The LCDproc driver API
This chapter describes the driver API of v0.5 of LCDproc.

Overview of Operation
The API defines functions which drivers may implement to provide certain services. If a driver implements a function, the function will be detected by the server. For some optional functions the server core provides default implementations.
The API consists of functions to:
print data to the display,
set display properties (backlight, brightness, contrast, extra LEDs),
read input from an input device (may be the display), and
inform the server of the driver/display capabilities.
The API is best described by starting with the struct lcd_logical_driver which is defined in server/drivers/lcd.h.
Below is a commented version of lcd_logical_driver. Each function is described more detailed in Functions in Detail.


typedef struct lcd_logical_driver {

	//////// Variables to be provided by the driver module
	// The driver loader will look for symbols with these names !

	// pointer to a string describing the API version
	char *api_version;

	// Does this driver require to be in foreground ?
	int *stay_in_foreground;

	/ Does this driver support multiple instances ?
	int *supports_multiple;

	// What should alternatively be prepended to the function names ?
	char **symbol_prefix;

	//////// Functions to be provided by the driver module

	//// Mandatory functions (necessary for all drivers)

	// initialize driver: returns >= 0 on success
	int (*init)		(Driver *drvthis);

	// close driver
	void (*close)		(Driver *drvthis);


	//// Essential output functions (necessary for output drivers)

	// get display width / height (in characters; 1-based)
	int (*width)		(Driver *drvthis);
	int (*height)		(Driver *drvthis);

	// clear screen
	void (*clear)		(Driver *drvthis);

	// flush screen contents to LCD
	void (*flush)		(Driver *drvthis);

	// write string s at position (x,y)
	void (*string)		(Driver *drvthis, int x, int y, const char *str);

	// write char c at position (x,y)
	void (*chr)		(Driver *drvthis, int x, int y, char c);


	//// essential input functions (necessary for input drivers)

	// get key from driver: returns a string denoting the key pressed
	const char *(*get_key)	(Driver *drvthis);


	//// Extended output functions (optional; core provides alternatives)

	// draw a bar from pos (x,y) upward / to the right filling promille of len chars
	void (*vbar)		(Driver *drvthis, int x, int y, int len, int promille, int options);
	void (*hbar)		(Driver *drvthis, int x, int y, int len, int promille, int options);

	// display (big) number num at horizontal position x
	void (*num)		(Driver *drvthis, int x, int num);

	// set heartbeat state; animate heartbeat
	void (*heartbeat)	(Driver *drvthis, int state);

	// draw named icon at position (x,y)
	int (*icon)		(Driver *drvthis, int x, int y, int icon);

	// set cursor type and move it to position (x,y)
	void (*cursor)		(Driver *drvthis, int x, int y, int type);


	//// User-defined character functions

	// set special character / get free characters
	// - It is currently unclear how this system should work exactly
	// - The set_char function expects a simple block of data with 1 byte for each pixel-line.
	//   (So that is 8 bytes for a 5x8 char)
	void (*set_char)	(Driver *drvthis, int n, unsigned char *dat);
	int (*get_free_chars)	(Driver *drvthis);

	// get width / height of a character cell (in pixels)
	// - necessary to provide info about cell size to clients
	// - if not defined, the core will provide alternatives returning default values
	int (*cellwidth)	(Driver *drvthis);
	int (*cellheight)	(Driver *drvthis);


	//// Hardware functions

	// get / set the display's contrast
	int (*get_contrast)	(Driver *drvthis);
	void (*set_contrast)	(Driver *drvthis, int promille);

	// get / set brightness for given backlight state
	int (*get_brightness)	(Driver *drvthis, int state);
	void (*set_brightness)	(Driver *drvthis, int state, int promille);

	// set backlight state
	void (*backlight)	(Driver *drvthis, int state);

	// set output
	void (*output)		(Driver *drvthis, int state);


	//// Informational functions
	// get a string describing the driver and it's features
	const char * (*get_info) (Driver *drvthis);



	//////// Variables in server core, available for drivers

	// name of the driver instance (name of the config file section)
	// - do not change from the driver; consider it read-only
	// - to be used to access the driver's own section in the config file
	char * name;

	// pointer to the driver instance's private data
	// - filled by the server by calling store_private_ptr()
	// - the driver should cast this to it's own private structure pointer
	void * private_data;


	//////// Functions in server core, available for drivers

	// store a pointer to the driver instance's private data
	int (*store_private_ptr) (struct lcd_logical_driver * driver, void * private_data);

	// Config file functions, cwprovided by the server
	// - see configfile.h on how to use these functions
	// - as sectionname, always use the driver name: drvthis->name
	short (*config_get_bool) (char * sectionname, char * keyname,
				int skip, short default_value);
	long int (*config_get_int) (char * sectionname, char * keyname,
				int skip, long int default_value);
	double (*config_get_float) (char * sectionname, char * keyname,
				int skip, double default_value);
	const char *(*config_get_string) (char * sectionname, char * keyname,
				int skip, const char * default_value);
				// Returns a string in server memory space.
				// Copy this string.
	int config_has_section	(const char *sectionname);
	int config_has_key	(const char *sectionname, const char *keyname);

	// Display properties functions (for drivers that adapt to other loaded drivers)
	// - the return the size of another already loaded driver
	// - if no driver is loaded yet, the return values will be 0
	int (*get_display_width) ();
	int (*get_display_height) ();
} Driver;

Private Data
To support multiple instances it is necessary to stop using global variables to store a driver's data in. Instead, you should store it in a structure, that you allocate and store on driver's init.
In the driver's private structure will probably at least be something like:

typedef struct MyDriver_private_data {
	int fd;				// file descriptor for the LCD device
        int width, height;		// dimension of the LCD (in characters, 1-based
        int cellwidth, cellheight;	// Size of each LCD cell, in pixels
        unsigned char *framebuf;	// Frame buffer...
} PrivateData;

You allocate and store this structure like this:

	PrivateData *p;

	// Allocate and store private data
	p = (PrivateData *) malloc(sizeof(PrivateData));
	if (p == NULL)
		return -1;
	if (drvthis->store_private_ptr( drvthis, p ) < 0)
		return -1;

	// initialize private data
	p->fd = -1;
	p->cellheight = 8;
	p->cellwidth = 6;

	(... continue with the rest of your init routine)

You retrieve this private data pointer by adding the following code to the beginning of your functions:

	PrivateData *p = (PrivateData *) drvthis->private_data;

Then you can access your data like:

	p->framebuf

Functions in Detail
int (*init) Driver *drvthis
The init() function. It starts up the LCD, initializes all variables, allocates private data space and stores the pointer by calling store_private_ptr(); Returns <0 on error.
void (*close) Driver *drvthis
Shut down the connection with the LCD. Called just before unloading the driver.
int (*width) Driver *drvthis
Get the screen width in characters. The result is 1-based.
int (*height) Driver *drvthis
Get the screen height in character lines. The result is 1-based.
void (*clear) Driver *drvthis
Clear the framebuffer.
void (*flush) Driver *drvthis
Flush the framebuffer to the LCD.
void (*string) Driver *drvthis int x int y const char *str
Place string str into position (x,y) in the framebuffer. All coordinates are 1-based, i.e. (1,1) is top left. The driver should check for overflows, i.e. that the positional parameters are within the screen's boundaries and cut off the part of the string that is out of bounds.
void (*chr) Driver *drvthis int x int y char c
Place a single character c into position (x,y) in the framebuffer. The driver should check for overflows, i.e. that the positional parameters are within the screen's boundaries and ignore the request if the character is out of bounds.
void (*vbar) Driver *drvthis int x int y int len int promille int options
Draw a vertical bar at position (x,y) that has maximal length len, where a fraction of (promille / 1000) is filled.
void (*hbar) Driver *drvthis int x int y int len int promille int options
Draw a horizontal bar at position (x,y) that has maximal length len, where a fraction of (promille / 1000) is filled.
void (*num) Driver *drvthis int x int num
Display big number num at horizontal position x.
void (*heartbeat) Driver *drvthis int state
Sets the heartbeat to the indicated state: 0=off, 1=on. Use HEARTBEAT_ON to say that we want to display/refresh the heartbeat. The driver choose how to do it.
int (*icon) Driver *drvthis int x int y int icon
Draw named icon icon at position (x,y). If the driver returns -1 the server core will draw an appropriate replacement character.
void (*cursor) Driver *drvthis int x int y int type
Move cursor to position (x,y), setting its type to type.
void (*set_char) Driver *drvthis char ch unsigned char *dat
The set_char function expects a simple block of data with 1 byte for each pixel-line. (So that is 8 bytes for a 5x8 char)
int (*get_free_chars) Driver *drvthis
Get total number of custom characters available.
int (*cellwidth) Driver *drvthis
Return the width of a character cell in pixels. The result is 1-based.
int (*cellheight) Driver *drvthis
Return the height of a character cell in pixels. The result is 1-based.
int (*get_contrast) Driver *drvthis
Get the contrast value from the driver. The return value is an integer in the range from 0 to 1000. Many displays do not support getting or setting contrast using software.
int (*set_contrast) Driver *drvthis int promille
Sets the contrast to the given value, which is an integer in the range from 0 to 1000. It is up to the driver to map the logical interval [0, 1000] into the interval that the hardware supports. Many displays do not support software setting of contrast.
int (*get_brightness) Driver *drvthis int state
Get the brightness value from the driver for the given backlight state. The parameter state determines which one is returned. The return value is an integer in the range from 0 to 1000. Many displays do not support getting or setting brightness using software.
int (*set_brightness) Driver *drvthis int state int promille
Set the brightness for the given backlight state to the value given. Value must be an integer in the range from 0 to 1000. It is up to the driver to map the logical interval [0, 1000] into the interval that the hardware supports. Many displays do not support software setting of brightness.
void (*backlight) Driver *drvthis int state
Sets the backlight to the given brightness state. Often hardware can only support two values for the backlight: on and off. In that case any value of state > 0 will switch the backlight on.
void (*output) Driver *drvthis int state
Sets the output value. Some displays/wirings have a general purpose output, which can be controlled by calling this function. See the 'output' command in the 'widget language'.
const char *(*get_key) Driver *drvthis
Checks if a key has been pressed on the device. Returns NULL for "no key pressed", or a string describing the pressed key. These characters should match the keypad-layout.
const char *(*get_info) Driver *drvthis
Returns a string describing the driver and its features.
short (*config_get_bool) char *sectionname char *keyname int skip short default_value
Call to server. Retrieve a bool from the config file. Sectionname should be the name of the driver (as in the struct). If the key cannot be found, the default value will be returned. skip should be 0 usually, but if you want to retrieve multiple identical keys, then increase skip to get every next value.
long int (*config_get_int) char *sectionname char *keyname int skip long int default_value
Call to server. Retrieve an integer from the config file.
double (*config_get_float) char *sectionname char *keyname int skip double default_value
Call to server. Retrieve a float from the config file.
const char *(*config_get_string) char *sectionname char *keyname int skip const char *default
Call to server. Retrieve a string from the config file. Fill result with a pointer to some available space. You can fill it with a default value. If the key is found, it will be overwritten with the value from the key. Note that you should always first copy the the returned string. It is in the address space of the server, and will be freed at the next call.
int config_has_section const char *sectionname
Returns whether a section exists. Does not need to be called prior to a call to a config_get_* function.
int config_has_key const char *sectionname const char *keyname
Returns the number of times a key exists. Does not need to be called prior to a call to a config_get_* function.

First version, Joris Robijn, 20011016
Corrected and expanded, Peter Marschall 20060411
Sync'd with lcd.h, Markus Dolze, 20090322