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       mq_open - open a message queue


       #include <mqueue.h>

       mqd_t mq_open(const char *name, int oflag);
       mqd_t mq_open(const char *name, int oflag, mode_t mode,
                     struct mq_attr *attr);

       Link with -lrt.


       mq_open() creates a new POSIX message queue or opens an existing queue.
       The queue is identified by name.  For details of  the  construction  of
       name, see mq_overview(7).

       The  oflag  argument  specifies flags that control the operation of the
       call.  Exactly one of the following must be specified in oflag:

              Open the queue to receive messages only.

              Open the queue to send messages only.

       O_RDWR Open the queue to both send and receive messages.

       Zero or more of the following flags can additionally be ORed in oflag:

              Open the queue in non-blocking  mode.   In  circumstances  where
              mq_receive(3)   and   mq_send(3)  would  normally  block,  these
              functions instead fail with the error EAGAIN.

              Create the message queue if it does not exist.  The owner  (user
              ID)  of the message queue is set to the effective user ID of the
              calling process.  The group ownership (group ID) is set  to  the
              effective group ID of the calling process.

       O_EXCL If  O_CREAT  was  specified in oflag, and a queue with the given
              name already exists, then fail with the error EEXIST.

       If O_CREAT is specified in oflag, then two additional arguments must be
       supplied.   The mode argument specifies the permissions to be placed on
       the new queue, as for open(2).  The  permissions  settings  are  masked
       against  the process umask.  The attr argument specifies attributes for
       the queue.  See mq_getattr(3) for details.  If attr is NULL,  then  the
       queue is created with implementation-defined default attributes.


       On  success,  mq_open()  returns  a message queue descriptor for use by
       other message queue functions.  On error, mq_open() returns (mqd_t) -1,
       with errno set to indicate the error.


       EACCES The  queue  exists,  but  the caller does not have permission to
              open it in the specified mode.

       EEXIST Both O_CREAT and O_EXCL were specified in  oflag,  but  a  queue
              with this name already exists.

       EINVAL O_CREAT  was  specified  in  oflag,  and  attr was not NULL, but
              attr->mq_maxmsg or attr->mq_msqsize was invalid.  Both of  these
              fields  must  be  greater  than  zero.   In  a  process  that is
              unprivileged (does not have  the  CAP_SYS_RESOURCE  capability),
              attr->mq_maxmsg must be less than or equal to the msg_max limit,
              and  attr->mq_msgsize  must  be  less  than  or  equal  to   the
              msgsize_max  limit.   In addition, even in a privileged process,
              attr->mq_maxmsg  cannot  exceed  the   HARD_MAX   limit.    (See
              mq_overview(7) for details of these limits.)

       EMFILE The  process already has the maximum number of files and message
              queues open.

              name was too long.

       ENFILE The system limit on the total number of open files  and  message
              queues has been reached.

       ENOENT The  O_CREAT  flag was not specified in oflag, and no queue with
              this name exists.

       ENOMEM Insufficient memory.

       ENOSPC Insufficient space for the creation  of  a  new  message  queue.
              This   probably   occurred  because  the  queues_max  limit  was
              encountered; see mq_overview(7).




       In kernels before 2.6.14, the process umask  was  not  applied  to  the
       permissions specified in mode.


       mq_close(3),  mq_getattr(3),  mq_notify(3),  mq_receive(3), mq_send(3),
       mq_unlink(3), mq_overview(7)


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