








		     The X Font Library


		       Keith Packard
		      MIT X Consortium

			David Lemke
		 Network Computing Devices


This document describes the data structures and interfaces
for using the X Font library.  It is intended as a reference
for programmers building X and Font servers.  You may want
to refer to the following documents:

o    "Definition of the Porting Layer for the X v11 Sample
     Server" for a discussion on how this library interacts
     with the X server

o    "Font Server Implementation Overview" which discusses
     the design of the font server.

o    "Bitmap Distribution Format" which covers the contents
     of the bitmap font files which this library reads;
     although the library is capable of reading other for-
     mats as well, including non-bitmap fonts.

o    "The X Font Service Protocol" for a description of the
     constraints placed on the design by including support
     for this font service mechanism.

This document assumes the reader is familiar with the X
server design, the X protocol as it relates to fonts and the
C programming language.  As with most MIT produced documen-
tation, this relies heavily on the source code, so have a
listing handy.

1.  Requirements for the Font library

To avoid miles of duplicate code in the X server, the font
server and the various font manipulation tools, the font
library should provide interfaces appropriate for all of
these tasks.  In particular, the X server and font server
should be able to both use the library to access disk based
fonts, and to communicate with a font server.  By providing
a general library, we hoped to avoid duplicating code
between the X server and font server.

Another requirement is that the X server (or even a font
server) be able to continue servicing requests from other
clients while awaiting a response from the font server on



Font Library Interface	    - 1 -	       July 27, 1991








behalf of one client.  This is the strongest requirement
placed on the font library, and has warped the design in
curious ways.  Because both the X server and font server are
single threaded, the font library must not suspend inter-
nally, rather it returns an indication of suspension to the
application which continues processing other things, until
the font data is ready, at which time it restarts the sus-
pended request.

Because the code for reading and manipulating bitmap font
data is used by the font applications "mkfontdir" and
"bdftopcf", the font library includes bitmap-font specific
interfaces which those applications use, instead of the more
general interfaces used by the X and font servers, which are
unaware of the source of the font data.  These routines will
be refered to as the bitmap font access methods.

2.  General Font Library Interface details.

To avoid collision between the #define name space for
errors, the Font library defines a new set of return values:

     #define AllocError      80
     #define StillWorking    81
     #define FontNameAlias   82
     #define BadFontName     83
     #define Suspended	     84
     #define Successful      85
     #define BadFontPath     86
     #define BadCharRange    87
     #define BadFontFormat   88
     #define FPEResetFailed  89

Whenever a routine returns Suspended, the font library will
notify the caller (via the ClientSignal interface described
below) who should then reinvoke the same routine again with
the same arguments.

3.  Font Path Elements

At the center of the general font access methods used by X
and fs is the Font Path Element data structure.  Like most
structures in the X server, this contains a collection of
data and some function pointers for manipulating this data:

     /* External view of font paths */
     typedef struct _FontPathElement {
	 int	     name_length;
	 char	    *name;
	 int	     type;
	 int	     refcount;
	 pointer     private;
     } FontPathElementRec, *FontPathElementPtr;




Font Library Interface	    - 2 -	       July 27, 1991








     typedef struct _FPEFunctions {
	 int	     (*name_check) ( /* name */ );
	 int	     (*init_fpe) ( /* fpe */ );
	 int	     (*reset_fpe) ( /* fpe */ );
	 int	     (*free_fpe) ( /* fpe */ );
	 int	     (*open_font) (  /* client, fpe, flags,
			     name, namelen, format,
			     fid,  ppfont, alias */ );
	 int	     (*close_font) ( /* pfont */ );
	 int	     (*list_fonts) ( /* client, fpe, pattern,
			     patlen, maxnames, paths */ );
	 int	     (*start_list_fonts_with_info) (
			     /* client, fpe, name, namelen,
				maxnames, data */ );
	 int	     (*list_next_font_with_info) (
			     /* client, fpe, name, namelen,
				info, num, data */ );
	 int	     (*wakeup_fpe) ( /* fpe, mask */ );
	 int	     (*client_died) ( /* client, fpe */ );
     } FPEFunctionsRec, FPEFunctions;

The function pointers are split out from the data structure
to save memory; additionally, this avoids any complications
when initializing the data structure as there would not be
any way to discover the appropriate function to call (a
chicken and egg problem).

When a font path type is initialized, it passes the function
pointers to the server which are then stored in an FPEFunc-
tionsRec.  Each function is described below in turn.

3.1.  (*name_check)

Each new font path member is passed to this function; if the
return value is Successful, then the FPE recognises the for-
mat of the string.  This does not guarantee that the FPE
will be able to successfully use this member.  For example,
the disk-based font directory file "fonts.dir" may be cor-
rupted, this will not be detected until the font path is
initialized.  This routine never returns Suspended.

3.2.  (*init_fpe)

Initialize a new font path element.  This function prepares
a new font path element for other requests:  the disk font
routine reads the "fonts.dir" and "fonts.alias" files into
the internal format, while the font server routine connects
to the requested font server and prepares for using it.
This routine returns Successful if everything went OK, oth-
erwise the return value indicates the source of the problem.
This routine never returns Suspended.






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3.3.  (*reset_fpe)

When the X font path is reset, and some of the new members
are also in the old font path, this function is called to
reinitialize those FPEs.  This routine returns Successful if
everything went OK.  It returns FPEResetFailed if (for some
reason) the reset failed, and the caller should remove the
old FPE and simply create a new one in its place.  This is
used by the disk-based fonts routine as resetting the inter-
nal directory structures would be more complicated than sim-
ply having destroying the old and creating a new.

3.4.  (*free_fpe)

When the server is finished with an FPE, this function is
called to dispose of any internal state.  It should return
Successful, unless something terrible happens.

3.5.  (*open_font)

This routine requests that a font be opened.  The client
argument is used by the font library only in connection with
suspending/restarting the request.  The flags argument spec-
ifies some behaviour for the library and can be any of:

     /* OpenFont flags */
     #define FontLoadInfo    0x0001
     #define FontLoadProps   0x0002
     #define FontLoadMetrics 0x0004
     #define FontLoadBitmaps 0x0008
     #define FontLoadAll     0x000f
     #define FontOpenSync    0x0010

The various fields specify which portions of the font should
be loaded at this time.  When FontOpenSync is specified,
this routine will not return until all of the requested por-
tions are loaded.  Otherwise, this routine may return Sus-
pended.  When the presented font name is actually an alias
for some other font name, FontName Alias is returned, and
the actual font name is stored in the location pointed to by
the alias argument as a null-terminated string.

3.6.  (*close_font)

When the server is finished with a font, this routine dis-
poses of any internal state and frees the font data struc-
ture.

3.7.  (*list_fonts)

The paths argument is a data structure which will be filled
with all of the font names from this directory which match
the specified pattern.	At most maxnames will be added.
This routine may return Suspended.



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3.8.  (*start_list_fonts_with_info)

This routine sets any internal state for a verbose listing
of all fonts matching the specified pattern.  This routine
may return Suspended.

3.9.  (*list_next_font_with_info)

To avoid storing huge amounts of data, the interface for
ListFontsWithInfo allows the server to get one reply at a
time and forward that to the client.  When the font name
returned is actually an alias for some other font, Font-
NameAlias will be returned.  The actual font name is return
instead, and the font alias which matched the pattern is
returned in the location pointed to by data as a null-termi-
nated string.  The caller can then get the information by
recursively listing that font name with a maxnames of 1.
When Successful is returned, the matching font name is
returned, and a FontInfoPtr is stored in the location
pointed to by data.  Data must be initialized with a pointer
to a FontInfoRec allocated by the caller.  When the pointer
pointed to by data is not left pointing at that storage, the
caller mustn't free the associated property data.  This rou-
tine may return Suspended.

3.10.  (*wakeup_fpe)

Whenever an FPE function has returned Suspended, this rou-
tine is called whenever the application wakes up from wait-
ing for input (from select(2)).  This mask argument should
be the value returned from select(2).

3.11.  (*client_died)

When an FPE function has returned Suspended and the associ-
ated client is being destroyed, this function allows the
font library to dispose of any state associated with that
client.

4.  Fonts

The data structure which actually contains the font informa-
tion has changed significantly since previous releases; it
now attempts to hide the actual storage format for the data
from the application, providing accessor functions to get at
the data.  This allows a range of internal details for dif-
ferent font sources.  The structure is split into two
pieces, so that ListFontsWithInfo can share information from
the font when it has been loaded.  The FontInfo structure,
then, contains only information germane to LFWI.

     typedef struct _FontInfo {
	 unsigned short firstCol;	     /* range of glyphs for this font */
	 unsigned short lastCol;



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	 unsigned short firstRow;
	 unsigned short lastRow;
	 unsigned short defaultCh;	     /* default character index */
	 unsigned int noOverlap:1;	     /* no combination of glyphs overlap */
	 unsigned int terminalFont:1;	     /* Character cell font */
	 unsigned int constantMetrics:1;     /* all metrics are the same */
	 unsigned int constantWidth:1;	     /* all character widths are the same*/
	 unsigned int inkInside:1;	     /* all ink inside character cell */
	 unsigned int inkMetrics:1;	     /* font has ink metrics */
	 unsigned int allExist:1;	     /* no missing chars in range */
	 unsigned int drawDirection:2;	     /* left-to-right/right-to-left*/
	 unsigned int cachable:1;	     /* font needn't be opened each time*/
	 unsigned int anamorphic:1;	     /* font is strangely scaled */
	 short	     maxOverlap;	     /* maximum overlap amount */
	 short	     pad;		     /* unused */
	 xCharInfo   maxbounds; 	     /* glyph metrics maximums */
	 xCharInfo   minbounds; 	     /* glyph metrics minimums */
	 xCharInfo   ink_maxbounds;	     /* ink metrics maximums */
	 xCharInfo   ink_minbounds;	     /* ink metrics minimums */
	 short	     fontAscent;	     /* font ascent amount */
	 short	     fontDescent;	     /* font descent amount */
	 int	     nprops;		     /* number of font properties */
	 FontPropPtr props;		     /* font properties */
	 char	    *isStringProp;	     /* boolean array */
     }		 FontInfoRec, *FontInfoPtr;

The font structure, then, contains a font info record, the
format of the bits in each bitmap and the functions which
access the font records (which are stored in an opaque for-
mat hung off of fontPrivate).

     typedef struct _Font {
	 int	     refcnt;
	 FontInfoRec info;
	 char	     bit;		     /* bit order: LSBFirst/MSBFirst */
	 char	     byte;		     /* byte order: LSBFirst/MSBFirst */
	 char	     glyph;		     /* glyph pad: 1, 2, 4 or 8 */
	 char	     scan;		     /* glyph scan unit: 1, 2 or 4 */
	 fsBitmapFormat format; 	     /* FS-style format (packed) */
	 int	     (*get_glyphs) ( /* font, count, chars, encoding, count, glyphs */ );
	 int	     (*get_metrics) ( /* font, count, chars, encoding, count, glyphs */ );
	 int	     (*get_bitmaps) (/* client, font, flags, format,
					flags, nranges, ranges, data_sizep,
					num_glyphsp, offsetsp, glyph_datap,
					free_datap */ );
	 int	     (*get_extents) (/* client, font, flags, nranges,
					 ranges, nextentsp, extentsp */);
	 void	     (*unload_font) ( /* font */ );
	 FontPathElementPtr fpe;	     /* FPE associated with this font */
	 pointer     svrPrivate;	     /* X/FS private data */
	 pointer     fontPrivate;	     /* private to font */
	 pointer     fpePrivate;	     /* private to FPE */
	 int	     maxPrivate;	     /* devPrivates (see below) */
	 pointer     *devPrivates;	     /*  ... */



Font Library Interface	    - 6 -	       July 27, 1991








     }		 FontRec, *FontPtr;

Yes, there are several different private pointers in the
Font structure; they were added haphazardly until the
devPrivate pointers were added.  Future releases may remove
some (or all) of the specific pointers, leaving only the
devPrivates mechanism.

There are two similar interfaces implemented -
get_glyphs/get_metrics and get_bitmaps/get_extents.  Too
little time caused the font-server specific interfaces to be
placed in the font library (and portions duplicated in each
renderer) instead of having them integrated into the font
server itself.	This may change.  The X server uses only
get_glyphs/get_metrics, and those will not change dramati-
cally.	Each of the routines is described below

4.1.  (*get_glyphs)

This routine returns CharInfoPtrs for each of the requested
characters in the font.  If the character does not exist in
the font, the default character will be returned, unless no
default character exists in which case that character is
skipped.  Thus, the number of glyphs returned will not
always be the same as the number of characters passed in.

4.2.  (*get_metrics)

This is similar to (*get_glyphs) except that pointers to
xCharInfo structures are returned, and, if the font has ink
metrics, those are returned instead of the bitmap metrics.

4.3.  (*get-bitmaps)

This packs the glyph image data in the requested format and
returns it.  The ranges/nranges argument specify the set of
glyphs from the font to pack together.

4.4.  (*get_extents)

This returns the metrics for the specified font from the
specified ranges.


4.5.  (*unload_font)

This is called from the FPE routine (*close_font), and so
should not ever be called from the application.

4.6.  maxPrivate

When initializing a new font structure, maxPrivate should be
set to -1 so that the FontSetPrivate() macro works properly
with an index of 0.  Initializing maxPrivate to 0 can cause



Font Library Interface	    - 7 -	       July 27, 1991








problems if the server tries to set something at index 0.
























































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