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MSGRCV(3P)                 POSIX Programmer's Manual                MSGRCV(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
       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 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  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 mes-
       sage  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 imple-
       mentation-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_NOER-
              ROR) is 0.
       EACCES Operation permission is denied to the calling process; see  Sec-
              tion 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 avail-
       able, but not if the message is too large. The message size  is  calcu-
       lated 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 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(), 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 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                           MSGRCV(3P)