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() function sends a message to the message queue specified in msqid.  The msgp argument 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 msgsz bytes.  The argument msgsz can range from 0 to a system-imposed maximum, 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.

ERRORS

       The msgsnd() function will fail if:

       [EINVAL]           The msqid argument 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.

                          The msgsz argument is greater than msg_qbytes.

                          The mtype argument 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]           The msgp argument 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.

BUGS

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

Debian                                            July 9, 2009                                         MSGSND(2)