APUE中sigpending代码解释不了解

jiangf

AUPE第二版的10.13节中讲到 sigpending 函数,对下面的这段代码不了解

#include "apue.h" static void sig_quit(int); int main(void) { &nbsp;&nbsp;&nbsp;&nbsp;sigset_t    newmask, oldmask, pendmask; &nbsp;&nbsp;&nbsp;&nbsp;if (signal(SIGQUIT, sig_quit) == SIG_ERR) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;err_sys("can't catch SIGQUIT"); &nbsp;&nbsp;&nbsp;&nbsp;/* &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;* Block SIGQUIT and save current signal mask. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;*/ &nbsp;&nbsp;&nbsp;&nbsp;sigemptyset(&newmask); &nbsp;&nbsp;&nbsp;&nbsp;sigaddset(&newmask, SIGQUIT); &nbsp;&nbsp;&nbsp;&nbsp;if (sigprocmask(SIG_BLOCK, &newmask, &oldmask) < 0) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;err_sys("SIG_BLOCK error"); &nbsp;&nbsp;&nbsp;&nbsp;sleep(5);   /* SIGQUIT here will remain pending */ &nbsp;&nbsp;&nbsp;&nbsp;if (sigpending(&pendmask) < 0) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;err_sys("sigpending error"); &nbsp;&nbsp;&nbsp;&nbsp;if (sigismember(&pendmask, SIGQUIT)) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;printf("\nSIGQUIT pending\n"); &nbsp;&nbsp;&nbsp;&nbsp;/* &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;* Reset signal mask which unblocks SIGQUIT. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;*/ &nbsp;&nbsp;&nbsp;&nbsp;if (sigprocmask(SIG_SETMASK, &oldmask, NULL) < 0) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;err_sys("SIG_SETMASK error"); &nbsp;&nbsp;&nbsp;&nbsp;printf("SIGQUIT unblocked\n"); &nbsp;&nbsp;&nbsp;&nbsp;sleep(5);   /* SIGQUIT here will terminate with core file */ &nbsp;&nbsp;&nbsp;&nbsp;exit(0); } static void sig_quit(int signo) { &nbsp;&nbsp;&nbsp;&nbsp;printf("caught SIGQUIT\n"); &nbsp;&nbsp;&nbsp;&nbsp;if (signal(SIGQUIT, SIG_DFL) == SIG_ERR) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;err_sys("can't reset SIGQUIT"); }

代码输出:
SIGQUIT pending          after return from sleep
caught SIGQUIT           in signal handler
SIGQUIT unblocked        after return from sigprocmask


在 if (sigprocmask(SIG_BLOCK, &newmask, &oldmask) < 0)这句代码中已经讲SIGQUIT信号阻塞了,在未解除阻塞之前这么还会捕
获到SIGQUIT这个信号呢.

jiangf

补充一下,AUPE中对这个情况的解释是SIGQUIT在进程睡眠期间产生,这个信号现在是未决的并且是非阻塞的,所以它会在sigprocmask 函数返回之前递送给该进程.
If we generate the quit signal during this sleep period, the signal is now pending and unblocked, so it is delivered before sigprocmask returns.

在sleep之前进程该信号都已经阻塞了呀,怎么还说是非阻塞

bobozhang

你没有搞懂吧，是这句if (sigprocmask(SIG_SETMASK, &oldmask, NULL) < 0) unblock后才收到的sigquit，因为sigprocmask是一个系统调用，在从它返回用户空间时内核会去检查信号的，这时，sigquit已经可以处理了，于是就马上处理后才回到用户空间执行那句printf("SIGQUIT unblocked\n");当然就是你看到的输入顺序了

[ 本帖最后由 bobozhang 于 2009-5-6 10:38 编辑 ]

wwdwwd

是这样的：在信号被阻塞期间发送给进程的信号会被推迟发送，直到进程解除对某信号的阻塞，这时候信号才会到来
请看apue p269页