How do I use mqueue in a c program on a Linux based system?

Question

Hi

How do I use mqueue (message queue) in a c program on a Linux based system?

I'm looking for some good code examples that can show how this is done in a correct and proper way, maybe a howto.

Thanks

Answer 1

#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);
    }

    if (status == 0)
    {
        status = mq_receive(mq, (char *)(&b), sizeof(int), NULL);
    }

    if ((status == 0) && (mq_close(mq) == -1))
    {
        printf("Error closing message queue.\n");
        status = 1;
    }

    if ((status == 0) && (mq_unlink("test_queue") == -1))
    {
        printf("Error deleting message queue.\n");
        status = 1;
    }

    printf("a = %d, b = %d\n", a, b);

    return status;
} 

Answer 2

The following is a simple example of a server that receives messages from clients until it receives an "exit" message telling it to stop.

The code for the server:

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <errno.h>
#include <mqueue.h>

#include "common.h"

int main(int argc, char **argv)
{
    mqd_t mq;
    struct mq_attr attr;
    char buffer[MAX_SIZE + 1];
    int must_stop = 0;

    /* initialize the queue attributes */
    attr.mq_flags = 0;
    attr.mq_maxmsg = 10;
    attr.mq_msgsize = MAX_SIZE;
    attr.mq_curmsgs = 0;

    /* create the message queue */
    mq = mq_open(QUEUE_NAME, O_CREAT | O_RDONLY, 0644, &attr);
    CHECK((mqd_t)-1 != mq);

    do {
        ssize_t bytes_read;

        /* receive the message */
        bytes_read = mq_receive(mq, buffer, MAX_SIZE, NULL);
        CHECK(bytes_read >= 0);

        buffer[bytes_read] = '\0';
        if (! strncmp(buffer, MSG_STOP, strlen(MSG_STOP)))
        {
            must_stop = 1;
        }
        else
        {
            printf("Received: %s\n", buffer);
        }
    } while (!must_stop);

    /* cleanup */
    CHECK((mqd_t)-1 != mq_close(mq));
    CHECK((mqd_t)-1 != mq_unlink(QUEUE_NAME));

    return 0;
}

The code for the client:

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <mqueue.h>

#include "common.h"


int main(int argc, char **argv)
{
    mqd_t mq;
    char buffer[MAX_SIZE];

    /* open the mail queue */
    mq = mq_open(QUEUE_NAME, O_WRONLY);
    CHECK((mqd_t)-1 != mq);


    printf("Send to server (enter \"exit\" to stop it):\n");

    do {
        printf("> ");
        fflush(stdout);

        memset(buffer, 0, MAX_SIZE);
        fgets(buffer, MAX_SIZE, stdin);

        /* send the message */
        CHECK(0 <= mq_send(mq, buffer, MAX_SIZE, 0));

    } while (strncmp(buffer, MSG_STOP, strlen(MSG_STOP)));

    /* cleanup */
    CHECK((mqd_t)-1 != mq_close(mq));

    return 0;
}

The common header:

#ifndef COMMON_H_
#define COMMON_H_

#define QUEUE_NAME  "/test_queue"
#define MAX_SIZE    1024
#define MSG_STOP    "exit"

#define CHECK(x) \
    do { \
        if (!(x)) { \
            fprintf(stderr, "%s:%d: ", __func__, __LINE__); \
            perror(#x); \
            exit(-1); \
        } \
    } while (0) \


#endif /* #ifndef COMMON_H_ */

Compiling:

gcc -o server server.c -lrt
gcc -o client client.c -lrt