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 Definitions 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 argument 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 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 message 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 interprocess 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 Definitions 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 .