CODEWITHSUNDEEP
clinuxcentosmqueue
John Duffy
John Duffy

Reputation: 103

Posix Message Queues QSIZE Query

When I examine the QSIZE of a message queue using "cat /dev/mqueue/myqueue" before and after doing a mq_send()/mq_receive() there appears to be some residual bytes left on the queue after the mq_recieve(). My little test program is below:

#include <stdio.h>
#include <stdlib.h>

#include <mqueue.h>


#define MSG_LEN 8192 // As per mq_msgsize as returned by mq_getattr() on CentOS 7 x86_64 


int main(void)
{
    mqd_t mq;


    /* Create a message queue */
    if ((mq = mq_open("/myqueue", O_RDWR|O_NONBLOCK|O_CREAT|O_EXCL, S_IRUSR|S_IWUSR, NULL)) == -1) {
        perror("Error calling mq_open()");
        exit(EXIT_FAILURE);
    }


    /* Create a message */

    char msg[MSG_LEN] = {" "};

    int msglen = snprintf(msg, MSG_LEN, "Hello, World!"); // msglen is 13 without the terminating '\0' 

    if (msglen < 0) {
        fprintf(stderr, "Error calling snprintf()\n");
        exit(EXIT_FAILURE);
    }


    /*
     * QSIZE here using "cat /dev/mqueue/myqueue" is: 0
     * OK
     */


    /* Send the message */

    if (mq_send(mq, msg, msglen + 1, 1) == -1) { // add 1 to msglen for the termining '\0'
        perror("Error calling mq_send()");
        exit(EXIT_FAILURE);
    }


    /*
     * QSIZE here using "cat /dev/mqueue/myqueue" is: 62
     * Why not 14?
     */


    /* Receive the message */

    if (mq_receive(mq, msg, MSG_LEN, NULL) == -1) {
        perror("Error calling mq_receive()");
        exit(EXIT_FAILURE);
    }


    /*
     * QSIZE here using "cat /dev/mqueue/myqueue" is: 48
     * Why not 0?
     */


    exit(EXIT_SUCCESS);
}

I don't understand why 62 bytes are initially put onto the queue, and a residual of 48 bytes is left behind after sending and receiving a 14 byte message. Any help would be much appreciated.

Kind regards

John Duffy

Upvotes: 1

Views: 941

Answers (1)

Charlie C
Charlie C

Reputation: 311

I believe the message queue stores some bookkeeping information within the file. From the mq_overview() man page:

The contents of each file in the directory consist of a single line containing information about the queue:

       $ cat /dev/mqueue/mymq
       QSIZE:129     NOTIFY:2    SIGNO:0    NOTIFY_PID:8260

http://man7.org/linux/man-pages/man7/mq_overview.7.html

Upvotes: 1

Related Questions