From 72a62ad88f82fe1ffee50024db4dd0f3fa5806f7 Mon Sep 17 00:00:00 2001 From: Chris Johns Date: Thu, 3 Nov 2016 16:58:08 +1100 Subject: Rename all manuals with an _ to have a -. It helps released naming of files. --- posix-users/message_passing.rst | 741 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 741 insertions(+) create mode 100644 posix-users/message_passing.rst (limited to 'posix-users/message_passing.rst') diff --git a/posix-users/message_passing.rst b/posix-users/message_passing.rst new file mode 100644 index 0000000..64c64a3 --- /dev/null +++ b/posix-users/message_passing.rst @@ -0,0 +1,741 @@ +.. comment SPDX-License-Identifier: CC-BY-SA-4.0 + +.. COMMENT: COPYRIGHT (c) 1988-2014. +.. COMMENT: On-Line Applications Research Corporation (OAR). +.. COMMENT: All rights reserved. + +Message Passing Manager +####################### + +Introduction +============ + +The message passing manager is the means to provide communication and +synchronization capabilities using POSIX message queues. + +The directives provided by the message passing manager are: + +- mq_open_ - Open a Message Queue + +- mq_close_ - Close a Message Queue + +- mq_unlink_ - Remove a Message Queue + +- mq_send_ - Send a Message to a Message Queue + +- mq_receive_ - Receive a Message from a Message Queue + +- mq_notify_ - Notify Process that a Message is Available + +- mq_setattr_ - Set Message Queue Attributes + +- mq_getattr_ - Get Message Queue Attributes + +Background +========== + +Theory +------ + +Message queues are named objects that operate with readers and writers. In +addition, a message queue is a priority queue of discrete messages. POSIX +message queues offer a certain, basic amount of application access to, and +control over, the message queue geometry that can be changed. + +Messages +-------- + +A message is a variable length buffer where information can be stored to +support communication. The length of the message and the information stored in +that message are user-defined and can be actual data, pointer(s), or +empty. There is a maximum acceptable length for a message that is associated +with each message queue. + +Message Queues +-------------- + +Message queues are named objects similar to the pipes of POSIX. They are a +means of communicating data between multiple processes and for passing messages +among tasks and ISRs. Message queues can contain a variable number of messages +from 0 to an upper limit that is user defined. The maximum length of the +message can be set on a per message queue basis. Normally messages are sent +and received from the message queue in FIFO order. However, messages can also +be prioritized and a priority queue established for the passing of +messages. Synchronization is needed when a task waits for a message to arrive +at a queue. Also, a task may poll a queue for the arrival of a message. + +.. index:: mqd_t + +The message queue descriptor ``mqd_t`` represents the message queue. It is +passed as an argument to all of the message queue functions. + +Building a Message Queue Attribute Set +-------------------------------------- + +The ``mq_attr`` structure is used to define the characteristics of the message +queue. + +.. index:: mq_attr + +.. code-block:: c + + typedef struct mq_attr{ + long mq_flags; + long mq_maxmsg; + long mq_msgsize; + long mq_curmsgs; + }; + +All of these attributes are set when the message queue is created using +mq_open. The mq_flags field is not used in the creation of a message queue, it +is only used by ``mq_setattr`` and ``mq_getattr``. The structure ``mq_attr`` is +passed as an argument to ``mq_setattr`` and ``mq_getattr``. + +The mq_flags contain information affecting the behavior of the message +queue. The ``O_NONBLOCK`` ``mq_flag`` is the only flag that is defined. In +``mq_setattr``, the ``mq_flag`` can be set to dynamically change the blocking +and non-blocking behavior of the message queue. If the non-block flag is set +then the message queue is non-blocking, and requests to send and receive +messages do not block waiting for resources. For a blocking message queue, a +request to send might have to wait for an empty message queue, and a request to +receive might have to wait for a message to arrive on the queue. Both +``mq_maxmsg`` and ``mq_msgsize`` affect the sizing of the message +queue. ``mq_maxmsg`` specifies how many messages the queue can hold at any one +time. ``mq_msgsize`` specifies the size of any one message on the queue. If +either of these limits is exceeded, an error message results. + +Upon return from ``mq_getattr``, the ``mq_curmsgs`` is set according to the +current state of the message queue. This specifies the number of messages +currently on the queue. + +Notification of a Message on the Queue +-------------------------------------- + +Every message queue has the ability to notify one (and only one) process +whenever the queue's state changes from empty (0 messages) to nonempty. This +means that the process does not have to block or constantly poll while it waits +for a message. By calling ``mq_notify``, you can attach a notification request +to a message queue. When a message is received by an empty queue, if there are +no processes blocked and waiting for the message, then the queue notifies the +requesting process of a message arrival. There is only one signal sent by the +message queue, after that the notification request is de-registered and another +process can attach its notification request. After receipt of a notification, a +process must re-register if it wishes to be notified again. + +If there is a process blocked and waiting for the message, that process gets +the message, and notification is not sent. It is also possible for another +process to receive the message after the notification is sent but before the +notified process has sent its receive request. + +Only one process can have a notification request attached to a message queue at +any one time. If another process attempts to register a notification request, +it fails. You can de-register for a message queue by passing a NULL to +mq_notify, this removes any notification request attached to the +queue. Whenever the message queue is closed, all notification attachments are +removed. + +POSIX Interpretation Issues +--------------------------- + +There is one significant point of interpretation related to the RTEMS +implementation of POSIX message queues: + + | What happens to threads already blocked on a message queue when the mode + | of that same message queue is changed from blocking to non-blocking? + +The RTEMS POSIX implementation decided to unblock all waiting tasks with an +``EAGAIN`` status just as if a non-blocking version of the same operation had +returned unsatisfied. This case is not discussed in the POSIX standard and +other implementations may have chosen alternative behaviors. + +Operations +========== + +Opening or Creating a Message Queue +----------------------------------- + +If the message queue already exists, ``mq_open()`` opens it, if the message +queue does not exist, ``mq_open()`` creates it. When a message queue is +created, the geometry of the message queue is contained in the attribute +structure that is passed in as an argument. This includes mq_msgsize that +dictates the maximum size of a single message, and the mq_maxmsg that dictates +the maximum number of messages the queue can hold at one time. The blocking or +non-blocking behavior of the queue can also specified. + +Closing a Message Queue +----------------------- + +The ``mq_close()`` function is used to close the connection made to a message +queue that was made during mq_open. The message queue itself and the messages +on the queue are persistent and remain after the queue is closed. + +Removing a Message Queue +------------------------ + +The ``mq_unlink()`` function removes the named message queue. If the message +queue is not open when mq_unlink is called, then the queue is immediately +eliminated. Any messages that were on the queue are lost, and the queue can not +be opened again. If processes have the queue open when mq_unlink is called, the +removal of the queue is delayed until the last process using the queue has +finished. However, the name of the message queue is removed so that no other +process can open it. + +Sending a Message to a Message Queue +------------------------------------ + +The ``mq_send()`` function adds the message in priority order to the message +queue. Each message has an assigned a priority. The highest priority message is +be at the front of the queue. + +The maximum number of messages that a message queue may accept is specified at +creation by the ``mq_maxmsg`` field of the attribute structure. If this amount +is exceeded, the behavior of the process is determined according to what +``oflag`` was used when the message queue was opened. If the queue was opened +with ``O_NONBLOCK`` flag set, the process does not block, and an error is +returned. If the ``O_NONBLOCK`` flag was not set, the process does block and +wait for space on the queue. + +Receiving a Message from a Message Queue +---------------------------------------- + +The ``mq_receive()`` function is used to receive the oldest of the highest +priority message(s) from the message queue specified by mqdes. The messages are +received in FIFO order within the priorities. The received message's priority +is stored in the location referenced by the ``msg_prio``. If the ``msg_prio`` +is a ``NULL``, the priority is discarded. The message is removed and stored in +an area pointed to by ``msg_ptr`` whose length is of ``msg_len``. The +``msg_len`` must be at least equal to the ``mq_msgsize`` attribute of the +message queue. + +The blocking behavior of the message queue is set by ``O_NONBLOCK`` at +``mq_open`` or by setting ``O_NONBLOCK`` in ``mq_flags`` in a call to +``mq_setattr``. If this is a blocking queue, the process does block and wait on +an empty queue. If this a non-blocking queue, the process does not block. Upon +successful completion, ``mq_receive`` returns the length of the selected +message in bytes and the message is removed from the queue. + +Notification of Receipt of a Message on an Empty Queue +------------------------------------------------------ + +The ``mq_notify()`` function registers the calling process to be notified of +message arrival at an empty message queue. Every message queue has the ability +to notify one (and only one) process whenever the queue's state changes from +empty (0 messages) to nonempty. This means that the process does not have to +block or constantly poll while it waits for a message. By calling +``mq_notify``, a notification request is attached to a message queue. When a +message is received by an empty queue, if there are no processes blocked and +waiting for the message, then the queue notifies the requesting process of a +message arrival. There is only one signal sent by the message queue, after that +the notification request is de-registered and another process can attach its +notification request. After receipt of a notification, a process must +re-register if it wishes to be notified again. + +If there is a process blocked and waiting for the message, that process gets +the message, and notification is not sent. Only one process can have a +notification request attached to a message queue at any one time. If another +process attempts to register a notification request, it fails. You can +de-register for a message queue by passing a ``NULL`` to ``mq_notify``, this +removes any notification request attached to the queue. Whenever the message +queue is closed, all notification attachments are removed. + +Setting the Attributes of a Message Queue +----------------------------------------- + +The ``mq_setattr()`` function is used to set attributes associated with the +open message queue description referenced by the message queue descriptor +specified by mqdes. The ``*omqstat`` represents the old or previous +attributes. If ``omqstat`` is non-``NULL``, the function ``mq_setattr()`` +stores, in the location referenced by omqstat, the previous message queue +attributes and the current queue status. These values are the same as would be +returned by a call to ``mq_getattr()`` at that point. + +There is only one ``mq_attr.mq_flag`` that can be altered by this call. This is +the flag that deals with the blocking and non-blocking behavior of the message +queue. If the flag is set then the message queue is non-blocking, and requests +to send or receive do not block while waiting for resources. If the flag is +not set, then message send and receive may involve waiting for an empty queue +or waiting for a message to arrive. + +Getting the Attributes of a Message Queue +----------------------------------------- + +The ``mq_getattr()`` function is used to get status information and attributes +of the message queue associated with the message queue descriptor. The results +are returned in the mq_attr structure referenced by the mqstat argument. All of +these attributes are set at create time, except the blocking/non-blocking +behavior of the message queue which can be dynamically set by using +mq_setattr. The attribute mq_curmsg is set to reflect the number of messages on +the queue at the time that ``mq_getattr`` was called. + +Directives +========== + +This section details the message passing manager's directives. A subsection is +dedicated to each of this manager's directives and describes the calling +sequence, related constants, usage, and status codes. + +.. _mq_open: + +mq_open - Open a Message Queue +------------------------------ +.. index:: mq_open +.. index:: open a message queue + +**CALLING SEQUENCE:** + +.. code-block:: c + + #include + mqd_t mq_open( + const char *name, + int oflag, + mode_t mode, + struct mq_attr *attr + ); + +**STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``EACCES`` + - Either the message queue exists and the permissions requested in + ``oflags`` were denied, or the message does not exist and permission to + create one is denied. + * - ``EEXIST`` + - You tried to create a message queue that already exists. + * - ``EINVAL`` + - An inappropriate name was given for the message queue, or the values of + ``mq-maxmsg`` or ``mq_msgsize`` were less than 0. + * - ``ENOENT`` + - The message queue does not exist, and you did not specify to create it. + * - ``EINTR`` + - The call to mq_open was interrupted by a signal. + * - ``EMFILE`` + - The process has too many files or message queues open. This is a process + limit error. + * - ``ENFILE`` + - The system has run out of resources to support more open message + queues. This is a system error. + * - ``ENAMETOOLONG`` + - ``mq_name`` is too long. + +**DESCRIPTION:** + +The ``mq_open()`` function establishes the connection between a process and a +message queue with a message queue descriptor. If the message queue already +exists, ``mq_open`` opens it, if the message queue does not exist, ``mq_open`` +creates it. Message queues can have multiple senders and receivers. If +``mq_open`` is successful, the function returns a message queue +descriptor. Otherwise, the function returns a -1 and sets ``errno`` to indicate +the error. + +The name of the message queue is used as an argument. For the best of +portability, the name of the message queue should begin with a "/" and no other +"/" should be in the name. Different systems interpret the name in different +ways. + +The ``oflags`` contain information on how the message is opened if the queue +already exists. This may be ``O_RDONLY`` for read only, ``O_WRONLY`` for write +only, of O_RDWR, for read and write. + +In addition, the ``oflags`` contain information needed in the creation of a message +queue. + +.. list-table:: + :class: rtems-table + + * - ``O_NONBLOCK`` + - If the non-block flag is set then the message queue is non-blocking, and + requests to send and receive messages do not block waiting for + resources. If the flag is not set then the message queue is blocking, and + a request to send might have to wait for an empty message + queue. Similarly, a request to receive might have to wait for a message to + arrive on the queue. + * - ``O_CREAT`` + - This call specifies that the call the mq_open is to create a new message + queue. In this case the mode and attribute arguments of the function call + are utilized. The message queue is created with a mode similar to the + creation of a file, read and write permission creator, group, and others. + The geometry of the message queue is contained in the attribute structure. + This includes mq_msgsize that dictates the maximum size of a single + message, and the mq_maxmsg that dictates the maximum number of messages + the queue can hold at one time. If a ``NULL`` is used in the mq_attr + argument, then the message queue is created with implementation defined + defaults. + * - ``O_EXCL`` + - is always set if ``O_CREAT`` flag is set. If the message queue already + exists, ``O_EXCL`` causes an error message to be returned, otherwise, the + new message queue fails and appends to the existing one. + +**NOTES:** + +The ``mq_open()`` function does not add or remove messages from the queue. +When a new message queue is being created, the ``mq_flag`` field of the +attribute structure is not used. + +.. _mq_close: + +mq_close - Close a Message Queue +-------------------------------- +.. index:: mq_close +.. index:: close a message queue + +**CALLING SEQUENCE:** + +.. code-block:: c + + #include + int mq_close( + mqd_t mqdes + ); + +**STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``EINVAL`` + - The descriptor does not represent a valid open message queue + +**DESCRIPTION:** + +The ``mq_close`` function removes the association between the message queue +descriptor, mqdes, and its message queue. If ``mq_close()`` is successfully +completed, the function returns a value of zero; otherwise, the function +returns a value of -1 and sets ``errno`` to indicate the error. + +**NOTES:** + +If the process had successfully attached a notification request to the message +queue via ``mq_notify``, this attachment is removed, and the message queue is +available for another process to attach for notification. ``mq_close`` has no +effect on the contents of the message queue, all the messages that were in the +queue remain in the queue. + +.. _mq_unlink: + +mq_unlink - Remove a Message Queue +---------------------------------- +.. index:: mq_unlink +.. index:: remove a message queue + +**CALLING SEQUENCE:** + +.. code-block:: c + + #include + int mq_unlink( + const char *name + ); + +**STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``EINVAL`` + - The descriptor does not represent a valid message queue + +**DESCRIPTION:** + +The ``mq_unlink()`` function removes the named message queue. If the message +queue is not open when ``mq_unlink`` is called, then the queue is immediately +eliminated. Any messages that were on the queue are lost, and the queue can not +be opened again. If processes have the queue open when ``mq_unlink`` is called, +the removal of the queue is delayed until the last process using the queue has +finished. However, the name of the message queue is removed so that no other +process can open it. Upon successful completion, the function returns a value +of zero. Otherwise, the named message queue is not changed by this function +call, and the function returns a value of +-1 and sets ``errno`` to indicate the error. + +**NOTES:** + +Calls to ``mq_open()`` to re-create the message queue may fail until the +message queue is actually removed. However, the ``mq_unlink()`` call need not +block until all references have been closed; it may return immediately. + +.. _mq_send: + +mq_send - Send a Message to a Message Queue +------------------------------------------- +.. index:: mq_send +.. index:: send a message to a message queue + +**CALLING SEQUENCE:** + +.. code-block:: c + + #include + int mq_send( + mqd_t mqdes, + const char *msg_ptr, + size_t msg_len, + unsigned int msg_prio + ); + +**STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``EBADF`` + - The descriptor does not represent a valid message queue, or the queue was + opened for read only ``O_RDONLY`` + * - ``EINVAL`` + - The value of msg_prio was greater than the ``MQ_PRIO_MAX``. + * - ``EMSGSIZE`` + - The msg_len is greater than the ``mq_msgsize`` attribute of the message + queue + * - ``EAGAIN`` + - The message queue is non-blocking, and there is no room on the queue for + another message as specified by the ``mq_maxmsg``. + * - ``EINTR`` + - The message queue is blocking. While the process was waiting for free + space on the queue, a signal arrived that interrupted the wait. + +**DESCRIPTION:** + +The ``mq_send()`` function adds the message pointed to by the argument +``msg_ptr`` to the message queue specified by mqdes. Each message is assigned a +priority , from 0 to ``MQ_PRIO_MAX``. ``MQ_PRIO_MAX`` is defined in +```` and must be at least 32. Messages are added to the queue in +order of their priority. The highest priority message is at the front of the +queue. + +The maximum number of messages that a message queue may accept is specified at +creation by the ``mq_maxmsg`` field of the attribute structure. If this amount is +exceeded, the behavior of the process is determined according to what oflag was +used when the message queue was opened. If the queue was opened with ``O_NONBLOCK`` +flag set, then the ``EAGAIN`` error is returned. If the ``O_NONBLOCK`` flag was not +set, the process blocks and waits for space on the queue, unless it is +interrupted by a signal. + +Upon successful completion, the ``mq_send()`` function returns a value of +zero. Otherwise, no message is enqueued, the function returns -1, and ``errno`` +is set to indicate the error. + +**NOTES:** + +If the specified message queue is not full, ``mq_send`` inserts the message at +the position indicated by the ``msg_prio`` argument. + +.. _mq_receive: + +mq_receive - Receive a Message from a Message Queue +--------------------------------------------------- +.. index:: mq_receive +.. index:: receive a message from a message queue + +**CALLING SEQUENCE:** + +.. code-block:: c + + #include + size_t mq_receive( + mqd_t mqdes, + char *msg_ptr, + size_t msg_len, + unsigned int *msg_prio + ); + +**STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``EBADF`` + - The descriptor does not represent a valid message queue, or the queue was + opened for write only ``O_WRONLY`` + * - ``EMSGSIZE`` + - The msg_len is less than the ``mq_msgsize`` attribute of the message queue + * - ``EAGAIN`` + - The message queue is non-blocking, and the queue is empty + * - ``EINTR`` + - The message queue is blocking. While the process was waiting for a message + to arrive on the queue, a signal arrived that interrupted the wait. + +**DESCRIPTION:** + +The ``mq_receive`` function is used to receive the oldest of the highest +priority message(s) from the message queue specified by mqdes. The messages are +received in FIFO order within the priorities. The received message's priority +is stored in the location referenced by the ``msg_prio``. If the ``msg_prio`` +is a ``NULL``, the priority is discarded. The message is removed and stored in +an area pointed to by ``msg_ptr`` whose length is of ``msg_len``. The +``msg_len`` must be at least equal to the mq_msgsize attribute of the message +queue. + +The blocking behavior of the message queue is set by ``O_NONBLOCK`` at +``mq_open`` or by setting ``O_NONBLOCK`` in ``mq_flags`` in a call to +``mq_setattr``. If this is a blocking queue, the process blocks and waits on an +empty queue. If this a non-blocking queue, the process does not block. + +Upon successful completion, ``mq_receive`` returns the length of the selected +message in bytes and the message is removed from the queue. Otherwise, no +message is removed from the queue, the function returns a value of -1, and sets +``errno`` to indicate the error. + +**NOTES:** + +If the size of the buffer in bytes, specified by the ``msg_len`` argument, is +less than the ``mq_msgsize`` attribute of the message queue, the function fails +and returns an error + +.. _mq_notify: + +mq_notify - Notify Process that a Message is Available +------------------------------------------------------ +.. index:: mq_notify +.. index:: notify process that a message is available + +**CALLING SEQUENCE:** + +.. code-block:: c + + #include + int mq_notify( + mqd_t mqdes, + const struct sigevent *notification + ); + +**STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``EBADF`` + - The descriptor does not refer to a valid message queue + * - ``EBUSY`` + - A notification request is already attached to the queue + +**DESCRIPTION:** + +If the argument notification is not ``NULL``, this function registers the +calling process to be notified of message arrival at an empty message queue +associated with the specified message queue descriptor, ``mqdes``. + +Every message queue has the ability to notify one (and only one) process +whenever the queue's state changes from empty (0 messages) to nonempty. This +means that the process does not have to block or constantly poll while it waits +for a message. By calling ``mq_notify``, a notification request is attached to +a message queue. When a message is received by an empty queue, if there are no +processes blocked and waiting for the message, then the queue notifies the +requesting process of a message arrival. There is only one signal sent by the +message queue, after that the notification request is de-registered and another +process can attach its notification request. After receipt of a notification, a +process must re-register if it wishes to be notified again. + +If there is a process blocked and waiting for the message, that process +gets the message, and notification is not be sent. Only one process can +have a notification request attached to a message queue at any one time. +If another process attempts to register a notification request, it fails. +You can de-register for a message queue by passing a NULL to mq_notify; +this removes any notification request attached to the queue. Whenever the +message queue is closed, all notification attachments are removed. + +Upon successful completion, mq_notify returns a value of zero; otherwise, the +function returns a value of -1 and sets ``errno`` to indicate the error. + +**NOTES:** + +It is possible for another process to receive the message after the +notification is sent but before the notified process has sent its receive +request. + +.. _mq_setattr: + +mq_setattr - Set Message Queue Attributes +----------------------------------------- +.. index:: mq_setattr +.. index:: set message queue attributes + +**CALLING SEQUENCE:** + +.. code-block:: c + + #include + int mq_setattr( + mqd_t mqdes, + const struct mq_attr *mqstat, + struct mq_attr *omqstat + ); + +**STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``EBADF`` + - The message queue descriptor does not refer to a valid, open queue. + * - ``EINVAL`` + - The mq_flag value is invalid. + +**DESCRIPTION:** + +The ``mq_setattr`` function is used to set attributes associated with the open +message queue description referenced by the message queue descriptor specified +by mqdes. The ``*omqstat`` represents the old or previous attributes. If +``omqstat`` is non-``NULL``, the function ``mq_setattr()`` stores, in the +location referenced by ``omqstat``, the previous message queue attributes and +the current queue status. These values are the same as would be returned by a +call to ``mq_getattr()`` at that point. + +There is only one mq_attr.mq_flag which can be altered by this call. This is +the flag that deals with the blocking and non-blocking behavior of the message +queue. If the flag is set then the message queue is non-blocking, and requests +to send or receive do not block while waiting for resources. If the flag is not +set, then message send and receive may involve waiting for an empty queue or +waiting for a message to arrive. + +Upon successful completion, the function returns a value of zero and the +attributes of the message queue have been changed as specified. Otherwise, the +message queue attributes is unchanged, and the function returns a value of -1 +and sets ``errno`` to indicate the error. + +**NOTES:** + +All other fields in the ``mq_attr`` are ignored by this call. + +.. _mq_getattr: + +mq_getattr - Get Message Queue Attributes +----------------------------------------- +.. index:: mq_getattr +.. index:: get message queue attributes + +**CALLING SEQUENCE:** + +.. code-block:: c + + #include + int mq_getattr( + mqd_t mqdes, + struct mq_attr *mqstat + ); + +**STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``EBADF`` + - The message queue descriptor does not refer to a valid, open message + queue. + +**DESCRIPTION:** + +The ``mqdes`` argument specifies a message queue descriptor. The ``mq_getattr`` +function is used to get status information and attributes of the message queue +associated with the message queue descriptor. The results are returned in the +``mq_attr`` structure referenced by the mqstat argument. All of these +attributes are set at create time, except the blocking/non-blocking behavior of +the message queue which can be dynamically set by using mq_setattr. The +attribute ``mq_curmsg`` is set to reflect the number of messages on the queue +at the time that ``mq_getattr`` was called. + +Upon successful completion, the ``mq_getattr`` function returns zero. +Otherwise, the function returns -1 and sets ``errno`` to indicate the error. + +**NOTES:** -- cgit v1.2.3