关于 sigwait() 的模棱两可的描述

Question

如果没有信号 set 在调用时处于挂起状态，线程应为 暂停直到一个或多个成为 待办的。由 set 定义的信号 应已被阻止 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。

这真是模棱两可，pending和block这里有什么区别？

它关于如何在sigwait和sigaction之间进行选择的结论根本不清楚：

总之，当需要 代码运行以响应 通知一个异步信号 线程，sigwait() 应该用于 处理信号。交替- tively，如果实现提供信号量，它们也可以是 使用，在 sigwait() 或 从信号处理例程中 以前注册 使用 sigaction()。

有人可以让sigwait的理由更合理吗？

Answer 1

每个进程都有一个与之关联的所谓信号掩码，它定义了被阻塞的一组信号。信号掩码可以通过setprocmask(2)（单线程代码）和pthread_sigmask(3)（多线程代码）查询或设置。

无论何时发出信号（通过kill(2) 或raise(3) 显式发出信号，或通过诸如分段错误引发SIGSEGV 之类的其他机制），都会根据当前信号掩码检查该信号。如果信号未被阻塞，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塞，则信号状态设置为pending，程序继续执行。

因此考虑以下示例程序：

#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.

“待处理”和“已阻止”并不相互排斥。

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.

所以sigaction() 用于允许其他代码运行，直到信号未决，而sigwait() 暂停线程的执行，直到信号未决但被阻塞。