使用C中的信号，如何在计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，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数据并对其进行处理（数据以GPS模块的频率进行流式传输）。

然而，我知道while(1)不是一个非常节能的解决方案，（这个RPi稍后将坐在无人机上）。

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，直到定时器结束，这样CPU就不会在上面浪费时间/精力

如果sigaction()调用中不允许SIGSTOP信号，我该如何执行上述操作？

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，并且只有在收到信号时才会返回，因此您可以运行代码，再次循环，然后重复。您需要一个信号处理程序来阻止SIGALRM终止您的程序，但它不必做任何事情，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，有时你可能会跳过信号。你可能在乎，也可能不在乎。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()来解除锁定，因为解除锁定和等待信号将是一个原子操作。以下是这种方法的一个例子：

#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，上面的示例中省略了错误检查，但您的代码当然应该为每个系统调用都包含它。