MSGGET(2)

NAME
msgget - get message queue

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

int msgget(key, msgflg)
key_t key;
int msgflg;

DESCRIPTION
msgget() returns the message queue identifier associated with key.

A message queue identifier and associated message queue and data structure (see intro(2)) are created for key() if one of the following is true:

•   key is equal to IPC_PRIVATE.

•   key does not already have a message queue identifier associated with it, and (msgflg & IPC_CREAT) is ``true''.

Upon creation, the data structure associated with the new message queue identifier is initialized as follows:

•   msg_perm.cuid, msg_perm.uid, msg_perm.cgid, and msg_perm.gid are set equal to the effective user ID and effective group ID, respectively, of the calling process.

•   The low-order 9 bits of msg_perm.mode are set equal to the low-order 9 bits of msgflg.

•   msg_qnum, msg_lspid, msg_lrpid, msg_stime, and msg_rtime are set equal to 0.

•   msg_ctime is set equal to the current time.

•   msg_qbytes is set equal to the system-wide standard value of the maximum number of bytes allowed on a message queue.

A message queue identifier (msqid) is a unique positive integer created by a msgget(2) system call. Each msqid has a message queue and a data structure associated with it. The data structure is referred to as msqid_ds() and contains the following members:

    struct	ipc_perm msg_perm;	/* operation permission struct */
    ushort	msg_qnum;	/* number of msgs on q */
    ushort	msg_qbytes;	/* max number of bytes on q */
    ushort	msg_lspid;	/* pid of last msgsnd operation */
    ushort	msg_lrpid;	/* pid of last msgrcv operation */
    time_t	msg_stime;	/* last msgsnd time */
    time_t	msg_rtime;	/* last msgrcv time */
    time_t	msg_ctime;	/* last change time */
    		/* Times measured in secs since */
    		/* 00:00:00 GMT, Jan. 1, 1970 */

msg_perm() is an ipc_perm structure that specifies the message operation permission (see below). This structure includes the following members:

    ushort	cuid;	/* creator user id */
    ushort	cgid;	/* creator group id */
    ushort	uid;	/* user id */
    ushort	gid;	/* group id */
    ushort	mode;	/* r/w permission */

msg_qnum is the number of messages currently on the queue. msg_qbytes is the maximum number of bytes allowed on the queue. msg_lspid is the process ID of the last process that performed a msgsnd operation. msg_lrpid is the process ID of the last process that performed a msgrcv operation. msg_stime is the time of the last msgsnd operation, msg_rtime is the time of the last msgrcv operation, and msg_ctime is the time of the last msgctl(2) operation that changed a member of the above structure.

RETURN VALUES
msgget() returns A non-negative message queue identifier on success. On failure, it returns -1 and sets errno to indicate the error.

ERRORS

EACCES
    A message queue identifier exists for key, but operation permission (see intro(2)) as specified by the low-order 9 bits of msgflg would not be granted.

EEXIST
    A message queue identifier exists for key() but ( (msgflg & IPC_CREAT) & (msgflg & IPC_EXCL)) is ``true''.

ENOENT
    A message queue identifier does not exist for key() and (msgflg & IPC_CREAT) is ``false''.

ENOSPC
    A message queue identifier is to be created but the system-imposed limit on the maximum number of allowed message queue identifiers system wide would be exceeded.

SEE ALSO
intro(2), msgctl(2), msgop(2)