当已经在信号处理程序中时接收到信号会发生什么? [英] What happens when a signal is received while already in a signal handler?

问题描述

我有一个父进程，它产生了几个子进程.我想知道何时通过注册SIGCHLD信号处理程序退出任何子进程.

I have a parent process spawning several child processes. I want to know when any child process exits by registering a SIGCHLD signal handler.

问题是，如果在父进程已经在信号处理程序中的情况下又收到另一个SIGCHLD(或任何其他信号)，会发生什么情况?

The question is, what happens if another SIGCHLD (or any other signal) is received, while the parent process is already in a signal handler?

我可以想到以下结果:

• 信号被忽略
• 该信号已排队，将在当前处理程序返回后立即进行处理
• 与主程序一样，当前处理程序又被中断

哪个是正确的?

推荐答案

在您的具体示例中(接收到相同的信号)，信号在信号处理程序完成之后被传递(因此，第2点是正确的).但是请注意，您可能会丢失"信号.

In your concrete example (the same signal being received), the signal is delivered after the signal handler has finished (so bullet point #2 is correct). Note, however, that you may "lose" signals.

这样做的原因是，当信号在其处理程序中时，它被阻止了.被阻止的信号被设置为未决，但未排队.术语待定"表示操作系统记住有一个信号等待下一次机会传递，而未排队"表示操作系统通过在某处设置标志来实现此目的，而不是通过保留有关如何操作的确切记录来进行.许多信号已经到达.

The reason for that is that while a signal is being inside its handler, it is blocked. Blocked signals are set to pending, but not queued. The term "pending" means that the operating system remembers that there is a signal waiting to be delivered at the next opportunity, and "not queued" means that it does this by setting a flag somewhere, but not by keeping an exact record of how many signals have arrived.

因此，在处理程序中，您可能会再收到2或3(或10)个SIGCHLD，但只能看到一个(所以在某些情况下，项目符号#1 也可以是) ).

Thus, you may receive 2 or 3 (or 10) more SIGCHLD while in your handler, but only see one (so in some cases, bullet point #1 can be correct, too).

请注意，可以传递给sigaction的几个标志会影响默认行为，例如SA_NODEFER(防止阻塞信号)和SA_NOCLDWAIT(在某些系统上可能根本不产生信号).

Note that several flags that you can pass to sigaction can affect the default behaviour, such as SA_NODEFER (prevents blocking signal) and SA_NOCLDWAIT (may not generate signal at all on some systems).

当然，现在，如果您收到其他类型的信号，则不能保证它不会中断您的处理程序.因此，最好不要使用非信号安全功能.

Now of course, if you receive a different type of signal, there's no guarantee that it won't interrupt your handler. For that reason, one preferrably doesn't use non signal safe functions.

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