急，关于posix消息队列！！！
文件包含<mqueue.h>,为什么用gcc编译显示函数未定义？？？

/tmp/ccc10ZBt.o: In function `main':
mqcreatel.c:(.text+0x93): undefined reference to `mq_open'
mqcreatel.c:(.text+0xa1): undefined reference to `mq_close'
collect2: ld returned 1 exit status

#include <mqueue.h>
#include <sys/stat.h>
#include <string.h>
#include <stdio.h>
#define MQ_FILE "/mq_test"
#define BUF_LEN 128

int main(void)
{
     mqd_t mqd;
    char buf[BUF_LEN];
    int  por = 0;
    int ret = 0;
    struct mq_attr attr;
    attr.mq_flags = 0;
    attr.mq_maxmsg = 3;
    attr.mq_msgsize = 50;
    attr.mq_curmsgs= 0;
    mqd = mq_open(MQ_FILE, O_WRONLY,0666,&attr);
    if (-1 == mqd)
    {
        printf("mq_open error.\n");
        return -1;
    }

    do{
        buf[BUF_LEN-1]='\0';
        printf("MQ_MSG : ");
        scanf("%s", buf);
        if(buf[BUF_LEN-1]!= '\0')
        {
            continue;
        }
        printf("strlen:%d\nMQ_POR : ",strlen(buf));
        scanf("%d", &por);
        ret== mq_send(mqd, buf, strlen(buf)+1, por);
        if (ret != 0)
        {
            perror("mq_send error.\n");
        }
        memset(buf,'\0',BUF_LEN);
    }while(strcmp(buf, "quit"));


    mq_close(mqd);
    mq_unlink(MQ_FILE);

    return 0;
} 
 

posix c
[解决办法]
NAME
mq_open - open a message queue

SYNOPSIS
#include <fcntl.h> /* For O_* constants */
#include <sys/stat.h> /* For mode constants */
#include <mqueue.h>

mqd_t mq_open(const char *name, int oflag);
mqd_t mq_open(const char *name, int oflag, mode_t mode,
struct mq_attr *attr);

Link with -lrt.