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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  speci-
       fies  attributes for the queue.	See mq_getattr(3) for details.	If attr is NULL, then the
       queue is created with implementation-defined default attributes.

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).

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 3.53 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at
       http://www.kernel.org/doc/man-pages/.

Linux					    2009-02-20				       MQ_OPEN(3)
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