msgsnd(2) - NetBSD Manual Pages

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MSGSND(2)                 NetBSD Programmer's Manual                 MSGSND(2)


NAME

     msgsnd - send a message to a message queue


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() function sends a message from 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 mes-
     sages (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 al-
     ready 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_cbytes is incremented by msgsz.

     +   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:

     [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 mes-
                   sage 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 sig-
                   nal.


SEE ALSO

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


BUGS

     NetBSD does not define the EIDRM error value, which should be used in the
     case of a removed message queue.


HISTORY

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

NetBSD 1.4                      August 17, 1995                              2