使用C中的信号，如何在计时器结束时停止和继续执行程序

Question

我正在开发一个在Raspberry-Pi（Linux）上运行的程序，该程序从GPS模块获取数据并进行处理。

现在，该程序以while(1)循环为中心，该循环获取GPS数据并对其进行填充（数据以GPS模块的频率进行流式传输）。

但是，我知道while(1)并不是一种非常省电的解决方案，（此RPi稍后将坐在无人机上）。

我想设置一个计时器，只有在计时器结束时，程序才会获取GPS数据。 理想情况下，程序将完全停止，直到计时器结束，这样CPU才不会浪费时间/精力。

如果在sigaction()调用中不允许SIGSTOP信号，该怎么办？

void timer_handler(int signum){
    /* what can I do to make a program stop and coninue,
       so that no CPU cycles are devoted to this program until
       the timer is elapsed ? (SIGSTOP is not allowed) */
}

int main(int argc, char** argv){

    struct sigaction sigAct;
    struct itimerval timer;
    sa.sa_handler = &timer_handler;

    // SIGALRM is only a place holder here.. 
    sigaction(SIGALRM, &sigAct, NULL);

    /* Here would be some timer.it_value, timer.it_interval stuff,
    setting it to some arbitrary time */

    setitimer(ITIMER_REAL, &timer, NULL);

    /* Main program loop here */
    while(1){
        // process GPS data.
        // instead of while(1), I'd like this loop to run only when
        // the timer ends.
    }

}

Answer 1

在您的循环中，只需调用pause() 。 它会阻塞并仅在收到信号时返回，因此您可以运行代码，再次循环并重复。 您需要一个信号处理程序来阻止SIGALRM终止程序，但是它不需要执行任何操作，您可以将函数体保留为空。

例如：

#define _POSIX_C_SOURCE 200809L

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <sys/time.h>

void timer_handler(int signum)
{
   (void) signum;       /*  Avoids warning for unused argument  */
}

int main(void)
{
    struct sigaction sa;
    sa.sa_handler = timer_handler;
    sa.sa_mask = 0;
    sa.sa_flags = 0;
    sigaction(SIGALRM, &sa, NULL);

    struct itimerval timer;
    timer.it_value.tv_sec = 1;
    timer.it_value.tv_usec = 0;
    timer.it_interval.tv_sec = 1;
    timer.it_interval.tv_usec = 0;
    setitimer(ITIMER_REAL, &timer, NULL);

    while ( 1 ) {
        pause();
        printf("Timer expired - get GPS data.\n");
    }

    return 0;
}

产生输出：

paul@horus:~/src/sandbox$ ./alarm
Timer expired - get GPS data.
Timer expired - get GPS data.
Timer expired - get GPS data.
Timer expired - get GPS data.
Timer expired - get GPS data.
Timer expired - get GPS data.
Timer expired - get GPS data.
Timer expired - get GPS data.
^C
paul@horus:~/src/sandbox$

这是一个很粗糙的解决方案。 如果有可能代码有时需要花费比计时器间隔更长的时间运行，那么事情就变得不可靠，您有时可能会跳过信号。 您可能会或可能不会在乎。 对于更复杂的方法，您可以阻止接收SIGALRM ，然后使用sigsuspend()屏蔽它的信号掩码调用sigsuspend() ，这是安全的，因为知道取消阻塞和等待信号将是原子操作。 这是该方法的示例：

#define _POSIX_C_SOURCE 200809L

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <sys/time.h>

void timer_handler(int signum)
{
    static const char msg[] = "Handler was called.\n";
    write(STDIN_FILENO, msg, sizeof(msg) - 1);
    (void) signum;
}

int main(void)
{
    struct sigaction sa;
    sa.sa_handler = timer_handler;
    sa.sa_mask = 0;
    sa.sa_flags = 0;
    sigaction(SIGALRM, &sa, NULL);

    struct itimerval timer;
    timer.it_value.tv_sec = 1;
    timer.it_value.tv_usec = 0;
    timer.it_interval.tv_sec = 1;
    timer.it_interval.tv_usec = 0;
    setitimer(ITIMER_REAL, &timer, NULL);

    sigset_t old_mask, block_mask;
    sigemptyset(&block_mask);
    sigaddset(&block_mask, SIGALRM);
    sigprocmask(SIG_BLOCK, &block_mask, &old_mask);

    sleep(3);   /*  To demonstrate signal handler won't be
                    called until sigsuspend() is called, timer
                    is firing every second while we're sleeping  */

    while ( 1 ) {
        sigsuspend(&old_mask);
        printf("Timer expired - get GPS data.\n");
    }

    return 0;
}

输出：

paul@horus:~/src/sandbox$ ./alarm2
Handler was called.
Timer expired - get GPS data.
Handler was called.
Timer expired - get GPS data.
Handler was called.
Timer expired - get GPS data.
Handler was called.
Timer expired - get GPS data.
Handler was called.
Timer expired - get GPS data.
Handler was called.
Timer expired - get GPS data.
^C
paul@horus:~/src/sandbox$

为了简洁起见，在上面的示例中省略了错误检查，但是您的代码当然应该在每个系统调用中都包括它。