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       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 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  functions  instead  fail with the error

              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.


       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.

       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

       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

       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 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),


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