每个进程都有与其相关的所谓 信号屏蔽 ，它定义了被 阻塞
的信号集。可以查询信号掩码或使用setprocmask(2)（对于单线程代码）和pthread_sigmask(3)（对于多线程代码）进行设置。

每当发出信号时（显式地通过kill(2)或raise(3)，或通过某种其他机制，例如分段故障升高SIGSEGV），都会根据当前信号掩码检查信号。如果未阻塞信号，则立即采取行动：如果设置了相应的信号处理程序，则将运行默认操作（通常以异常状态退出或忽略它）。如果信号被信号掩码阻止，则信号状态将设置为
待处理 ，程序将继续执行。

因此，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