BrlAPI Reference manual Sébastien Hinderer Samuel Thibault V1.5, November 2019 This document describes BrlAPI. ______________________________________________________________________ Table of Contents 1. Introduction 1.1 Concepts 1.2 How to read this manual 2. General description of BrlAPI 2.1 Historical notes. 2.2 Why BrlAPI is part of brltty . 2.3 How it works. 2.3.1 Connection. 2.3.2 Authorization. 2.3.3 Real use of the braille terminal. 2.3.3.1 Braille display and braille key presses processing. 2.3.3.2 Parameter handling The server exposes some parameters to the client. Some parameters are global to all clients (e.g. the braille display size), while others are local per client (e.g. retaindots, i.e. whether to send Perkins presses as dot patterns or as letters). Some parameters are read-only (e.g. the braille display size), while others are read-write (e.g. retaindots). Some parameters may change during execution, while others change only when a client set it. Clients can either request the current value of a parameter, or set its value (if it is read-write), or request the server to notify on value change. 2.3.3.3 Raw mode. 2.3.3.4 Suspend Mode. 2.3.3.5 Remarks. 2.3.4 Disconnection. 3. Concurrency management between BrlAPI clients 3.1 VT switching 3.2 A pile of "paper sheets" 3.3 Hierarchy 3.4 The X-window case 3.5 Detaching 4. Installation and configuration of BrlAPI 5. Library description 5.1 Connecting to BrlAPI 5.2 Getting driver name 5.3 Getting display size 5.4 Entering raw mode, immediately leaving raw mode. 5.5 Getting tty control 5.6 Writing something on the display 5.7 Waiting for a key press 5.8 Understanding commands 5.9 Leaving tty control 5.10 Disconnecting from BrlAPI 5.11 Putting everything together... 6. Writing ( BrlAPI -compliant) drivers for brltty 6.1 Overview of the driver's structure 6.2 Basic driver structure 6.3 Enhancements for BrlAPI 6.3.1 Exchanging raw data packets 6.3.1.1 Remarks. 7. Protocol reference 7.1 Reliable packet transmission channel 7.2 Responses from the server 7.3 Operating modes 7.4 Details for each type of packet 7.4.1 BRLAPI_PACKET_VERSION This must be the first packet ever transmitted from the server to the client and from the client to the server. The server sends one first for letting the client know its protocol version. Data is an integer indicating the protocol version. Then client must then respond the same way for giving its version. If the protocol version can't be handled by the server, a BRLAPI_ERROR_PROTOCOL_VERSION error packet is returned and the connection is closed. 7.4.2 BRLAPI_PACKET_AUTH 7.4.3 BRLAPI_PACKET_GETDRIVERNAME (see brlapi_getDriverName() ) 7.4.4 BRLAPI_PACKET_GETMODELID (see brlapi_getModelIdentifier() ) 7.4.5 BRLAPI_PACKET_GETDISPLAYSIZE (see brlapi_getDisplaySize() ) 7.4.6 BRLAPI_PACKET_ENTERTTYMODE (see brlapi_enterTtyMode() and brlapi_enterTtyModeWithPath() ) 7.4.7 BRLAPI_PACKET_KEY (see brlapi_readKey() ) 7.4.8 BRLAPI_PACKET_SETFOCUS (see brlapi_setFocus() ) 7.4.9 BRLAPI_PACKET_LEAVETTYMODE (see brlapi_leaveTtyMode() ) 7.4.10 BRLAPI_PACKET_IGNOREKEYRANGE and BRLAPI_PACKET_ACCEPTKEYRANGE (see brlapi_ignoreKeyRange() and brlapi_acceptKeyRange() ) 7.4.11 BRLAPI_PACKET_WRITE (see brlapi_write() ) 7.4.12 BRLAPI_PACKET_ENTERRAWMODE (see brlapi_enterRawMode() ) 7.4.13 BRLAPI_PACKET_LEAVERAWMODE (see brlapi_leaveRawMode() ) 7.4.14 BRLAPI_PACKET_PACKET (see brlapi_sendRaw() and brlapi_recvRaw() ) 7.4.15 BRLAPI_PACKET_SUSPENDDRIVER (see brlapi_suspendDriver() ) 7.4.16 BRLAPI_PACKET_PARAM_REQUEST 7.4.17 BRLAPI_PACKET_PARAM_VALUE 7.4.18 BRLAPI_PACKET_PARAM_VALUE This packet is sent asynchronously by the server to provide an update of a value of a parameter. This is sent only if the client has previously sent a BRLAPI_PACKET_PARAM_REQUEST packet with the BRLAPI_PARAMF_SUBSCRIBE for the corresponding parameter. It is structured exactly like a BRLAPI_PACKET_PARAM_VALUE packet. 7.4.19 BRLAPI_PACKET_SYNCHRONIZE This packet is sent by the client and just acknowledged by the server. This allows the client to perform a round-try with the server, thus collecting any pending exception notification. ______________________________________________________________________ 1. Introduction BrlAPI is a service provided by the brltty daemon. Its purpose is to allow programmers to write applications that take advantage of a braille terminal in order to deliver a blind user suitable information for his/her specific needs. While an application communicates with the braille terminal, everything brltty sends to the braille terminal in the application's console is ignored, whereas each piece of data coming from the braille terminal is sent to the application, rather than to brltty. 1.1. Concepts All throughout this manual, a few terms will be used which are either specific to braille terminals, or introduced because of BrlAPI. They are defined below. Taking a few minutes to go through this glossary will save a lot of time and questions later. Authorization key A file containing arbitrary data, that has to be sent to the server by the client, to prove it is allowed to establish a connection and then control the braille terminal. Braille display The small screen on the braille terminal that is able to display braille text. Braille keyboard The keyboard of the braille terminal. Braille terminal A computer designed to display text in braille. In this case, the text is supposed to come from another computer running Linux or any other Unix system. Brltty The background process that gives a blind person access to the console screen thanks to a braille terminal or speech synthetizer. Client An application designed to handle a braille terminal thanks to BrlAPI. Command A code returned by the driver, indicating an action to do, for instance "go to previous line", "go to next line", etc. Driver A library that has functions to communicate with a braille terminal. Basically, a driver has functions to open communication with the braille terminal, close the communication, write on the braille display, and read keypresses from the braille keyboard, plus some special functions that will be described in detail in this manual. Key A code that is returned by the driver when a key is pressed. This is different from a command, because the command concept is driver-independent (all drivers use the same command codes - those defined by brltty), whereas codes used for returning keypresses may vary between drivers. BrlAPI's Library This library helps clients to connect and use BrlAPI's server thanks to a series of brlapi_-prefixed functions. Packet A sequence of bytes making up the atomic unit in communications, either between braille drivers and braille terminals or between the server and clients. Raw mode Mode in which the client application exchanges packets with the driver. Normal operations like sending text for display or reading keypresses are not available in this mode. It lets applications take advantage of advanced functionalities of the driver's communication protocol. Server The part of brltty that controls incoming connections and communication between clients and braille drivers. Suspend mode Mode in which the server keeps the device driver closed, so that the client can connect directly to the device. Tty Synonym for console, terminal, ... Linux' console consist of several Virtual Ttys (VTs). The screen program's windows also are Ttys. X-window system's xterms emulate Ttys as well. 1.2. How to read this manual This manual is split in five parts. ``General description'' Describes more precisely what BrlAPI is and how it works in collaboration with brltty's core, the braille driver and clients. In this part, a "connection-use-disconnection" scenario will be described step by step, explaining for each step what BrlAPI does in reaction to client instructions. These explanations will take place at a user level. ``Concurrency management'' This part explains how concurrency between BrlAPI clients is handled thanks to focus tellers. ``Installation and configuration'' This part explains in detail how to install and configure the API. For instructions on how to install and configure brltty, please report to the brltty documentation. ``Library description'' This part describes how client applications can communicate with the server using the BrlAPI library that comes with brltty. Each function will be briefly described, classified by categories. More exhaustive descriptions of every function are available in the corresponding online manual pages. ``Writing braille drivers'' This part describes how the braille drivers included in brltty should be written in order to take advantage of BrlAPI's services. ``Protocol reference'' This part describes in detail the communication protocol that is used to communicate between server and clients. What should be read probably depends on what should be done by applications with BrlAPI. Reading chapters ``General description'', ``Concurrency management'' and ``Installation and configuration'' is recommended, since they provide useful information and (hopefully) lead to a good understanding of BrlAPI, for an efficient use. Chapter ``Library description'' concerns writing applications that take advantage of braille terminals so as to bring specific (and more useful) information to blind people. Chapter ``Drivers'' is for braille driver implementation: either adding a braille driver to brltty or modifying an existing one so that it can benefit from BrlAPI's features, this chapter will be of interest, since it describes exactly what is needed to write a driver for brltty: the core of drivers interface for instance. Finally, chapter ``Protocol reference'' is for not using the library, but using the BrlAPI server directly, when the library might not be sufficient: it describes the underlying protocol that will have to be used to do so. 2. General description of BrlAPI Here is explained what BrlAPI is, and what it precisely does. These explanations should be simple enough to be accessible to every user. For a more technical review of BrlAPI's functionalities, please see chapter ``Libary description''. 2.1. Historical notes. Originally, brltty was designed to give access to the Linux console to visually impaired people, through a braille terminal or a speech synthetizer. At that time, applications running in the console were not taking care of the presence of a braille terminal (most applications didn't even know what a braille terminal was). This situation where applications are not aware of the presence of a special device is elegant of course, since it lets use an unlimited number of applications which don't need to be specially designed for visually impaired people. However, it appeared that applications specially designed to take advantage of a braille terminal could be wanted, to provide the suitable information to blind users, for instance. The idea of BrlAPI is to propose an efficient communication mechanism, to control the braille display, read keys from the braille keyboard, or to exchange data with the braille terminal at a lower level (e.g. to write file transfer protocols between braille terminals and Linux systems). 2.2. Why BrlAPI is part of brltty . Instead of rewriting a whole communication program from scratch, we chose to add communication mechanisms to brltty. This choice has two main justifications. On the one hand, integration to brltty allows us to use the increasing number of drivers written for brltty, thus handling a large number of braille terminals without having to rewrite any piece of existing code. On the other hand, if an application chooses to send its own information to the braille display, and to process braille keys, brltty has to be warned, so that it won't try to communicate with the braille terminal while the application already does. To make this synchronzation between brltty and client applications possible, it seemed easier to add the communication mechanisms to brltty's core, instead of writing an external program providing them. 2.3. How it works. We are now going to describe the steps an application should go through to get control of the braille terminal, and what happens on brltty's side at each step. This step-by-step description will let us introduce more precisely some concepts that are useful for every BrlAPI user. 2.3.1. Connection. The first thing any client application has to do is to connect (in the Unix sense of the word) to BrlAPI which is an mere application server. If this is not clear, the only thing to be remembered is that this step allows the client application to let the server know about its presence. At this stage, nothing special is done on brltty's side. 2.3.2. Authorization. Since Unix is designed to allow many users to work on the same machine, it's quite possible that there are more than one user accounts on the system. Most probably, one doesn't want any user with an account on the machine to be able to communicate with the braille terminal (just imagine what would happen if, while somebody was working with the braille terminal, another user connected to the system began to communicate with it, preventing the first one from doing his job...). That's why BrlAPI has to provide a way to determine whether a user who established a connection is really allowed to communicate with the braille terminal. To achieve this, BrlAPI requires that each application that wants to control a braille terminal sends an authorization key before doing anything else. The control of the braille terminal will only be possible for the client once it has sent the proper authorization key. What is called authorization key is in fact a Unix file containing data (it must be non-empty) on your system. All the things you have to do is to give read permissions on this file to users that are allowed to communicate with the braille terminal, and only to them. This way, only authorized users will have access to the authorization key and then be able to send it to BrlAPI. To see how to do that, please see chapter ``Installation and configuration''. At the end of this step, the user is authorized to take control of the braille terminal. On brltty's side, some data structures are allocated to store information on the client, but this has no user-level side- effect. 2.3.3. Real use of the braille terminal. Once the client is properly connected and authorized, there are two possible types of communication with the braille terminal. The chosen type of communication depends on what the client plans to do. If its purpose is to display information on the braille display or to process braille keys, it will have to take control of the Linux tty on which it is running. If its purpose is to exchange data with the braille terminal (e.g. for file transfer), it will enter what is called "raw mode". 2.3.3.1. Braille display and braille key presses processing. If the client wants to display something on the braille display or to process braille keys itself, rather than letting brltty process them, it has to take control of the Linux terminal it is running on. Once a client has obtained the control of his tty, BrlAPI will completely discard brltty's display on this tty (and only this one), leaving the braille display free for the client. At the same time, if a key is pressed on the braille keyboard, BrlAPI checks whether the client application is interested in this key or not. If it is, the key is passed to it, either as a key code or as a brltty command. If it is not, the key code is converted into a brltty command and returned to brltty. Once the client is not interested in displaying text or reading braille keys any more, it has to leave the tty, so that either brltty can continue its job, or another client can take control of it. 2.3.3.2. Parameter handling The server exposes some parameters to the client. Some parameters are global to all clients (e.g. the braille display size), while others are local per client (e.g. retaindots, i.e. whether to send Perkins presses as dot patterns or as letters). Some parameters are read-only (e.g. the braille display size), while others are read-write (e.g. retaindots). Some parameters may change during execution, while others change only when a client set it. Clients can either request the current value of a parameter, or set its value (if it is read-write), or request the server to notify on value change. 2.3.3.3. Raw mode. Only one client can be in raw mode at the same time. In this mode, data coming from the braille terminal are checked by the driver (to ensure they are valid), but instead of being processed, they are delivered "as-is" to the client that is in raw mode. In the other direction, packets sent to BrlAPI by the client that is in raw mode are passed to the driver which is expected to deliver them to the braille terminal without any modification. 2.3.3.4. Suspend Mode. Only one client can be in suspend mode at the same time. This mode is also exclusive with raw mode. In this mode, the server keeps the device driver closed, and thus the client can open the device directly by itself. This mode is not recommended, since the client will then have to reimplement device access. Raw mode should really be preferred, since it lets the client take advantage of server's ability to talk with the device (USB/bluetooth support for instance). 2.3.3.5. Remarks. ˇ The operations described in the three previous subsections are not completely mutually exclusive. An application that controls its current tty can enter raw or suspend mode, provided that no other application already is in this mode. ˇ Not every braille driver supports raw mode. It has to be specially (re)written to support it, since it has to provide special functions to process incoming and outgoing packets. The same restriction is true (but less strong) concerning the ability to deliver/convert keycodes into commands: not every driver has this ability, it has to be modified to get it. ˇ Operations previously described can be repeated. You can, for instance, use raw mode to transfer data onto your braille terminal, display text in braille, return to raw mode..., all that without having to reconnect to BrlAPI before each operation. 2.3.4. Disconnection. Once the client has finished using the braille terminal, it has to disconnect from the API, so that the memory structures allocated for the connection can be freed and eventually used by another client. This step is transparent for the user, in the sense that it involves no change on the braille display. 3. Concurrency management between BrlAPI clients An essential purpose of BrlAPI is to manage concurrent access to the braille display between the brltty daemon and applications. This concurrency is managed "per Tty". We first describe this with a flat view, and then consider Tty hierarchy. 3.1. VT switching Let's first describe how things work with the simple case of a single series of Virtual Ttys (VTs), the linux console for instance. As described in ``General Description'', before being able to write output, a BrlAPI client has to "get" a tty, i.e. it sends to the BrlAPI server the number of the linux' Virtual Tty on which it is running. The BrlAPI server uses this information so as to know which client's output should be shown on the braille display, according to the focus teller's information. Let's say some client A is running on VT 2. It "got" VT 2 and wrote some output on its BrlAPI connection. The focus teller is brltty here: it always tells to the BrlAPI server which VT is currently shown on the screen and gets usual keyboard presses (it is "active"). Let's say VT 1 is active, then the BrlAPI server shows brltty's output on the braille display. I.e. the usual brltty screen reading appears. Moreover, when braille keys are pressed, they are passed to brltty, so that usual screen reading can be performed. When the user switches to VT 2, brltty (as focus teller) tells it to the BrlAPI server, which then remembers that client A has got it and has produced some output. The server then displays this output on the braille display. Note that A doesn't need to re-submit its output: the server had recorded it so as to be able to show it as soon as the focus switches to VT 2. Whenever some key of the braille device is pressed, BrlAPI looks whether it is in the list of keys that client A said to be of his interest. If it is, it is passed to A (and not to brltty). If it isn't, it is passed to brltty (and not to A). As a consequence, whenever clients get and release Ttys and the user switches between Ttys, either the brltty screen reading or the client's output is automatically shown according to rather natural rules. 3.2. A pile of "paper sheets" Let's look at VT 2 by itself. What is shown on the braille display can be seen as the result of a pile of two paper sheets. brltty is represented by the bottom sheet on which its screen reading is written, and client A by the top sheet on which its output is written. A's sheet hence "covers" brltty's sheet: A's output "mask" brltty's screen reading. A may yet want to temporarily let brltty's screen reading appear on VT 2, while still receiving some key presses, for instance. For this, it sends a "void" write. The server then clears the recorded output for this connection (in the sheet representation, the sheet becomes "transparent"). As a consequence, brltty's output is automatically shown (by transparency in the sheet representation), just like if A had released the Tty. Keypresses are handled in a similar way: A's desire to get key presses is satisfied first before brltty. Let's say some other client B (probably launched by A) also gets VT 2 and outputs some text on its BrlAPI connection. This adds a third sheet, on top of the two previous ones. It means that the BrlAPI server will show B's output on the braille device. If A then outputs some text, the server will record it (on A's sheet which hence becomes opaque again), but it won't be displayed on the braille device, since B's sheet is still at the top and opaque (i.e. with some text on it). But if B issues a void write, the server clears its ouput buffer (i.e. B's sheet becomes transparent), and as a result A's output appear on the braille display (by transparency through B's sheet). The sheet order is by default determined by the Tty "get"ting order. Clients can however change their priority (which by default is 50) to a higher value in order to show up higher in the pile, or to a lower value in order to hide lower in the pile. 3.3. Hierarchy Now, what happens when running some screen program on, say, VT 3? It emulates a series of Ttys, whose output actually appear on the same VT 3. That's where a hierarchy level appears: the focus information is not only the VT number but also, in the case of VT 3, which screen window is active. This hence forms a tree of Ttys: the "root" being the vga driver's output, whose sons are VTs, and VT 3 has the screen windows as sons. Brltty is a focus teller for the root, screen will have to be a focus teller for VT 3. Screen should then get VT 3, not display anything (so that the usual brltty screen reading will be shown by transparency), and tell the BrlAPI server which screen window is active (at startup and at each window switch). This is not implemented directly in screen yet, but this may be achieved via a second brltty daemon running the Screen driver (but it isn't yet able to get the current window number though) and the BrlAPI driver. A BrlAPI client C running in some screen window number 1 would then have to get the Tty path "VT 3 then window 1", which is merely expressed as "3 1". The window number is available in the WINDOW environment variable, set by screen. The VT number, which actually represents the "path to screen's output" should be available in the WINDOWPATH environment variable, also set by screen. The client can thus merely concatenate the content of WINDOWPATH (which could hold many levels of window numbers) and of WINDOW and give the result as tty path to the BrlAPI server, which then knows precisely where the client's usual output resides. In practice, applications just need to call brlapi_enterTtyMode(BRLAPI_TTY_DEFAULT), and the the BrlAPI client library will automatically perform all that. Whenever the user switches to VT 3, the BrlAPI server remembers the window that screen told to be active. If it was window 1, it then displays C's output (if any). Else brltty's usual screen reading is shown. Of course, several clients may be run in window 1 as well, and the "sheet pile" mecanism applies: brltty's sheet first (at the root of the Ttys tree), then screen's sheet (which is transparent, on VT 3), then C's sheet (on window 1 of VT 3), then other clients' sheets (on the same window). Ttys are hence organized in a tree, each client adding its sheet at some tty in the tree. 3.4. The X-window case Let's say some X server is running on VT 7 of a Linux system. Xorg's xinit and xdm commands automatically set the X session's WINDOWPATH environment variable to "7", so that X11 BrlAPI clients started from the session just need to call brlapi_enterTtyMode(xid) where xid is the X-window ID of the window of the client. The BrlAPI library will automatically prepend the content of WINDOWPATH to it. For text-based BrlAPI clients running in an xterm (which should just call brlapi_enterTtyMode(BRLAPI_TTY_DEFAULT) as explained in the previous section), BrlAPI detects the window id thanks to the WINDOWID variable set by xterm. Screen readers are not bound to a particular window, so they should call brlapi_enterTtyModeWithPath(NULL, 0) to let the BrlAPI library only send the content of WINDOWPATH, expressing that screen readers take the whole tty. The user should notably launch xbrlapi, which is a focus teller for X-window as well as a keyboard simulator (brltty can't reliably simulate them at the kernel level in such situation). For accessing AT-SPI contents (like gnome or kde applications), Orca should also be launched. For accessing AT-SPI terminals (like gnome- terminal) in the same way as in the console, a second brltty daemon running the at-spi screen driver and the BrlAPI driver can also be launched. All three would get the VT of the X session, in that order (for now): xbrlapi first, then orca and brltty at last. When the X focus is on an AT-SPI terminal, brltty will hence be able to grab the braille display and key presses. Else orca would get them. And xbrlapi would finally get remaining key presses and simulate them. Note: old versions of xinit, xdm, kdm or gdm do not automatically set the WINDOWPATH variable. The user can set it by hand in his ~/.xsession, ~/.xinitrc, ~/.gdmrc... to "7" Note: some Operating Systems like Solaris do not have VTs. In that case WINDOWPATH is empty or not even set. Everything explained above still work fine. 3.5. Detaching Several programs allow detaching: screen and VNC for instance. In such situation, an intermediate BrlAPI server should be run for each such session. Clients would connect to it, and it would prepend the "current tty" path on the fly while forwarding things to the root BrlAPI server. This intermediate server is yet to be written (but it is actually relatively close to be). 4. Installation and configuration of BrlAPI make install will install libbrlapi.so in /lib, and include files in /usr/include/brltty. An authorization key will also typically be set in /etc/brlapi.key (if it is not, just create it and put arbitrary data in it), but it won't be readable by anybody else than root. It is up to you to define a group of users who will have the right to read it and hence be able to connect to the server. For instance, you may want to do: ______________________________________________________________________ # addgroup brlapi # chgrp brlapi /etc/brlapi.key # chmod g+r /etc/brlapi.key # addgroup user1 brlapi # addgroup user2 brlapi ... ______________________________________________________________________ 5. Library description Let's now see how one can write dedicated applications. Basic notions will be seen, along with a very simple client. Greater details are given as online manual pages. The historical test program for BrlAPI was something like: ˇ connect to BrlAPI ˇ get driver id ˇ get driver name ˇ get display size ˇ try entering raw mode, immediately leave raw mode. ˇ get tty control ˇ write something on the display ˇ wait for a key press ˇ leave tty control ˇ disconnect from BrlAPI It is here rewritten, its working briefly explained. 5.1. Connecting to BrlAPI Connection to BrlAPI is needed first, thanks to the brlapi_openConnection call. For this, a brlapi_connectionSettings_t variable must be filled which will hold the settings the library needs to connect to the server. Just giving NULL will work for local use. The other parameter lets you get back the parameters which were actually used to initialize connection. NULL will also be nice for now. ______________________________________________________________________ if (brlapi_openConnection(NULL, NULL)<0) { brlapi_perror("brlapi_openConnection"); exit(1); } ______________________________________________________________________ The connection might fail, so testing is needed. 5.2. Getting driver name Knowing the type of the braille device might be useful: ______________________________________________________________________ char name[BRLAPI_MAXNAMELENGTH+1]; if (brlapi_getDriverName(name, sizeof(name)) < 0) brlapi_perror("brlapi_getDriverName"); else fprintf(stderr, "Driver name: %s\n", name); ______________________________________________________________________ This is particularly useful before entering raw mode to achieve file transfers for instance, just to check that the device is really the one expected. 5.3. Getting display size Before writing on the braille display, the size should be always first checked to be sure everything will hold on it: ______________________________________________________________________ if (brlapi_getDisplaySize(&x, &y) < 0) brlapi_perror("brlapi_getDisplaySize"); else fprintf(stderr, "Braille display has %d line%s of %d column%s\n", y, y>1?"s":"", x, x>1?"s":""); ______________________________________________________________________ 5.4. Entering raw mode, immediately leaving raw mode. Entering raw mode is very simple: ______________________________________________________________________ fprintf(stderr, "Trying to enter in raw mode... "); if (brlapi_enterRawMode(name) < 0) brlapi_perror("brlapi_enterRawMode"); else { fprintf(stderr, "Ok, leaving raw mode immediately\n"); brlapi_leaveRawMode(); } ______________________________________________________________________ Not every driver supports raw mode, so testing is needed. While in raw mode, brlapi_sendRaw and brlapi_recvRaw can be used to send and get data directly to and from the device. It should be used with care, improper use might completely thrash the device! 5.5. Getting tty control Let's now display something on the device. control of the tty must be get first: ______________________________________________________________________ fprintf(stderr, "Taking control of the tty... "); if (brlapi_enterTtyMode(BRLAPI_TTY_DEFAULT, NULL) >= 0) { fprintf(stderr, "Ok\n"); ______________________________________________________________________ The first parameter tells the server the number of the tty to take control of. Setting BRLAPI_TTY_DEFAULT lets the library determine it for us. The server is asked to send brltty commands, which are device- independent. Getting control might fail if, for instance, another application already took control of this tty, so testing is needed. From now on, the braille display is detached from the screen. 5.6. Writing something on the display The application can now write things on the braille display without altering the screen display: ______________________________________________________________________ fprintf(stderr, "Writing to braille display... "); if (brlapi_writeText(0, "Press a braille key to continue...") >= 0) { fprintf(stderr, "Ok\n"); ______________________________________________________________________ The cursor is also asked not to be shown: its position is set to 0. "Writing to braille display... Ok" is now displayed on the screen, and "Press a braille key to continue..." on the braille display. 5.7. Waiting for a key press To have a break for the user to be able to read these messages, a key press (a command here, which is driver-independent) may be waited for: ______________________________________________________________________ fprintf(stderr, "Waiting until a braille key is pressed to continue... "); if (brlapi_readKey(1, &key) > 0) fprintf(stderr, "got it! (code=%"BRLAPI_PRIxKEYCODE")\n", key); ______________________________________________________________________ The command is returned, as described in and . It is not transmitted to brltty: it is up to the application to define the behavior, here cleanly exitting, as described below. The first parameter tells the lib to block until a key press is indeed read. 5.8. Understanding commands There are two kinds of commands: braille commands (line up/down, top/bottom, etc.) and X Keysyms (i.e. regular keyboard keys). One way to discover which key was pressed is to just use a switch statement: ______________________________________________________________________ switch(key) { case BRLAPI_KEY_TYPE_CMD|BRLAPI_KEY_CMD_LNUP: fprintf(stderr, "line up\n"); break; case BRLAPI_KEY_TYPE_CMD|BRLAPI_KEY_CMD_LNDN: fprintf(stderr, "line down\n"); break; case BRLAPI_KEY_TYPE_SYM|XK_Tab: fprintf(stderr, "tab\n"); break; default: fprintf(stderr, "unknown key\n"); break; } ______________________________________________________________________ Another way is to ask BrlAPI to expand the keycode into separate information parts: ______________________________________________________________________ brlapi_expandedKeyCode_t ekey; brlapi_expandKeyCode(key, &ekey); fprintf(stderr, "type %u, command %u, argument %u, flags %u\n", ekey.type, ekey.command, ekey.argument, ekey.flags); ______________________________________________________________________ Eventually, named equivalents are provided: ______________________________________________________________________ brlapi_describedKeyCode_t dkey; int i; brlapi_describeKeyCode(key, &dkey); fprintf(stderr, "type %s, command %s, argument %u, flags", dkey.type, dkey.command, dkey.argument); for (i = 0; i < dkey.flags; i++) fprintf(stderr, " %s", dkey.flag[i]); fprintf(stderr, "\n"); ______________________________________________________________________ 5.9. Leaving tty control Let's now leave the tty: ______________________________________________________________________ fprintf(stderr, "Leaving tty... "); if (brlapi_leaveTtyMode() >= 0) fprintf(stderr, "Ok\n"); ______________________________________________________________________ But control of another tty can still be get for instance, by calling brlapi_enterTtyMode() again... 5.10. Disconnecting from BrlAPI Let's disconnect from BrlAPI: ______________________________________________________________________ brlapi_closeConnection(); ______________________________________________________________________ The application can as well still need to connect to another server on another computer for instance, by calling brlapi_openConnection() again... 5.11. Putting everything together... ______________________________________________________________________ #include #include #include int main() { brlapi_keyCode_t key; char name[BRLAPI_MAXNAMELENGTH+1]; unsigned int x, y; /* Connect to BrlAPI */ if (brlapi_openConnection(NULL, NULL)<0) { brlapi_perror("brlapi_openConnection"); exit(1); } /* Get driver name */ if (brlapi_getDriverName(name, sizeof(name)) < 0) brlapi_perror("brlapi_getDriverName"); else fprintf(stderr, "Driver name: %s\n", name); /* Get display size */ if (brlapi_getDisplaySize(&x, &y) < 0) brlapi_perror("brlapi_getDisplaySize"); else fprintf(stderr, "Braille display has %d line%s of %d column%s\n", y, y>1?"s":"", x, x>1?"s":""); /* Try entering raw mode, immediately go out from raw mode */ fprintf(stderr, "Trying to enter in raw mode... "); if (brlapi_enterRawMode(name) < 0) brlapi_perror("brlapi_enterRawMode"); else { fprintf(stderr, "Ok, leaving raw mode immediately\n"); brlapi_leaveRawMode(); } /* Get tty control */ fprintf(stderr, "Taking control of the tty... "); if (brlapi_enterTtyMode(BRLAPI_TTY_DEFAULT, NULL) >= 0) { fprintf(stderr, "Ok\n"); /* Write something on the display */ fprintf(stderr, "Writing to braille display... "); if (brlapi_writeText(0, "Press a braille key to continue...") >= 0) { fprintf(stderr, "Ok\n"); /* Wait for a key press */ fprintf(stderr, "Waiting until a braille key is pressed to continue... "); if (brlapi_readKey(1, &key) > 0) { brlapi_expandedKeyCode_t ekey; brlapi_describedKeyCode_t dkey; int i; fprintf(stderr, "got it! (code=%"BRLAPI_PRIxKEYCODE")\n", key); brlapi_expandKeyCode(key, &ekey); fprintf(stderr, "type %u, command %u, argument %u, flags %u\n", ekey.type, ekey.command, ekey.argument, ekey.flags); brlapi_describeKeyCode(key, &dkey); fprintf(stderr, "type %s, command %s, argument %u, flags", dkey.type, dkey.command, dkey.argument); for (i = 0; i < dkey.flags; i++) fprintf(stderr, " %s", dkey.flag[i]); fprintf(stderr, "\n"); } else brlapi_perror("brlapi_readKey"); } else brlapi_perror("brlapi_writeText"); /* Leave tty control */ fprintf(stderr, "Leaving tty... "); if (brlapi_leaveTtyMode() >= 0) fprintf(stderr, "Ok\n"); else brlapi_perror("brlapi_leaveTtyMode"); } else brlapi_perror("brlapi_enterTtyMode"); /* Disconnect from BrlAPI */ brlapi_closeConnection(); return 0; } ______________________________________________________________________ This should compile well thanks to gcc apiclient.c -o apiclient -lbrlapi 6. Writing ( BrlAPI -compliant) drivers for brltty In this chapter, we will describe in details how to write a driver for brltty. We begin with a general description of the structure the driver should have, before explaining more precisely what each function is supposed to do. 6.1. Overview of the driver's structure A braille driver is in fact a library that is either dynamically loaded by brltty at startup, or statically linked to it during the compilation, depending on the options given to the ./configure script. This library has to provide every function needed by the core, plus some additional functions, that are not mandatory, but which improve communication with BrlAPI and the service level provided to client applications. Basically, a driver library needs to provide a function to open the communication with the braille terminal, one to close this communication, one to read key codes from the braille keyboard, and one to write text on the braille display. As we will see in a moment, other functions are required. Moreover, a driver can provide additional functionalities, by defining some macros asserting that it has these functionalities, and by defining associated functions. 6.2. Basic driver structure Every brltty driver must consist in at least a file called braille.c, located in an appropriate subdirectory of the BrailleDrivers subdirectory. This braille.c file must have the following layout #include "prologue.h" /* Include standard C headers */ #include "Programs/brl.h" #include "Programs/misc.h" #include "Programs/scr.h" #include "Programs/message.h" /* Include other files */ static void brl_identify() { } static int brl_open(BrailleDisplay *brl, char **parameters, const char *tty) { ... } static void brl_close(BrailleDisplay *brl) { ... } static void brl_writeWindow(BrailleDisplay *brl) { ... } static void brl_writeStatus(BrailleDisplay *brl) { ... } static int brl_readCommand(BrailleDisplay *brl, DriverCommandContext context) { ... } Before giving a detailed description of what each function is supposed to do, we define the BrailleDisplay structure, since each function has an argument of type BrailleDisplay *. The BrailleDisplay structure is defined like this: typedef struct { int x, y; /* The dimensions of the display */ int helpPage; /* The page number within the help file */ unsigned char *buffer; /* The contents of the display */ unsigned isCoreBuffer:1; /* The core allocated the buffer */ unsigned resizeRequired:1; /* The display size has changed */ unsigned int writeDelay; void (*bufferResized)(int rows, int columns); } BrailleDisplay; We now describe each function's semantics and calling convention. The brl_identify() function takes no argument and returns nothing. It is called as soon as the driver is loaded, and its purpose is to print some information about the driver in the system log. To achieve this, the only thing this function has to do is to call LOG_PRINT with appropriate arguments (log level and string to put in the syslog). The brl_open() function takes 3 arguments and returns an int. Its purpose is to initialize the communication with the braille terminal. Generally, this function has to open the file referred to by the tty argument, and to configure the associated communication port. The parameters argument contains parameters passed to the driver with the -B command-line option. It's up to the driver's author to decide wether or not he/she wants to use this argument, and what for. The function can perform some additional tasks such as trying to identify precisely which braille terminal model is connected to the computer, by sending it a request and analyzing its answer. The value that is finally returned depends on the success of the initialization process. If it fails, th function has to return -1. The function returns 0 on success. The brl_close() function takes just one argument, and returns nothing. The name of this function should be self-explanatory; it's goal is to close (finish) the communication between the computer and the braille terminal. In general, the only thing this function has to do is to close the file descriptor associated to the braille terminal's communication port. The brl_writeWindow() function takes just one argument of type BrailleDisplay, and returns nothing. This function displays the specified text on the braille window. This routine is the right place to check if the text that has to be displayed is not already on the braille display, to send it only if necessary. More generally, if the braille terminal supports partial refresh of the display, the calculus of what exactly has to be sent to the braille display to have a proper display, according to what was previously displayed should be done in this function. The brl_writeStatus() function is very similar to brl_writeWindow(). The only difference is that whereas brl_writeWindow() writes on the main braille display, brl_writeStatus() writes on an auxiliary braille display, which occasionaly appears on some braille terminals. The remarks that have been done concerning optimizations for refreshing the display still apply here. The brl_readCommand() function takes two arguments, and returns an integer. Its purpose is to read commands from the braille keyboard and to pass them to brltty's core, which in turn will process them. The first argument, of type BrailleDisplay, is for future use, and can safely be ignored for the moment. The second argument indicates in which context (state) brltty is. For instance, it specifies if brltty is in a menu, displays a help screen, etc. This information can indeed be of some interest when translating a key into a command, especially if the keys can have different meanings, depending on the context. So, this function has to read keypresses from the braille keyboard, and to convert them into commands, according to the given context, these commands then being returned to brltty. For a complete list of available command codes, please have a look at brl.h in the Programs subdirectory. Two codes have special meanings: eof specifies that no command is available now, and that no key is waiting to be converted into command in a near future. CMD_NOOP specifies that no command is available, but that one will be, soon. As a consequence, brl_readCommand will be called again immediately. Returning CMD_NOOP is appropriate for instance when a key is composed of two consecutive data packets. When the first of them is received, one can expect that the second will arrive quickly, so that trying to read it as soon as possible is a good idea. 6.3. Enhancements for BrlAPI To improve the level of service provided to client applications communicating with braille drivers through BrlAPI, the drivers should declare some additional functions that will then be called by the API when needed. For each additional feature that has to be implemented in a driver, a specific macro must be defined, in addition to the functions implementing that feature. For the moment, two features are supported by BrlAPI: ˇ reading braille terminal specific key codes, ˇ exchanging raw data packets between the braille terminal and a client application running on the PC. For each feature presented below, only a short description of each concerned macro and function will be given. For a more complete description of concepts used here, please refer to chapters ``Introduction'' and ``General description''. 6.3.1. Exchanging raw data packets Under some circumstances, an application running on the PC can be interested in a raw level communication with the braille terminal. For instance, to implement a file transfer protocol, commands to display braille or to read keys are not enough. In such a case, one must have a way to send raw data to the terminal, and to receive them from it. A driver that wants to provide such a mechanism has to define three functions: one to send packets, another one to receive them, and the last one to reset the communication when problems occur. The macro that declares that a driver is able to transmit packets is: #define BRL_HAVE_PACKET_IO The prototypes of the functions the driver should define are: static int brl_writePacket(BrailleDisplay *brl, const unsigned char *packet, int size); static int brl_readPacket(BrailleDisplay *brl, unsigned char *p, int size); static void brl_rescue(BrailleDisplay *brl) brl_writePacket() sends a packet of size bytes, stored at packet, to the braille terminal. If the communication protocol allows to determined if a packet has been send properly (e.g. the terminal sends back an acknowledgement for each packet he receives), then this function should wait the acknowledgement, and, if it is not received, retransmission of the packet should take place. brl_readPacket() reads a packet of at most size bytes, and stores it at the specified address. The read must not block. I.e., if no packet is available, the function should return immediately, returning 0. brl_rescue() is called by BrlAPI when a client application terminates without properly leaving the raw mode. This function should restore the terminal's state, so that it is able to display text in braille again. 6.3.1.1. Remarks. ˇ If the driver provides such functions, every other functions should use them, instead of trying to communicate directly with the braille terminal. For instance, readCommand() should call readPacket(), and then extract a key from the packet, rather than reading directly from the communication port's file descriptor. The same applies for brl_writeWindow(), which should use brl_writePacket(), rather than writing on the communication port's file descriptor. ˇ For the moment, the argument of type BrailleDisplay can safely be ignored by the functions described here. 7. Protocol reference Under some circumstances, it may be preferable to communicate directly with BrlAPI's server rather than using BrlAPI's library. Here are the needed details to be able to do this. This chapter is also of interest if a precise understanding of how the communication stuff works is desired, to be sure to understand how to write multithreaded clients, for instance. In all the following, integer will mean an unsigned 32 bits integer in network byte order (ie most significant bytes first). 7.1. Reliable packet transmission channel The protocol between BrlAPI's server and clients is based on exchanges of packets. So as to avoid locks due to packet loss, these exchanges are supposed reliable, and ordering must be preserved, thus BrlAPI needs a reliable packet transmission channel. To achieve this, BrlAPI uses a TCP-based connection, on which packets are transmitted this way: ˇ the size in bytes of the packet is transmitted first as an integer, ˇ then the type of the packet, as an integer, ˇ and finally the packet data. The size does not include the { size, type } header, so that packets which don't need any data have a size of 0 byte. The type of the packet can be either of BRLAPI_PACKET_* constants defined in api_protocol.h. Each type of packet will be further discussed below. BrlAPI's library ships two functions to achieve packets sending and receiving using this protocol: brlapi_writePacket and brlapi_readPacket. It is a good idea to use these functions rather than rewriting them, since this protocol might change one day in favor of a real reliable packet transmission protocol such as the experimental RDP. 7.2. Responses from the server As described below, many packets are `acknowledged'. It means that upon reception, the server sends either: ˇ a BRLAPI_PACKET_ACK packet, with no data, which means the operation corresponding to the received packet was successful, ˇ or a BRLAPI_PACKET_ERROR packet, the data being an integer which should be one of BRLAPI_ERROR_* constants. This means the operation corresponding to the received packet failed. Some other packets need some information as a response. Upon reception, the server will send either: ˇ a packet of the same type, its data being the response, ˇ or a BRLAPI_PACKET_ERROR packet. If at some point an ill-formed or non-sense packet is received by the server, and BRLAPI_PACKET_EXCEPTION is returned, holding the guilty packet for further analysis. 7.3. Operating modes The connection between the client and the server can be in either of the four following modes: ˇ authorization mode: this is the initial mode, when the client hasn't got the authorization to use the server yet. The server first sends a BRLAPI_PACKET_VERSION packet that announces the server version. The client must send back a BRLAPI_PACKET_VERSION for announcing its own version too. The server then sends a BRLAPI_PACKET_AUTH packet that announces which authorization methods are allowed. The client can then send BRLAPI_PACKET_AUTH packets, which makes the connection enter normal mode. If no authorization is needed, the server can announce the NONE method, the client then doesn't need to send a BRLAPI_PACKET_AUTH packet. ˇ normal mode: the client is authorized to use the server, but didn't ask for a tty or raw mode. The client can send either of these types of packet: ˇ BRLAPI_PACKET_GETDRIVERNAME or BRLAPI_PACKET_GETDISPLAYSIZE to get pieces of information from the server, ˇ BRLAPI_PACKET_ENTERTTYMODE to enter tty handling mode, ˇ BRLAPI_PACKET_ENTERRAWMODE to enter raw mode, ˇ tty handling mode: the client holds the control of a tty: brltty has no power on it any more, masked keys excepted. It's up to the client to manage display and keypresses. For this, it can send either of these types of packet: ˇ BRLAPI_PACKET_LEAVETTYMODE to leave tty handling mode and go back to normal mode, ˇ BRLAPI_PACKET_IGNOREKEYRANGE and BRLAPI_PACKET_ACCEPTKEYRANGE to mask and unmask keys, ˇ BRLAPI_PACKET_WRITE to display text on this tty, ˇ BRLAPI_PACKET_ENTERRAWMODE to enter raw mode, ˇ BRLAPI_PACKET_GETDRIVERNAME or BRLAPI_PACKET_GETDISPLAYSIZE to get pieces of information from the server, And the server might send BRLAPI_PACKET_KEY packets to signal key presses. ˇ raw mode: the client wants to exchange packets directly with the braille terminal. Only these types of packet will be accepted. ˇ BRLAPI_PACKET_LEAVERAWMODE to get back to previous mode, either normal or tty handling mode. ˇ BRLAPI_PACKET_PACKET to send a packet to the braille terminal. And the server might send BRLAPI_PACKET_PACKET packets to give re- ceived packets from the terminal to the client. ˇ suspend mode: the client wants to completely drive the braille terminal. The device driver is hence kept closed. No type of packet is allowed except BRLAPI_PACKET_RESUME Termination of the connection is initiated by the client in normal mode by simply closing its side of the socket. The server will then close the connection. 7.4. Details for each type of packet Here is described the semantics of each type of packet. Most of them are directly linked to some of BrlAPI's library's functions. Reading their online manual page as well will hence be of good help for understanding. 7.4.1. BRLAPI_PACKET_VERSION This must be the first packet ever transmitted from the server to the client and from the client to the server. The server sends one first for letting the client know its protocol version. Data is an integer indicating the protocol version. Then client must then respond the same way for giving its version. If the protocol version can't be handled by the server, a BRLAPI_ER- ROR_PROTOCOL_VERSION error packet is returned and the connection is closed. 7.4.2. BRLAPI_PACKET_AUTH This must be the second packet ever transmitted from the server to the client and from the client to the server. The server sends one first for letting the client know which authorization methods are available. Data is the allowed authorization types, as integers. If the NONE method is not announced by the server, the client can then try to get authorized by sending packets whose data is the type of authorization that is tried (as an integer), and eventually some data (if the authorization type needs it). If the authorization is successful, the server acknowledges the packet, and other types of packets might be used, other BRLAPI_PACKET_AUTH shouldn't be sent by the client. If the authorization is not successful, the server sends a BRLAPI_ERROR_AUTHENTICATION error, and the client can try another authorization method. Authorization methods are as follow: ˇ NONE: the client doesn't need to send an authorization packet. ˇ KEY: data holds a secret key, the authorization is successful only if the key matches the server secret key. ˇ CREDENTIALS: Operating-System-specific credentials are explicitely sent over the socket, the authorization is successful if the server considers the credentials sufficient. Note: when the Operating system permits it, the server may use implicit credential check, and then advertise the none method. 7.4.3. BRLAPI_PACKET_GETDRIVERNAME (see brlapi_getDriverName() ) This should be sent by the client when it needs the full name of the current brltty driver. The returned string is \0 terminated. 7.4.4. BRLAPI_PACKET_GETMODELID (see brlapi_getModelIdentifier() ) This should be sent by the client when it needs to identify which model of braille display is currently used by brltty. The returned string is \0 terminated. 7.4.5. BRLAPI_PACKET_GETDISPLAYSIZE (see brlapi_getDisplaySize() ) This should be sent by the client when it needs to know the braille display size. The returned data are two integers: width and then height. 7.4.6. BRLAPI_PACKET_ENTERTTYMODE (see brlapi_enterTtyMode() and br- lapi_enterTtyModeWithPath() ) This should be sent by the client to get control of a tty. Sent data are first a series of integers: the first one gives the number of following integers, which are the numbers of ttys that leads to the tty that the application wants to take control of (it can be empty if the tty is one of the machine's VT). The last integer of this series tells the number of the tty to get control of. Finaly, how key presses should be reported is sent: either a driver name or "", preceded by the number of caracters in the driver name (0 in the case of ""), as an unsigned byte. This packet is then acknowledged by the server. 7.4.7. BRLAPI_PACKET_KEY (see brlapi_readKey() ) As soon as the client gets a tty, it must be prepared to handle BRLAPI_PACKET_KEY incoming packets at any time (as soon as the key was pressed on the braille terminal, hopefuly). The data holds a key code as 2 integers, or the key flags then the command code as 2 integers, depending on what has been request in the BRLAPI_PACKET_ENTERTTYMODE packet. 7.4.8. BRLAPI_PACKET_SETFOCUS (see brlapi_setFocus() ) For the server to know which tty is active, one particular client is responsible for sending BRLAPI_PACKET_SETFOCUS packets. They hold a single integer telling the new current tty. For instance, when running an X server on VT 7, the xbrlapi client would have sent a BRLAPI_PACKET_ENTERTTYMODE(7) and will send window IDs whenever X focus changes, allowing display and keypresses switching between xterms. 7.4.9. BRLAPI_PACKET_LEAVETTYMODE (see brlapi_leaveTtyMode() ) This should be sent to free the tty and masked keys lists. This is acknowledged by the server. 7.4.10. BRLAPI_PACKET_IGNOREKEYRANGE and BRLAPI_PACKET_ACCEPTKEYRANGE (see brlapi_ignoreKeyRange() and brlapi_acceptKeyRange() ) If the client doesn't want every key press to be signaled to it, but some of them to be given to brltty for normal processing, it can send BRLAPI_PACKET_IGNOREKEYRANGE packets to tell ranges of key codes which shouldn't be sent to it, but given to brltty, and BRLAPI_PACKET_ACCEPTKEYRANGE packets to tell ranges of key codes which should be sent to it, and not given to brltty. The server keeps a dynamic list of ranges, so that arbitrary sequences of such packets can be sent. A range is composed of 2 keycodes: the "first" and the "last" boundaries. Each keycode is composed of 2 integers: the key flags then the command code. The range expressed by these two keycodes is the set of keycodes whose command codes are between the command code of the "first" keycode and the "last" keycode (inclusive), and whose flags contain at least the flags of the "first" keycode and at most the flags of the "last" keycode. Setting the "first" and "last" keycode to the same value express only one keycode, for instance. Setting the first and last keycode to the same command code but setting no flags in the "first" keycode and setting one flag in the "last" keycode expresses only two keycode, with the same command code and no flags set except possibly the flag that is set in the "last" keycode. Setting one flag i in the "first" keycode and setting the same flag plus another flag j in the "last" keycode expresses that the keycodes in the range have flag i set and possibly flag j set, but no other flag. Several such ranges can be provided one after the other. 7.4.11. BRLAPI_PACKET_WRITE (see brlapi_write() ) To display text on the braille terminal and set the position of the cursor, the client can send a BRLAPI_PACKET_WRITE packet. The packet begins with an integer holding flags (see BRLAPI_WF_*). These flags indicate which data will then be available, in the following order (corresponding to flag weight): ˇ A display number can be given as a integer, in case the braille display has several. If not given, usual display is used. ˇ A region must be given as two integers indicating the beginning and the number of characters of the part of the braille display which is to be updated, the first cell of the display being numbered 1. For braille displays that have several lines, the first cell of the second line of the display is numbered the length of lines plus one, etc., in other words the display is handled as the concatenation of the lines of the display. If the number is negative, its absolute value is taken into account, and the update is padded or truncated to fill the rest of the display. ˇ The text to display can then be given, preceded by its size in bytes expressed as an integer. It will erase the corresponding region in the AND and OR fields. If the region size is positive, the text's length in characters must exactly match the region size. For multibyte text, this is the number of wide characters. Notably, combining and double-width characters count for 1. ˇ Then an AND field can be given, one byte per character: the 8-dot representation of the above text will be AND-ed with this field, hence allowing to erase some unwanted parts of characters. Dots are coded as described in ISO/TR 11548-1: dot 1 is set iff bit 0 is set, dot 2 is set iff bit 1 is set, ... dot i+1 is set if bit i is set. This also corresponds to the low-order byte of the coding of unicode's braille row U+2800. ˇ As well, an OR field may be given, one byte per character: the 8-dot result of the AND operation above (or the 8-dot representation of the text if no AND operation was performed) is OR-ed with this field, hence allowing to set some dots, to underline characters for instance. ˇ A cursor position can be specified. 1 representing the first character of the display, 0 turning the cursor off. If not given, the cursor (if any) is left unmodified. ˇ Last but not least, the charset of the text can be specified: the length of the name first in one byte, then the name itself in ASCII characters. If the charset is not specified, an 8-bit charset is assumed, and it is assumed to be the same as the server's. Multibyte charsets may be used, AND and OR fields' bytes will correspond to each text's wide character, be it a combining or a double-width character. A BRLAPI_PACKET_WRITE packet without any flag (and hence no data) means a "void" WRITE: the server clears the output buffer for this connection. 7.4.12. BRLAPI_PACKET_ENTERRAWMODE (see brlapi_enterRawMode() ) To enter raw mode, the client must send a BRLAPI_PACKET_ENTERRAWMODE packet, which is acknowledged. Once in raw mode, no other packet than BRLAPI_PACKET_LEAVERAWMODE or BRLAPI_PACKET_PACKET will be accepted. The data must hold the special value BRLAPI_DEVICE_MAGIC: 0xdeadbeef, then the name of the driver (one byte for the length, then the name) to avoid erroneous raw mode activating. 7.4.13. BRLAPI_PACKET_LEAVERAWMODE (see brlapi_leaveRawMode() ) To leave raw mode, the client must send a BRLAPI_PACKET_LEAVERAWMODE packet, which is acknowledged. 7.4.14. BRLAPI_PACKET_PACKET (see brlapi_sendRaw() and br- lapi_recvRaw() ) While in raw mode, only BRLAPI_PACKET_PACKET packets can be exchanged between the client and the server: to send a packet to the braille terminal, the client merely sends a BRLAPI_PACKET_PACKET packet, its data being the packet to send to the terminal. Whenever its receives a packet from the terminal, the server does exactly the same, so that packet exchanges between the terminal and the server are exactly reproduced between the server and the client. 7.4.15. BRLAPI_PACKET_SUSPENDDRIVER (see brlapi_suspendDriver() ) To enter suspend mode, the client must send a BRLAPI_PACKET_SUSPEND packet, which is acknowledge. Once in suspend mode, no other packet than BRLAPI_PACKET_RESUME will be accepted. The data must hold the special value BRLAPI_DEVICE_MAGIC: 0xdeadbeef, then the name of the driver (one byte for the length, then the name) to avoid erroneous raw mode activating. 7.4.16. BRLAPI_PACKET_PARAM_REQUEST This packet is sent by the client to request values of parameters. The packet begins with an integer which holds flags (see BRLAPI_PARAMF_*) which describe which, how, and when the value should be returned by the server: ˇ When the BRLAPI_PARAMF_GLOBAL flag is set, the server will return/subscribe the global value instead of the local value. ˇ When the BRLAPI_PARAMF_GET flag is set, the server acknowledges the request by returning the latest value with a BRLAPI_PACKET_PARAM_VALUE packet. Otherwise the server acknowledges the request with a BRLAPI_PACKET_ACK packet, without providing the value. ˇ When the BRLAPI_PARAMF_SUBSCRIBE flag is set, the server will keep sending asynchronously the value of the parameter whenever it changes, with BRLAPI_PACKET_PARAM_UPDATE packets, until another request packet has the BRLAPI_PARAMF_UNSUBSCRIBE flag set for this parameter. ˇ When the BRLAPI_PARAMF_SELF flag is set along BRLAPI_PARAMF_SUBSCRIBE, the server will send the value of the parameter when it is changed even by the client itself. ˇ When the BRLAPI_PARAMF_UNSUBSCRIBE flag is set, the server will stop sending asynchronously the value of the parameter with BRLAPI_PACKET_PARAM_UPDATE packets. It does not make sense to set both the BRLAPI_PARAMF_SUBSCRIBE and BRLAPI_PARAMF_UNSUBSCRIBE flags. Then an integer representing the parameter to be requested. Then two integers that form (in big-endian order) a 64bit value used to subspecify the precise parameter to be requested (e.g. a keycode number). If several BRLAPI_PARAMF_SUBSCRIBE packets are sent by the client, as many BRLAPI_PARAMF_UNSUBSCRIBE packets have to be sent by the client before the server stops sending BRLAPI_PACKET_PARAM_UPDATE packets. 7.4.17. BRLAPI_PACKET_PARAM_VALUE This packet is sent by the client or the server to provide the value of a parameter. The packet begins with an integer which holds flags (see BRLAPI_PVF_*) which describe which value is being transmitted: ˇ When the BRLAPI_PVF_GLOBAL flag is set, the value is the global value instead of the local value. Then an integer representing the parameter being transmitted. Then two integers that form (in big-endian order) a 64bit value used to subspecify the precise parameter being transmitted (e.g. a keycode number). Eventually, the packet contains the value. When the packet is sent by the client, it defines the new value of the parameter, and if it is a global value, the server broadcasts the new value to all clients which have subscribed to updates. The packet is then acknowledged by the server on success. If the value can not be changed, the server returns an error (e.g. BRLAPI_ERROR_READONLY_PARAMETER). 7.4.18. BRLAPI_PACKET_PARAM_VALUE This packet is sent asynchronously by the server to provide an update of a value of a parameter. This is sent only if the client has previously sent a BRLAPI_PACKET_PARAM_RE- QUEST packet with the BRLAPI_PARAMF_SUBSCRIBE for the corresponding parameter. It is structured exactly like a BRLAPI_PACKET_PARAM_VALUE packet. 7.4.19. BRLAPI_PACKET_SYNCHRONIZE This packet is sent by the client and just acknowledged by the server. This allows the client to perform a round-try with the server, thus collecting any pending exception no- tification.