Linux下C編程：sigsuspend執行過程分析

用於在接受到某個信號之前，臨時用mask替換進程的信號掩碼，並暫停進程執行，直到收到信號為止。

/*The sigsuspend() function replaces the current signal mask of the calling thread with the set of signals pointed     
   to by sigmask and then suspends the thread until delivery of a signal whose action is either to execute a signal-catching     
  function or to terminate the process. This will not cause any other signals that may have been pending on the process to     
  become pending on the thread.    
If the action is to terminate the process then sigsuspend() will never return. If the action is to execute a signal-catching     
  function, thensigsuspend() will return after the signal-catching function returns, with the signal mask restored to the set     
  that existed prior to thesigsuspend() call.    
It is not possible to block signals that cannot be ignored. This is enforced by the system without causing an error to be indicated.*/

也就是說，sigsuspend後，進程就掛在那裡，等待著開放的信號的喚醒。系統在接受到信號後，馬上就把現在的信號集還原為原來的，然後調用處理函數。

Stevens在《Unix環境高級編程》一書中是如是回答的“If a signal is caught and if the signal handler returns, then sigsuspend returns and the signal mask of the process is set to its value before the call to sigsuspend.”，由於sigsuspend是原子操作，所以這句給人的感覺就是先調用signal handler先返回，然後sigsuspend再返回。

int main(void) {     
   sigset_t   newmask, oldmask, zeromask;     
         
   if (signal(SIGINT, sig_int) == SIG_ERR)     
      err_sys("signal(SIGINT) error");     
         
   sigemptyset(&zeromask);     
         
   sigemptyset(&newmask);     
   sigaddset(&newmask, SIGINT);     
   /* block SIGINT and save current signal mask */ 
   if (sigprocmask(SIG_BLOCK, &newmask, &oldmask) < 0)     
      err_sys("SIG_BLOCK error");     
         
   /* critical region of code */ 
   pr_mask("in critical region: ");     
         
   /* allow all signals and pause */ 
   if (sigsuspend(&zeromask) != -1)     
      err_sys("sigsuspend error");     
   pr_mask("after return from sigsuspend: ");     
         
   /* reset signal mask which unblocks SIGINT */ 
   if (sigprocmask(SIG_SETMASK, &oldmask, NULL) < 0)     
      err_sys("SIG_SETMASK error");     
         
   /* and continue processing ... */ 
   exit(0);     
}     
         
static void sig_int(int signo) {     
   pr_mask("\nin sig_int: ");     
   return;     
}

結果：

$a.out 
in critical region: SIGINT     
^C     
in sig_int: SIGINT     
after return from sigsuspend: SIGINT

如果按照sig_handler先返回，那麼SIGINT是不該被打印出來的，因為那時屏蔽字還沒有恢復，所有信號都是不阻塞的。那麼是Stevens說錯了麼？當然沒有，只是Stevens沒有說請在sigsuspend的原子操作中到底做了什麼？

sigsuspend的整個原子操作過程為：

(1) 設置新的mask阻塞當前進程；

(2) 收到信號，恢復原先mask；

(3) 調用該進程設置的信號處理函數；

(4) 待信號處理函數返回後，sigsuspend返回。

大致就是上面這個過程，噢，原來signal handler是原子操作的一部分，而且是在恢復屏蔽字後執行的，所以上面的例子是沒有問題的，Stevens說的也沒錯。由於Linux和Unix的千絲萬縷的聯系，所以在兩個平台上絕大部分的系統調用的語義是一致的。上面的sigsuspend的原子操作也是從《深入理解Linux內核》一書中揣度出來的。書中的描述如下：

/*The sigsuspend( ) system call puts the process in the TASK_INTERRUPTIBLE state, after having blocked the standard signals specified    
 by a bit mask array to which the mask parameter points. The process will wake up only when a nonignored, nonblocked signal is sent     
 to it. The corresponding sys_sigsuspend( ) service routine executes these statements:    
*/ 
         
mask &= ~(sigmask(SIGKILL) | sigmask(SIGSTOP));     
spin_lock_irq(¤t->sigmask_lock);     
saveset = current->blocked;     
siginitset(¤t->blocked, mask);     
recalc_sigpending(current);     
spin_unlock_irq(¤t->sigmask_lock);     
regs->eax = -EINTR;     
while (1) {     
    current->state = TASK_INTERRUPTIBLE;     
    schedule(  );     
    if (do_signal(regs, &saveset))     
        return -EINTR;     
}