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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

       msgrcv — XSI message receive operation

SYNOPSIS

       #include <sys/msg.h>

       ssize_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp,
           int msgflg);

DESCRIPTION

       The  msgrcv()  function operates on XSI message queues (see the Base Definitions 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 msgrcv() function shall read a message from the  queue  associated  with  the  message
       queue  identifier specified by msqid and place it in the user-defined buffer pointed to by
       msgp.

       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  the received message's type as specified by the sending
       process.

       The structure member mtext is the text of the message.

       The argument msgsz specifies the size in bytes of mtext.  The received  message  shall  be
       truncated  to  msgsz  bytes  if it is larger than msgsz and (msgflg & MSG_NOERROR) is non-
       zero.  The truncated part of the message shall be lost and no indication of the truncation
       shall be given to the calling process.

       If the value of msgsz is greater than {SSIZE_MAX}, the result is implementation-defined.

       The argument msgtyp specifies the type of message requested as follows:

        *  If msgtyp is 0, the first message on the queue shall be received.

        *  If msgtyp is greater than 0, the first message of type msgtyp shall be received.

        *  If  msgtyp  is  less than 0, the first message of the lowest type that is less than or
           equal to the absolute value of msgtyp shall be received.

       The argument msgflg specifies the action to be taken if a message of the desired  type  is
       not on the queue. These are as follows:

        *  If (msgflg & IPC_NOWAIT) is non-zero, the calling thread shall return immediately with
           a return value of −1 and errno set to [ENOMSG].

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

           --  A message of the desired type is placed on the queue.

           --  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 a  message
               is  not received 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:

        *  msg_qnum shall be decremented by 1.

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

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

RETURN VALUE

       Upon successful completion, msgrcv() shall return a value equal to  the  number  of  bytes
       actually  placed into the buffer mtext.  Otherwise, no message shall be received, msgrcv()
       shall return −1, and errno shall be set to indicate the error.

ERRORS

       The msgrcv() function shall fail if:

       E2BIG  The value of mtext is greater than msgsz and (msgflg & MSG_NOERROR) is 0.

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

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

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

       EINVAL msqid is not a valid message queue identifier.

       ENOMSG The  queue does not contain a message of the desired type and (msgflg & IPC_NOWAIT)
              is non-zero.

       The following sections are informative.

EXAMPLES

   Receiving a Message
       The following example receives the first message on the queue (based on the value  of  the
       msgtyp  argument,  0).  The  queue  is identified by the msqid argument (assuming that the
       value has previously been set). This call specifies that an error should be reported if no
       message  is available, but not if the message is too large. 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;
           long msgtyp = 0;
           ...
           result = msgrcv(msqid, (void *) &msg, sizeof(msg.text),
                    msgtyp, MSG_NOERROR | 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 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(), msgsnd(), 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 Electrical 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.kernel.org/doc/man-pages/reporting_bugs.html .