msgrcv(3p) — Linux manual page


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.1‐2017, 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

       The argument msgtyp specifies the type of message requested as

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

        *  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

        *  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

       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);


       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

RATIONALE         top




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.1‐2017, Section 3.226,
       Message Queue, sys_msg.h(0p)

COPYRIGHT         top

       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 .

       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 .

IEEE/The Open Group               2017                        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)