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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 IEEE Std 1003.1-2001, Section 3.224, Message Queue).
       It is unspecified whether this function interoperates with the realtime
       interprocess communication facilities defined in 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 exe-
          cution 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 equal 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  equal  to  the  process ID of the calling
          process.
        * msg_stime shall be set equal to the current time.
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  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 less
              than 0 or 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 XSI Interprocess Communication can be easily  modified  to
       use the alternative interfaces.
RATIONALE
       None.
FUTURE DIRECTIONS
       None.
SEE ALSO
       XSI  Interprocess  Communication,  Realtime,  mq_close(), mq_getattr(),
       mq_notify(),   mq_open(),   mq_receive(),   mq_send(),    mq_setattr(),
       mq_unlink(),  msgctl(), msgget(), msgrcv(), sigaction(), the Base Defi-
       nitions volume of IEEE Std 1003.1-2001, <sys/msg.h>
COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
       -- 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                           MSGSND(3P)