# 18.8. `signal` — 设置异步事件处理程序

该模块提供了在 Python 中使用信号处理程序的机制。

## 18.8.1. 一般规则

The `signal.signal()` function allows defining custom handlers to be executed when a signal is received. A small number of default handlers are installed: `SIGPIPE` is ignored (so write errors on pipes and sockets can be reported as ordinary Python exceptions) and `SIGINT` is translated into a `KeyboardInterrupt` exception.

一旦设置，特定信号的处理程序将保持安装，直到它被显式重置（ Python 模拟 BSD 样式接口而不管底层实现），但 `SIGCHLD` 的处理程序除外，它遵循底层实现。

### 18.8.1.1. 执行 Python 信号处理程序

Python 信号处理程序不会在低级（ C ）信号处理程序中执行。相反，低级信号处理程序设置一个标志，告诉 virtual machine 稍后执行相应的 Python 信号处理程序（例如在下一个 bytecode 指令）。这会导致：

- 捕获同步错误是没有意义的，例如 `SIGFPE` 或 `SIGSEGV` ，它们是由 C 代码中的无效操作引起的。Python 将从信号处理程序返回到 C 代码，这可能会再次引发相同的信号，导致 Python 显然的挂起。 从Python 3.3开始，你可以使用 `faulthandler` 模块来报告同步错误。

- 纯 C 中实现的长时间运行的计算（例如在大量文本上的正则表达式匹配）可以在任意时间内不间断地运行，而不管接收到任何信号。计算完成后将调用 Python 信号处理程序。

### 18.8.1.2. 信号与线程

Python 信号处理程序总是在主 Python 线程中执行，即使信号是在另一个线程中接收的。这意味着信号不能用作线程间通信的手段。 你可以使用 `threading` 模块中的同步原函数。

此外，只允许主线程设置新的信号处理程序。

## 18.8.2. 模块内容

3.5 版更變: 信号（ SIG* ），处理程序（ `SIG_DFL` ， `SIG_IGN`）和 sigmask（ `SIG_BLOCK` ， `SIG_UNBLOCK` ， `SIG_SETMASK` ）下面列出的相关常量变成了 `enums` 。 `getsignal()` ， `pthread_sigmask()` ， `sigpending()` 和 `sigwait()` 函数返回人类可读的 `enums` 。

在 `signal` 模块中定义的变量是：

`signal.``SIG_DFL`

  这是两种标准信号处理选项之一；它只会执行信号的默认函数。 例如，在大多数系统上，对于 `SIGQUIT` 的默认操作是转储核心并退出，而对于 `SIGCHLD` 的默认操作是简单地忽略它。

`signal.``SIG_IGN`

  这是另一个标准信号处理程序，它将简单地忽略给定的信号。

`SIG*`

  All the signal numbers are defined symbolically. For example, the hangup signal is defined as `signal.SIGHUP`; the variable names are identical to the names used in C programs, as found in `<signal.h>`. The Unix man page for 『`signal()`』 lists the existing signals (on some systems this is signal(2), on others the list is in signal(7)). Note that not all systems define the same set of signal names; only those names defined by the system are defined by this module.

`signal.``CTRL_C_EVENT`

  对应于 `Ctrl+C` 击键事件的信号。此信号只能用于 `os.kill()` 。

  Availability: Windows.

  3.2 版新加入.

`signal.``CTRL_BREAK_EVENT`

  对应于 `Ctrl+Break` 击键事件的信号。此信号只能用于 `os.kill()` 。

  Availability: Windows.

  3.2 版新加入.

`signal.``NSIG`

  比最高信号数多一。

`signal.``ITIMER_REAL`

  实时递减间隔计时器，并在到期时发送 `SIGALRM` 。

`signal.``ITIMER_VIRTUAL`

  仅在进程执行时递减间隔计时器，并在到期时发送 SIGVTALRM 。

`signal.``ITIMER_PROF`

  Decrements interval timer both when the process executes and when the system is executing on behalf of the process. Coupled with ITIMER_VIRTUAL, this timer is usually used to profile the time spent by the application in user and kernel space. SIGPROF is delivered upon expiration.

`signal.``SIG_BLOCK`

  A possible value for the how parameter to `pthread_sigmask()` indicating that signals are to be blocked.

  3.3 版新加入.

`signal.``SIG_UNBLOCK`

  A possible value for the how parameter to `pthread_sigmask()` indicating that signals are to be unblocked.

  3.3 版新加入.

`signal.``SIG_SETMASK`

  A possible value for the how parameter to `pthread_sigmask()` indicating that the signal mask is to be replaced.

  3.3 版新加入.

The `signal` module defines one exception:

exception `signal.``ItimerError`

  Raised to signal an error from the underlying `setitimer()` or `getitimer()` implementation. Expect this error if an invalid interval timer or a negative time is passed to `setitimer()`. This error is a subtype of `OSError`.

  3.3 版新加入: This error used to be a subtype of `IOError`, which is now an alias of `OSError`.

The `signal` module defines the following functions:

`signal.``alarm`(time)

  If time is non-zero, this function requests that a `SIGALRM` signal be sent to the process in time seconds. Any previously scheduled alarm is canceled (only one alarm can be scheduled at any time). The returned value is then the number of seconds before any previously set alarm was to have been delivered. If time is zero, no alarm is scheduled, and any scheduled alarm is canceled. If the return value is zero, no alarm is currently scheduled. (See the Unix man page alarm(2).) Availability: Unix.

`signal.``getsignal`(signalnum)

  Return the current signal handler for the signal signalnum. The returned value may be a callable Python object, or one of the special values `signal.SIG_IGN`, `signal.SIG_DFL` or `None`. Here, `signal.SIG_IGN` means that the signal was previously ignored, `signal.SIG_DFL` means that the default way of handling the signal was previously in use, and `None` means that the previous signal handler was not installed from Python.

`signal.``pause`()

  Cause the process to sleep until a signal is received; the appropriate handler will then be called. Returns nothing. Not on Windows. (See the Unix man page signal(2).)

  See also `sigwait()`, `sigwaitinfo()`, `sigtimedwait()` and `sigpending()`.

`signal.``pthread_kill`(thread_id, signalnum)

  Send the signal signalnum to the thread thread_id, another thread in the same process as the caller. The target thread can be executing any code (Python or not). However, if the target thread is executing the Python interpreter, the Python signal handlers will be executed by the main thread. Therefore, the only point of sending a signal to a particular Python thread would be to force a running system call to fail with `InterruptedError`.

  Use `threading.get_ident()` or the `ident` attribute of `threading.Thread` objects to get a suitable value for thread_id.

  If signalnum is 0, then no signal is sent, but error checking is still performed; this can be used to check if the target thread is still running.

  Availability: Unix (see the man page pthread_kill(3) for further information).

  See also `os.kill()`.

  3.3 版新加入.

`signal.``pthread_sigmask`(how, mask)

  Fetch and/or change the signal mask of the calling thread. The signal mask is the set of signals whose delivery is currently blocked for the caller. Return the old signal mask as a set of signals.

  The behavior of the call is dependent on the value of how, as follows.

  - `SIG_BLOCK`: The set of blocked signals is the union of the current set and the mask argument.

  - `SIG_UNBLOCK`: The signals in mask are removed from the current set of blocked signals. It is permissible to attempt to unblock a signal which is not blocked.

  - `SIG_SETMASK`: The set of blocked signals is set to the mask argument.

  mask is a set of signal numbers (e.g. {`signal.SIGINT`, `signal.SIGTERM`}). Use `range(1, signal.NSIG)` for a full mask including all signals.

  For example, `signal.pthread_sigmask(signal.SIG_BLOCK, [])` reads the signal mask of the calling thread.

  Availability: Unix. See the man page sigprocmask(3) and pthread_sigmask(3) for further information.

  See also `pause()`, `sigpending()` and `sigwait()`.

  3.3 版新加入.

`signal.``setitimer`(which, seconds[, interval])

  Sets given interval timer (one of `signal.ITIMER_REAL`, `signal.ITIMER_VIRTUAL` or `signal.ITIMER_PROF`) specified by which to fire after seconds (float is accepted, different from `alarm()`) and after that every interval seconds. The interval timer specified by which can be cleared by setting seconds to zero.

  When an interval timer fires, a signal is sent to the process. The signal sent is dependent on the timer being used; `signal.ITIMER_REAL` will deliver `SIGALRM`, `signal.ITIMER_VIRTUAL` sends `SIGVTALRM`, and `signal.ITIMER_PROF` will deliver `SIGPROF`.

  The old values are returned as a tuple: (delay, interval).

  Attempting to pass an invalid interval timer will cause an `ItimerError`. Availability: Unix.

`signal.``getitimer`(which)

  Returns current value of a given interval timer specified by which. Availability: Unix.

`signal.``set_wakeup_fd`(fd)

  Set the wakeup file descriptor to fd. When a signal is received, the signal number is written as a single byte into the fd. This can be used by a library to wakeup a poll or select call, allowing the signal to be fully processed.

  The old wakeup fd is returned (or -1 if file descriptor wakeup was not enabled). If fd is -1, file descriptor wakeup is disabled. If not -1, fd must be non-blocking. It is up to the library to remove any bytes from fd before calling poll or select again.

  Use for example `struct.unpack('%uB' % len(data), data)` to decode the signal numbers list.

  When threads are enabled, this function can only be called from the main thread; attempting to call it from other threads will cause a `ValueError` exception to be raised.

  3.5 版更變: On Windows, the function now also supports socket handles.

`signal.``siginterrupt`(signalnum, flag)

  Change system call restart behaviour: if flag is `False`, system calls will be restarted when interrupted by signal signalnum, otherwise system calls will be interrupted. Returns nothing. Availability: Unix (see the man page siginterrupt(3) for further information).

  Note that installing a signal handler with `signal()` will reset the restart behaviour to interruptible by implicitly calling `siginterrupt()` with a true flag value for the given signal.

`signal.``signal`(signalnum, handler)

  Set the handler for signal signalnum to the function handler. handler can be a callable Python object taking two arguments (see below), or one of the special values `signal.SIG_IGN` or `signal.SIG_DFL`. The previous signal handler will be returned (see the description of `getsignal()` above). (See the Unix man page signal(2).)

  When threads are enabled, this function can only be called from the main thread; attempting to call it from other threads will cause a `ValueError` exception to be raised.

  The handler is called with two arguments: the signal number and the current stack frame (`None` or a frame object; for a description of frame objects, see the description in the type hierarchy or see the attribute descriptions in the `inspect` module).

  On Windows, `signal()` can only be called with `SIGABRT`, `SIGFPE`, `SIGILL`, `SIGINT`, `SIGSEGV`, `SIGTERM`, or `SIGBREAK`. A `ValueError` will be raised in any other case. Note that not all systems define the same set of signal names; an `AttributeError` will be raised if a signal name is not defined as `SIG*` module level constant.

`signal.``sigpending`()

  Examine the set of signals that are pending for delivery to the calling thread (i.e., the signals which have been raised while blocked). Return the set of the pending signals.

  Availability: Unix (see the man page sigpending(2) for further information).

  See also `pause()`, `pthread_sigmask()` and `sigwait()`.

  3.3 版新加入.

`signal.``sigwait`(sigset)

  Suspend execution of the calling thread until the delivery of one of the signals specified in the signal set sigset. The function accepts the signal (removes it from the pending list of signals), and returns the signal number.

  Availability: Unix (see the man page sigwait(3) for further information).

  See also `pause()`, `pthread_sigmask()`, `sigpending()`, `sigwaitinfo()` and `sigtimedwait()`.

  3.3 版新加入.

`signal.``sigwaitinfo`(sigset)

  Suspend execution of the calling thread until the delivery of one of the signals specified in the signal set sigset. The function accepts the signal and removes it from the pending list of signals. If one of the signals in sigset is already pending for the calling thread, the function will return immediately with information about that signal. The signal handler is not called for the delivered signal. The function raises an `InterruptedError` if it is interrupted by a signal that is not in sigset.

  The return value is an object representing the data contained in the `siginfo_t` structure, namely: `si_signo`, `si_code`, `si_errno`, `si_pid`, `si_uid`, `si_status`, `si_band`.

  Availability: Unix (see the man page sigwaitinfo(2) for further information).

  See also `pause()`, `sigwait()` and `sigtimedwait()`.

  3.3 版新加入.

  3.5 版更變: The function is now retried if interrupted by a signal not in sigset and the signal handler does not raise an exception (see PEP 475 for the rationale).

`signal.``sigtimedwait`(sigset, timeout)

  Like `sigwaitinfo()`, but takes an additional timeout argument specifying a timeout. If timeout is specified as `0`, a poll is performed. Returns `None` if a timeout occurs.

  Availability: Unix (see the man page sigtimedwait(2) for further information).

  See also `pause()`, `sigwait()` and `sigwaitinfo()`.

  3.3 版新加入.

  3.5 版更變: The function is now retried with the recomputed timeout if interrupted by a signal not in sigset and the signal handler does not raise an exception (see PEP 475 for the rationale).

## 18.8.3. 示例

Here is a minimal example program. It uses the `alarm()` function to limit the time spent waiting to open a file; this is useful if the file is for a serial device that may not be turned on, which would normally cause the `os.open()` to hang indefinitely. The solution is to set a 5-second alarm before opening the file; if the operation takes too long, the alarm signal will be sent, and the handler raises an exception.

    import signal, os

    def handler(signum, frame):
        print('Signal handler called with signal', signum)
        raise OSError("Couldn't open device!")

    # Set the signal handler and a 5-second alarm
    signal.signal(signal.SIGALRM, handler)
    signal.alarm(5)

    # This open() may hang indefinitely
    fd = os.open('/dev/ttyS0', os.O_RDWR)

    signal.alarm(0)          # Disable the alarm