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

Question

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

I am looking for some good code examples that can show how this is done correctly and properly, maybe a howto.

Answer 1

The following is a simple example of your 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

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