c - 如何在信号驱动的恶魔中暂停主循环？

Question

我正在为树莓派开发电子电话线仿真。为此，我需要一个函数，它每 20 毫秒(=50Hz，振铃电压频率的一半)调用一次，检查电流并更新 HW PWM 的占空比。为此，我在相应的计时器上设置了一个信号处理程序(因为我会在纯嵌入式环境中将其挂入调度程序)并让它完成工作。请注意，下面的代码被缩短只是为了关注这个问题。

这已经很好用了。我很惊讶，时间保持得如此准确(抖动低于 10µs，用示波器测量)。

但是，到目前为止有一个缺陷:当这样工作时，实际上不需要主循环。我无法从中返回，因为那会终止进程。当我让它成为一个空循环时，一切正常。但我在考虑不必要地消耗 CPU 负载。

我试过sleep、wait(我知道这两个都不应该再用了)，和 sigsuspend。但是所有这些都让警报处理程序不再被调用。

#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <syslog.h>
#include <string.h>
#include <signal.h>
#include <syslog.h>
#include <sys/time.h>
#include <bcm2835.h>

#define PIN_PWM RPI_V2_GPIO_P1_35

bool b_Alive;

/** Signal-Handlers: ******************************************************************/

void SIG_Alarm (int signum) {
    /** Just toggle a pin for now:                                                    */
    static bool bOn;
    if (bOn) {
        bOn = 0;
        bcm2835_gpio_write(PIN_PWM, LOW);
    }else{
        bOn = 1;
        bcm2835_gpio_write(PIN_PWM, HIGH);
    }
}

void SIG_Quit (int signum) {
    /** Shutdown the bcm-library:                                                     */
    bcm2835_close();
    /** Close the sys-log handler:                                                    */
    syslog(LOG_NOTICE + LOG_DAEMON, "Received SigInt and closed.");
    closelog();
    b_Alive = false;    
}

/** Main-Function: ********************************************************************/


int main(void) {
    /** Variables:                                                                    */
    struct sigaction sa;
    struct itimerval timer;
    /** Open syslog instead: */
    openlog( "PhoneLined", LOG_PID | LOG_CONS | LOG_NDELAY, LOG_LOCAL0 );
    /** Setup pins:                                                                   */
    if (!bcm2835_init()) return 1;
    bcm2835_gpio_fsel(PIN_PWM, BCM2835_GPIO_FSEL_OUTP);
    /** Setup signal handler for kill:                                                */
    memset(&sa, 0, sizeof (sa));
    sigemptyset(&sa.sa_mask);
    sa.sa_handler = &SIG_Quit;
    sigaction(SIGINT , &sa, NULL);
    sigaction(SIGQUIT, &sa, NULL);
    sigaction(SIGKILL, &sa, NULL);
    sigaction(SIGTERM, &sa, NULL);
    /** Setup signal handler for timer:                                               */
    memset(&sa, 0, sizeof (sa));
    sigemptyset(&sa.sa_mask);
    sa.sa_handler = &SIG_Alarm;
    sigaction(SIGVTALRM, &sa, NULL);
    /** Configure the timer of a start- and cycle-time of 20ms (=50Hz):               */
    timer.it_value.tv_sec = 0;
    timer.it_value.tv_usec = 20000;
    timer.it_interval.tv_sec = 0;
    timer.it_interval.tv_usec = 20000;
    setitimer(ITIMER_VIRTUAL, &timer, NULL);
    /** Prepare main-loop:                                                            */
    b_Alive = true;
    syslog(LOG_NOTICE + LOG_DAEMON, "Sucessfully initialized.");
    /** ... and do nothing, while the timer works:                                    */
    while (b_Alive) {
        //pause(), suspend, wait or anything?
    }
    exit(EXIT_SUCCESS);
}

如果在此循环中实际上什么都不做，是否有任何提示，如何继续？

Answer 1

至少将volatile 添加到您的标志定义中，否则循环将被优化器(for-O2 或更高版本)删除

(用cc -Wall -O4 -S signal.c检查)

---

#include <stdbool.h>
// volatile sig_atomic_t
volatile bool
        b_Alive=false;

---

主要():

...
timer.it_value.tv_sec = 0;
timer.it_value.tv_usec = 20000;
timer.it_interval.tv_sec = 0;
timer.it_interval.tv_usec = 20000;
setitimer(ITIMER_REAL, &timer, NULL);

// Prepare main-loop:            

b_Alive = true;
    syslog(LOG_NOTICE + LOG_DAEMON, "Sucessfully initialized.");
    /** ... and do nothing, while the timer works:                                    */
    while (b_Alive) {
        pause(); //, suspend, wait or anything?
    }
    exit(EXIT_SUCCESS);

---

还有，定时器类型(来自精美的手册):

---

   ITIMER_REAL    This timer counts down in real (i.e., wall clock) time.  At each expiration, a SIGALRM signal is generated.

   ITIMER_VIRTUAL This timer counts down against the user-mode CPU time consumed by the process.  (The measurement includes CPU time consumed by all threads in the process.)  At each
                  expiration, a SIGVTALRM signal is generated.

---

并且，由于 pause() 不消耗任何 CPU 滴答，因此使用 ITIMER_VIRTUAL 时计时器永远不会过期。 (还有:传递了不同的信号)