I need simple example of function mq_close in C programming language.

mq_close() closes the message queue descriptor mqdes.

On success mq_close() returns 0; on error, -1 is returned, with errno set to indicate the error.

#include <stdio.h>
#include <fcntl.h>
#include <mqueue.h>

int main(int argc, char *argv[])
{
    mqd_t mq;               // message queue
    struct mq_attr ma;      // message queue attributes
    int status = 0;
    int a = 5;
    int b = 0;
    printf("a = %d, b = %d\n", a, b);
    // Specify message queue attributes.
    ma.mq_flags = 0;                // blocking read/write
    ma.mq_maxmsg = 16;              // maximum number of messages allowed in queue
    ma.mq_msgsize = sizeof(int);    // messages are contents of an int
    ma.mq_curmsgs = 0;              // number of messages currently in queue

    // Create the message queue with some default settings.
    mq = mq_open("/test_queue", O_RDWR | O_CREAT, 0700, &ma);
    // -1 indicates an error.
    if (mq == -1)
    {
        printf("Failed to create queue.\n");
        status = 1;
    }
    if (status == 0)
    {
        status = mq_send(mq, (char *)(&a), sizeof(int), 1);
        status = mq_receive(mq, (char *)(&b), sizeof(int), NULL);
    }
    if ((status == 0) && (mq_close(mq) == -1))
    {
        printf("Error closing message queue.\n");
        printf("Error deleting message queue.\n");
        status = 1;
    }
    printf("a = %d, b = %d\n", a, b);
    return status;
}

ERRORS:

undefined reference to `mq_open'

undefined reference to `mq_send'

undefined reference to `mq_receive'

undefined reference to `mq_close'

undefined reference to `mq_unlink'

Linux is new to me, and I'm beginner in operating systems in general. Could you explain me why this code can't compile?

CodePudding user response：

As per the man page https://man7.org/linux/man-pages/man3/mq_open.3.html you need to

Link with -lrt.

ie

  gcc mq.c -lrt