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