#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:

              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.

              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).  (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 spec-
       ifies attributes for the queue.  See  mq_getattr(3)  for  details.   If
       attr  is  NULL,  then  the queue is created with implementation-defined
       default attributes.

              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 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 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.

       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).


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

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

       This  page  is  part of release 3.54 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

Linux                             2009-02-20                        MQ_OPEN(3)
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