c - 了解 struct itimerval 字段 tv_usec

Question

你好，我正在研究这段代码:

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


volatile sig_atomic_t print_flag = true;
static int count = 0;

void timer_handler (int signum)
{

 printf ("timer expired %d times\n", ++count);
 if(count>20) {
    print_flag = false;
 } 
}

int main ()
{
 struct sigaction sa;
 struct itimerval timer;

 /* Install timer_handler as the signal handler for SIGVTALRM. */
 memset (&sa, 0, sizeof (sa));
 sa.sa_handler = &timer_handler;
 sigaction (SIGALRM, &sa, NULL);

 /* Configure the timer to expire after 250 msec... */
 timer.it_value.tv_sec = 0;
 timer.it_value.tv_usec = 250000;
 /* ... and every 250 msec after that. */
 timer.it_interval.tv_sec = 0;
 timer.it_interval.tv_usec = 250000;
 /* Start a virtual timer. It counts down whenever this process is
   executing. */
 setitimer (ITIMER_REAL, &timer, NULL);

 /* Do busy work. */
 while (print_flag) {
    sleep(1);
 }

printf("job done bye bye\n");
    exit(0);

}

有了这个设置，一切正常，我得到了这个输出

...
timer expired 17 times
timer expired 18 times
timer expired 19 times
timer expired 20 times
timer expired 21 times
job done bye bye

如果我尝试更改代码注释 timer.it_interval.tv_usec 和 timer.it_interval.tv_usec 并同时设置 timer.it_value.tv_sec 和 timer.it_value.tv_sec 等于，例如，3 它没有完成他的工作。

但是，如果我像这样维护 tv_usec 的显式声明，它会起作用:

 timer.it_value.tv_sec = 3;
 timer.it_value.tv_usec = 0;
 timer.it_interval.tv_sec = 3;
 timer.it_interval.tv_usec = 0;

为什么我要为这两个字段明确声明 tv_usec？

Answer 1

只要你不初始化 *.tv_usec字段，它们的值是未定义。如果它包含一个大于 999999 的值或小于 0 , setitimer ()只会失败 EINVAL如手册页所述。

您应该自己初始化所有数据。如果你愿意节省一行代码，你可以memset timer结构为 0与您为 sa 所做的方式相同.