undefined reference timer_create  undefined  reference timer_settimer

#include <stdio.h> #include <signal.h> #include <time.h> #include <string.h> #include <stdlib.h> #include <unistd.h>   #define CLOCKID CLOCK_REALTIME   void timer_thread(union sigval v) {     printf("timer_thread function! %d\n", v.sival_int); }   int main() {     // XXX int timer_create(clockid_t clockid, struct sigevent *evp, timer_t *timerid);     // clockid--值：CLOCK_REALTIME,CLOCK_MONOTONIC，CLOCK_PROCESS_CPUTIME_ID,CLOCK_THREAD_CPUTIME_ID     // evp--存放环境值的地址,结构成员说明了定时器到期的通知方式和处理方式等     // timerid--定时器标识符     timer_t timerid;     struct sigevent evp;     memset(&evp, 0, sizeof(struct sigevent));        //清零初始化       evp.sigev_value.sival_int = 111;            //也是标识定时器的，这和timerid有什么区别？回调函数可以获得     evp.sigev_notify = SIGEV_THREAD;            //线程通知的方式，派驻新线程     evp.sigev_notify_function = timer_thread;        //线程函数地址       if (timer_create(CLOCKID, &evp, &timerid) == -1)     {         perror("fail to timer_create");         exit(-1);     }       // XXX int timer_settime(timer_t timerid, int flags, const struct itimerspec *new_value,struct itimerspec *old_value);     // timerid--定时器标识     // flags--0表示相对时间，1表示绝对时间     // new_value--定时器的新初始值和间隔，如下面的it     // old_value--取值通常为0，即第四个参数常为NULL,若不为NULL，则返回定时器的前一个值           //第一次间隔it.it_value这么长,以后每次都是it.it_interval这么长,就是说it.it_value变0的时候会装载it.it_interval的值     struct itimerspec it;     it.it_interval.tv_sec = 1;     it.it_interval.tv_nsec = 0;     it.it_value.tv_sec = 1;     it.it_value.tv_nsec = 0;       if (timer_settime(timerid, 0, &it, NULL) == -1)     {         perror("fail to timer_settime");         exit(-1);     }       pause();       return 0; } /*  * int timer_gettime(timer_t timerid, struct itimerspec *curr_value);  * 获取timerid指定的定时器的值，填入curr_value  *  */

编译后为什么是提示undefined reference timer_create  undefined  reference timer_settimer？
我知道是这个提示是库里找不到相关函数定义，但是现在我要怎么才能去找到他呢

加 链接选项 -lrt

TIMER_SETTIME(2)           Linux Programmer's Manual          TIMER_SETTIME(2)   NAME        timer_settime, timer_gettime - arm/disarm and fetch state of POSIX per-        process timer   SYNOPSIS        #include <time.h>          int timer_settime(timer_t timerid, int flags,                          const struct itimerspec *new_value,                          struct itimerspec * old_value);        int timer_gettime(timer_t timerid, struct itimerspec *curr_value);          Link with -lrt.      Feature Test Macro Requirements for glibc (see feature_test_macros(7)):          timer_settime(), timer_gettime(): _POSIX_C_SOURCE >= 199309L

++，man timer_create或man timer_settime