IPC: Messgae queues with message priority

In the previous post we saw how we can create a message queue and use the same to communicate between processes.

The message queues also have the ability to assign priorities to messages that are sent, so that while recieving the messages with higher priority are recieved first even if they are sent after a low priority message.

Let us look at this operation of the message queues using an example.

We will use the example similar to the one used in the previous post, only difference being the queue is being created by the sender where as in the previous example it was created by the reciever.

To set the priority of a message we pass the priority as an integer number to the call mq_send. For eg :

mq_send(q1,buf1,strlen(buf1),1);

The last argument "1" is the priority of the message, higher the integer number higher the priority.

Let us send 4 messages each with a different priority and send the least prioirty message first. i.e.

mq_send(q1,buf1,strlen(buf1),1); mq_send(q1,buf2,strlen(buf2),2); mq_send(q1,buf3,strlen(buf3),3); mq_send(q1,buf4,strlen(buf4),4);

And in the recieve program we will recieve using mq_recieve 4 times, and while recieveing we will read the priority of the message and display the same.

mq_receive(q2,buf,attr1->mq_msgsize,&prio); printf("Priority= %d",prio); printf("\n Message = %s\n",buf);

The complete codes for the two programs will be as below.

prio_send.c

#include "fcntl.h" /* For O_* constants */ #include "sys/stat.h" /* For mode constants */ #include "mqueue.h" #include <stdio.h> main() { mqd_t q1; char *buf1="hello1"; char *buf2="hello2"; char *buf3="hello3"; char *buf4="hello4"; q1 = mq_open("/test_q",O_CREAT|O_RDWR,0666,NULL); if(q1 == -1) { printf("Error"); } mq_send(q1,buf1,strlen(buf1),1); mq_send(q1,buf2,strlen(buf2),2); mq_send(q1,buf3,strlen(buf3),3); mq_send(q1,buf4,strlen(buf4),4); }

recieve.c

#include "fcntl.h" /* For O_* constants */ #include "sys/stat.h" /* For mode constants */ #include "mqueue.h" #include <stdio.h> main() { mqd_t q2; char *buf; struct mq_attr *attr1; int prio; attr1 = malloc(sizeof(struct mq_attr)); q2 = mq_open("/test_q",O_RDWR); if(q2 == -1) { printf("Error"); } buf = malloc(10*sizeof(char)); mq_getattr(q2, attr1); mq_receive(q2,buf,attr1->mq_msgsize,&prio); printf("Priority= %d",prio); printf("\n Message = %s\n",buf); mq_receive(q2,buf,attr1->mq_msgsize,&prio); printf("Priority= %d",prio); printf("\n Message = %s\n",buf); mq_receive(q2,buf,attr1->mq_msgsize,&prio); printf("Priority= %d",prio); printf("\n Message = %s\n",buf); mq_receive(q2,buf,attr1->mq_msgsize,&prio); printf("Priority= %d",prio); printf("\n Message = %s\n",buf); }

Compile the two programs

cc -lrt prio_send.c -o prio_send cc -lrt prio_recieve.c -o prio_recieve

First execute the send program, so that we send messages of four different priorities in the queue

./prio_send

Now to read the message execute the receive program

./prio_recieve

The output should be :

Priority= 4 Message = hello4 Priority= 3 Message = hello3 Priority= 2 Message = hello2 Priority= 1 Message = hello1

Thus we can see that the message queue does not perform as a fifo and the message with higher priority is read before message with lower priority .Messages of the same prioirty are read in the fifo manner.