17. Message Passing Manager¶

17.1. 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

17.2. Background¶

17.2.1. 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.

17.2.2. 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.

17.2.3. 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.

The message queue descriptor mqd_t represents the message queue. It is passed as an argument to all of the message queue functions.

17.2.4. Building a Message Queue Attribute Set¶

The mq_attr structure is used to define the characteristics of the message queue.

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.

17.2.5. 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.

17.2.6. 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.

17.3. Operations¶

17.3.1. 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.

17.3.2. 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.

17.3.3. 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.

17.3.4. 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.

17.3.5. 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.

17.3.6. 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.

17.3.7. 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.

17.3.8. 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.

17.4. 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.

17.4.1. mq_open - Open a Message Queue¶

CALLING SEQUENCE:

#include <mqueue.h>
mqd_t mq_open(
    const char     *name,
    int             oflag,
    mode_t          mode,
    struct mq_attr *attr
);

STATUS CODES:

`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.

`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.

17.4.2. mq_close - Close a Message Queue¶

CALLING SEQUENCE:

#include <mqueue.h>
int mq_close(
    mqd_t mqdes
);

STATUS CODES:

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.

17.4.3. mq_unlink - Remove a Message Queue¶

CALLING SEQUENCE:

#include <mqueue.h>
int mq_unlink(
    const char *name
);

STATUS CODES:

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.

17.4.4. mq_send - Send a Message to a Message Queue¶

CALLING SEQUENCE:

#include<mqueue.h>
int mq_send(
    mqd_t        mqdes,
    const char  *msg_ptr,
    size_t       msg_len,
    unsigned int msg_prio
);

STATUS CODES:

`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 <limits.h> 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.

17.4.5. mq_receive - Receive a Message from a Message Queue¶

CALLING SEQUENCE:

#include <mqueue.h>
size_t mq_receive(
    mqd_t         mqdes,
    char         *msg_ptr,
    size_t        msg_len,
    unsigned int *msg_prio
);

STATUS CODES:

`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
`EAGAIN`	The operation would block but has been called from an ISR
`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

17.4.6. mq_notify - Notify Process that a Message is Available¶

CALLING SEQUENCE:

#include <mqueue.h>
int mq_notify(
    mqd_t                  mqdes,
    const struct sigevent *notification
);

STATUS CODES:

`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.

17.4.7. mq_setattr - Set Message Queue Attributes¶

CALLING SEQUENCE:

#include <mqueue.h>
int mq_setattr(
    mqd_t                 mqdes,
    const struct mq_attr *mqstat,
    struct mq_attr       *omqstat
);

STATUS CODES:

`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.

17.4.8. mq_getattr - Get Message Queue Attributes¶

CALLING SEQUENCE:

#include <mqueue.h>
int mq_getattr(
    mqd_t           mqdes,
    struct mq_attr *mqstat
);

STATUS CODES:

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: