Name

Library

Synopsis

Return values

Detailed description

    long mtype;    /* message type */
    char mtext[1]; /* body of message */

         

mtype is an integer greater than 0 that can be used for selecting messages, mtext is an array of bytes, with a size up to that of the system limit (Dv MSGMAX).

The value of msgtyp has one of the following meanings:

• The msgtyp argument is greater than 0. The first message of type msgtyp will be received.
• The msgtyp argument is equal to 0. The first message on the queue (of any message type) will be received.
• The msgtyp argument is less than 0. The first message of the lowest message type that is less than or equal to the absolute value of msgtyp will be received.

The msgsz argument specifies the maximum length of the requested message. If the received message has a length greater than msgsz it will be silently truncated if the MSG_NOERROR flag is set in msgflg, otherwise an error will be returned.

If no matching message is present on the message queue specified by msqid, the behavior of msgrcv depends on whether the IPC_NOWAIT flag is set in msgflg or not. If IPC_NOWAIT is set, msgrcv will immediately return a value of -1, and set errno to ENOMSG. If IPC_NOWAIT is not set, the calling process will be blocked until:

• A message of the requested type becomes available on the message queue.
• The message queue is removed, in which case -1 will be returned, and errno set to EINVAL.

If a message is successfully received, the data structure associated with msqid is updated as follows:

• msg_cbytes is decremented by the size of the message.
• msg_lrpid is set to the pid of the caller.
• msg_lrtime is set to the current time.
• msg_qnum is decremented by 1.

Examples

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

         
#define MESSAGE_Q_KEY 1000

         
int main(void)
{
   int msq_id;
   int msg_len = 128;
   int msg_type = 0; /* Any type of message */
   struct {
       long mtype;
       char mtext[128];
   } msg_buf;
   int ret_val = -1;
   /*
    * Get the message queue id for the given key
    */
    if ((msq_id = msgget(MESSAGE_Q_KEY, IPC_CREAT)) == -1) {
       printf("Message Q  get id failed with errno %d\n", errno);
       return -1;
    }
    /*
     * Get the message from the queue
     */
    if ((ret_val = msgrcv(msq_id, (struct msgbuf *)&msg_buf, msg_len, msg_type, 0)) == -1) {
        printf("Message Q recv failed with errno %d\n", errno);
    }
    /*
     * Remove the message queue
     */
     if (msgctl(msq_id, IPC_RMID, NULL) == -1) {
       printf("Message Q  delete failed with errno %d\n", errno);
       return -1;
     }
     return ret_val;
}

         

         
 Send a message to the queue

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

         
#define MESSAGE_Q_KEY 1000

         
int main(void)
{
   int msq_id, len;
   struct {
       long mtype;
       char mtext[128];
   } msg_buf;
   /*
    * Create a message queue with the given key
    */
    if ((msq_id = msgget(MESSAGE_Q_KEY, IPC_CREAT | IPC_EXCL | 0666)) == -1) {
       printf("Message Q create failed with errno %d0, errno);
       return -1;
    }
    msg_buf.mtype = 1; /* message identifier */
    strcpy(msg_buf.mtext, "some_data_to_send"); /* data */
    len = strlen(msg_buf.mtext)+1;
    /*
     * Put the message in the queue
     */
    if (msgsnd(msq_id, (struct msgbuf *)&msg_buf, len, 0) == -1) {
        printf("Message Q send failed with errno %d\n", errno);
    }
    return 0;
}

         

Errors

See also

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