msgsnd(2) - NetBSD Manual Pages

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MSGSND(2)                 NetBSD System Calls Manual                 MSGSND(2)


NAME

     msgsnd -- send a message to a message queue


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <sys/msg.h>

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


DESCRIPTION

     The msgsnd() function sends a message from the message queue specified in
     msqid.  The msgp argument is a pointer to a user-defined structure con-
     taining the message.  This structure must contain a first field of type
     long that will indicate the user-defined type of the message.  The
     remaining fields will contain the contents of the message.  The following
     is an example of what this user-defined structure might look like:

           struct mymsg {
               long mtype;    /* message type */
               char mtext[1]; /* body of message */
           };

     The mtype field is an integer greater than 0 that can be used for select-
     ing messages (see msgrcv(2)).  The mtext field is an array of bytes of
     length msgsz, with size up to the system limit MSGMAX.

     If the number of bytes already on the message queue plus msgsz is greater
     than the maximum number of bytes in the message queue (msg_qbytes, see
     msgctl(2)), or if the number of messages on all queues system-wide is
     already equal to the system limit, msgflg determines the action of
     msgsnd().  If msgflg has IPC_NOWAIT mask set in it, the call will return
     immediately.  If msgflg does not have IPC_NOWAIT set in it, the call will
     block until:

           ·   The condition which caused the call to block no longer exists.
               The message was sent.

           ·   The message queue is removed, in which case -1 will be returned
               and errno set to EINVAL.

           ·   The caller catches a signal.  The call returns with errno set
               to EINTR.

     After a successful call, the data structure associated with the message
     queue is updated in the following way:

           ·   msg_qnum is incremented by 1.

           ·   msg_lspid is set to the pid of the calling process.

           ·   msg_stime is set to the current time.


RETURN VALUES

     Upon successful completion, 0 is returned.  Otherwise, -1 is returned and
     errno is set to indicate the error.


ERRORS

     msgsnd() will fail if:

     [EACCES]           The calling process does not have write access to the
                        message queue.

     [EAGAIN]           There was no space for this message either on the
                        queue or in the whole system, and IPC_NOWAIT was set
                        in msgflg.

     [EFAULT]           msgp points to an invalid address.

     [EINTR]            The system call was interrupted by the delivery of a
                        signal.

     [EINVAL]           The msqid argument is not a valid message queue iden-
                        tifier, or the value of mtype is less than 1.

                        The message queue was removed while msgsnd() was wait-
                        ing for a resource to become available in order to
                        deliver the message.

                        The msgsz argument is greater than msg_qbytes or
                        SSIZE_MAX.


SEE ALSO

     msgctl(2), msgget(2), msgrcv(2)


STANDARDS

     The msgsnd system call conforms to X/Open System Interfaces and Headers
     Issue 5 (``XSH5'') and IEEE Std 1003.1-2001 (``POSIX.1'').


HISTORY

     Message queues appeared in the first release of AT&T System V UNIX.

NetBSD 8.1                       July 24, 2013                      NetBSD 8.1