msgsnd(3p) — Linux manual page


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

       msgsnd — XSI message send operation

SYNOPSIS         top

       #include <sys/msg.h>

       int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);

DESCRIPTION         top

       The msgsnd() 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 msgsnd() function shall send a message to the queue associated
       with the message queue identifier specified by msqid.

       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 a non-zero positive type long that can
       be used by the receiving process for message selection.

       The structure member mtext is any text of length msgsz bytes. The
       argument msgsz can range from 0 to a system-imposed maximum.

       The argument msgflg specifies the action to be taken if one or more
       of the following is true:

        *  The number of bytes already on the queue is equal to msg_qbytes;
           see <sys/msg.h>.

        *  The total number of messages on all queues system-wide is equal
           to the system-imposed limit.

       These actions are as follows:

        *  If (msgflg & IPC_NOWAIT) is non-zero, the message shall not be
           sent and the calling thread shall return immediately.

        *  If (msgflg & IPC_NOWAIT) is 0, the calling thread shall suspend
           execution until one of the following occurs:

           --  The condition responsible for the suspension no longer
               exists, in which case the message is sent.

           --  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 the message is not sent 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; see <sys/msg.h>:

        *  msg_qnum shall be incremented by 1.

        *  msg_lspid shall be set to the process ID of the calling process.

        *  msg_stime shall be set to the current time, as described in
           Section 2.7.1, IPC General Description.

RETURN VALUE         top

       Upon successful completion, msgsnd() shall return 0; otherwise, no
       message shall be sent, msgsnd() shall return −1, and errno shall be
       set to indicate the error.

ERRORS         top

       The msgsnd() function shall fail if:

       EACCES Operation permission is denied to the calling process; see
              Section 2.7, XSI Interprocess Communication.

       EAGAIN The message cannot be sent for one of the reasons cited above
              and (msgflg & IPC_NOWAIT) is non-zero.

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

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

       EINVAL The value of msqid is not a valid message queue identifier, or
              the value of mtype is less than 1; or the value of msgsz is
              greater than the system-imposed limit.

       The following sections are informative.

EXAMPLES         top

   Sending a Message
       The following example sends a message to the queue identified by the
       msqid argument (assuming that value has previously been set). This
       call specifies that an error should be reported if no message is
       available.  The message size is calculated directly using the sizeof

           #include <sys/msg.h>
           int result;
           int msqid;
           struct message {
               long type;
               char text[20];
           } msg;

           msg.type = 1;
           strcpy(msg.text, "This is message 1");
           result = msgsnd(msqid, (void *) &msg, sizeof(msg.text), 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 interfaces.

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), msgrcv(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 .

       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                 2013                          MSGSND(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)msgrcv(3p)