Linux : Check if message queue is empty

Question

i want to know if a queue message is empty or not . i have used msg_ctl() as follows it doesn't work :

struct msqid_ds buf;
int num_messages;

rc = msgctl(msqid, IPC_STAT, &buf);

and i've used this peek function :

int peek_message( int qid, long type )
{
    int result, length;
    if((result = msgrcv( qid, NULL, 0, type, IPC_NOWAIT)) == -1) {
        if(errno==E2BIG)
            return(1);
    }

    return(0);
}

in both cases i get the same result before and after sending a message to the queue.

the message gets to the queue successfully , i've tested that with reading what i've sent.

Answer 1

I wrote the sample code that does seem to work properly:

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <stdio.h>
#include <errno.h>

struct msgbuf {
    long mtype;       /* message type, must be > 0 */
    char mtext[1];    /* message data */
};

int main(void) {
    int msqid;
    //msqid = msgget(IPC_PRIVATE, (IPC_CREAT | IPC_EXCL | 0600));
    msqid = msgget((key_t)1235, 0600 | IPC_CREAT);

    printf("Using message queue %d\n", msqid);
    struct msqid_ds buf;

    int rc = msgctl(msqid, IPC_STAT, &buf);

    uint msg = (uint)(buf.msg_qnum);
    printf("# messages before post: %u\n", msg);

    printf("Posting message to queue...\n");
    struct msgbuf qmsg;
    qmsg.mtype = 100;
    qmsg.mtext[0] = 'T';

    int res = msgsnd(msqid, &qmsg, 1, MSG_NOERROR);

    rc = msgctl(msqid, IPC_STAT, &buf);

    msg = (uint)(buf.msg_qnum);
    printf("# messages after post: %u\n", msg);

    return 0;
}

Maybe that will be helpful to you? The number of messages on the queue does seem to increment correctly when using this code.