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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:

       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
              functions 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
       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.  Since Linux  3.5,  two
       /proc  files  can be used to control these defaults; see mq_overview(7)
       for details.


       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 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
              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 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
              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
              encountered; see mq_overview(7).


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

       │InterfaceAttributeValue   │
       │mq_open() │ Thread safety │ MT-Safe │


       POSIX.1-2001, POSIX.1-2008.


   C library/kernel differences
       The  mq_open()  library function is implemented on top of a system call
       of the same name.  The library function performs  the  check  that  the
       name  starts  with a slash (/), giving the EINVAL error if it does not.
       The kernel system call expects name to contain no preceding  slash,  so
       the  C  library function passes name without the preceding slash (i.e.,
       name+1) to the system call.


       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), mq_overview(7)


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