.. _timerfd-howto:

*****************************
  timer file descriptor HOWTO
*****************************

:Release: 1.13

This HOWTO discusses Python's support for the linux timer file descriptor.


Examples
========

The following example shows how to use a timer file descriptor
to execute a function twice a second:

.. code-block:: python

   # 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)

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

   # 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()

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

   # 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