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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

       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  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

           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

       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

       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 attributes(7).

       │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),


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