timer file descriptor HOWTO

Version:

1.13

Dieses HOWTO bespricht Pythons Unterstützung für den Linux Timer File Descriptor.

Beispiele

Das folgende Beispiel zeigt, wie ein Timer File Descriptor verwendet wird, um eine Funktion zweimal pro Sekunde auszuführen.

# Practical scripts should use really use a non-blocking timer,
# we use a blocking timer here for simplicity.
import os, time

# Create the timer file descriptor
fd = os.timerfd_create(time.CLOCK_REALTIME)

# Start the timer in 1 second, with an interval of half a second
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)

Um den Präzisionsverlust durch den Typ float zu vermeiden, erlauben Timer File Descriptors die Angabe der anfänglichen Fälligkeit und des Intervalls in Nanosekunden mit den _ns Varianten der Funktionen.

Dieses Beispiel zeigt, wie epoll() mit Timer File Descriptors verwendet werden kann, um zu warten, bis der File Descriptor zum Lesen bereit ist.

import os, time, select, socket, sys

# Create an epoll object
ep = select.epoll()

# In this example, use loopback address to send "stop" command to the server.
#
# $ telnet 127.0.0.1 1234
# Trying 127.0.0.1...
# Connected to 127.0.0.1.
# Escape character is '^]'.
# stop
# Connection closed by foreign host.
#
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)

# Create timer file descriptors in non-blocking mode.
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)

# Start the timer with os.timerfd_settime_ns() in nanoseconds.
# Timer 1 fires every 0.25 seconds; timer 2 every 0.5 seconds; etc
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()

Dieses Beispiel zeigt, wie select() mit Timer File Descriptors verwendet werden kann, um zu warten, bis der File Descriptor zum Lesen bereit ist.

import os, time, select, socket, sys

# In this example, use loopback address to send "stop" command to the server.
#
# $ telnet 127.0.0.1 1234
# Trying 127.0.0.1...
# Connected to 127.0.0.1.
# Escape character is '^]'.
# stop
# Connection closed by foreign host.
#
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.bind(("127.0.0.1", 1234))
sock.setblocking(False)
sock.listen(1)

# Create timer file descriptors in non-blocking mode.
num = 3
fds = [os.timerfd_create(time.CLOCK_REALTIME, flags=os.TFD_NONBLOCK)
       for _ in range(num)]
select_fds = fds + [sock]

# Start the timers with os.timerfd_settime() in seconds.
# Timer 1 fires every 0.25 seconds; timer 2 every 0.5 seconds; etc
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