msqid_ds(4)						     Kernel Interfaces Manual						       msqid_ds(4)

NAME

       msqid_ds - Defines a message queue

SYNOPSIS

       #include <sys/msg.h>
       struct  msqid_ds{       struct  ipc_perm  msg_perm;	 struct  msg  *msg_first;	struct	msg  *msg_last;       u_short  msg_cbytes;
	    u_short  msg_qnum;	     u_short  msg_qbytes;	u_short  msg_lspid;	  ushort  msg_lrpid;	   time_t  msg_stime;	    time_t
       msg_rtime;      time_t msg_ctime; };

DESCRIPTION

       The msqid_ds structure defines a message queue associated with a message queue ID.  There is one queue per message queue ID.  Collectively,
       the queues are stored as an array, with message queue IDs serving as an index into the array.

       A message queue is implemented as a linked list of messages, with msg_first amd msg_last pointing to the first and  last  messages  on  the
       queue.

       The IPC permissions for the message queue are implemented in a separate, but associated, ipc_perm structure.

       A  message queue is created indirectly via the msgget() call.  If msgget() is called with a non-existent message queue ID, the kernel allo-
       cates a new msqid_ds structure, initializes it, and returns the message queue ID that is to be associated with the message queue.

FIELDS

       The ipc_perm structure that defines permissions for message operations.	See NOTES.  A pointer to  the  first  message  on  the	queue.	 A
       pointer	to  the last message on the queue.  The current number of bytes on the queue.  The number of messages currently on the queue.  The
       maximum number of bytes allowed on the queue.  The process ID of the last process that called msgsnd() for the queue.  The  process  ID	of
       the  last  process  that called msgrcv() for the queue.	The time of the last msgsnd() operation.  The time of the last msgrcv() operation.
       The time of the last msgctl() operation that changed a member of the msqid_ds structure.

NOTES

       The msg_perm field identifies the associated ipc_perm structure that defines the permissions for operations  on	the  message  queue.   The
       ipc_perm structure (from the sys/ipc.h header file) is shown here.  struct ipc_perm {
	     ushort    uid;	 /* owner's user id	       */
	     ushort    gid;	 /* owner's group id	       */
	     ushort    cuid;	 /* creator's user id	       */
	     ushort    cgid;	 /* creator's group id	       */
	     ushort    mode;	 /* access modes	       */
	     ushort    seq;	 /* slot usage sequence number */
	     key_t	key;	   /*  key			   */ }; The mode field is a 9-bit field that contains the permissions for message
       operations.  The first three bits identify owner permissions; the second three bits identify group permissions; and  the  last  three  bits
       identify  other	permissions.   In  each group, the first bit indicates read permission; the second bit indicates write permission; and the
       third bit is not used.

RELATED INFORMATION

       Functions: msgctl(2), msgget(2), msgrcv(2), msgsnd(2) delim off

																       msqid_ds(4)