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")
event_fd = int(sys.argv[3])
print("Eventfd received by Python")
print(event_fd)
ep = select.epoll(-1)
ep.register(event_fd, select.EPOLLIN | select.EPOLLOUT)
event_write_value = 100
while True:
print("Waiting in Python for event")
ep.poll(timeout=None, maxevents=- 1)
v = os.eventfd_read(event_fd)
if v != 99:
print("found")
print(v)
os.eventfd_write(event_fd, event_write_value)
if v == 99:
os.close(event_fd)
This is the C code that writes to Python, then waits for Python to write back:
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;
}
This is the screen output when I run with gdb:
Inside Python
Eventfd received by Python
5
Waiting in Python for event
Traceback (most recent call last):
File "/usr/local/lib/python3.10/runpy.py", line 196, in
_run_module_as_main
return _run_code(code, main_globals, None,
File "/usr/local/lib/python3.10/runpy.py", line 86, in _run_code
exec(code, run_globals)
File "/opt/P01_SH/NPC_CPython.py", line 36, in <module>
v = os.eventfd_read(event_fd)
BlockingIOError: [Errno 11] Resource temporarily unavailable
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 0.000548
Failed to close epoll file descriptor
Unlink_shm status: Bad file descriptor
fn() took 0.000648 seconds to execute
[Inferior 1 (process 12618) exited normally]
(gdb)
So my question is why do I get "BlockingIOError: [Errno 11] Resource
temporarily unavailable" and "Failed epoll_wait Bad file descriptor" from
Python?
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