NAME    [Toc]    [Back]

     msgsnd - send a message to a message queue

LIBRARY    [Toc]    [Back]

     Standard C Library (libc, -lc)

SYNOPSIS    [Toc]    [Back]

     #include <sys/msg.h>

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

DESCRIPTION    [Toc]    [Back]

     The msgsnd() function sends a message from the message queue specified in
     msqid.  msgp points to a user-defined structure containing 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 */
     };

     mtype is an integer greater than 0 that can be used for selecting messages
 (see msgrcv(2)).  mtext is an array of bytes, 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:

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

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

     +o   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:

     +o   msg_qnum is incremented by 1.

     +o   msg_lspid is set to the pid of the calling process.

     +o   msg_stime is set to the current time.

RETURN VALUES    [Toc]    [Back]

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

ERRORS    [Toc]    [Back]

     msgsnd() will fail if:

     [EINVAL]           msqid is not a valid message queue identifier, or the
                        value of mtype is less than 1.

                        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.

     [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.

SEE ALSO    [Toc]    [Back]

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

STANDARDS    [Toc]    [Back]

     The msgsnd system call conforms to X/Open System Interfaces and Headers
     Issue 5 (``XSH5'').

HISTORY    [Toc]    [Back]

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

BSD                             August 17, 1995                            BSD