diff options
Diffstat (limited to '')
| -rw-r--r-- | doc/posix_users/message.t | 692 |
1 files changed, 0 insertions, 692 deletions
diff --git a/doc/posix_users/message.t b/doc/posix_users/message.t deleted file mode 100644 index 9e63f7f440..0000000000 --- a/doc/posix_users/message.t +++ /dev/null @@ -1,692 +0,0 @@ -@c -@c COPYRIGHT(c) 1988-2002. -@c On-Line Applications Research Corporation(OAR). -@c All rights reserved. - -@chapter Message Passing Manager - -@section 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: - -@itemize @bullet -@item @code{mq_open} - Open a Message Queue -@item @code{mq_close} - Close a Message Queue -@item @code{mq_unlink} - Remove a Message Queue -@item @code{mq_send} - Send a Message to a Message Queue -@item @code{mq_receive} - Receive a Message from a Message Queue -@item @code{mq_notify} - Notify Process that a Message is Available -@item @code{mq_setattr} - Set Message Queue Attributes -@item @code{mq_getattr} - Get Message Queue Attributes -@end itemize - -@section Background - -@subsection 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. - -@subsection 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. - -@subsection 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. - -@findex mqd_t -The message queue descriptor @code{mqd_t} represents the message queue. It is -passed as an argument to all of the message queue functions. - -@subsection Building a Message Queue Attribute Set - -The mq_attr structure is used to define the characteristics of the message -queue. - -@findex mq_attr -@example -@group -typedef struct mq_attr@{ - long mq_flags; - long mq_maxmsg; - long mq_msgsize; - long mq_curmsgs; -@}; -@end group -@end example - -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. - -@subsection 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. - -@subsection POSIX Interpretation Issues - -There is one significant point of interpretation related to -the RTEMS implementation of POSIX message queues: - -@cite{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 @code{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. - -@section Operations - -@subsection 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. - -@subsection 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. - -@subsection 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. - -@subsection 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. - -@subsection 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. - -@subsection 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. - -@subsection 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. - -@subsection 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. - -@section 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. - -@c -@c -@c -@page -@subsection mq_open - Open a Message Queue - -@findex mq_open -@cindex open a message queue - -@subheading CALLING SEQUENCE: - -@example -#include <mqueue.h> - -mqd_t mq_open( - const char *name, - int oflag, - mode_t mode, - struct mq_attr *attr -); -@end example - -@subheading STATUS CODES: - -@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. - -@code{EEXIST} - You tried to create a message queue that already exists. - -@code{EINVAL} - An inappropriate name was given for the message queue, or -the values of mq-maxmsg or mq_msgsize were less than 0. - -@code{ENOENT} - The message queue does not exist, and you did not specify -to create it. - -@code{EINTR} - The call to mq_open was interrupted by a signal. - -@code{EMFILE} - The process has too many files or message queues open. -This is a process limit error. - -@code{ENFILE} - The system has run out of resources to support more open -message queues. This is a system error. - -@code{ENAMETOOLONG} - mq_name is too long. - -@subheading 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. @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. @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. - -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. @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. - -@subheading 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. - -@c -@c -@c -@page -@subsection mq_close - Close a Message Queue - -@findex mq_close -@cindex close a message queue - -@subheading CALLING SEQUENCE: - -@example -#include <mqueue.h> - -int mq_close( - mqd_t mqdes -); -@end example - -@subheading STATUS CODES: - -@code{EINVAL} - The descriptor does not represent a valid open message -queue - -@subheading 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. - -@subheading 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. - -@c -@c -@c -@page -@subsection mq_unlink - Remove a Message Queue - -@findex mq_unlink -@cindex remove a message queue - -@subheading CALLING SEQUENCE: - -@example -#include <mqueue.h> - -int mq_unlink( - const char *name -); -@end example - -@subheading STATUS CODES: - -@code{EINVAL} - The descriptor does not represent a valid message queue - -@subheading 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. - -@subheading 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. - -@c -@c -@c -@page -@subsection mq_send - Send a Message to a Message Queue - -@findex mq_send -@cindex send a message to a message queue - -@subheading CALLING SEQUENCE: - -@example -#include<mqueue.h> -int mq_send( - mqd_t mqdes, - const char *msg_ptr, - size_t msg_len, - unsigned int msg_prio -); -@end example - -@subheading STATUS CODES: - -@code{EBADF} - The descriptor does not represent a valid message queue, or the queue was opened for read only O_RDONLY -@code{EINVAL} - The value of msg_prio was greater than the MQ_PRIO_MAX. -@code{EMSGSIZE} - The msg_len is greater than the mq_msgsize attribute of the message queue -@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. -@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. - -@subheading 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. - -@subheading NOTES: - -If the specified message queue is not full, mq_send inserts the message at -the position indicated by the msg_prio argument. - -@c -@c -@c -@page -@subsection mq_receive - Receive a Message from a Message Queue - -@findex mq_receive -@cindex receive a message from a message queue - -@subheading CALLING SEQUENCE: - -@example -#include <mqueue.h> - -size_t mq_receive( - mqd_t mqdes, - char *msg_ptr, - size_t msg_len, - unsigned int *msg_prio -); -@end example - -@subheading STATUS CODES: - -@code{EBADF} - The descriptor does not represent a valid message queue, or the queue was opened for write only O_WRONLY -@code{EMSGSIZE} - The msg_len is less than the mq_msgsize attribute of the message queue -@code{EAGAIN} - The message queue is non-blocking, and the queue is empty -@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. - -@subheading 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. - -@subheading 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 - -@c -@c -@c -@page -@subsection mq_notify - Notify Process that a Message is Available - -@findex mq_notify -@cindex notify process that a message is available - -@subheading CALLING SEQUENCE: - -@example -#include <mqueue.h> - -int mq_notify( - mqd_t mqdes, - const struct sigevent *notification -); -@end example - -@subheading STATUS CODES: - -@code{EBADF} - The descriptor does not refer to a valid message queue -@code{EBUSY} - A notification request is already attached to the queue - -@subheading 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. - -@subheading 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. - -@c -@c -@c -@page -@subsection mq_setattr - Set Message Queue Attributes - -@findex mq_setattr -@cindex set message queue attributes - -@subheading CALLING SEQUENCE: - -@example -#include <mqueue.h> - -int mq_setattr( - mqd_t mqdes, - const struct mq_attr *mqstat, - struct mq_attr *omqstat -); -@end example - -@subheading STATUS CODES: - -@code{EBADF} - The message queue descriptor does not refer to a valid, open queue. -@code{EINVAL} - The mq_flag value is invalid. - -@subheading 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. - -@subheading NOTES: - -All other fields in the mq_attr are ignored by this call. - -@c -@c -@c -@page -@subsection mq_getattr - Get Message Queue Attributes - -@findex mq_getattr -@cindex get message queue attributes - -@subheading CALLING SEQUENCE: - -@example -#include <mqueue.h> -int mq_getattr( - mqd_t mqdes, - struct mq_attr *mqstat -); -@end example - -@subheading STATUS CODES: - -@code{EBADF} - The message queue descriptor does not refer to a valid, -open message queue. - -@subheading 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. - -@subheading NOTES: - |