MSGRCV(3)                  POSIX Programmer's Manual                 MSGRCV(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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.1-2017, Section 3.226, 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 aa 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.1-2017, Section 3.226, Message
       Queue, <sys_msg.h>

COPYRIGHT
       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(3)
