c - RT 计时器奇怪的行为

Question

我正在学习如何使用 RT 计时器创建周期性事件。我尝试使用这个基于 timer_create 文档示例的示例。

预期的行为是每 5 秒产生一个周期性事件，同时主执行休眠 30 秒，但我得到以下行为。

定时器被创建并在主执行休眠时被初始化。 5 秒后，定时器事件产生，调用相应的处理程序，当处理程序函数完成时，主执行被唤醒，耗时为 5 秒，但必须为 30 秒。

/* gcc main.c -Wall -Wextra -lrt -o timer */ 
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <signal.h>
#include <time.h>

/* Macros *********************************************************************/
#define CLOCKID CLOCK_REALTIME
/* Real-time signals number */
#define RT_SIGNAL_NUM SIGRTMIN

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE);} while (0)

#define MAIN_SLEEP 30
#define TIMER_TIME 5


/* Global data ****************************************************************/
struct itimerspec gIts;

timer_t gTimerId;

/* Functions ******************************************************************/

static void handler(int sig, siginfo_t *si, void *uc)
{
    int or;
    timer_t *tidp;
    /* Note: calling printf() from a signal handler is not
       strictly correct, since printf() is not async-signal-safe;
       see signal(7) */

    printf("Caught signal %d\n", sig);
    signal(sig, SIG_IGN);

}

void main(int argc, char *argv[])
{
    struct sigevent sev;
    long long freq_nanosecs;
    sigset_t mask;
    struct sigaction sa;
    time_t begin, end;
    double time_spent;

    (void) argc;
    (void) argv;

    /* Establish handler for  Real-time signal */
    printf("Establishing handler for signal %d\n", RT_SIGNAL_NUM);
    /* If SA_SIGINFO is specified in sa_flags, then sa_sigaction (instead of
     * sa_handler) specifies the signal-handling function for signum.
     */
    sa.sa_flags = SA_SIGINFO;
    sa.sa_sigaction = handler;
    /* the Signal no would be masked. */
    sigemptyset(&sa.sa_mask);
    /* Change the action taken by a process on receipt of a specific signal. */
    if (-1 == sigaction(RT_SIGNAL_NUM, &sa, NULL))
    {
        errExit("sigaction");
    }

    /* Configure timer:
     * SIGEV_SIGNAL: Upon timer expiration, generate the signal sigev_signo for
     * the process.
     */
    sev.sigev_notify = SIGEV_SIGNAL;
    sev.sigev_signo = RT_SIGNAL_NUM;
    sev.sigev_value.sival_ptr = &gTimerId;
    /* Create and init the timer */
    if (-1 == timer_create(CLOCKID, &sev, &gTimerId))
    {
        errExit("timer_create");
    }

    printf("timer ID is 0x%lx\n", (long) gTimerId);
    /* Start the timer */
    gIts.it_value.tv_sec = TIMER_TIME ;
    gIts.it_value.tv_nsec = 0;
    gIts.it_interval.tv_sec = TIMER_TIME;
    gIts.it_interval.tv_nsec = 0;
    if (-1 == timer_settime(gTimerId, 0, &gIts, NULL))
    {
         errExit("timer_settime");
    }

    /* Sleep for a while; meanwhile, the timer may expire multiple times */
    struct itimerspec currTime;
    printf("Main Sleep for %d seconds\n", MAIN_SLEEP);
    begin = time(NULL);
    sleep(MAIN_SLEEP);
    end = time(NULL);
    printf("Main Wake up\n");
    time_spent = (double)(end - begin);
    timer_gettime(gTimerId, &currTime);
    printf("Main sleep elapsed time = %f s %d %d\n",time_spent);
    printf("Timer time = %ld s\n",currTime.it_value.tv_sec);

    exit(EXIT_SUCCESS);
}

这是执行输出:

Establishing handler for signal 34
timer ID is 0x9522008
Main Sleep for 30 seconds
Caught signal 34
Main Wake up
Main sleep elapsed time = 5.000000 s
Timer time = 4 s

谁能帮我弄清楚这里发生了什么？

提前致谢。

Answer 1

当信号产生时，sleep函数被中断并立即返回。引用 sleep

的手册页

The sleep() function suspends execution of the calling thread until either [the time has] elapsed or a signal is delivered to the thread and its action is to invoke a signal-catching function or to terminate the thread or process.

幸运的是，sleep 函数的返回值是剩余 sleep 时间，因此您可以像这样简单地在循环中调用 sleep

int t = 30;
while ( t > 0 )
    t = sleep( t );