Linux定时函数timerfd的使用问题以及解决办法

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在Linux下使用定时器时，大都会使用timerfd系列函数，本文记录一个使用时遇到的一个问题以及解决办法。

文章目录

- 一基本代码
- 二问题
- 三解决办法
- 四结语

一基本代码

以下代码是timerfd的一个基本使用，

#include <sys/timerfd.h>
#include <sys/time.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <unistd.h>
#include <pthread.h>
#include <signal.h>
#include <sys/epoll.h>#define handle_error(msg) do { perror(msg); exit(EXIT_FAILURE); } while (0)bool fRunning = true;int gTimerfd = -1;int initTimer(void); // 初始化定时器
void startTimer(int timerfd); // 开启定时器
void stopTimer(int timerfd); // 停止定时器
void print_elapsed_time(void); // 打印逝去的时间// 当收到ctrl+c时的信号处理函数
void sig_handler(int signum)
{stopTimer(gTimerfd);close(gTimerfd);gTimerfd = -1;fRunning = false;
}void *thr_func(void *arg)
{int timerfd = *(int*)(arg);startTimer(timerfd);print_elapsed_time();printf("timer startedn");uint64_t exp = 0;while (fRunning){int ret = read(timerfd, &exp, sizeof(uint64_t));if (ret == sizeof(uint64_t)) {print_elapsed_time();}}return NULL;
}int main(void)
{signal(SIGINT, sig_handler);gTimerfd = initTimer();if (gTimerfd < 0){return -1;}pthread_t tid;pthread_create(&tid, NULL, thr_func, &gTimerfd);pthread_join(tid, NULL);return 0;}int initTimer(void)
{int timerfd = timerfd_create(CLOCK_MONOTONIC, 0);if (timerfd == -1){printf("timerfd_create failn");}return timerfd;
}void startTimer(int timerfd)
{struct itimerspec new_value = {};new_value.it_value.tv_sec  = 0;new_value.it_value.tv_nsec = 50000000; // 50msnew_value.it_interval.tv_sec  = 0;new_value.it_interval.tv_nsec = 50000000; // 50msif (timerfd_settime(timerfd, 0, &new_value, NULL) == -1){printf("timerfd_settime failn");}
}void stopTimer(int timerfd)
{if (timerfd > 0){struct itimerspec new_value = {};new_value.it_value.tv_sec  = 0; new_value.it_value.tv_nsec = 0;new_value.it_interval.tv_sec  = 0;new_value.it_interval.tv_nsec = 0;if (timerfd_settime(timerfd, 0, &new_value, NULL) == -1){printf("timerfd_settime failn");}}
}void print_elapsed_time(void)
{static struct timeval start = {};static int first_call = 1;if (first_call == 1){first_call = 0;if (gettimeofday(&start, NULL) == -1){handle_error("gettimeofday");}}struct timeval current = {};if (gettimeofday(&current, NULL) == -1){handle_error("gettimeofday");}static int old_secs = 0, old_usecs = 0;int secs  = current.tv_sec - start.tv_sec;int usecs = current.tv_usec - start.tv_usec;if (usecs < 0){--secs;usecs += 1000000;}usecs = (usecs + 500)/1000; // 四舍五入if (secs != old_secs || usecs != old_usecs){printf("%d.%03dn", secs, usecs);old_secs = secs;old_usecs = usecs;}}

代码比较简单，使用阻塞的timerfd (参见函数initTimer())，每隔50ms打印一下程序运行后累计的时间，按ctrl+c去停止定时器，并把全局变量fRunning置为false，这样可以让线程自然结束（不需要调用pthread_cancel）

二问题

上面的程序编译并运行，

g++ timerfd_test.cpp -pthread

效果如下，

运行时没问题，按ctrl+c时，发现程序卡主了，无法结束，

原因是：信号处理函数sig_handler里停止定时器后，read()函数阻塞住了，导致线程无法结束，这样main函数里的pthread_join()无法返回，归根到底是因为创建timerfd时选择了阻塞类型

既然是因为使用阻塞式timerfd造成的，那么使用非阻塞式的timerfd不就行了，把initTimer()改为如下，在创建时传递非阻塞的标志，

int initTimer(void)
{int timerfd = timerfd_create(CLOCK_MONOTONIC, TFD_NONBLOCK);if (timerfd == -1){printf("timerfd_create failn");}return timerfd;
}

重新运行并按ctrl+c，效果如下，

确实正常结束了，但是CPU的使用率就飙升了…，可以使用top或htop进行查看，如下，达到74.8%

阻塞式调用可以降低CPU的使用率，如果把代码改回阻塞式，查看运行时的CPU使用率如下，只有0.3%…

三解决办法

代码再改回阻塞式的timerfd，如何解决无法结束的问题呢？经过资料查询，发现可以使用epoll+timerfd+eventfd这种方式来解决。

思路如下：

timerfd使用阻塞式的（也可以非阻塞，因为使用了epoll来监测，epoll是阻塞的）
使用epoll来监测timerfd
添加eventfd，也使用epoll来监测
当需要结束时，先停止定时器（会造成阻塞），然后通过eventfd来发送通知，最后结束线程

改进后代码如下，

#include <sys/timerfd.h>
#include <sys/time.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <unistd.h>
#include <pthread.h>
#include <signal.h>
#include <sys/epoll.h>
#include <sys/eventfd.h>#define handle_error(msg) do { perror(msg); exit(EXIT_FAILURE); } while (0)bool fRunning = true;int gTimerfd = -1;int gEventfd = -1;int initTimer(void);
void startTimer(int timerfd);
void stopTimer(int timerfd);
void print_elapsed_time(void);void sig_handler(int signum)
{stopTimer(gTimerfd);close(gTimerfd);gTimerfd = -1;uint64_t u = 100;write(gEventfd, &u, sizeof(uint64_t));fRunning = false;
}void *thr_func(void *arg)
{int timerfd = *(int*)(arg);startTimer(timerfd);int epollfd = epoll_create1(EPOLL_CLOEXEC); // or epoll_create(1)if (epollfd == -1){handle_error("epoll_create1");}struct epoll_event evTimer;evTimer.events = EPOLLIN; evTimer.data.fd = timerfd;epoll_ctl(epollfd, EPOLL_CTL_ADD, timerfd, &evTimer);struct epoll_event evEvent;evEvent.events = EPOLLIN; evEvent.data.fd = gEventfd;epoll_ctl(epollfd, EPOLL_CTL_ADD, gEventfd, &evEvent);const int maxEvents = 2; // 2 eventsstruct epoll_event events[maxEvents]; print_elapsed_time();printf("timer startedn");uint64_t exp = 0;int result = 0;while (fRunning){int nfd = epoll_wait(epollfd, events, maxEvents, -1);if (nfd > 0){for (int i = 0; i < nfd; ++i){exp = 0;result = 0;if (events[i].data.fd == timerfd){result = read(timerfd, &exp, sizeof(uint64_t));if (result == sizeof(uint64_t)) {print_elapsed_time();}}else if (events[i].data.fd == gEventfd){result = read(gEventfd, &exp, sizeof(uint64_t));if (result == sizeof(uint64_t)) {if (exp == 100){fRunning = false;}}}}}}return NULL;
}int main(void)
{signal(SIGINT, sig_handler);gTimerfd = initTimer();if (gTimerfd < 0){return -1;}gEventfd = eventfd(0, 0);if (gEventfd < 0){return -1;}pthread_t tid;pthread_create(&tid, NULL, thr_func, &gTimerfd);pthread_join(tid, NULL);return 0;}int initTimer(void)
{//int timerfd = timerfd_create(CLOCK_MONOTONIC, TFD_NONBLOCK);int timerfd = timerfd_create(CLOCK_MONOTONIC, 0);if (timerfd == -1){printf("timerfd_create failn");}return timerfd;
}void startTimer(int timerfd)
{struct itimerspec new_value = {};new_value.it_value.tv_sec  = 0;new_value.it_value.tv_nsec = 50000000; // 50msnew_value.it_interval.tv_sec  = 0;new_value.it_interval.tv_nsec = 50000000; // 50msif (timerfd_settime(timerfd, 0, &new_value, NULL) == -1){printf("timerfd_settime failn");}
}void stopTimer(int timerfd)
{if (timerfd > 0){struct itimerspec new_value = {};new_value.it_value.tv_sec  = 0; new_value.it_value.tv_nsec = 0;new_value.it_interval.tv_sec  = 0;new_value.it_interval.tv_nsec = 0;if (timerfd_settime(timerfd, 0, &new_value, NULL) == -1){printf("timerfd_settime failn");}}
}void print_elapsed_time(void)
{static struct timeval start = {};static int first_call = 1;if (first_call == 1){first_call = 0;if (gettimeofday(&start, NULL) == -1){handle_error("gettimeofday");}}struct timeval current = {};if (gettimeofday(&current, NULL) == -1){handle_error("gettimeofday");}static int old_secs = 0, old_usecs = 0;int secs  = current.tv_sec - start.tv_sec;int usecs = current.tv_usec - start.tv_usec;if (usecs < 0){--secs;usecs += 1000000;}usecs = (usecs + 500)/1000; // 四舍五入if (secs != old_secs || usecs != old_usecs){printf("%d.%03dn", secs, usecs);old_secs = secs;old_usecs = usecs;}}