source: rtems/doc/posix_users/message.t @ 3d7e9240

4.104.114.84.95
Last change on this file since 3d7e9240 was 3d7e9240, checked in by Joel Sherrill <joel.sherrill@…>, on 10/11/99 at 19:51:56

Now build.

  • Property mode set to 100644
File size: 26.0 KB
Line 
1@c
2@c  COPYRIGHT(c) 1988-1998.
3@c  On-Line Applications Research Corporation(OAR).
4@c  All rights reserved.
5@c
6@c  $Id$
7@c
8
9@chapter Message Passing Manager
10
11@section Introduction
12
13The message passing manager is the means to provide communication and synchronization capabilities using POSIX message queues.
14
15The directives provided by the message passing manager are:
16
17@itemize @bullet
18@item @code{mq_open} - to begin using a message queue
19@item @code{mq_close} - to close the queue after a message has been sent.
20@item @code{mq_unlink} - to remove a message queue from the system entirely
21@item @code{mq_send} - to send messages to a queue you have accessed
22@item @code{mq_receive} - to receive messages from a queue you have accessed
23@item @code{mq_notify} - to notify that a message is on the queue
24@item @code{mq_setattr} - to change the blocking/non-blocking behavior of the message queue
25@item @code{mq_getattr} - to retrieve the message queue attributes for the named queue
26@end itemize
27
28@section Background
29
30@subsection Theory
31
32Message 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.
33
34@subsection Messages
35
36A 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.
37
38@subsection Message Queues
39
40Message 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.
41The message queue descriptor mqd_t mq represents the message queue.  It is passed as an argument to all of the message queue functions.
42
43@subsection Building a Message Queue Attribute Set
44
45 The mq_attr structure is used to define the characteristics of the message queue.
46
47@example
48@group
49/*
50 *  Create Message Queue (mq) Structure
51 */
52
53typedef struct mq_attr@{
54        long mq_flags;                  /* Message queue flags */
55        long mq_maxmsg;                 /* Maximum number of messages for the queue */
56        long mq_msgsize;                /* Maximum message size */
57        long mq_curmsgs;                /* Number of messages currently queued */
58@};
59@end group
60@end example
61
62All 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.
63The 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.
64Upon 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.
65
66@subsection Notification of a Message on the Queue
67
68Every 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.
69If 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.
70Only 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.
71
72@section Operations
73
74@subsection Opening or Creating a Message Queue
75
76If 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.
77
78@subsection Closing a Message Queue
79
80The 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.
81
82@subsection Removing a Message Queue
83
84The 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.
85
86@subsection Sending a Message to a Message Queue
87
88The 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.
89The 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.
90
91@subsection Receiving a Message from a Message Queue
92
93The 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.
94The 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.
95Upon successful completion, mq_receive returns the length of the selected message in bytes and the message is removed from the queue.
96
97@subsection Notification of Receipt of a Message on an Empty Queue
98
99The 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.
100If 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.
101
102@subsection Setting the Attributes of a Message Queue
103
104The 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.
105There 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.
106
107@subsection Getting the Attributes of a Message Queue
108
109The 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.
110
111@section Directives
112
113This 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.
114
115@subsection mq_open -to create or open a message queue
116
117@subheading CALLING SEQUENCE:
118
119@example
120#include <mqueue.h>
121mqd_t mq_open(const char *name, int oflag, mode_t mode, struct mq_attr *attr);   
122@end example
123
124@subheading STATUS CODES:
125
126@code{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.
127
128@code{EEXIST} - You tried to create a message queue that already exists.
129
130@code{EINVAL} - An inappropriate name was given for the message queue, or the values of mq-maxmsg or mq_msgsize were less than 0.
131
132@code{ENOENT} - The message queue does not exist, and you did not specify to create it.
133
134@code{EINTR} - The call to mq_open was interrupted by a signal.
135
136@code{EMFILE} - The process has too many files or message queues open.  This is a process limit error.
137
138@code{ENFILE} - The system has run out of resources to support more open message queues.  This is a system error.
139
140@code{ENAMETOOLONG} - mq_name is too long.
141
142@subheading DESCRIPTION:
143
144The 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.
145
146The 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.
147
148The 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.
149
150In addition, the oflags contain information needed in the creation of a message queue.
151@code{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.
152@code{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.
153
154The 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.
155@code{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.
156
157@subheading NOTES:
158
159The 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.
160
161@subsection mq_close - to close the queue after a message has been sent
162
163@subheading CALLING SEQUENCE:
164
165@example
166#include <mqueue.h>
167int mq_close(mqd_t mqdes);
168@end example
169
170@subheading STATUS CODES:
171
172@code{EINVAL} - The descriptor does not represent a valid open message queue
173
174@subheading DESCRIPTION:
175
176The 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.
177
178@subheading NOTES:
179
180If 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.
181
182@subsection mq_unlink - to remove a message queue from the system entirely
183
184@subheading CALLING SEQUENCE:
185
186@example
187#include <mqueue.h>
188int mq_unlink(const char *name);
189@end example
190
191@subheading STATUS CODES:
192
193@code{EINVAL} - The descriptor does not represent a valid message queue
194
195@subheading DESCRIPTION:
196
197The 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.
198
199@subheading NOTES:
200
201Calls 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.
202
203subsection mq_send - to send messages to a queue you have accessed
204
205@subheading CALLING SEQUENCE:
206
207@example
208#include<mqueue.h>
209int mq_send(mqd_t mqdes, const char *msg_ptr, size_t msg_len, unsigned int msg_prio);
210@end example
211
212@subheading STATUS CODES:
213
214@code{EBADF} - The descriptor does not represent a valid message queue, or the queue was opened for read only O_RDONLY
215@code{EINVAL} - The value of msg_prio was greater than the MQ_PRIO_MAX.
216@code{EMSGSIZE} - The msg_len is greater than the mq_msgsize attribute of the message queue
217@code{EAGAIN} - The message queue is non-blocking, and there is no room on the queue for another message as specified by the mq_maxmsg.
218@code{EINTR} - The message queue is blocking.  While the process was waiting for free space on the queue, a signal arrived that interrupted the wait.
219
220@subheading DESCRIPTION:
221
222The 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.
223The 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.
224Upon 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.
225
226@subheading NOTES:
227
228If the specified message queue is not full, mq_send inserts the message at the position indicated by the msg_prio argument.
229
230@subsection mq_receive - to receive messages from a queue you have accessed
231
232@subheading CALLING SEQUENCE:
233
234@example
235#include <mqueue.h>
236size_t mq_receive(mqd_t mqdes, char *msg_ptr, size_t msg_len, unsigned int *msg_prio);
237@end example
238
239@subheading STATUS CODES:
240
241@code{EBADF} - The descriptor does not represent a valid message queue, or the queue was opened for write only O_WRONLY
242@code{EMSGSIZE} - The msg_len is less than the mq_msgsize attribute of the message queue
243@code{EAGAIN} - The message queue is non-blocking, and the queue is empty
244@code{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.
245
246@subheading DESCRIPTION:
247
248The 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.
249The 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.
250Upon 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.
251
252@subheading NOTES:
253
254If 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
255
256@subsection mq_notify - to notify that a message is on the queue
257
258@subheading CALLING SEQUENCE:
259
260@example
261#include <mqueue.h>
262int mq_notify (mqd_t mqdes, const struct sigevent * notification);
263@end example
264
265@subheading STATUS CODES:
266
267@code{EBADF} - The descriptor does not refer to a valid message queue
268@code{EBUSY} - A notification request is already attached to the queue
269
270@subheading DESCRIPTION:
271
272If 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.
273Every 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.
274If 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.
275Upon successful completion, mq_notify returns a value of zero; otherwise, the function returns a value of -1 and sets errno to indicate the error.
276
277@subheading NOTES:
278
279It is possible for another process to receive the message after the notification is sent but before the notified process has sent its receive request.
280
281@subsection mq_setattr - to change the blocking and non-blocking behavior of the message queue
282
283@subheading CALLING SEQUENCE:
284
285@example
286#include <mqueue.h>
287int mq_setattr(mqd_t mqdes, const struct mq_attr *mqstat, struct mq_attr *omqstat);
288@end example
289
290@subheading STATUS CODES:
291
292@code{EBADF} - The message queue descriptor does not refer to a valid, open queue.
293@code{EINVAL} - The mq_flag value is invalid.
294
295@subheading DESCRIPTION:
296
297The 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.
298There 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.
299Upon 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.
300
301@subheading NOTES:
302
303All other fields in the mq_attr are ignored by this call.
304
305@subsection mq_getattr - retrieves the message queue attributes
306
307@subheading CALLING SEQUENCE:
308
309@example
310#include <mqueue.h>
311int mq_getattr(mqd_t mqdes, struct mq_attr *mqstat);
312@end example
313
314@subheading STATUS CODES:
315
316@code{EBADF} - The message queue descriptor does not refer to a valid, open message queue.
317
318@subheading DESCRIPTION:
319
320The 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.
321Upon successful completion, the mq_getattr function returns zero.  Otherwise, the function returns -1 and sets errno to indicate the error.
322
323@subheading NOTES:
324
Note: See TracBrowser for help on using the repository browser.