关于sigwait（）的模棱两可的描述

pthread_sigmask(3)

kill(2)

raise(3)

SIGSEGV

#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;
}

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

signal(7)

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.

signal(7)

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.

sigaction()

sigwait()