mq_open(3) — Linux manual page


mq_open(3)              Library Functions Manual              mq_open(3)

NAME         top

       mq_open - open a message queue

LIBRARY         top

       Real-time library (librt, -lrt)

SYNOPSIS         top

       #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);

DESCRIPTION         top

       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

       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

       O_EXCL If O_CREAT was specified in oflag, and a queue with the
              given name already exists, then fail with the error

              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.

       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 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

       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

RETURN VALUE         top

       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         top

       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 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

       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 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         top

       For an explanation of the terms used in this section, see
       │ Interface                           Attribute     Value   │
       │ mq_open()                           │ Thread safety │ MT-Safe │

VERSIONS         top

   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.

STANDARDS         top


HISTORY         top


BUGS         top

       Before Linux 2.6.14, the process umask was not applied to the
       permissions specified in mode.

SEE ALSO         top

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

Linux man-pages (unreleased)     (date)                       mq_open(3)

Pages that refer to this page: getrlimit(2)syscalls(2)umask(2)mq_close(3)mq_getattr(3)mq_notify(3)mq_receive(3)mq_send(3)mq_unlink(3)mq_overview(7)