Doc/howto/timerfd.rst
.. _timerfd-howto:
timer file descriptor HOWTO
:Release: 1.13
This HOWTO discusses Python's support for the linux timer file descriptor.
The following example shows how to use a timer file descriptor to execute a function twice a second:
.. code-block:: python
import os, time
fd = os.timerfd_create(time.CLOCK_REALTIME)
os.timerfd_settime(fd, initial=1, interval=0.5)
try: # Process timer events four times. for _ in range(4): # read() will block until the timer expires _ = os.read(fd, 8) print("Timer expired") finally: # Remember to close the timer file descriptor! os.close(fd)
To avoid the precision loss caused by the :class:float type,
timer file descriptors allow specifying initial expiration and interval
in integer nanoseconds with _ns variants of the functions.
This example shows how :func:~select.epoll can be used with timer file
descriptors to wait until the file descriptor is ready for reading:
.. code-block:: python
import os, time, select, socket, sys
ep = select.epoll()
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM) sock.bind(("127.0.0.1", 1234)) sock.setblocking(False) sock.listen(1) ep.register(sock, select.EPOLLIN)
num = 3 fds = [] for _ in range(num): fd = os.timerfd_create(time.CLOCK_REALTIME, flags=os.TFD_NONBLOCK) fds.append(fd) # Register the timer file descriptor for read events ep.register(fd, select.EPOLLIN)
for i, fd in enumerate(fds, start=1): one_sec_in_nsec = 10**9 i = i * one_sec_in_nsec os.timerfd_settime_ns(fd, initial=i//4, interval=i//4)
timeout = 3 try: conn = None is_active = True while is_active: # Wait for the timer to expire for 3 seconds. # epoll.poll() returns a list of (fd, event) pairs. # fd is a file descriptor. # sock and conn[=returned value of socket.accept()] are socket objects, not file descriptors. # So use sock.fileno() and conn.fileno() to get the file descriptors. events = ep.poll(timeout)
# If more than one timer file descriptors are ready for reading at once,
# epoll.poll() returns a list of (fd, event) pairs.
#
# In this example settings,
# 1st timer fires every 0.25 seconds in 0.25 seconds. (0.25, 0.5, 0.75, 1.0, ...)
# 2nd timer every 0.5 seconds in 0.5 seconds. (0.5, 1.0, 1.5, 2.0, ...)
# 3rd timer every 0.75 seconds in 0.75 seconds. (0.75, 1.5, 2.25, 3.0, ...)
#
# In 0.25 seconds, only 1st timer fires.
# In 0.5 seconds, 1st timer and 2nd timer fires at once.
# In 0.75 seconds, 1st timer and 3rd timer fires at once.
# In 1.5 seconds, 1st timer, 2nd timer and 3rd timer fires at once.
#
# If a timer file descriptor is signaled more than once since
# the last os.read() call, os.read() returns the number of signaled
# as host order of class bytes.
print(f"Signaled events={events}")
for fd, event in events:
if event & select.EPOLLIN:
if fd == sock.fileno():
# Check if there is a connection request.
print(f"Accepting connection {fd}")
conn, addr = sock.accept()
conn.setblocking(False)
print(f"Accepted connection {conn} from {addr}")
ep.register(conn, select.EPOLLIN)
elif conn and fd == conn.fileno():
# Check if there is data to read.
print(f"Reading data {fd}")
data = conn.recv(1024)
if data:
# You should catch UnicodeDecodeError exception for safety.
cmd = data.decode()
if cmd.startswith("stop"):
print(f"Stopping server")
is_active = False
else:
print(f"Unknown command: {cmd}")
else:
# No more data, close connection
print(f"Closing connection {fd}")
ep.unregister(conn)
conn.close()
conn = None
elif fd in fds:
print(f"Reading timer {fd}")
count = int.from_bytes(os.read(fd, 8), byteorder=sys.byteorder)
print(f"Timer {fds.index(fd) + 1} expired {count} times")
else:
print(f"Unknown file descriptor {fd}")
finally: for fd in fds: ep.unregister(fd) os.close(fd) ep.close()
This example shows how :func:~select.select can be used with timer file
descriptors to wait until the file descriptor is ready for reading:
.. code-block:: python
import os, time, select, socket, sys
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM) sock.bind(("127.0.0.1", 1234)) sock.setblocking(False) sock.listen(1)
num = 3 fds = [os.timerfd_create(time.CLOCK_REALTIME, flags=os.TFD_NONBLOCK) for _ in range(num)] select_fds = fds + [sock]
for i, fd in enumerate(fds, start=1): os.timerfd_settime(fd, initial=i/4, interval=i/4)
timeout = 3 try: conn = None is_active = True while is_active: # Wait for the timer to expire for 3 seconds. # select.select() returns a list of file descriptors or objects. rfd, wfd, xfd = select.select(select_fds, select_fds, select_fds, timeout) for fd in rfd: if fd == sock: # Check if there is a connection request. print(f"Accepting connection {fd}") conn, addr = sock.accept() conn.setblocking(False) print(f"Accepted connection {conn} from {addr}") select_fds.append(conn) elif conn and fd == conn: # Check if there is data to read. print(f"Reading data {fd}") data = conn.recv(1024) if data: # You should catch UnicodeDecodeError exception for safety. cmd = data.decode() if cmd.startswith("stop"): print(f"Stopping server") is_active = False else: print(f"Unknown command: {cmd}") else: # No more data, close connection print(f"Closing connection {fd}") select_fds.remove(conn) conn.close() conn = None elif fd in fds: print(f"Reading timer {fd}") count = int.from_bytes(os.read(fd, 8), byteorder=sys.byteorder) print(f"Timer {fds.index(fd) + 1} expired {count} times") else: print(f"Unknown file descriptor {fd}") finally: for fd in fds: os.close(fd) sock.close() sock = None