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MQ_OPEN(3)                 Linux Programmer's Manual                MQ_OPEN(3)

       mq_open - open a message queue

       #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

       The  oflag argument specifies flags that control the operation of the call.  (Defi-
       nitions 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 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

       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 includ-
       ing <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.

       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  indi-
       cate the error.

       EACCES The  queue exists, but the caller does not have permission to 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 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 lim-

       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

       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 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),

       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-

Linux                             2009-02-20                        MQ_OPEN(3)

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