Name

Library

Synopsis

Return values

Detailed description

A message queue is created if either key is equal to IPC_PRIVATE, or key does not have a message queue identifier associated with it, and the IPC_CREAT bit is set in msgflg.

If a new message queue is created, the data structure associated with it (the msqid_ds structure, see msgctl is initialized as follows:

• msg_perm.cuid and msg_perm.uid are set to the effective uid of the calling process. In the current implementation, uid is set to root.
• msg_perm.gid and msg_perm.cgid are set to the effective gid of the calling process. In the current implementation, gid is set to root.
• msg_perm.mode is set to the lower 9 bits of msgflg.
• msg_cbytes, msg_qnum, msg_lspid, msg_lrpid, msg_rtime, and msg_stime are set to 0.
• msg_qbytes is set to the system wide maximum value for the number of bytes in a queue (Dv MSGMNB).
• msg_ctime is set to the current time.

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, len;
   struct {
       long mtype;
       char mtext[128];
   } msg_buf;
   /*
    * Create a message queue with a given key
    */
    if ((msq_id = msgget(MESSAGE_Q_KEY, IPC_CREAT | IPC_EXCL | 0666)) == -1) {
       printf("Message Q create failed with errno %d\n", 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;
}

         

Example code to return an existing message queue with msgget

#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;
   /*
    * Get the message queue id for the given key
    */
    if ((msq_id = msgget(MESSAGE_Q_KEY, 0)) == -1) {
       printf("Message Q  get id failed with errno %d\n", errno);
       return -1;
    }
    /*
     * Get the message from the queue
     */
    if (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 0;
}

         

Errors

See also

Feedback