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 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 func-
              tions 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  permis-
       sions bits can be obtained by including <sys/stat.h>.)  The permissions
       settings are masked against the process umask.

       The fields of the struct mq_attr pointed to attr  specify  the  maximum
       number of messages and the maximum size of messages that the queue will
       allow.  This structure is defined as follows:

           struct mq_attr {
               long mq_flags;       /* Flags (ignored for mq_open()) */
               long mq_maxmsg;      /* Max. # of messages on queue */
               long mq_msgsize;     /* Max. message size (bytes) */
               long mq_curmsgs;     /* # of messages currently in queue
                                       (ignored for mq_open()) */

       Only the mq_maxmsg and mq_msgsize  fields  are  employed  when  calling
       mq_open(); the values in the remaining fields are ignored.

       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 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  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 mes-
              sage 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).

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

       |Interface | Attribute     | Value   |
       |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 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 5.05 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at

Linux                             2017-09-15                        MQ_OPEN(3)
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