DragonFly On-Line Manual Pages

MSGSND(2)                DragonFly System Calls Manual               MSGSND(2)

NAME
     msgsnd - send a message to a message queue

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <sys/types.h>
     #include <sys/ipc.h>
     #include <sys/msg.h>

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

DESCRIPTION
     The msgsnd() system call sends a message to the message queue specified
     in msqid.  msgp points to a structure containing the message.  This
     structure should consist of the following members:

         long mtype;    /* message type */
         char mtext[1]; /* body of message */

     mtype is an integer greater than 0 that can be used for selecting
     messages (see msgrcv(2)), mtext is an array of bytes, with a size up to
     that of the system limit (MSGMAX).

     If the number of bytes already on the message queue plus msgsz is bigger
     than the maximum number of bytes on the message queue (msg_qbytes, see
     msgctl(2)), or 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 does no longer exist.
         The message will be sent.

     *   The message queue is removed, in which case -1 will be returned, and
         errno is 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_cbytes is incremented by the size of the message.

     *   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
     The msgsnd() function returns the value 0 if successful; otherwise the
     value -1 is returned and the global variable errno is set to indicate the
     error.

ENVIRONMENT
     The XSI Interprocess Communication family of functions is also available
     as an implementation in userspace.  To use it, the sysvipcd(8) daemon has
     to be running.

     If the USR_SYSVIPC variable is set in a process' environment, the process
     and its children will use the userspace implementation.

ERRORS
     msgsnd() will fail if:

     [EINVAL]           msqid is not a valid message queue identifier

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

                        msgsz is less than 0, or greater than msg_qbytes.

                        mtype is not greater than 0.

     [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.

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

AUTHORS
     The DragonFly specific userspace implementation (see ENVIRONMENT) was
     written by Larisa Grigore.

BUGS
     NetBSD, DragonFly, and FreeBSD do not define the EIDRM error value, which
     should be used in the case of a removed message queue.

DragonFly 6.3-DEVELOPMENT       January 4, 2015      DragonFly 6.3-DEVELOPMENT