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  EEX-
              IST.

       If  O_CREAT  is specified in oflag, then two additional arguments must be supplied.  The mode argument specifies the per-
       missions 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  implementa-
       tion-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 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 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 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 3.25 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)