This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       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 Section2.7, XSIInterprocessCommunication can  be  easily  modified  to  use  the
       alternative interfaces.

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
       for Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group  Base
       Specifications  Issue  7, 2018 Edition, Copyright (C) 2018 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 .

       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 .

IEEE/The Open Group                                   2017                                        MSGRCV(3POSIX)

       The  msgrcv()  function  operates on XSI message queues (see the Base Definitions volume of POSIX.1‐2017,
       Section3.226, MessageQueue).  It is unspecified whether this function interoperates with  the  realtime
       interprocess communication facilities defined in Section2.8, 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 Section2.7.1, IPCGeneralDescription.

       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  Section2.7,  XSIInterprocessCommunication.

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

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

       EINVALmsqid 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.

       Thefollowingsectionsareinformative.

ReceivingaMessage
       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);

       None.

       msgrcv — XSI message receive operation

       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.

       None.

       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.

Section2.7,   XSIInterprocessCommunication,  Section2.8,  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.1‐2017, Section3.226, MessageQueue, <sys_msg.h>

       #include <sys/msg.h>

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