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.1‐2017, 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 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

        *  msg_qnum shall be incremented by 1.

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

        *  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

       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 operator.

           #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

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.1‐2017, 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                        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)