sigsuspend 函数释疑「建议收藏」

手册：
#include <signal.h>

int sigsuspend(const sigset_t *sigmask);

The sigsuspend() function shall replace the current signal mask of the calling thread with the set of signals pointed to by sigmask and then suspend the thread until delivery of a signal whose action is either to execute a signal-catching function or to terminate the process. This shall 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() shall never return. If the action is to execute a signal-catching function, then sigsuspend() shall return after the signal-catching function returns, with the signal mask restored to the set that existed prior to the sigsuspend() 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.

也就是说，sigsuspend后，进程就挂在那里，等待着开放的信号的唤醒。系统在接受到信号后，马上就把现在的信号集还原为原来的，然后调用处理函数。

Unix提供了等待信号的系统调用，sigsuspend就是其中一个，下面我摘录的解释不错

CU 网友讨论的问题的核心就是到底sigsuspend先返回还是signal handler先返回。这个问题Stevens在《Unix环境高级编程》一书中是如是回答的“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.”，由于sigsuspend是原子操作，所以这句给人的感觉就是先调用signal handler先返回，然后sigsuspend再返回。但其第一个例子这么讲又说不通，看下面的代码：
CU上讨论该问题起于中的该例子：

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;
}

结果：
$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，那么SIGINT是不该被打印出来的，因为那时屏蔽字还没有恢复，所有信号都是不阻塞的。那么是Stevens说错了么？当然没有，只是Stevens没有说请在sigsuspend的原子操作中到底做了什么？
sigsuspend的整个原子操作过程为：
(1) 设置新的mask阻塞当前进程；
(2) 收到信号，调用该进程设置的信号处理函数；
(3) 待信号处理函数返回后，恢复原先mask；
(4) sigsuspend返回。
大致就是上面这个过程，噢，原来signal handler是原子操作的一部分，所以上面的例子是没有问题的，Stevens说的也没错。由于Linux和Unix的千丝万缕的联系，所以在两个平台上绝大部分的系统调用的语义是一致的。上面的sigsuspend的原子操作也是从《深入理解Linux内核》一书中揣度出来的。书中的描述如下：
The sigsuspend( ) system call puts the process in the TASK_INTERRUPTIBLE state, after having signaspecifie blocked the standard sid 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;
}
而最后的do_signal函数调用则是负责调用User Signal Handler的家伙。我想到这CU上的那个问题该被解疑清楚了吧。