Linux : Check if message queue is empty - c

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.