Thanks very much for your reply.
I am now getting a single event returned in Python, but it's not the right
event, as I'll explain below.
I rearranged the Python code based on your comments:
#!/usr/bin/python3
import sys
import os
import select
print("Inside Python")
event_fd = int(sys.argv[3])
print("Eventfd received by Python")
print(event_fd)
event_write_value = 100
ep = select.epoll(-1)
ep.register(event_fd, select.EPOLLIN | select.EPOLLOUT )
os.set_blocking(event_fd, False)
#__
print("Starting poll loop")
for fd_event in ep.poll():
print("Python fd_event")
print(fd_event)
fd_received = fd_event[0]
event_received = fd_event[1]
You advised to leave off select.EPOLLOUT from the line ep.register(event_fd,
select.EPOLLIN | select.EPOLLOUT ) -- which makes sense because I'm not waiting
for that event -- but without it both processes freeze in the for loop (below
print("Starting poll loop")) so we never receive an EPOLLIN event. So I
included it, and here is the screen output from gdb:
Inside Python
Eventfd received by Python
5
Everything OK in Python
Starting poll loop
Python fd_event
(5, 4)
Writing to Python
5 Received from Python
8 Writing to Python
Failed epoll_wait Bad file descriptor
5 Received from Python
8 Writing to Python
Failed epoll_wait Bad file descriptor
5 Received from Python
-1time taken 0.000629
Failed to close epoll file descriptor
Unlink_shm status: Bad file descriptor
fn() took 0.000717 seconds to execute
[Inferior 1 (process 26718) exited normally]
(gdb) q
The Python fd_event tuple is 5, 4 -- 5 is the correct file descriptor and 4 is
an EPOLLOUT event, which is not what I want.
The eventfd is created in C as nonblocking:
int eventfd_initialize() {
int efd = eventfd(0, EFD_NONBLOCK);
return efd; }
When C writes it calls epoll_wait:
ssize_t epoll_write(int event_fd, int epoll_fd, struct epoll_event *
event_struc, int action_code)
{
int64_t ewbuf[1];
ewbuf[0] = (int64_t)action_code;
int maxevents = 1;
int timeout = -1;
fprintf(stdout, " Writing to Python \n%d", event_fd);
write(event_fd, &ewbuf, 8);
if (epoll_wait(epoll_fd, event_struc, maxevents, timeout) == -1)
{
fprintf(stderr, "Failed epoll_wait %s\n", strerror(errno));
}
ssize_t rdval = read(event_fd, &ewbuf, 8);
fprintf(stdout, " Received from Python \n%ld", rdval);
return 0;
}
The C side initializes its epoll this way:
int epoll_initialize(int efd, int64_t * output_array)
{
struct epoll_event ev = {};
int epoll_fd = epoll_create1(0);
struct epoll_event * ptr_ev = &ev;
if(epoll_fd == -1)
{
fprintf(stderr, "Failed to create epoll file descriptor\n");
return 1;
}
ev.events = EPOLLIN | EPOLLOUT;
ev.data.fd = efd; //was 0
if(epoll_ctl(epoll_fd, EPOLL_CTL_ADD, efd, &ev) == -1)
{
fprintf(stderr, "Failed to add file descriptor to epoll\n");
close(epoll_fd);
return 1;
}
output_array[0] = epoll_fd;
output_array[1] = (int64_t)ptr_ev; //&ev;
return 0;
}
Technically C is not waiting for an EPOLLIN event, but again without it both
processes freeze unless either C or Python includes both events. So that
appears to be where the problem is.
The Linux epoll man page says, "epoll_wait waits for I/O events, blocking the
calling thread if no events are currently available."
https://man7.org/linux/man-pages/man7/epoll.7.html. That may be the clue to
why both processes freeze when I poll on only one event in each one.
Thanks for any ideas based on this update, and thanks again for your earlier
reply.
Jen
--
Sent with Tutanota, the secure & ad-free mailbox.
Nov 25, 2021, 06:34 by ba...@barrys-emacs.org:
>
>
>
>> On 24 Nov 2021, at 22:42, Jen via Python-list <>> python-list@python.org>> >
>> wrote:
>>
>> I have a C program that uses fork-execv to run Python 3.10 in a child
>> process, and I am using eventfd with epoll for IPC between them. The
>> eventfd file descriptor is created in C and passed to Python through execv.
>> Once the Python child process starts I print the file descriptor to verify
>> that it is correct (it is).
>>
>> In this scenario C will write to the eventfd at intervals and Python will
>> read the eventfd and take action based on the value in the eventfd. But in
>> the Python while True loop I get "BlockingIOError: [Errno 11] Resource
>> temporarily unavailable" then with each new read it prints "Failed
>> epoll_wait Bad file descriptor."
>>
>> This is the Python code:
>>
>> #!/usr/bin/python3
>> import sys
>> import os
>> import select
>>
>> print("Inside Python")
>>
&
