1，使用sleep()和usleep()
其中sleep精度是1秒，usleep精度是1微妙，具体代码就不写了。使用这种方法缺点比较明显，在Linux系统中，sleep类函数不能保证精度，尤其在系统负载比较大时，sleep一般都会有超时现象。
2，使用信号量SIGALRM + alarm()
这种方式的精度能达到1秒，其中利用了*nix系统的信号量机制，首先注册信号量SIGALRM处理函数，调用alarm()，设置定时长度，代码如下：
[cpp] view plain copy
#include <stdio.h>
#include <signal.h>
void timer(int sig)
{
if(SIGALRM == sig)
{
printf("timer\n")
alarm(1) //we contimue set the timer
}
return
}
int main()
{
signal(SIGALRM, timer)//relate the signal and function
alarm(1) //trigger the timer
getchar()
return 0
}
alarm方式虽然很好，但是无法首先低于1秒的精度。
3，使用RTC机制
RTC机制利用系统硬件提供的Real Time Clock机制，通过读取RTC硬件/dev/rtc，通过ioctl()设置RTC频率，代码如下：
[cpp] view plain copy
#include <stdio.h>
#include <linux/rtc.h>
#include <sys/ioctl.h>
#include <sys/time.h>
#include <sys/types.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <stdlib.h>
int main(int argc, char* argv[])
{
unsigned long i = 0
unsigned long data = 0
int retval = 0
int fd = open ("/dev/rtc", O_RDONLY)
if(fd <0)
{
perror("open")
exit(errno)
}
/*Set the freq as 4Hz*/
if(ioctl(fd, RTC_IRQP_SET, 1) <0)
{
perror("ioctl(RTC_IRQP_SET)")
close(fd)
exit(errno)
}
/* Enable periodic interrupts */
if(ioctl(fd, RTC_PIE_ON, 0) <0)
{
perror("ioctl(RTC_PIE_ON)")
close(fd)
exit(errno)
}
for(i = 0i <100i++)
{
if(read(fd, &data, sizeof(unsigned long)) <0)
{
perror("read")
close(fd)
exit(errno)
}
printf("timer\n")
}
/* Disable periodic interrupts */
ioctl(fd, RTC_PIE_OFF, 0)
close(fd)
return 0
}
这种方式比较方便，利用了系统硬件提供的RTC，精度可调，而且非常高。
4，使用select()
这种方法在看APUE神书时候看到的，方法比较冷门，通过使用select()，来设置定时器；原理利用select()方法的第5个参数，第一个参数设置为0，三个文件描述符集都设置为NULL，第5个参数为时间结构体，代码如下：
[cpp] view plain copy
#include <sys/time.h>
#include <sys/select.h>
#include <time.h>
#include <stdio.h>
/*seconds: the secondsmseconds: the micro seconds*/
void setTimer(int seconds, int mseconds)
{
struct timeval temp
temp.tv_sec = seconds
temp.tv_usec = mseconds
select(0, NULL, NULL, NULL, &temp)
printf("timer\n")
return
}
int main()
{
int i
for(i = 0 i <100i++)
setTimer(1, 0)
return 0
}
这种方法精度能够达到微妙级别，网上有很多基于select()的多线程定时器，说明select()稳定性还是非常好。
总结：如果对系统要求比较低，可以考虑使用简单的sleep()，毕竟一行代码就能解决；如果系统对精度要求比较高，则可以考虑RTC机制和select()机制。
个人解决了，以下是一个实现：#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <signal.h>
#include <string.h>
#include <pthread.h>
#include <time.h>
#if 1
pthread_attr_t attr
timer_t hard_timer, software_timer
struct sigevent hard_evp, software_evp
static void watchdog_hard_timeout(union sigval v)
{
time_t t
char p[32]
timer_t *q
struct itimerspec ts
int ret
time(&t)
strftime(p, sizeof(p), "%T", localtime(&t))
printf("watchdog hard timeout!\n")
printf("%s thread %d, val = %u, signal captured.\n", p, (unsigned int)pthread_self(), v.sival_int)
q = (timer_t *)(v.sival_ptr)
printf("hard timer_t:%d add:%p, q:%p!\n", (int)hard_timer, &hard_timer, q)
ts.it_interval.tv_sec = 0
ts.it_interval.tv_nsec = 0
ts.it_value.tv_sec = 6
ts.it_value.tv_nsec = 0
ret = timer_settime(*q, CLOCK_REALTIME, &ts, NULL)
if (ret != 0) {
printf("settime err(%d)!\n", ret)
}
}
static void watchdog_software_timeout(union sigval v)
{
time_t t
char p[32]
timer_t *q
struct itimerspec ts
int ret
time(&t)
strftime(p, sizeof(p), "%T", localtime(&t))
printf("watchdog software timeout!\n")
printf("%s thread %d, val = %u, signal captured.\n", p, (unsigned int)pthread_self(), v.sival_int)
q = (timer_t *)(v.sival_ptr)
printf("hard timer_t:%d add:%p, q:%p!\n", (int)hard_timer, &hard_timer, q)
ts.it_interval.tv_sec = 0
ts.it_interval.tv_nsec = 0
ts.it_value.tv_sec = 10
ts.it_value.tv_nsec = 0
ret = timer_settime(*q, CLOCK_REALTIME, &ts, NULL)
if (ret != 0) {
printf("settime err(%d)!\n", ret)
}
}
static void dcmi_sol_pthread_attr_destroy(pthread_attr_t *attr)
{
pthread_attr_destroy(attr)
}
static int dcmi_sol_pthread_attr_init(pthread_attr_t *attr)
{
int ret
if ((ret = pthread_attr_init(attr) != 0)) {
goto err
}
if ((ret = pthread_attr_setdetachstate(attr, PTHREAD_CREATE_DETACHED)) != 0) {
dcmi_sol_pthread_attr_destroy(attr)
goto err
}
/* 设置线程的栈大小,失败则用系统默认值 */
pthread_attr_setstacksize(attr, 128 * 1024)
return 0
err:
printf("set ptread attr failed(ret:%d)!\n", ret)
return -1
}
int main(void)
{
struct itimerspec ts
int ret
ret = dcmi_sol_pthread_attr_init(&attr)
if (ret != 0) {
printf("init pthread attributes fail(%d)!\n", ret)
exit(-1)
}
memset(&hard_evp, 0, sizeof(struct sigevent))
hard_evp.sigev_value.sival_ptr = &hard_timer
hard_evp.sigev_notify = SIGEV_THREAD
hard_evp.sigev_notify_function = watchdog_hard_timeout
hard_evp.sigev_notify_attributes = NULL//&attr
memset(&software_evp, 0, sizeof(struct sigevent))
software_evp.sigev_value.sival_ptr = &software_timer
software_evp.sigev_notify = SIGEV_THREAD
software_evp.sigev_notify_function = watchdog_software_timeout
software_evp.sigev_notify_attributes = NULL//&attr
ret = timer_create(CLOCK_REALTIME, &hard_evp, &hard_timer)
if(ret != 0) {
perror("hard timer_create fail!")
exit(-1)
}
ret = timer_create(CLOCK_REALTIME, &software_evp, &software_timer)
if (ret != 0) {
timer_delete(hard_timer)
perror("software timer_create fail!")
exit(-1)
}
ts.it_interval.tv_sec = 0
ts.it_interval.tv_nsec = 0
ts.it_value.tv_sec = 6
ts.it_value.tv_nsec = 0
ret = timer_settime(hard_timer, CLOCK_REALTIME, &ts, NULL)
if(ret != 0) {
perror("hard timer_settime fail!")
timer_delete(hard_timer)
timer_delete(software_timer)
exit(-1)
}
ts.it_value.tv_sec = 10
ret = timer_settime(software_timer, CLOCK_REALTIME, &ts, NULL)
if(ret != 0) {
perror("hard timer_settime fail!")
timer_delete(hard_timer)
timer_delete(software_timer)
exit(-1)
}
while(1) {
printf("main ready sleep!\n")
sleep(15)
printf("main sleep finish!\n")
}
return 0
}
#endif
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