（一） 函数原型：

int setitimer(int which, const struct itimerval *value,
                     struct itimerval *ovalue);

DESCRIPTION
       The  system  provides each process with three interval timers, each decrementing in a distinct time domain.  When any timer expires, a
       signal is sent to the process, and the timer (potentially) restarts.

       ITIMER_REAL    decrements in real time, and delivers SIGALRM upon expiration.

       ITIMER_VIRTUAL decrements only when the process is executing, and delivers SIGVTALRM upon expiration.

       ITIMER_PROF    decrements both when the process executes and when the system is executing on behalf  of  the  process.   Coupled  with
                      ITIMER_VIRTUAL, this timer is usually used to profile the time spent by the application in user and kernel space.  SIG-
                      PROF is delivered upon expiration.

       Timer values are defined by the following structures:

            struct itimerval {
                struct timeval it_interval; /* next value */
                struct timeval it_value;    /* current value */
            };
            struct timeval {
                long tv_sec;                /* seconds */
                long tv_usec;               /* microseconds */
            };

参数：
which：间歇计时器类型，有三种选择

ITIMER_REAL //数值为0，计时器的值实时递减，发送的信号是SIGALRM。
ITIMER_VIRTUAL //数值为1，进程执行时递减计时器的值，发送的信号是SIGVTALRM。
ITIMER_PROF //数值为2，进程和系统执行时都递减计时器的值，发送的信号是SIGPROF。

value，ovalue：时间参数，原型如下

struct itimerval {
    struct timeval it_interval; /* 计时器重启动的间歇值 */
    struct timeval it_value;    /* 计时器安装后首先启动的初始值 */
};

struct timeval {
    long tv_sec;                /* 秒 */
    long tv_usec;               /* 微妙(1/1000000) */
};

getitimer()用计时器的当前值填写value指向的结构体。
setitimer()将value指向的结构体设为计时器的当前值，如果ovalue不是NULL，将返回计时器原有值。

返回说明：
成功执行时，返回0。失败返回-1，errno被设为以下的某个值
EFAULT：value或ovalue是不有效的指针
EINVAL：其值不是ITIMER_REAL，ITIMER_VIRTUAL 或 ITIMER_PROF之一

（二）setitimer用法示例：

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

void PrintMsg(int Num)
{
    printf("%s\n", "Hello World");

    return;
}

int main(int argc, char* argv[])
{
    signal(SIGALRM, PrintMsg);

    struct itimerval tick;
    tick.it_value.tv_sec = 10;  //十秒钟后将启动定时器
    tick.it_value.tv_usec = 0;
    tick.it_interval.tv_sec  =1; //定时器启动后，每隔1秒将执行相应的函数
    tick.it_interval.tv_usec = 0;

    //setitimer将触发SIGALRM信号
    int ret = setitimer(ITIMER_REAL, &tick, NULL);

    if ( ret != 0)
    {
        printf("Set timer error. %s \n", strerror(errno) );

        return -1;
    }

    printf("Wait!\n");

    getchar();

    return 0;
}