struct mq_attr

{

    long mq_flags;      /* message queue flag : 0, O_NONBLOCK */

    long mq_maxmsg;     /* max number of messages allowed on queue*/

    long mq_msgsize;    /* max size of a message (in bytes)*/

    long mq_curmsgs;    /* number of messages currently on queue */

};

/* Receive the oldest from highest priority messages in message queue

   MQDES.  */

ssize_t mq_receive (mqd_t __mqdes, char *__msg_ptr, size_t __msg_len,

                    unsigned int *__msg_prio);

/* Add message pointed by MSG_PTR to message queue MQDES.  */

int mq_send (mqd_t __mqdes, __const char *__msg_ptr, size_t __msg_len,

             unsigned int __msg_prio);

// mqsend.c

#include    "unpipc.h"

int

main(int argc, char **argv)

{

    mqd_t   mqd;

    void    *ptr;

    size_t  len;

    unsigned int  prio;

    if (argc != )

        err_quit("usage: mqsend <name> <#bytes> <priority>");

    len = atoi(argv[]);

    prio = atoi(argv[]);

    mqd = Mq_open(argv[], O_WRONLY);

    ptr = Calloc(len, sizeof(char)); // 使用calloc函数分配一个长度为len的缓冲区，默认被初始化为0

    Mq_send(mqd, ptr, len, prio); // 写入消息队列

    exit();

}

// mqreveive.c

#include    "unpipc.h"

int

main(int argc, char **argv)

{

    int     c, flags;

    mqd_t   mqd;

    ssize_t n;

    unsigned int prio;

    void    *buff;

    struct mq_attr  attr;

    flags = O_RDONLY;

    while ( (c = Getopt(argc, argv, "n")) != -)

    {

        switch (c)

        {

            case 'n': // 命令行中带-n表示以非阻塞模式

                flags |= O_NONBLOCK;

                break;

        }

    }

    if (optind != argc - )

        err_quit("usage: mqreceive [ -n ] <name>");

    mqd = Mq_open(argv[optind], flags);

    Mq_getattr(mqd, &attr);

    // 这里开辟的缓冲区需要>=消息的最大长度，以便能够容纳接收到的消息

    buff = Malloc(attr.mq_msgsize); // 根据最大消息的大小开辟缓冲区以接收消息

    n = Mq_receive(mqd, buff, attr.mq_msgsize, &prio); // 这里的len长度需要>=消息的最大长度，否则mq_receive会立即返回EMSGSIZE错误

    printf("read %ld bytes, priority = %u\n", (long) n, prio);

    exit();

}