mq_open(3) - Linux manual page (original) (raw)

mqopen(3) Library Functions Manual mqopen(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)](../man7/mq%5Foverview.7.html).

   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 _OR_ed in
   _oflag_:

   **O_CLOEXEC** (since Linux 2.6.26)
          Set the close-on-exec flag for the message queue
          descriptor.  See [open(2)](../man2/open.2.html) for a discussion of why this flag
          is useful.

   **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**.

   **O_NONBLOCK**
          Open the queue in nonblocking mode.  In circumstances where
          [mq_receive(3)](../man3/mq%5Freceive.3.html) and [mq_send(3)](../man3/mq%5Fsend.3.html) 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)](../man2/open.2.html).  (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 mqattr_ 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 _mqmaxmsg_ and _mqmsgsize_ 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)](../man7/mq%5Foverview.7.html) for details.

RETURN VALUE top

   On success, **mq_open**() returns a message queue descriptor for use
   by other message queue functions.  On error, **mq_open**() returns
   _(mqdt) -1_, with _[errno](../man3/errno.3.html)_ 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)](../man7/mq%5Foverview.7.html).

   **EINVAL O_CREAT** was specified in _oflag_, and _attr_ was not NULL, but
          _attr->mqmaxmsg_ or _attr->mqmsqsize_ 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->mqmaxmsg_ must be less than or equal to
          the _msgmax_ limit, and _attr->mqmsgsize_ must be less than
          or equal to the _msgsizemax_ limit.  In addition, even in a
          privileged process, _attr->mqmaxmsg_ cannot exceed the
          **HARD_MAX** limit.  (See [mq_overview(7)](../man7/mq%5Foverview.7.html) 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)](../man2/getrlimit.2.html)).

   **ENAMETOOLONG**
          _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 _queuesmax_ limit was
          encountered; see [mq_overview(7)](../man7/mq%5Foverview.7.html).

ATTRIBUTES top

   For an explanation of the terms used in this section, see
   [attributes(7)](../man7/attributes.7.html).
   ┌──────────────────────────────────────┬───────────────┬─────────┐
   │ **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

   POSIX.1-2008.

HISTORY top

   POSIX.1-2001.

BUGS top

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

COLOPHON top

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Linux man-pages 6.10 2024-07-23 mqopen(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)