PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

PROLOG         top

       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         top

       msgrcv — XSI message receive operation

SYNOPSIS         top

       #include <sys/msg.h>

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

DESCRIPTION         top

       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(3p).

       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         top

       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         top

       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         top

   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         top

       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         top

       None.

FUTURE DIRECTIONS         top

       None.

SEE ALSO         top

       Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime,
       mq_close(3p), mq_getattr(3p), mq_notify(3p), mq_open(3p),
       mq_receive(3p), mq_send(3p), mq_setattr(3p), mq_unlink(3p),
       msgctl(3p), msgget(3p), msgsnd(3p), sigaction(3p)

       The Base Definitions volume of POSIX.1‐2008, Section 3.225, Message
       Queue, sys_msg.h(0p)

COPYRIGHT         top

       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 .

IEEE/The Open Group                 2013                          MSGRCV(3P)

Pages that refer to this page: sys_msg.h(0p)ipcs(1p)mq_close(3p)mq_getattr(3p)mq_notify(3p)mq_open(3p)mq_receive(3p)mq_setattr(3p)mq_unlink(3p)msgctl(3p)msgget(3p)msgsnd(3p)