xref: /xsrc/external/mit/fontconfig/dist/doc/fontconfig-user.txt

                                   fonts-conf

Name

   fonts.conf -- Font configuration files

Synopsis

      /etc/fonts/fonts.conf
      /etc/fonts/fonts.dtd
      /etc/fonts/conf.d
      $XDG_CONFIG_HOME/fontconfig/conf.d
      $XDG_CONFIG_HOME/fontconfig/fonts.conf
      ~/.fonts.conf.d
      ~/.fonts.conf

Description

   Fontconfig is a library designed to provide system-wide font
   configuration, customization and application access.

Functional Overview

   Fontconfig contains two essential modules, the configuration module which
   builds an internal configuration from XML files and the matching module
   which accepts font patterns and returns the nearest matching font.

  Font Configuration

   The configuration module consists of the FcConfig datatype, libexpat and
   FcConfigParse which walks over an XML tree and amends a configuration with
   data found within. From an external perspective, configuration of the
   library consists of generating a valid XML tree and feeding that to
   FcConfigParse. The only other mechanism provided to applications for
   changing the running configuration is to add fonts and directories to the
   list of application-provided font files.

   The intent is to make font configurations relatively static, and shared by
   as many applications as possible. It is hoped that this will lead to more
   stable font selection when passing names from one application to another.
   XML was chosen as a configuration file format because it provides a format
   which is easy for external agents to edit while retaining the correct
   structure and syntax.

   Font configuration is separate from font matching; applications needing to
   do their own matching can access the available fonts from the library and
   perform private matching. The intent is to permit applications to pick and
   choose appropriate functionality from the library instead of forcing them
   to choose between this library and a private configuration mechanism. The
   hope is that this will ensure that configuration of fonts for all
   applications can be centralized in one place. Centralizing font
   configuration will simplify and regularize font installation and
   customization.

  Font Properties

   While font patterns may contain essentially any properties, there are some
   well known properties with associated types. Fontconfig uses some of these
   properties for font matching and font completion. Others are provided as a
   convenience for the applications' rendering mechanism.

  Property        Type    Description
  --------------------------------------------------------------
  family          String  Font family names
  familylang      String  Languages corresponding to each family
  style           String  Font style. Overrides weight and slant
  stylelang       String  Languages corresponding to each style
  fullname        String  Font full names (often includes style)
  fullnamelang    String  Languages corresponding to each fullname
  slant           Int     Italic, oblique or roman
  weight          Int     Light, medium, demibold, bold or black
  size            Double  Point size
  width           Int     Condensed, normal or expanded
  aspect          Double  Stretches glyphs horizontally before hinting
  pixelsize       Double  Pixel size
  spacing         Int     Proportional, dual-width, monospace or charcell
  foundry         String  Font foundry name
  antialias       Bool    Whether glyphs can be antialiased
  hinting         Bool    Whether the rasterizer should use hinting
  hintstyle       Int     Automatic hinting style
  verticallayout  Bool    Use vertical layout
  autohint        Bool    Use autohinter instead of normal hinter
  globaladvance   Bool    Use font global advance data (deprecated)
  file            String  The filename holding the font
  index           Int     The index of the font within the file
  ftface          FT_Face Use the specified FreeType face object
  rasterizer      String  Which rasterizer is in use (deprecated)
  outline         Bool    Whether the glyphs are outlines
  scalable        Bool    Whether glyphs can be scaled
  color           Bool    Whether any glyphs have color
  scale           Double  Scale factor for point->pixel conversions (deprecated)
  dpi             Double  Target dots per inch
  rgba            Int     unknown, rgb, bgr, vrgb, vbgr,
                          none - subpixel geometry
  lcdfilter       Int     Type of LCD filter
  minspace        Bool    Eliminate leading from line spacing
  charset         CharSet Unicode chars encoded by the font
  lang            String  List of RFC-3066-style languages this
                          font supports
  fontversion     Int     Version number of the font
  capability      String  List of layout capabilities in the font
  fontformat      String  String name of the font format
  embolden        Bool    Rasterizer should synthetically embolden the font
  embeddedbitmap  Bool    Use the embedded bitmap instead of the outline
  decorative      Bool    Whether the style is a decorative variant
  fontfeatures    String  List of the feature tags in OpenType to be enabled
  namelang        String  Language name to be used for the default value of
                          familylang, stylelang, and fullnamelang
  prgname         String  String  Name of the running program
  postscriptname  String  Font family name in PostScript


  Font Matching

   Fontconfig performs matching by measuring the distance from a provided
   pattern to all of the available fonts in the system. The closest matching
   font is selected. This ensures that a font will always be returned, but
   doesn't ensure that it is anything like the requested pattern.

   Font matching starts with an application constructed pattern. The desired
   attributes of the resulting font are collected together in a pattern. Each
   property of the pattern can contain one or more values; these are listed
   in priority order; matches earlier in the list are considered "closer"
   than matches later in the list.

   The initial pattern is modified by applying the list of editing
   instructions specific to patterns found in the configuration; each
   consists of a match predicate and a set of editing operations. They are
   executed in the order they appeared in the configuration. Each match
   causes the associated sequence of editing operations to be applied.

   After the pattern has been edited, a sequence of default substitutions are
   performed to canonicalize the set of available properties; this avoids the
   need for the lower layers to constantly provide default values for various
   font properties during rendering.

   The canonical font pattern is finally matched against all available fonts.
   The distance from the pattern to the font is measured for each of several
   properties: foundry, charset, family, lang, spacing, pixelsize, style,
   slant, weight, antialias, rasterizer and outline. This list is in priority
   order -- results of comparing earlier elements of this list weigh more
   heavily than later elements.

   There is one special case to this rule; family names are split into two
   bindings; strong and weak. Strong family names are given greater
   precedence in the match than lang elements while weak family names are
   given lower precedence than lang elements. This permits the document
   language to drive font selection when any document specified font is
   unavailable.

   The pattern representing that font is augmented to include any properties
   found in the pattern but not found in the font itself; this permits the
   application to pass rendering instructions or any other data through the
   matching system. Finally, the list of editing instructions specific to
   fonts found in the configuration are applied to the pattern. This modified
   pattern is returned to the application.

   The return value contains sufficient information to locate and rasterize
   the font, including the file name, pixel size and other rendering data. As
   none of the information involved pertains to the FreeType library,
   applications are free to use any rasterization engine or even to take the
   identified font file and access it directly.

   The match/edit sequences in the configuration are performed in two passes
   because there are essentially two different operations necessary -- the
   first is to modify how fonts are selected; aliasing families and adding
   suitable defaults. The second is to modify how the selected fonts are
   rasterized. Those must apply to the selected font, not the original
   pattern as false matches will often occur.

  Font Names

   Fontconfig provides a textual representation for patterns that the library
   can both accept and generate. The representation is in three parts, first
   a list of family names, second a list of point sizes and finally a list of
   additional properties:

           <families>-<point sizes>:<name1>=<values1>:<name2>=<values2>...


   Values in a list are separated with commas. The name needn't include
   either families or point sizes; they can be elided. In addition, there are
   symbolic constants that simultaneously indicate both a name and a value.
   Here are some examples:

     Name                            Meaning
     ----------------------------------------------------------
     Times-12                        12 point Times Roman
     Times-12:bold                   12 point Times Bold
     Courier:italic                  Courier Italic in the default size
     Monospace:matrix=1 .1 0 1       The users preferred monospace font
                                     with artificial obliquing


   The '\', '-', ':' and ',' characters in family names must be preceded by a
   '\' character to avoid having them misinterpreted. Similarly, values
   containing '\', '=', '_', ':' and ',' must also have them preceded by a
   '\' character. The '\' characters are stripped out of the family name and
   values as the font name is read.

Debugging Applications

   To help diagnose font and applications problems, fontconfig is built with
   a large amount of internal debugging left enabled. It is controlled by
   means of the FC_DEBUG environment variable. The value of this variable is
   interpreted as a number, and each bit within that value controls different
   debugging messages.

     Name         Value    Meaning
     ---------------------------------------------------------
     MATCH            1    Brief information about font matching
     MATCHV           2    Extensive font matching information
     EDIT             4    Monitor match/test/edit execution
     FONTSET          8    Track loading of font information at startup
     CACHE           16    Watch cache files being written
     CACHEV          32    Extensive cache file writing information
     PARSE           64    (no longer in use)
     SCAN           128    Watch font files being scanned to build caches
     SCANV          256    Verbose font file scanning information
     MEMORY         512    Monitor fontconfig memory usage
     CONFIG        1024    Monitor which config files are loaded
     LANGSET       2048    Dump char sets used to construct lang values
     MATCH2        4096    Display font-matching transformation in patterns


   Add the value of the desired debug levels together and assign that (in
   base 10) to the FC_DEBUG environment variable before running the
   application. Output from these statements is sent to stdout.

Lang Tags

   Each font in the database contains a list of languages it supports. This
   is computed by comparing the Unicode coverage of the font with the
   orthography of each language. Languages are tagged using an RFC-3066
   compatible naming and occur in two parts -- the ISO 639 language tag
   followed a hyphen and then by the ISO 3166 country code. The hyphen and
   country code may be elided.

   Fontconfig has orthographies for several languages built into the library.
   No provision has been made for adding new ones aside from rebuilding the
   library. It currently supports 122 of the 139 languages named in ISO
   639-1, 141 of the languages with two-letter codes from ISO 639-2 and
   another 30 languages with only three-letter codes. Languages with both two
   and three letter codes are provided with only the two letter code.

   For languages used in multiple territories with radically different
   character sets, fontconfig includes per-territory orthographies. This
   includes Azerbaijani, Kurdish, Pashto, Tigrinya and Chinese.

Configuration File Format

   Configuration files for fontconfig are stored in XML format; this format
   makes external configuration tools easier to write and ensures that they
   will generate syntactically correct configuration files. As XML files are
   plain text, they can also be manipulated by the expert user using a text
   editor.

   The fontconfig document type definition resides in the external entity
   "fonts.dtd"; this is normally stored in the default font configuration
   directory (/etc/fonts). Each configuration file should contain the
   following structure:

           <?xml version="1.0"?>
           <!DOCTYPE fontconfig SYSTEM "fonts.dtd">
           <fontconfig>
           ...
           </fontconfig>


  <fontconfig>

   This is the top level element for a font configuration and can contain
   <dir>, <cachedir>, <include>, <match> and <alias> elements in any order.

  <dir prefix="default">

   This element contains a directory name which will be scanned for font
   files to include in the set of available fonts. If 'prefix' is set to
   "xdg", the value in the XDG_DATA_HOME environment variable will be added
   as the path prefix. please see XDG Base Directory Specification for more
   details.

  <cachedir prefix="default">

   This element contains a directory name that is supposed to be stored or
   read the cache of font information. If multiple elements are specified in
   the configuration file, the directory that can be accessed first in the
   list will be used to store the cache files. If it starts with '~', it
   refers to a directory in the users home directory. If 'prefix' is set to
   "xdg", the value in the XDG_CACHE_HOME environment variable will be added
   as the path prefix. please see XDG Base Directory Specification for more
   details. The default directory is ``$XDG_CACHE_HOME/fontconfig'' and it
   contains the cache files named ``<hash
   value>-<architecture>.cache-<version>'', where <version> is the fontconfig
   cache file version number (currently 7).

  <include ignore_missing="no" prefix="default">

   This element contains the name of an additional configuration file or
   directory. If a directory, every file within that directory starting with
   an ASCII digit (U+0030 - U+0039) and ending with the string ``.conf'' will
   be processed in sorted order. When the XML datatype is traversed by
   FcConfigParse, the contents of the file(s) will also be incorporated into
   the configuration by passing the filename(s) to FcConfigLoadAndParse. If
   'ignore_missing' is set to "yes" instead of the default "no", a missing
   file or directory will elicit no warning message from the library. If
   'prefix' is set to "xdg", the value in the XDG_CONFIG_HOME environment
   variable will be added as the path prefix. please see XDG Base Directory
   Specification for more details.

  <config>

   This element provides a place to consolidate additional configuration
   information. <config> can contain <blank> and <rescan> elements in any
   order.

  <blank>

   Fonts often include "broken" glyphs which appear in the encoding but are
   drawn as blanks on the screen. Within the <blank> element, place each
   Unicode characters which is supposed to be blank in an <int> element.
   Characters outside of this set which are drawn as blank will be elided
   from the set of characters supported by the font.

  <rescan>

   The <rescan> element holds an <int> element which indicates the default
   interval between automatic checks for font configuration changes.
   Fontconfig will validate all of the configuration files and directories
   and automatically rebuild the internal datastructures when this interval
   passes.

  <selectfont>

   This element is used to black/white list fonts from being listed or
   matched against. It holds acceptfont and rejectfont elements.

  <acceptfont>

   Fonts matched by an acceptfont element are "whitelisted"; such fonts are
   explicitly included in the set of fonts used to resolve list and match
   requests; including them in this list protects them from being
   "blacklisted" by a rejectfont element. Acceptfont elements include glob
   and pattern elements which are used to match fonts.

  <rejectfont>

   Fonts matched by an rejectfont element are "blacklisted"; such fonts are
   excluded from the set of fonts used to resolve list and match requests as
   if they didn't exist in the system. Rejectfont elements include glob and
   pattern elements which are used to match fonts.

  <glob>

   Glob elements hold shell-style filename matching patterns (including ? and
   *) which match fonts based on their complete pathnames. This can be used
   to exclude a set of directories (/usr/share/fonts/uglyfont*), or
   particular font file types (*.pcf.gz), but the latter mechanism relies
   rather heavily on filenaming conventions which can't be relied upon. Note
   that globs only apply to directories, not to individual fonts.

  <pattern>

   Pattern elements perform list-style matching on incoming fonts; that is,
   they hold a list of elements and associated values. If all of those
   elements have a matching value, then the pattern matches the font. This
   can be used to select fonts based on attributes of the font (scalable,
   bold, etc), which is a more reliable mechanism than using file extensions.
   Pattern elements include patelt elements.

  <patelt name="property">

   Patelt elements hold a single pattern element and list of values. They
   must have a 'name' attribute which indicates the pattern element name.
   Patelt elements include int, double, string, matrix, bool, charset and
   const elements.

  <match target="pattern">

   This element holds first a (possibly empty) list of <test> elements and
   then a (possibly empty) list of <edit> elements. Patterns which match all
   of the tests are subjected to all the edits. If 'target' is set to "font"
   instead of the default "pattern", then this element applies to the font
   name resulting from a match rather than a font pattern to be matched. If
   'target' is set to "scan", then this element applies when the font is
   scanned to build the fontconfig database.

  <test qual="any" name="property" target="default" compare="eq">

   This element contains a single value which is compared with the target
   ('pattern', 'font', 'scan' or 'default') property "property" (substitute
   any of the property names seen above). 'compare' can be one of "eq",
   "not_eq", "less", "less_eq", "more", "more_eq", "contains" or
   "not_contains". 'qual' may either be the default, "any", in which case the
   match succeeds if any value associated with the property matches the test
   value, or "all", in which case all of the values associated with the
   property must match the test value. 'ignore-blanks' takes a boolean value.
   if 'ignore-blanks' is set "true", any blanks in the string will be ignored
   on its comparison. this takes effects only when compare="eq" or
   compare="not_eq". When used in a <match target="font"> element, the
   target= attribute in the <test> element selects between matching the
   original pattern or the font. "default" selects whichever target the outer
   <match> element has selected.

  <edit name="property" mode="assign" binding="weak">

   This element contains a list of expression elements (any of the value or
   operator elements). The expression elements are evaluated at run-time and
   modify the property "property". The modification depends on whether
   "property" was matched by one of the associated <test> elements, if so,
   the modification may affect the first matched value. Any values inserted
   into the property are given the indicated binding ("strong", "weak" or
   "same") with "same" binding using the value from the matched pattern
   element. 'mode' is one of:

     Mode                    With Match              Without Match
     ---------------------------------------------------------------------
     "assign"                Replace matching value  Replace all values
     "assign_replace"        Replace all values      Replace all values
     "prepend"               Insert before matching  Insert at head of list
     "prepend_first"         Insert at head of list  Insert at head of list
     "append"                Append after matching   Append at end of list
     "append_last"           Append at end of list   Append at end of list
     "delete"                Delete matching value   Delete all values
     "delete_all"            Delete all values       Delete all values


  <int>, <double>, <string>, <bool>

   These elements hold a single value of the indicated type. <bool> elements
   hold either true or false. An important limitation exists in the parsing
   of floating point numbers -- fontconfig requires that the mantissa start
   with a digit, not a decimal point, so insert a leading zero for purely
   fractional values (e.g. use 0.5 instead of .5 and -0.5 instead of -.5).

  <matrix>

   This element holds four numerical expressions of an affine transformation.
   At their simplest these will be four <double> elements but they can also
   be more involved expressions.

  <range>

   This element holds the two <int> elements of a range representation.

  <charset>

   This element holds at least one <int> element of an Unicode code point or
   more.

  <langset>

   This element holds at least one <string> element of a RFC-3066-style
   languages or more.

  <name>

   Holds a property name. Evaluates to the first value from the property of
   the pattern. If the 'target' attribute is not present, it will default to
   'default', in which case the property is returned from the font pattern
   during a target="font" match, and to the pattern during a target="pattern"
   match. The attribute can also take the values 'font' or 'pattern' to
   explicitly choose which pattern to use. It is an error to use a target of
   'font' in a match that has target="pattern".

  <const>

   Holds the name of a constant; these are always integers and serve as
   symbolic names for common font values:

     Constant        Property        Value
     -------------------------------------
     thin            weight          0
     extralight      weight          40
     ultralight      weight          40
     light           weight          50
     demilight       weight          55
     semilight       weight          55
     book            weight          75
     regular         weight          80
     normal          weight          80
     medium          weight          100
     demibold        weight          180
     semibold        weight          180
     bold            weight          200
     extrabold       weight          205
     black           weight          210
     heavy           weight          210
     roman           slant           0
     italic          slant           100
     oblique         slant           110
     ultracondensed  width           50
     extracondensed  width           63
     condensed       width           75
     semicondensed   width           87
     normal          width           100
     semiexpanded    width           113
     expanded        width           125
     extraexpanded   width           150
     ultraexpanded   width           200
     proportional    spacing         0
     dual            spacing         90
     mono            spacing         100
     charcell        spacing         110
     unknown         rgba            0
     rgb             rgba            1
     bgr             rgba            2
     vrgb            rgba            3
     vbgr            rgba            4
     none            rgba            5
     lcdnone         lcdfilter       0
     lcddefault      lcdfilter       1
     lcdlight        lcdfilter       2
     lcdlegacy       lcdfilter       3
     hintnone        hintstyle       0
     hintslight      hintstyle       1
     hintmedium      hintstyle       2
     hintfull        hintstyle       3


  <or>, <and>, <plus>, <minus>, <times>, <divide>

   These elements perform the specified operation on a list of expression
   elements. <or> and <and> are boolean, not bitwise.

  <eq>, <not_eq>, <less>, <less_eq>, <more>, <more_eq>, <contains>,
  <not_contains

   These elements compare two values, producing a boolean result.

  <not>

   Inverts the boolean sense of its one expression element

  <if>

   This element takes three expression elements; if the value of the first is
   true, it produces the value of the second, otherwise it produces the value
   of the third.

  <alias>

   Alias elements provide a shorthand notation for the set of common match
   operations needed to substitute one font family for another. They contain
   a <family> element followed by optional <prefer>, <accept> and <default>
   elements. Fonts matching the <family> element are edited to prepend the
   list of <prefer>ed families before the matching <family>, append the
   <accept>able families after the matching <family> and append the <default>
   families to the end of the family list.

  <family>

   Holds a single font family name

  <prefer>, <accept>, <default>

   These hold a list of <family> elements to be used by the <alias> element.

EXAMPLE CONFIGURATION FILE

  System configuration file

   This is an example of a system-wide configuration file

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<!-- /etc/fonts/fonts.conf file to configure system font access -->
<fontconfig>
<!--
        Find fonts in these directories
-->
<dir>/usr/share/fonts</dir>
<dir>/usr/X11R6/lib/X11/fonts</dir>

<!--
        Accept deprecated 'mono' alias, replacing it with 'monospace'
-->
<match target="pattern">
        <test qual="any" name="family"><string>mono</string></test>
        <edit name="family" mode="assign"><string>monospace</string></edit>
</match>

<!--
        Names not including any well known alias are given 'sans-serif'
-->
<match target="pattern">
        <test qual="all" name="family" compare="not_eq"><string>sans-serif</string></test>
        <test qual="all" name="family" compare="not_eq"><string>serif</string></test>
        <test qual="all" name="family" compare="not_eq"><string>monospace</string></test>
        <edit name="family" mode="append_last"><string>sans-serif</string></edit>
</match>

<!--
        Load per-user customization file, but don't complain
        if it doesn't exist
-->
<include ignore_missing="yes" prefix="xdg">fontconfig/fonts.conf</include>

<!--
        Load local customization files, but don't complain
        if there aren't any
-->
<include ignore_missing="yes">conf.d</include>
<include ignore_missing="yes">local.conf</include>

<!--
        Alias well known font names to available TrueType fonts.
        These substitute TrueType faces for similar Type1
        faces to improve screen appearance.
-->
<alias>
        <family>Times</family>
        <prefer><family>Times New Roman</family></prefer>
        <default><family>serif</family></default>
</alias>
<alias>
        <family>Helvetica</family>
        <prefer><family>Arial</family></prefer>
        <default><family>sans</family></default>
</alias>
<alias>
        <family>Courier</family>
        <prefer><family>Courier New</family></prefer>
        <default><family>monospace</family></default>
</alias>

<!--
        Provide required aliases for standard names
        Do these after the users configuration file so that
        any aliases there are used preferentially
-->
<alias>
        <family>serif</family>
        <prefer><family>Times New Roman</family></prefer>
</alias>
<alias>
        <family>sans</family>
        <prefer><family>Arial</family></prefer>
</alias>
<alias>
        <family>monospace</family>
        <prefer><family>Andale Mono</family></prefer>
</alias>

<--
        The example of the requirements of OR operator;
        If the 'family' contains 'Courier New' OR 'Courier'
        add 'monospace' as the alternative
-->
<match target="pattern">
        <test name="family" compare="eq">
                <string>Courier New</string>
        </test>
        <edit name="family" mode="prepend">
                <string>monospace</string>
        </edit>
</match>
<match target="pattern">
        <test name="family" compare="eq">
                <string>Courier</string>
        </test>
        <edit name="family" mode="prepend">
                <string>monospace</string>
        </edit>
</match>

</fontconfig>


  User configuration file

   This is an example of a per-user configuration file that lives in
   $XDG_CONFIG_HOME/fontconfig/fonts.conf

 <?xml version="1.0"?>
 <!DOCTYPE fontconfig SYSTEM "fonts.dtd">
 <!-- $XDG_CONFIG_HOME/fontconfig/fonts.conf for per-user font configuration -->
 <fontconfig>

 <!--
         Private font directory
 -->
 <dir prefix="xdg">fonts</dir>

 <!--
         use rgb sub-pixel ordering to improve glyph appearance on
         LCD screens.  Changes affecting rendering, but not matching
         should always use target="font".
 -->
 <match target="font">
         <edit name="rgba" mode="assign"><const>rgb</const></edit>
 </match>
 <!--
         use WenQuanYi Zen Hei font when serif is requested for Chinese
 -->
 <match>
         <!--
                 If you don't want to use WenQuanYi Zen Hei font for zh-tw etc,
                 you can use zh-cn instead of zh.
                 Please note, even if you set zh-cn, it still matches zh.
                 if you don't like it, you can use compare="eq"
                 instead of compare="contains".
         -->
         <test name="lang" compare="contains">
                 <string>zh</string>
         </test>
         <test name="family">
                 <string>serif</string>
         </test>
         <edit name="family" mode="prepend">
                 <string>WenQuanYi Zen Hei</string>
         </edit>
 </match>
 <!--
         use VL Gothic font when sans-serif is requested for Japanese
 -->
 <match>
         <test name="lang" compare="contains">
                 <string>ja</string>
         </test>
         <test name="family">
                 <string>sans-serif</string>
         </test>
         <edit name="family" mode="prepend">
                 <string>VL Gothic</string>
         </edit>
 </match>
 </fontconfig>


Files

   fonts.conf contains configuration information for the fontconfig library
   consisting of directories to look at for font information as well as
   instructions on editing program specified font patterns before attempting
   to match the available fonts. It is in XML format.

   conf.d is the conventional name for a directory of additional
   configuration files managed by external applications or the local
   administrator. The filenames starting with decimal digits are sorted in
   lexicographic order and used as additional configuration files. All of
   these files are in XML format. The master fonts.conf file references this
   directory in an <include> directive.

   fonts.dtd is a DTD that describes the format of the configuration files.

   $XDG_CONFIG_HOME/fontconfig/conf.d and ~/.fonts.conf.d is the conventional
   name for a per-user directory of (typically auto-generated) configuration
   files, although the actual location is specified in the global fonts.conf
   file. please note that ~/.fonts.conf.d is deprecated now. it will not be
   read by default in the future version.

   $XDG_CONFIG_HOME/fontconfig/fonts.conf and ~/.fonts.conf is the
   conventional location for per-user font configuration, although the actual
   location is specified in the global fonts.conf file. please note that
   ~/.fonts.conf is deprecated now. it will not be read by default in the
   future version.

   $XDG_CACHE_HOME/fontconfig/*.cache-* and ~/.fontconfig/*.cache-* is the
   conventional repository of font information that isn't found in the
   per-directory caches. This file is automatically maintained by fontconfig.
   please note that ~/.fontconfig/*.cache-* is deprecated now. it will not be
   read by default in the future version.

Environment variables

   FONTCONFIG_FILE is used to override the default configuration file.

   FONTCONFIG_PATH is used to override the default configuration directory.

   FONTCONFIG_SYSROOT is used to set a default sysroot directory.

   FC_DEBUG is used to output the detailed debugging messages. see
   [1]Debugging Applications section for more details.

   FC_DBG_MATCH_FILTER is used to filter out the patterns. this takes a
   comma-separated list of object names and effects only when FC_DEBUG has
   MATCH2. see [2]Debugging Applications section for more details.

   FC_LANG is used to specify the default language as the weak binding in the
   query. if this isn't set, the default language will be determined from
   current locale.

   FONTCONFIG_USE_MMAP is used to control the use of mmap(2) for the cache
   files if available. this take a boolean value. fontconfig will checks if
   the cache files are stored on the filesystem that is safe to use mmap(2).
   explicitly setting this environment variable will causes skipping this
   check and enforce to use or not use mmap(2) anyway.

   SOURCE_DATE_EPOCH is used to ensure fc-cache(1) generates files in a
   deterministic manner in order to support reproducible builds. When set to
   a numeric representation of UNIX timestamp, fontconfig will prefer this
   value over using the modification timestamps of the input files in order
   to identify which cache files require regeneration. If SOURCE_DATE_EPOCH
   is not set (or is newer than the mtime of the directory), the existing
   behaviour is unchanged.

See Also

   fc-cat(1), fc-cache(1), fc-list(1), fc-match(1), fc-query(1),
   [3]SOURCE_DATE_EPOCH.

Version

   Fontconfig version 2.13.1

References

   Visible links
   1. file:///tmp/html-cm5uzN#DEBUG
   2. file:///tmp/html-cm5uzN#DEBUG
   3. https://reproducible-builds.org/specs/source-date-epoch/