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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 by the msgget() call.  If msgget() is
  called with a non-existent message queue ID, the kernel allocates a new
  msqid_ds structure, initializes it, and returns the message queue ID that
  is to be associated with the message queue.

  The following sections describe the fields in the msqid_ds structure and
  show the ipc_perm structure identified by the msg_perm field.

FIELDS
  msg_perm  The ipc_perm structure that defines permissions for message
	    operations.

  msg_first A pointer to the first message on the queue.

  msg_last  A pointer to the last message on the queue.

  msg_cbytes
	    The current number of bytes on the queue.

  msg_qnum  The number of messages currently on the queue.

  msg_qbytes
	    The maximum number of bytes allowed on the queue.

  msg_lspid The process ID of the last process that called msgsnd() for the
	    queue.

  msg_lrpid The process ID of the last process that called msgrcv() for the
	    queue.

  msg_stime The time of the last msgsnd() operation.

  msg_rtime The time of the last msgrcv() operation.

  msg_ctime The time of the last msgctl() operation that changed a member of
	    the ipc_perm structure designated by the msg_perm field.

THE ipc_perm STRUCTURE
  The msg_perm field identifies the associated ipc_perm structure that
  defines the permissions for operations on the message queue.	The ipc_perm
  structure (defined in the sys/ipc.h header file) is as follows:

       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 of the ipc_perm structure 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.

SEE ALSO
  Functions: msgctl(2), msgget(2), msgrcv(2), msgsnd(2)

Index for Section 4

Alphabetical listing for M

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