msg.h(7posix) [posix man page]

<sys/msg.h>(P)						     POSIX Programmer's Manual						    <sys/msg.h>(P)

NAME

       sys/msg.h - XSI message queue structures

SYNOPSIS

       #include <sys/msg.h>

DESCRIPTION

       The <sys/msg.h> header shall define the following data types through typedef:

       msgqnum_t
	      Used for the number of messages in the message queue.

       msglen_t
	      Used for the number of bytes allowed in a message queue.

       These types shall be unsigned integer types that are able to store values at least as large as a type unsigned short.

       The <sys/msg.h> header shall define the following constant as a message operation flag:

       MSG_NOERROR
	      No error if big message.

       The msqid_ds structure shall contain the following members:

	      struct ipc_perm msg_perm	 Operation permission structure.
	      msgqnum_t       msg_qnum	 Number of messages currently on queue.
	      msglen_t	      msg_qbytes Maximum number of bytes allowed on queue.
	      pid_t	      msg_lspid  Process ID of last msgsnd
	       ().
	      pid_t	      msg_lrpid  Process ID of last msgrcv
	       ().
	      time_t	      msg_stime  Time of last msgsnd
	       ().
	      time_t	      msg_rtime  Time of last msgrcv
	       ().
	      time_t	      msg_ctime  Time of last change.

       The pid_t, time_t, key_t, size_t, and ssize_t types shall be defined as described in <sys/types.h> .

       The following shall be declared as functions and may also be defined as macros. Function prototypes shall be provided.

	      int	msgctl(int, int, struct msqid_ds *);
	      int	msgget(key_t, int);
	      ssize_t	msgrcv(int, void *, size_t, long, int);
	      int	msgsnd(int, const void *, size_t, int);

       In addition, all of the symbols from <sys/ipc.h> shall be defined when this header is included.

       The following sections are informative.

APPLICATION USAGE

       None.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       <sys/ipc.h> , <sys/types.h> , msgctl(), msgget(), msgrcv(), msgsnd()

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technol-
       ogy -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
       Electrical  and	Electronics  Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE
       and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can  be  obtained
       online at http://www.opengroup.org/unix/online.html .

IEEE
/The Open Group						       2003							    <sys/msg.h>(P)

MSGCTL(2)						      BSD System Calls Manual							 MSGCTL(2)

NAME

     msgctl -- message control operations

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

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

     int
     msgctl(int msqid, int cmd, struct msqid_ds *buf);

DESCRIPTION

     The msgctl() system call performs some control operations on the message queue specified by msqid.

     Each message queue has a data structure associated with it, parts of which may be altered by msgctl() and parts of which determine the
     actions of msgctl().  The data structure is defined in <sys/msg.h> and contains (amongst others) the following members:

     struct msqid_ds {
	     struct  ipc_perm msg_perm;      /* msg queue permission bits */
	     struct  msg *msg_first; /* first message in the queue */
	     struct  msg *msg_last;  /* last message in the queue */
	     msglen_t msg_cbytes;    /* number of bytes in use on the queue */
	     msgqnum_t msg_qnum;     /* number of msgs in the queue */
	     msglen_t msg_qbytes;    /* max # of bytes on the queue */
	     pid_t   msg_lspid;      /* pid of last msgsnd() */
	     pid_t   msg_lrpid;      /* pid of last msgrcv() */
	     time_t  msg_stime;      /* time of last msgsnd() */
	     time_t  msg_rtime;      /* time of last msgrcv() */
	     time_t  msg_ctime;      /* time of last msgctl() */
     };

     The ipc_perm structure used inside the msqid_ds structure is defined in <sys/ipc.h> and looks like this:

     struct ipc_perm {
	     uid_t	     cuid;   /* creator user id */
	     gid_t	     cgid;   /* creator group id */
	     uid_t	     uid;    /* user id */
	     gid_t	     gid;    /* group id */
	     mode_t	     mode;   /* r/w permission */
	     unsigned short  seq;    /* sequence # (to generate unique ipcid) */
	     key_t	     key;    /* user specified msg/sem/shm key */
     };

     The operation to be performed by msgctl() is specified in cmd and is one of:

     IPC_STAT	Gather information about the message queue and place it in the structure pointed to by buf.

     IPC_SET	Set the value of the msg_perm.uid, msg_perm.gid, msg_perm.mode and msg_qbytes fields in the structure associated with msqid.  The
		values are taken from the corresponding fields in the structure pointed to by buf.  This operation can only be executed by the
		super-user, or a process that has an effective user id equal to either msg_perm.cuid or msg_perm.uid in the data structure associ-
		ated with the message queue.  The value of msg_qbytes can only be increased by the super-user.	Values for msg_qbytes that exceed
		the system limit (MSGMNB from <sys/msg.h>) are silently truncated to that limit.

     IPC_RMID	Remove the message queue specified by msqid and destroy the data associated with it.  Only the super-user or a process with an
		effective uid equal to the msg_perm.cuid or msg_perm.uid values in the data structure associated with the queue can do this.

     The permission to read from or write to a message queue (see msgsnd(2) and msgrcv(2)) is determined by the msg_perm.mode field in the same
     way as is done with files (see chmod(2)), but the effective uid can match either the msg_perm.cuid field or the msg_perm.uid field, and the
     effective gid can match either msg_perm.cgid or msg_perm.gid.

RETURN VALUES

     The msgctl() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate
     the error.

ERRORS

     The msgctl() function will fail if:

     [EPERM]		The cmd argument is equal to IPC_SET or IPC_RMID and the caller is not the super-user, nor does the effective uid match
			either the msg_perm.uid or msg_perm.cuid fields of the data structure associated with the message queue.

			An attempt is made to increase the value of msg_qbytes through IPC_SET but the caller is not the super-user.

     [EACCES]		The command is IPC_STAT and the caller has no read permission for this message queue.

     [EINVAL]		The msqid argument is not a valid message queue identifier.

			cmd is not a valid command.

     [EFAULT]		The buf argument specifies an invalid address.

SEE ALSO

     msgget(2), msgrcv(2), msgsnd(2)

HISTORY

     Message queues appeared in the first release of AT&T System V UNIX.

BSD
								   July 9, 2009 							       BSD