Question

如果没有信号在调用时，set是挂起的，线程应该是暂停直到一个或多个成为悬而未决。由set定义的信号应该被封锁了调用sigwait（）的时间;否则，行为未定义。 sigwait（）对信号的影响集合中信号的动作是未指定的。

这真的很模糊，pending和block之间的区别是什么？

关于如何在sigwait和sigaction之间进行选择的结论尚不清楚：

总之，当有必要时代码运行以响应异步信号通知应该使用thread，sigwait（）处理信号。 Alterna- 如果实现提供了信号量，它们也可以使用，遵循sigwait（）或来自信号处理程序以前注册过与sigaction（）。

有人可以说sigwait更合理吗？

Answer 1

每个进程都有一个与之关联的信号掩码，它定义了阻塞的信号集。可以使用setprocmask(2)（对于单线程代码）和pthread_sigmask(3)（对于多线程代码）查询或设置信号掩码。

无论何时发出信号（通过kill(2)或raise(3)明确提出，或通过其他一些机制，如分段故障引发SIGSEGV），都会针对当前信号检查信号面具。如果信号未被阻止，则立即对其进行操作：如果设置则调用相应的信号处理程序，否则运行默认操作（通常以异常状态退出或忽略它）。如果信号被信号掩码阻止，则信号状态设置为挂起，程序继续执行。

请考虑以下示例程序：

#include <signal.h>
#include <stdio.h>

void on_sigusr1(int sig)
{
  // Note: Normally, it's not safe to call almost all library functions in a
  // signal handler, since the signal may have been received in a middle of a
  // call to that function.
  printf("SIGUSR1 received!\n");
}

int main(void)
{
  // Set a signal handler for SIGUSR1
  signal(SIGUSR1, &on_sigusr1);

  // At program startup, SIGUSR1 is neither blocked nor pending, so raising it
  // will call the signal handler
  raise(SIGUSR1);

  // Now let's block SIGUSR1
  sigset_t sigset;
  sigemptyset(&sigset);
  sigaddset(&sigset, SIGUSR1);
  sigprocmask(SIG_BLOCK, &sigset, NULL);

  // SIGUSR1 is now blocked, raising it will not call the signal handler
  printf("About to raise SIGUSR1\n");
  raise(SIGUSR1);
  printf("After raising SIGUSR1\n");

  // SIGUSR1 is now blocked and pending -- this call to sigwait will return
  // immediately
  int sig;
  int result = sigwait(&sigset, &sig);
  if(result == 0)
    printf("sigwait got signal: %d\n", sig);

  // SIGUSR1 is now no longer pending (but still blocked).  Raise it again and
  // unblock it
  raise(SIGUSR1);
  printf("About to unblock SIGUSR1\n");
  sigprocmask(SIG_UNBLOCK, &sigset, NULL);
  printf("Unblocked SIGUSR1\n");

  return 0;
}

输出：

SIGUSR1 received!
About to raise SIGUSR1
After raising SIGUSR1
sigwait got signal: 30
About to unblock SIGUSR1
SIGUSR1 received!
Unblocked SIGUSR1

Answer 2

来自signal(7)手册页：

Signal Mask and Pending Signals
    A  signal  may  be  blocked,  which means that it will not be delivered
    until it is later unblocked.  Between the time when it is generated and
    when it is delivered a signal is said to be pending.

“等待”和“被阻止”不是相互排斥的。

同样来自signal(7)手册页：

Synchronously Accepting a Signal
    Rather than asynchronously catching a signal via a signal  handler,  it
    is  possible to synchronously accept the signal, that is, to block exe-
    cution until the signal is delivered, at which point the kernel returns
    information about the signal to the caller.  There are two general ways
    to do this:

    * sigwaitinfo(2), sigtimedwait(2),  and  sigwait(3)  suspend  execution
      until  one  of  the signals in a specified set is delivered.  Each of
      these calls returns information about the delivered signal.

因此sigaction()用于允许其他代码在信号挂起之前运行，而sigwait()暂停执行该线程，直到信号挂起但被阻止。

关于sigwait（）的含糊描述

2 个答案: