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MSGSND(3P)		   POSIX Programmer's Manual		    MSGSND(3P)

PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the	 corresponding
       Linux  manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.

NAME
       msgsnd — XSI message send operation

SYNOPSIS
       #include <sys/msg.h>

       int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);

DESCRIPTION
       The msgsnd() function operates on XSI message queues (see the Base Def‐
       initions	 volume of POSIX.1‐2008, Section 3.225, Message Queue).	 It is
       unspecified whether  this  function  interoperates  with	 the  realtime
       interprocess communication facilities defined in Section 2.8, Realtime.

       The msgsnd() function shall send a message to the queue associated with
       the message queue identifier specified by msqid.

       The application shall ensure that the argument msgp points to  a	 user-
       defined	buffer that contains first a field of type long specifying the
       type of the message, and then a data portion that holds the data	 bytes
       of  the	message.  The structure below is an example of what this user-
       defined buffer might look like:

	   struct mymsg {
	       long   mtype;	   /* Message type. */
	       char   mtext[1];	   /* Message text. */
	   }

       The structure member mtype is a non-zero positive type long that can be
       used by the receiving process for message selection.

       The structure member mtext is any text of length msgsz bytes. The argu‐
       ment msgsz can range from 0 to a system-imposed maximum.

       The argument msgflg specifies the action to be taken if one or more  of
       the following is true:

	*  The	number	of  bytes already on the queue is equal to msg_qbytes;
	   see <sys/msg.h>.

	*  The total number of messages on all queues system-wide is equal  to
	   the system-imposed limit.

       These actions are as follows:

	*  If (msgflg & IPC_NOWAIT) is non-zero, the message shall not be sent
	   and the calling thread shall return immediately.

	*  If (msgflg & IPC_NOWAIT) is 0, the  calling	thread	shall  suspend
	   execution until one of the following occurs:

	   --  The  condition responsible for the suspension no longer exists,
	       in which case the message is sent.

	   --  The message queue identifier msqid is removed from the  system;
	       when this occurs, errno shall be set to [EIDRM] and −1 shall be
	       returned.

	   --  The calling thread receives a signal that is to be  caught;  in
	       this  case  the	message	 is  not  sent	and the calling thread
	       resumes execution in the manner prescribed in sigaction().

       Upon successful	completion,  the  following  actions  are  taken  with
       respect to the data structure associated with msqid; see <sys/msg.h>:

	*  msg_qnum shall be incremented by 1.

	*  msg_lspid shall be set to the process ID of the calling process.

	*  msg_stime shall be set to the current time, as described in Section
	   2.7.1, IPC General Description.

RETURN VALUE
       Upon successful completion, msgsnd() shall return 0; otherwise, no mes‐
       sage shall be sent, msgsnd() shall return −1, and errno shall be set to
       indicate the error.

ERRORS
       The msgsnd() function shall fail if:

       EACCES Operation permission is denied to the calling process; see  Sec‐
	      tion 2.7, XSI Interprocess Communication.

       EAGAIN The  message  cannot  be sent for one of the reasons cited above
	      and (msgflg & IPC_NOWAIT) is non-zero.

       EIDRM  The message queue identifier msqid is removed from the system.

       EINTR  The msgsnd() function was interrupted by a signal.

       EINVAL The value of msqid is not a valid message queue  identifier,  or
	      the  value  of  mtype  is	 less than 1; or the value of msgsz is
	      greater than the system-imposed limit.

       The following sections are informative.

EXAMPLES
   Sending a Message
       The following example sends a message to the queue  identified  by  the
       msqid argument (assuming that value has previously been set). This call
       specifies that an error should be reported if no message is  available.
       The message size is calculated directly using the sizeof operator.

	   #include <sys/msg.h>
	   ...
	   int result;
	   int msqid;
	   struct message {
	       long type;
	       char text[20];
	   } msg;

	   msg.type = 1;
	   strcpy(msg.text, "This is message 1");
	   ...
	   result = msgsnd(msqid, (void *) &msg, sizeof(msg.text), IPC_NOWAIT);

APPLICATION USAGE
       The  POSIX Realtime Extension defines alternative interfaces for inter‐
       process communication (IPC). Application developers who need to use IPC
       should design their applications so that modules using the IPC routines
       described in Section 2.7, XSI Interprocess Communication can be	easily
       modified to use the alternative interfaces.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       Section	2.7,  XSI  Interprocess	 Communication, Section 2.8, Realtime,
       mq_close(),   mq_getattr(),   mq_notify(),   mq_open(),	 mq_receive(),
       mq_send(),  mq_setattr(),  mq_unlink(),	msgctl(),  msgget(), msgrcv(),
       sigaction()

       The Base Definitions volume of  POSIX.1‐2008,  Section  3.225,  Message
       Queue, <sys_msg.h>

COPYRIGHT
       Portions	 of  this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       --  Portable  Operating	System	Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
       cal  and	 Electronics  Engineers,  Inc  and  The	 Open Group.  (This is
       POSIX.1-2008 with the 2013 Technical Corrigendum	 1  applied.)  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.unix.org/online.html .

       Any typographical or formatting errors that appear  in  this  page  are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors,	 see  https://www.ker‐
       nel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group		     2013			    MSGSND(3P)
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