c - 使用C中的信号，如何在定时器结束时停止和继续程序

Question

我正在开发一个在 Raspberry-Pi (linux) 上运行的程序，它从 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 数据并对其进行处理(数据以 GPS 模块的频率传输)。

但是，我知道 while(1) 并不是一个非常节能的解决方案(这个 RPi 稍后将安装在无人机上)。

我想设置一个计时器，只有当计时器结束时，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 就不会在其上浪费时间/精力。

How can I do the above mentioned, if SIGSTOP signal is not allowed in sigaction() calls ?

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

在循环中，只需调用 pause()。它会阻塞并且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 终止您的程序，但它不必执行任何操作，您只需将函数体留空即可。

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;
}

产量输出:

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$

这是一个相当粗糙的解决方案。如果代码有时可能需要比计时器间隔更长的时间来运行，那么事情就会变得不可靠，有时您可能会跳过信号。您可能关心也可能不关心这一点。对于更复杂的方法，您可以阻止接收 SIGALRM，然后使用信号掩码调用 sigsuspend() 来解锁它，因为知道解锁和等待是安全的。因为信号将是一个原子操作。下面是该方法的一个示例:

#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包含它。