IPC:Message Queues:<sys/msg.h>

Two (or more) processes can exchange information via access to a common system message queue.

Before a process can send or receive a message, the queue must be initialized (through the msgget function see below) Operations to send and receive messages are performed by the msgsnd() and msgrcv() functions, respectively.

When a message is sent, its text is copied to the message queue. The msgsnd() and msgrcv() functions can be performed as either blocking or non-blocking operations.

IPC signal and other mechanisms can be employed to implement such transfer.

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Initialising the Message Queue

The msgget() function initializes a new message queue:

int msgget(key_t key, int msgflg)

Example:
#include <sys/ipc.h>; 
#include <sys/msg.h>; 

... 


key_t key; /* key to be passed to msgget() */ 
int msgflg /* msgflg to be passed to msgget() */ 
int msqid; /* return value from msgget() */ 

...
key = ...
msgflg = ...

if ((msqid = msgget(key, msgflg)) == &ndash;1)
  {
    perror("msgget: msgget failed");
    exit(1);
   } else
    (void) fprintf(stderr, &ldquo;msgget succeeded");
...




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Controlling message queues


The msgctl() function alters the permissions and other characteristics of a message queue.




int msgctl(int msqid, int cmd, struct msqid_ds *buf )




cmd:

IPC_STAT

-- Place information about the status of the queue in the data structure pointed to by buf. The process must have read permission for this call to succeed.

IPC_SET

-- Set the owner's user and group ID, the permissions, and the size (in number of bytes) of the message queue. A process must have the effective user ID of the owner, creator, or superuser for this call to succeed.

IPC_RMID

-- Remove the message queue specified by the msqid argument.

Example:

#include<sys/types.h>
#include <sys/ipc.h> 
#include <sys/msg.h>
...
if (msgctl(msqid, IPC_STAT, &buf) == -1) {
perror("msgctl: msgctl failed");
exit(1);
}
...
if (msgctl(msqid, IPC_SET, &buf) == -1) {
perror("msgctl: msgctl failed");
exit(1);
}
...

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Sending and Receiving Messages

The msgsnd() and msgrcv() functions send and receive messages
int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);

int msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp, int msgflg);


Example:



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

...

int msgflg; /* message flags for the operation */
struct msgbuf *msgp; /* pointer to the message buffer */
int msgsz; /* message size */
long msgtyp; /* desired message type */
int msqid /* message queue ID to be used */

...

msgp = (struct msgbuf *)malloc((unsigned)(sizeof(struct msgbuf)
- sizeof msgp->mtext + maxmsgsz));

if (msgp == NULL) {
(void) fprintf(stderr, "msgop: %s %d byte messages.\n",
"could not allocate message buffer for", maxmsgsz);
exit(1);

...

msgsz = ...
msgflg = ...

if (msgsnd(msqid, msgp, msgsz, msgflg) == -1)
perror("msgop: msgsnd failed");
...
msgsz = ...
msgtyp = first_on_queue;
msgflg = ...
if (rtrn = msgrcv(msqid, msgp, msgsz, msgtyp, msgflg) == -1)
perror("msgop: msgrcv failed");
...
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Example:





Send.c




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

#define MSGSZ     128


/*
 * Declare the message structure.
 */

typedef struct msgbuf {
         long    mtype;
         char    mtext[MSGSZ];
         } message_buf;

main()
{
    int msqid;
    int msgflg = IPC_CREAT | 0666;
    key_t key;
    message_buf sbuf;
    size_t buf_length;

    /*
     * Get the message queue id for the
     * "name" 1234, which was created by
     * the server.
     */
    key = 1234;

(void) fprintf(stderr, "\nmsgget: Calling msgget(%#lx,\
%#o)\n",
key, msgflg);

    if ((msqid = msgget(key, msgflg )) < 0) {
        perror("msgget");
        exit(1);
    }
    else 
     (void) fprintf(stderr,"msgget: msgget succeeded: msqid = %d\n", msqid);


    /*
     * We'll send message type 1
     */
     
    sbuf.mtype = 1;
    
    (void) fprintf(stderr,"msgget: msgget succeeded: msqid = %d\n", msqid);
    
    (void) strcpy(sbuf.mtext, "Did you get this?");
    
    (void) fprintf(stderr,"msgget: msgget succeeded: msqid = %d\n", msqid);
    
    buf_length = strlen(sbuf.mtext) + 1 ;
    
    

    /*
     * Send a message.
     */
    if (msgsnd(msqid, &sbuf, buf_length, IPC_NOWAIT) < 0) {
       printf ("%d, %d, %s, %d\n", msqid, sbuf.mtype, sbuf.mtext, buf_length);
        perror("msgsnd");
        exit(1);
    }

   else 
      printf("Message: \"%s\" Sent\n", sbuf.mtext);
      
    exit(0);
}





Recv.c



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

#define MSGSZ     128


/*
 * Declare the message structure.
 */

typedef struct msgbuf {
    long    mtype;
    char    mtext[MSGSZ];
} message_buf;


main()
{
    int msqid;
    key_t key;
    message_buf  rbuf;

    /*
     * Get the message queue id for the
     * "name" 1234, which was created by
     * the server.
     */
    key = 1234;

    if ((msqid = msgget(key, 0666)) < 0) {
        perror("msgget");
        exit(1);
    }

    
    /*
     * Receive an answer of message type 1.
     */
    if (msgrcv(msqid, &rbuf, MSGSZ, 1, 0) < 0) {
        perror("msgrcv");
        exit(1);
    }

    /*
     * Print the answer.
     */
    printf("%s\n", rbuf.mtext);
    exit(0);
}