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



NAME
       mq_open - open a message queue

SYNOPSIS
       #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.

DESCRIPTION
       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:

       O_RDONLY
              Open the queue to receive messages only.

       O_WRONLY
              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_NONBLOCK
              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.

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

RETURN VALUE
       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.

ERRORS
       EACCES The queue exists, but the caller does not have permission to open it in the  speci‐
              fied 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_msg‐
              size 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.

       ENAMETOOLONG
              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 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).

ATTRIBUTES
   Multithreading (see pthreads(7))
       The mq_open() function is thread-safe.

CONFORMING TO
       POSIX.1-2001.

BUGS
       In kernels before 2.6.14, the process umask was not applied to the  permissions  specified
       in mode.

SEE ALSO
       mq_close(3),   mq_getattr(3),   mq_notify(3),   mq_receive(3),  mq_send(3),  mq_unlink(3),
       mq_overview(7)

COLOPHON
       This page is part of release 3.74 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 http://www.kernel.org/doc/man-pages/.



Linux                                       2014-10-02                                 MQ_OPEN(3)