36, Jen Kris via Python-list <python-list@python.org>
> wrote:
>
> I have a C program that forks to create a child process and uses execv to
> call a Python program. The Python program communicates with the parent
> process (in C) through a FIFO pipe monitored with epoll().
>
> The Python child process is in a while True loop, which is intended to keep
> it running while the parent process proceeds, and perform functions for the C
> program only at intervals when the parent sends data to the child -- similar
> to a daemon process.
>
> The C process writes to its end of the pipe and the child process reads it,
> but then the child process continues to loop, thereby blocking the parent.
>
> This is the Python code:
>
> #!/usr/bin/python3
> import os
> import select
>
> #Open the named pipes
> pr = os.open('/tmp/Pipe_01', os.O_RDWR)
> Why open rdwr if you are only going to read the pipe?
> pw = os.open('/tmp/Pipe_02', os.O_RDWR)
> Only need to open for write.
>
> ep = select.epoll(-1)
> ep.register(pr, select.EPOLLIN)
>
> Is the only thing that the child does this:
> 1. Read message from pr
> 2. Process message
> 3. Write result to pw.
> 4. Loop from 1
>
> If so as Cameron said you do not need to worry about the poll.
> Do you plan for the child to become more complex?
>
> while True:
>
> events = ep.poll(timeout=2.5, maxevents=-1)
> #events = ep.poll(timeout=None, maxevents=-1)
>
> print("child is looping")
>
> for fileno, event in events:
> print("Python fileno")
> print(fileno)
> print("Python event")
> print(event)
> v = os.read(pr,64)
> print("Pipe value")
> print(v)
>
> The child process correctly receives the signal from ep.poll and correctly
> reads the data in the pipe, but then it continues looping. For example, when
> I put in a timeout:
>
> child is looping
> Python fileno
> 4
> Python event
> 1
> Pipe value
> b'10\x00'
> The C code does not need to write a 0 bytes at the end.
> I assume the 0 is from the end of a C string.
> UDS messages have a length.
> In the C just write 2 byes in the case.
>
> Barry
> child is looping
> child is looping
>
> That suggests that a while True loop is not the right thing to do in this
> case. My question is, what type of process loop is best for this situation?
> The multiprocessing, asyncio and subprocess libraries are very extensive, and
> it would help if someone could suggest the best alternative for what I am
> doing here.
>
> Thanks very much for any ideas.
>
>
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