POSIX message queue linux

Question

i am trying to receive mq_receive() the data from the sender mq_sender() it works when i used to run the send file first .. but if i run receive first it will block even though after send the data to the queue..i attached my code below

    //sender
#include<stdio.h>
#include<sys/types.h>
#include<fcntl.h>
#include<sys/stat.h>
#include<mqueue.h>
#include<string.h>
#include<unistd.h>
#define NAME "/queone"
#define MAX_SIZE 1021
void main()
{
        int i;
        struct mq_attr attr;
        attr.mq_maxmsg = 10;
        attr.mq_msgsize = MAX_SIZE;
        int fd;
        char s[MAX_SIZE]="hello";
        int r1,r2;


                mq_unlink(NAME);
                perror("mq_unlink");
                fd=mq_open(NAME,O_CREAT | O_WRONLY ,0664,&attr);
                printf("fd=%d\n",fd);
                for(i=0;i<5;i++)
                r1=mq_send(fd,s,sizeof(s),0);
                printf("%d",r1);
}
//receiver
#include<stdio.h>
#include<sys/types.h>
#include<fcntl.h>
#include<sys/stat.h>
#include<mqueue.h>
#include<string.h>
#include<unistd.h>
#define NAME "/queone"
#define MAX_SIZE 1021
void main()
{
        struct mq_attr attr;
        attr.mq_maxmsg = 10;
        attr.mq_msgsize = MAX_SIZE;

        int fd,fd1,r;
        char s[MAX_SIZE];
                fd=mq_open(NAME,O_CREAT|O_RDONLY ,0664,&attr);
                perror("mq_open");
                printf("fd=%d\n",fd);
                mq_getattr(fd,&attr);
                printf("%d\n",attr.mq_curmsgs);
                printf("%d\n",attr.mq_msgsize);

                r=mq_receive(fd,s,sizeof(s),0);
                printf("%d",r);
                if(r==-1)
                perror("mq_receive");
                printf("%s\n",s);
                mq_unlink(NAME);


        }

how to handle this

Answer 1

The problem is that you unconditionally call mq_unlink in the sender. Here's what's happening because of that:

1. The receiver creates a message queue
2. The receiver listens for a message on it
3. The receiver goes to sleep
4. The sender deletes the message queue
5. The sender creates a new message queue with the same name
6. The sender sends a message to the new message queue

Since the message queue that the sender sent the message to isn't the one that the receiver is listening on, the receiver obviously doesn't receive the message.