#include <fcntl.h>           /* For O_* constants */
       #include <sys/stat.h>        /* For mode constants */
       #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.  (Definitions of the flags values can be  obtained  by  including
       <fcntl.h>.)  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:

       O_CLOEXEC (since Linux 2.6.26)
              Set  the  close-on-exec  flag  for the message queue descriptor.
              See open(2) for a discussion of why this flag is useful.

              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.

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

       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).  (Symbolic definitions for  the  permis-
       sions bits can be obtained by including <sys/stat.h>.)  The permissions
       settings are masked against the process umask.

       The  attr  argument  specifies   attributes   for   the   queue.    See
              open it in the specified mode.

       EACCES name contained more than one slash.

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

       EINVAL name doesn't follow the format in mq_overview(7).

       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 unprivi-
              leged   (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 msg-
              size_max limit.  In addition,  even  in  a  privileged  process,
              attr->mq_maxmsg cannot exceed the HARD_MAX limit.  (See mq_over-
              view(7) for details of these limits.)

       EMFILE The per-process limit on the number of  open  file  and  message
              queue  descriptors  has  been  reached  (see  the description of
              RLIMIT_NOFILE in getrlimit(2)).

              name was too long.

       ENFILE The system-wide limit on the total number of open files and mes-
              sage queues has been reached.

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

       ENOENT name was just "/" followed by no other characters.

       ENOMEM Insufficient memory.

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

       For  an  explanation  of  the  terms  used   in   this   section,   see

       |Interface | Attribute     | Value   |
       |mq_open() | Thread safety | MT-Safe |
       POSIX.1-2001, POSIX.1-2008.

   C library/kernel differences
       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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Linux                             2015-12-28                        MQ_OPEN(3)
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