sigsuspend应用理解

下面是保护临界区代码不受相关信号干扰的代码片段
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sigprocmask(SIG_BLOCK, &newmask, &oldmask);

Critical region

sigsuspend(&waitmask);
sigprocmask(SIG_SETMASK, &oldmask, NULL);
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man sigsuspend:
1.sigsuspend() temporarily replaces the signal mask of the calling process;
2.and then suspends the process
3.then  sigsuspend() returns  after  the  signal  handler  returns；
4.and the signal    mask is restored to the state before the call to sigsuspend().



分析代码
============================
#include "apue.h"

static void    sig_int(int);

int
main(void)
{
    sigset_t    newmask, oldmask, waitmask;

    pr_mask("program start: ");

    if (signal(SIGINT, sig_int) == SIG_ERR)
        err_sys("signal(SIGINT) error");
    sigemptyset(&waitmask);
    sigaddset(&waitmask, SIGUSR1);
    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: ");

    /*
     * Pause, allowing all signals except SIGUSR1.
     */
    if (sigsuspend(&waitmask) != -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 ...
     */
    pr_mask("program exit: ");

    exit(0);
}

static void
sig_int(int signo)
{
    pr_mask("\nin sig_int: ");
}
=================================================

打印出
program start:
in critical region: SIGINT
^C
in sig_int: SIGINT SIGUSR1
after return from sigsuspend: SIGINT
program exit:

可见：在调用sigsuspend之前只有SIGINT被阻塞，调用sigsuspend后，信号屏蔽字集被临时替换成SIGUSR1，此时可以递送SIGINT，当SIGINT发生，去执行sig_int，注意：在sig_int里会打出
in sig_int: SIGINT SIGUSR1 ，是因为操作系统会自动给正在被递送的信号加入屏蔽字集里，当sig_int返回，sigsuspend也将返回，此时屏蔽字就是SIGINT。