mq_receive: message too long

Question

I am implementing a communication between 2 processes using a queue. The problem is that when I call the function mq_receive, I get this error: Message too long.

I h

Answer 1

When debugging realtime POSIX queues, you should start with a sample program which works and go forward from there. Once you have the sample program running, it should be a simple matter of ensuring that your own code follows all the steps.

The following program has been tested successfully under Ubuntu 11.04:

#include 
#include 
#include 
#include 
#include 

#define MQNAME "/pax"
#define MQMESG "Hello there!"

static mqd_t serverUp (void) {
    int rc;
    mqd_t svrHndl;
    struct mq_attr mqAttr;

    printf ("Bringing up server.\n");
    rc = mq_unlink (MQNAME);
    if (rc < 0) {
        printf ("   Warning %d (%s) on server mq_unlink.\n",
            errno, strerror (errno));
    }

    mqAttr.mq_maxmsg = 10;
    mqAttr.mq_msgsize = 1024;
    svrHndl = mq_open (MQNAME, O_RDWR|O_CREAT, S_IWUSR|S_IRUSR, &mqAttr);
    if (svrHndl < 0) {
        printf ("   Error %d (%s) on server mq_open.\n",
            errno, strerror (errno));
        exit (1);
    }
    printf ("   Server opened mqd_t of %d.\n", svrHndl);
    return svrHndl;
}

static void serverReceive (mqd_t svrHndl) {
    int rc;
    char buffer[2048];
    printf ("Server receiving on mqd_t %d.\n", svrHndl);
    rc = mq_receive (svrHndl, buffer, sizeof (buffer), NULL);
    if (rc < 0) {
        printf ("   Error %d (%s) on server mq_receive.\n",
            errno, strerror (errno));
        exit (1);
    }
    printf ("   Received [%s].\n", buffer);
}

static void serverDown (mqd_t svrHndl) {
    printf ("Bringing down server with mqd_t %d.\n", svrHndl);
    mq_close (svrHndl);
}
static void clientSend (void) {
    mqd_t cliHndl;
    int rc;
    printf ("Client sending.\n");
    cliHndl = mq_open (MQNAME, O_RDWR);
    if (cliHndl < 0) {
        printf ("   Error %d (%s) on client mq_open.\n",
            errno, strerror (errno));
        exit (1);
    }
    printf ("   Client opened mqd_t of %d.\n", cliHndl);

    rc = mq_send (cliHndl, MQMESG, sizeof (MQMESG), 1);
    if (rc < 0) {
        printf ("   Error %d (%s) on client mq_send.\n",
            errno, strerror (errno));
        exit (1);
    }

    mq_close (cliHndl);
}

int main (void) {
    mqd_t svrHndl;

    svrHndl = serverUp ();
    clientSend ();
    serverReceive (svrHndl);
    serverDown (svrHndl);

    return 0;
}

The output on my system is:

Bringing up server.
   Server opened mqd_t of 3.
Client sending.
   Client opened mqd_t of 4.
Server receiving on mqd_t 3.
   Received [Hello there!].
Bringing down server with mqd_t 3.