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NAME

       mq_open - open a message queue

SYNOPSIS

       #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.

DESCRIPTION

       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:

       O_RDONLY
              Open the queue to receive messages only.

       O_WRONLY
              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_NONBLOCK
              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.

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

RETURN VALUE

       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.

ERRORS

       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.

       ENAMETOOLONG
              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).

CONFORMING TO

       POSIX.1-2001.

BUGS

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

SEE ALSO

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

COLOPHON

       This  page  is  part of release 3.15 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.