X Version 11, Release 7.7
Copyright © 1993 Network Computing Devices
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Copyright © 1993, 1994 X Consortium
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July 27, 1991
Table of Contents
This document has not been updated since X11R6, and is likely to be somewhat out of date for the current libXfont.
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:
“Definition of the Porting Layer for the X v11 Sample Server” for a discussion on how this library interacts with the X server
“Font Server Implementation Overview” which discusses the design of the font server.
“Bitmap Distribution Format” which covers the contents of the bitmap font files which this library reads; although the library is capable of reading other formats as well, including non-bitmap fonts.
“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 documentation, this relies heavily on the source code, so have a listing handy.
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 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 internally, 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 suspended 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.
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.
At the center of the general font access methods used by X and xfs 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;
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 FPEFunctionsRec. Each function is described below in turn.
Each new font path member is passed to this function; if
the return value is Successful, then
the FPE recognises the format 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 corrupted, this will
not be detected until the font path is initialized. This
routine never returns Suspended.
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, otherwise the return value indicates the
source of the problem. This routine never returns
Suspended.
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 internal directory structures would be more complicated than simply having destroying the old and creating a new.
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.
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 specifies 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 portions are loaded. Otherwise, this routine may
return Suspended. When the presented
font name is actually an alias for some other font name,
FontNameAlias is returned, and the
actual font name is stored in the location pointed to by the
alias argument as a
null-terminated string.
When the server is finished with a font, this routine disposes of any internal state and frees the font data structure.
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.
This routine sets any internal state for a verbose listing of all fonts matching the specified pattern. This routine may return Suspended.
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,
FontNameAlias 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-terminated 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 routine may return
Suspended.
Whenever an FPE function has returned
Suspended, this routine is called
whenever the application wakes up from waiting for input
(from select(2)). This
mask argument should be
the value returned from select(2).
The data structure which actually contains the font information 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 different 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;
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 format 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; /* ... */
} 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 devPrivatesmechanism.
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 dramatically. Each of the routines
is described below.
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.
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.
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.
This is called from the FPE routine
(*close_font), and so should not ever be
called from the application.