RISCI_ATOM
/
harvey
镜像自地址 https://github.com/Harvey-OS/harvey.git


			
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<title>Information for Ghostscript developers</title>
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<h1>Information for Ghostscript developers</h1>

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<h2>Table of contents</h2>

<blockquote><ul>
<li><a href="#Introduction">Introduction</a>
<li><a href="#Architecture">Architecture</a>
<ul>
<li><a href="#Design_goals">Design goals</a>
<li><a href="#Design_principles">Design principles</a>
<li><a href="#Large_scale_structure">Large-scale structure</a>
<li><a href="#Object_oriented_constructs">Object-oriented constructs</a>
</ul>
<li><a href="#File_roadmap">File roadmap</a>
<ul>
<li><a href="#Substrate">Substrate</a>
<li><a href="#Graphics_library">Graphics library</a>
<ul>
<li><a href="#Library_support">Support</a>,
    <a href="#Paths">Paths</a>,
    <a href="#Text">Text</a>,
    <a href="#Images">Images</a>,
    <a href="#Paint">Paint</a>,
    <a href="#Clipping">Clipping</a>,
    <a href="#Other_graphics">Other graphics</a>,
    <a href="#Driver_support">Driver support</a>
</ul>
<li><a href="#Device_drivers">Device drivers</a>
<ul>
<li><a href="#Internal_devices">Internal devices</a>,
    <a href="#PS_and_PDF_writers">PostScript and PDF writers</a>,
    <a href="#High_level_devices">Other high-level devices</a>,
    <a href="#Other_maintained_drivers">Other maintained drivers</a>,
    <a href="#Contributed_drivers">Contributed drivers</a>
</ul>
<li><a href="#PostScript_interpreter">PostScript interpreter</a>
<ul>
<li><a href="#Main_program">Main program</a>,
    <a href="#Data_structures">Data structures</a>,
    <a href="#Interpreter_loop">Interpreter loop</a>,
    <a href="#Scanning_parsing">Scanning/parsing</a>,
    <a href="#Standard_operators">Standard operators</a>,
    <a href="#Non_standard_operators">Non-standard operators</a>,
    <a href="#Interpreter_support">Interpreter support</a>,
    <a href="#PostScript_code">PostScript code</a>
</ul>
<li><a href="#PDF_interpreter">PDF interpreter</a>
<li><a href="#Build_process">Build process</a>
<ul>
<li><a href="#Makefile_structure">Makefile structure</a>,
    <a href="#dev_files">.dev files</a>,
    <a href="#Generators">Generators</a>,
    <a href="#Build_support">Support</a>
</ul>
<li><a href="#Utilities">Utilities</a>
<ul>
<li><a href="#Utilities_in_PostScript">Utilities in PostScript</a>
<li><a href="#Utility_scripts">Utility scripts</a>
</ul>
</ul>
<li><a href="#Memory_management">Memory management</a>
<ul>
<li><a href="#Memory_manager_architecture">Memory manager architecture</a>
<ul>
<li><a href="#Objects_vs_strings">Objects vs strings</a>,
    <a href="#Structure_descriptors">Structure descriptors</a>,
    <a href="#Garbage_collection">Garbage collection</a>,
    <a href="#Movability">Movability</a>,
    <a href="#Parent_hierarchy">Parent hierarchy</a>,
    <a href="#Allocator_API">Allocator API</a>
</ul>
<li><a href="#Freeing_storage">Freeing storage</a>
<ul>
<li><a href="#Explicit_freeing">Explicit freeing</a>,
    <a href="#Reference_counting">Reference counting</a>,
    <a href="#Real_garbage_collection">(Real) garbage collection</a>
</ul>
<li><a href="#Special_implementations">Special implementations</a>
<ul>
<li><a href="#malloc">malloc</a>,
    <a href="#Locking">Locking</a>,
    <a href="#Retrying">Retrying</a>
</ul>
<li><a href="#Standard_implementation">Standard implementation</a>
<li><a href="#PostScript_interpreter_extensions">PostScript interpreter extensions</a>
<ul>
<li><a href="#Refs">Refs (PostScript "objects")</a>,
    <a href="#save_forgetsave_restore">save/.forgetsave/restore</a>,
    <a href="#Stable_allocators">Stable allocators</a>,
    <a href="#Interpreter_GC">Garbage collection</a>
</ul>
</ul>
<li><a href="#Portability">Portability</a>
<ul>
<li><a href="#Structural">Structural</a>
<ul>
<li><a href="#CPU_and_compiler">CPU and compiler</a>,
    <a href="#Library_headers">Library headers</a>,
    <a href="#Cross_platform_APIs">Cross-platform APIs</a>,
    <a href="#Makefiles">Makefiles</a>
</ul>
<li><a href="#Coding">Coding</a>
<ul>
<li><a href="#Explicit_dependencies">Explicit dependencies</a>,
    <a href="#Implicit_dependencies">Implicit dependencies</a>
</ul>
<li><a href="#Platform_specific_code">Platform-specific code</a>
</ul>
<li><a href="#Adding_features_and_options">Adding features and options</a>
<li><a href="#Troubleshooting">Troubleshooting</a>
</ul></blockquote>

<!-- [1.2 end table of contents] =========================================== -->

<!-- [1.3 begin hint] ====================================================== -->

<p>For other information, see the <a href="Readme.htm">Ghostscript
overview</a> and the documentation related to <a
href="Maintain.htm">maintaining Ghostscript</a>.

<!-- [1.3 end hint] ======================================================== -->

<hr>

<!-- [1.0 end visible header] ============================================== -->

<!-- [2.0 begin contents] ================================================== -->

<h2><a name="Introduction"></a>Introduction</h2>

<p>
This document provides a wealth of information about Ghostscript's
internals, primarily for developers actively working on Ghostscript.  It is
primarily <strong>descriptive</strong>, documenting the way things are; the
companion <a href="C-style.htm">C style guide</a> is primarily
<strong>prescriptive</strong>, documenting what developers should do when
writing new code.

<p>
THIS FILE IS A WORK IN PROGRESS.  MANY SECTIONS ARE PLACE-HOLDERS.

<h2><a name="Architecture"></a>Architecture</h2>

<h3><a name="Design_goals"></a>Design goals</h3>

<p>
Ghostscript has the following high-level design goals (not listed in order
of importance):

<ul>
<li>Functionality
<ul>
<li>Ability to interpret the current PostScript and PDF languages, as
defined (and occasionally, in the case of conflict, as implemented) by
Adobe.
<li>Ability to convert PostScript to and from PDF, comparable to
Adobe products.
<li>Ability to produce output for a wide range of resolutions (from
TV-resolution displays to imagesetters) and color models (black and white,
multilevel gray, bilevel or multi-level RGB and CMYK, 6- or 8-color
inkjet printers, spot color).
</ul>
<li>Performance
<ul>
<li>Ability to render PostScript and PDF with commercial-quality performance
(memory usage, speed, and output quality) on all platforms.
<li>Specifically, ability to render PostScript effectively in embedded
environments with constrained RAM, including the ability to put the code and
supporting data in ROM.
</ul>
<li>Licensing
<ul>
<li>Licensing that supports both the Open Source / Free software communities
and a commercial licensing business.
<li>Freedom from licensing restrictions or fees imposed by third parties.
</ul>
<li>Other
<ul>
<li>Easy source portability to any platform (CPU, operating system, and
development tools) that has an ANSI or K&amp;R C compiler.
<li>Support for writing new interpreters and new drivers with no change to
any existing code; specifically, ability to support PCL 5e, PCL 5c, and PCL
XL interpreters, and the ever-changing roster of inkjet printers.
</ul>
</ul>

<p>
These goals often conflict: part of Ghostscript's claim to quality is that
the conflicts have been resolved well.

<h3><a name="Design_principles"></a>Design principles</h3>

<p>
Part of what has kept Ghostscript healthy through many years of major code
revisions and functional expansion is consistent and conscientious adherence
to a set of design principles.  We hope the following list captures the most
important ones.

<h4>Non-preemption</h4>

<p>
Ghostscript is designed to be used as a component.  As such, it must share
its environment with other components.  Therefore, it must not require
ownership of, or make decisions about, inherently shared resources.
Specifically, it must not assume that it can "own" either the locus of
control or the management of the address space.

<p>
Not owning control means that whenever Ghostscript passes control to its
caller, it must do so in a way that doesn't constrain what the caller can do
next.  The caller must be able to call any other piece of software, wait for
an external event, execute another task, etc., without having to worry about
Ghostscript being in an unknown state.  While this is easy to arrange in a
multi-threaded environment (by running Ghostscript in a separate thread),
multi-threading APIs are not well standardized at this time (December 2000),
and may not be implemented efficiently, or at all, on some platforms.
Therefore, Ghostscript must choose between only two options for interacting
with its caller: to <em>return</em>, preserving its own state in data
structures, or to <em>call back</em> through a caller-supplied procedure.
Calling back constrains the client program unacceptably: the callback
procedure only has the options of either returning, or aborting Ghostscript.
In particular, if it wants (for whatever reason) to multi-task Ghostscript
with another program, it cannot do so in general, especially if the other
program also uses callback rather than suspension.  Therefore, Ghostscript
tries extremely hard to return, rather than calling back, for all caller
interaction.  In particular:

<ul>

<li>For callers that want to pass input to Ghostscript piece by piece,
Ghostscript returns with an <b><tt>e_NeedInput</tt></b> code rather than
using a callback.  This allows the caller complete flexibility in its
control structure for managing the source of input.  (It might, for example,
be generating the input dynamically.)

<li>In the future, the same arrangement should be used for input from
<b><tt>stdin</tt></b> and output to <b><tt>stdout</tt></b> and
<b><tt>stderr</tt></b>.

<li>Likewise, scheduling of Ghostscript's own threads (contexts), currently
done with a callback, should be done with suspension.  The Display
Ghostscript project (GNU DGS) is working on this.

</ul>

<p>
The one area where suspension is not feasible with Ghostscript's current
architecture is device output.  Device drivers are called from deep within
the graphics library.  (If Ghostscript were being redesigned from scratch,
we might try to do this with suspension as well, or at least optional
suspension.)

<p>
Not owning management of the address space means that even though
Ghostscript supports garbage collection for its own data, it must not do any
of the things that garbage collection schemes for C often require: it must
not replace 'malloc' and 'free', must not require its clients to use its own
allocator, must not rely on manipulating the read/write status of memory
pages, must not require special compiler or run-time support (e.g., APIs for
scanning the C stack), must not depend on the availability of
multi-threading, and must not take possession of one of a limited number of
timer interrupts.  However, in order not to constrain its own code unduly,
it must also not require using special macros or calls to enter or leave
procedures or assign pointers, and must not constrain the variety of C data
structures any more than absolutely necessary.  It achieves all of these
goals, at the expense of some complexity, some performance cost (mostly for
garbage collection), and some extra manual work required for each structure
type allocated by its allocator.  The details appear in the <a
href="#Memory_management">Memory management</a> section below.

<h4>Multi-instantiability</h4>

<p>
From many years of experience with the benefits of object-oriented design,
we have learned that when the word "the" appears in a software design --
"the" process scheduler, "the" memory manager, "the" output device, "the"
interpreter, "the" stack -- it often flags an area in which the software
will have difficulty adapting to future needs.  For this reason, Ghostscript
attempts to make every internal structure capable of existing in multiple
instances.  For example, Ghostscript's memory manager is not a one-of-a-kind
entity with global state and procedures: it is (or rather they are, since
Ghostscript has multiple memory managers, some of which have multiple
instances) objects with their own state and (virtual) procedures.
Ghostscript's PostScript interpreter has no writable non-local data
(necessary, but not sufficient, to allow multiple instances), and in the
future will be extended to be completely reentrant and instantiable.  The
device driver API is designed to make this easy for drivers as well.  The
graphics library is currently not completely reentrant or instantiable: we
hope this will occur in the future.

<h4>Late configuration binding</h4>

<p>
Ghostscript is designed to make configuration choices as late as possible,
subject to simplicity and performance considerations.  The major binding
times for such choices are compilation, linking, startup, and dynamic.

<ul>

<li>Compilation binds only CPU and compiler characteristics (including data
type size, presence of floating point hardware, and data alignment), and
whether the code will be used for production, debugging, or profiling.

<li>Linking binds the choice of what features and device drivers will be
included in the executable.  (Work is underway to make the choice of drivers
dynamic.)

<li>Startup binds essentially nothing.  Almost every option and parameter
that can appear on the command line can also be changed dynamically.

<li>The selection of output device, all parameters associated with the
device, the selection of debugging printout and self-checking (in debugging
configurations), the macro-allocation of memory, and almost all other
operational parameters are dynamic.

</ul>

<p>
In addition, a number of major implementation decisions are made dynamically
depending on the availability of resources.  For example, Ghostscript
chooses between banded and non-banded rendering depending on memory
availability.

<h3><a name="Large_scale_structure"></a>Large-scale structure</h3>

<p>
At the largest design scale, Ghostscript consists of 4 layers.  Layer N
is allowed to use the facilities of all layers M &lt;= N.

<ol>

<li>The bottom layer is called the <a href="#Substrate">substrate</a>.  It
includes facilities like memory management, streams, fixed-point arithmetic,
and low-level interfaces to the operating system.  The substrate is written
in C, with a little C++ and/or assembler code for some platforms.

<li>The layer above the substrate is the graphics layer.  It consists of two
separate sub-parts.  The graphics layer is written in C.

<ul>

<li>The <a href="#Graphics_library">graphics library</a> manages graphics
state information for, and decomposes and renders 2-D images described
using, a graphics model that is approximately the union of those of
PostScript, PDF, and PCL 5e/5c/XL.

<li>The <a href="#Device_drivers">device drivers</a> are called by the
graphics library to produce actual output.  The graphics library, and all
higher layers, call device driver procedures only through virtual functions.

</ul>

<li>The principal clients of the graphics layer are language interpreters.
Ghostscript as distributed includes the <a
href="#PostScript_interpreter">PostScript interpreter</a>; there are also
interpreters for PCL 5e, PCL 5c, and PCL XL, which are not currently freely
redistributable and are not included in the standard Ghostscript package.
The PostScript interpreter is written partly in C and partly in PostScript.

<li>The <a href="#PDF_interpreter">PDF interpreter</a> is actually a client
of the PostScript interpreter: it is written entirely in PostScript.

</ol>

<p>
The most important interface in Ghostscript is the API between the graphics
library and the device drivers: new printers (and, to a lesser extent,
window systems, displays, plotters, film recorders, and graphics file
formats) come on the scene frequently, and it must be possible to produce
output for them with a minimum of effort and distruption.  This API is the
only one that is extensively documented (see <a
href="Drivers.htm">Drivers.htm</a>) and kept stringently backward-compatible
through successive releases.

<h3><a name="Object_oriented_constructs"></a>Object-oriented constructs</h3>

<p>
Ghostscript makes heavy use of object-oriented constructs, including
analogues of classes, instances, subclassing, and class-associated
procedures.  Since Ghostscript is written in C, not C++, implementing these
constructs requires following coding conventions.  The <a
href="C-style.htm#Objects">"Objects"</a> section of the C style guide
explains these.

<p>
The memory manager API provides run-time type information about each class,
but this information does not include anything about subclassing.  See under
<a href="#Structure_descriptors">Structure descriptors</a> below.

<hr>

<h2><a name="File_roadmap"></a>File roadmap</h2>

<p>
This section of the document provides a roadmap to all of the Ghostscript
source files.

<h3><a name="Substrate"></a>Substrate</h3>

<h4>Memory manager</h4>

<p>
See <a href="#Memory_management">below</a>.

<h4>Streams</h4>

<dl>

<dt>
Framework, file and string streams:
<dd>
<a href="../src/gsdsrc.c">src/gsdsrc.c</a>,
<a href="../src/gsdsrc.h">src/gsdsrc.h</a>,
<a href="../src/scommon.h">src/scommon.h</a>,
<a href="../src/sfxboth.c">src/sfxboth.c</a>,
<a href="../src/sfxfd.c">src/sfxfd.c</a>,
<a href="../src/sfxstdio.c">src/sfxstdio.c</a>,
<a href="../src/stream.h">src/stream.h</a>,
<a href="../src/stream.c">src/stream.c</a>,
<a href="../src/strimpl.h">src/strimpl.h</a>.

<dt>
Standard filters:
<dd>

<dl>

<dt>
CCITTFax:
<dd>
<a href="../src/scf.h">src/scf.h</a>,
<a href="../src/scfd.c">src/scfd.c</a>,
<a href="../src/scfdgen.c">src/scfdgen.c</a>,
<a href="../src/scfdtab.c">src/scfdtab.c</a>,
<a href="../src/scfe.c">src/scfe.c</a>,
<a href="../src/scfetab.c">src/scfetab.c</a>,
<a href="../src/scfparam.c">src/scfparam.c</a>,
<a href="../src/scfx.h">src/scfx.h</a>.

<dt>
DCT (JPEG):
<dd>
<a href="../src/gsjconf.h">src/gsjconf.h</a>,
<a href="../src/gsjmorec.h">src/gsjmorec.h</a>,
<a href="../src/sdcparam.c">src/sdcparam.c</a>,
<a href="../src/sdcparam.h">src/sdcparam.h</a>,
<a href="../src/sdct.h">src/sdct.h</a>,
<a href="../src/sdctc.c">src/sdctc.c</a>,
<a href="../src/sdctd.c">src/sdctd.c</a>,
<a href="../src/sdcte.c">src/sdcte.c</a>,
<a href="../src/sddparam.c">src/sddparam.c</a>,
<a href="../src/sdeparam.c">src/sdeparam.c</a>,
<a href="../src/sjpeg.h">src/sjpeg.h</a>,
<a href="../src/sjpegc.c">src/sjpegc.c</a>,
<a href="../src/sjpegd.c">src/sjpegd.c</a>,
<a href="../src/sjpege.c">src/sjpege.c</a>.

<dt>
Other compression/decompression:
<dd>
<a href="../src/slzwc.c">src/slzwc.c</a>,
<a href="../src/slzwce.c">src/slzwce.c</a>,
<a href="../src/slzwd.c">src/slzwd.c</a>,
<a href="../src/slzwx.h">src/slzwx.h</a>,
<a href="../src/srld.c">src/srld.c</a>,
<a href="../src/srle.c">src/srle.c</a>,
<a href="../src/srlx.h">src/srlx.h</a>.

<dt>
Other:
<dd>
<a href="../src/sa85d.c">src/sa85d.c</a>,
<a href="../src/sa85d.h">src/sa85d.h</a>,
<a href="../src/sa85x.h">src/sa85x.h</a>,
<a href="../src/sfilter1.c">src/sfilter1.c</a>,
<a href="../src/sfilter2.c">src/sfilter2.c</a>,
<a href="../src/sstring.c">src/sstring.c</a>,
<a href="../src/sstring.h">src/sstring.h</a>.

</dl>

<dt>
Non-standard filters used to implement standard filters:
<dd>
<a href="../src/seexec.c">src/seexec.c</a>,
<a href="../src/sfilter.h">src/sfilter.h</a>,
<a href="../src/shc.c">src/shc.c</a>,
<a href="../src/shc.h">src/shc.h</a>,
<a href="../src/shcgen.c">src/shcgen.c</a>,
<a href="../src/shcgen.h">src/shcgen.h</a>,
<a href="../src/spdiff.c">src/spdiff.c</a>,
<a href="../src/spdiffx.h">src/spdiffx.h</a>,
<a href="../src/spngp.c">src/spngp.c</a>,
<a href="../src/spngpx.h">src/spngpx.h</a>,
<a href="../src/szlibc.c">src/szlibc.c</a>,
<a href="../src/szlibd.c">src/szlibd.c</a>,
<a href="../src/szlibe.c">src/szlibe.c</a>,
<a href="../src/szlibx.h">src/szlibx.h</a>,
<a href="../src/szlibxx.h">src/szlibxx.h</a>.

<dt>
Non-standard filters:
<dd>
<a href="../src/sbcp.c">src/sbcp.c</a>,
<a href="../src/sbcp.h">src/sbcp.h</a>,
<a href="../src/sbhc.c">src/sbhc.c</a>,
<a href="../src/sbhc.h">src/sbhc.h</a>,
<a href="../src/sbtx.h">src/sbtx.h</a>,
<a href="../src/sbwbs.c">src/sbwbs.c</a>,
<a href="../src/sbwbs.h">src/sbwbs.h</a>,
<a href="../src/smd5.c">src/smd5.c</a>,
<a href="../src/smd5.h">src/smd5.h</a>,
<a href="../src/sarc4.c">src/sarc4.c</a>,
<a href="../src/sarc4.h">src/sarc4.h</a>,
<a href="../src/smtf.c">src/smtf.c</a>,
<a href="../src/smtf.h">src/smtf.h</a>.

<dt>
Internal filters:
<dd>
a<a href="../src/siinterp.c">src/siinterp.c</a>,
<a href="../src/siinterp.h">src/siinterp.h</a>,
<a href="../src/siscale.c">src/siscale.c</a>,
<a href="../src/siscale.h">src/siscale.h</a>,
<a href="../src/sisparam.h">src/sisparam.h</a>.

<dt>
Higher-level stream support:
<dd>
<a href="../src/spprint.c">src/spprint.c</a>,
<a href="../src/spprint.h">src/spprint.h</a>,
<a href="../src/spsdf.c">src/spsdf.c</a>,
<a href="../src/spsdf.h">src/spsdf.h</a>,
<a href="../src/srdline.h">src/srdline.h</a>.

</dl>

<h4>Platform-specific code</h4>

See <a href="#Cross_platform_APIs">below</a>.

<h4>Miscellaneous</h4>

<dl>

<dt>
Library top level:
<dd>
<a href="../src/gsinit.c">src/gsinit.c</a>,
<a href="../src/gslib.h">src/gslib.h</a>.

<dt>
Configuration-related:
<dd>
<a href="../src/gconf.c">src/gconf.c</a>,
<a href="../src/gconf.h">src/gconf.h</a>,
<a href="../src/gscdef.c">src/gscdef.c</a>,
<a href="../src/gscdefs.h">src/gscdefs.h</a>.

<dt>
Arithmetic:
<dd>
<a href="../src/gsfemu.c">src/gsfemu.c</a>,
<a href="../src/gxarith.h">src/gxarith.h</a>,
<a href="../src/gxdda.h">src/gxdda.h</a>,
<a href="../src/gxfarith.h">src/gxfarith.h</a>,
<a href="../src/gxfixed.h">src/gxfixed.h</a>,
<a href="../src/gxfrac.h">src/gxfrac.h</a>.

<dt>
Operating system interface:
<dd>
<a href="../src/gserror.h">src/gserror.h</a>,
<a href="../src/gsexit.h">src/gsexit.h</a>,
<a href="../src/gxstdio.h">src/gxstdio.h</a>,
<a href="../src/gxsync.c">src/gxsync.c</a>,
<a href="../src/gxsync.h">src/gxsync.h</a>.

<dt>
Other:
<dd>
<a href="../src/gsargs.c">src/gsargs.c</a>,
<a href="../src/gsargs.h">src/gsargs.h</a>,
<a href="../src/gserrors.h">src/gserrors.h</a>,
<a href="../src/gsnotify.c">src/gsnotify.c</a>,
<a href="../src/gsnotify.h">src/gsnotify.h</a>,
<a href="../src/gsrect.h">src/gsrect.h</a>,
<a href="../src/gstypes.h">src/gstypes.h</a>,
<a href="../src/gsuid.h">src/gsuid.h</a>,
<a href="../src/gsutil.h">src/gsutil.h</a>,
<a href="../src/gsutil.c">src/gsutil.c</a>,
<a href="../src/gx.h">src/gx.h</a>,
<a href="../src/md5.c">src/md5.c</a>,
<a href="../src/md5.h">src/md5.h</a>.

</dl>

<h3><a name="Graphics_library"></a>Graphics library</h3>

<h4><a name="Library_support"></a>Support</h4>

<dl>

<dt>
Bitmap processing:
<dd>
<a href="../src/gsbitcom.c">src/gsbitcom.c</a>,
<a href="../src/gsbitmap.h">src/gsbitmap.h</a>,
<a href="../src/gsbitops.c">src/gsbitops.c</a>,
<a href="../src/gsbitops.h">src/gsbitops.h</a>,
<a href="../src/gsbittab.c">src/gsbittab.c</a>,
<a href="../src/gsbittab.h">src/gsbittab.h</a>,
<a href="../src/gsflip.c">src/gsflip.c</a>,
<a href="../src/gsflip.h">src/gsflip.h</a>,
<a href="../src/gxbitmap.h">src/gxbitmap.h</a>,
<a href="../src/gxbitops.h">src/gxbitops.h</a>,
<a href="../src/gxsample.c">src/gxsample.c</a>,
<a href="../src/gxsample.h">src/gxsample.h</a>.

<dt>
Functions:
<dd>
<a href="../src/gsfunc.c">src/gsfunc.c</a>,
<a href="../src/gsfunc.h">src/gsfunc.h</a>,
<a href="../src/gsfunc0.c">src/gsfunc0.c</a>,
<a href="../src/gsfunc0.h">src/gsfunc0.h</a>,
<a href="../src/gsfunc3.c">src/gsfunc3.c</a>,
<a href="../src/gsfunc3.h">src/gsfunc3.h</a>,
<a href="../src/gsfunc4.c">src/gsfunc4.c</a>,
<a href="../src/gsfunc4.h">src/gsfunc4.h</a>,
<a href="../src/gsfuncv.c">src/gsfuncv.c</a>,
<a href="../src/gsfuncv.h">src/gsfuncv.h</a>,
<a href="../src/gxfunc.h">src/gxfunc.h</a>.

<dt>
Parameter lists:
<dd>
<a href="../src/gscparam.c">src/gscparam.c</a>,
<a href="../src/gsparam.c">src/gsparam.c</a>,
<a href="../src/gsparam.h">src/gsparam.h</a>,
<a href="../src/gsparam2.c">src/gsparam2.c</a> (not used),
<a href="../src/gsparams.c">src/gsparams.c</a>,
<a href="../src/gsparams.h">src/gsparams.h</a>,
<a href="../src/gsparamx.c">src/gsparamx.c</a>,
<a href="../src/gsparamx.h">src/gsparamx.h</a>.

<dt>
I/O-related:
<dd>
<a href="../src/gdevpipe.c">src/gdevpipe.c</a>,
<a href="../src/gsfname.c">src/gsfname.c</a>,
<a href="../src/gsfname.h">src/gsfname.h</a>,
<a href="../src/gsio.h">src/gsio.h</a>,
<a href="../src/gsiodev.c">src/gsiodev.c</a>,
<a href="../src/gsiodevs.c">src/gsiodevs.c</a>,
<a href="../src/gxiodev.h">src/gxiodev.h</a>.

</dl>

<h4><a name="Paths"></a>Paths</h4>

<dl>

<dt>
Coordinate transformation:
<dd>
<a href="../src/gscoord.c">src/gscoord.c</a>,
<a href="../src/gscoord.h">src/gscoord.h</a>,
<a href="../src/gsmatrix.c">src/gsmatrix.c</a>,
<a href="../src/gsmatrix.h">src/gsmatrix.h</a>,
<a href="../src/gxcoord.h">src/gxcoord.h</a>,
<a href="../src/gxmatrix.h">src/gxmatrix.h</a>.

<dt>
Path building:
<dd>
<a href="../src/gsdps1.c">src/gsdps1.c</a>,
<a href="../src/gspath.c">src/gspath.c</a>,
<a href="../src/gspath.h">src/gspath.h</a>,
<a href="../src/gspath1.c">src/gspath1.c</a>,
<a href="../src/gspath2.h">src/gspath2.h</a>,
<a href="../src/gxpath.c">src/gxpath.c</a>,
<a href="../src/gxpath.h">src/gxpath.h</a>,
<a href="../src/gxpath2.c">src/gxpath2.c</a>,
<a href="../src/gxpcopy.c">src/gxpcopy.c</a>,
<a href="../src/gxpdash.c">src/gxpdash.c</a>,
<a href="../src/gxpflat.c">src/gxpflat.c</a>,
<a href="../src/gzpath.h">src/gzpath.h</a>.

<dt>
Path rendering:
<dd>
<a href="../src/gdevddrw.c">src/gdevddrw.c</a>,
<a href="../src/gsdps1.c">src/gsdps1.c</a>,
<a href="../src/gspaint.c">src/gspaint.c</a>,
<a href="../src/gspaint.h">src/gspaint.h</a>,
<a href="../src/gspenum.h">src/gspenum.h</a>,
<a href="../src/gxfill.c">src/gxfill.c</a>,
<a href="../src/gxpaint.c">src/gxpaint.c</a>,
<a href="../src/gxpaint.h">src/gxpaint.h</a>,
<a href="../src/gxstroke.c">src/gxstroke.c</a>.

<dt>
Clipping:
<dd>
See under <a href="#Clipping">Clipping</a> below.

</dl>

<h4><a name="Text"></a>Text</h4>

<dl>

<dt>
Fonts, generic:
<dd>
<a href="../src/gsfont.c">src/gsfont.c</a>,
<a href="../src/gsfont.h">src/gsfont.h</a>,
<a href="../src/gxfont.h">src/gxfont.h</a>.

<dt>
Fonts, specific FontTypes:
<dd>
<a href="../src/gsfcid.c">src/gsfcid.c</a>,
<a href="../src/gsfcmap.c">src/gsfcmap.c</a>,
<a href="../src/gsfcmap.h">src/gsfcmap.h</a>,
<a href="../src/gsfont0.c">src/gsfont0.c</a>,
<a href="../src/gxcid.h">src/gxcid.h</a>,
<a href="../src/gxfcid.h">src/gxfcid.h</a>,
<a href="../src/gxfcmap.h">src/gxfcmap.h</a>,
<a href="../src/gxfont0.h">src/gxfont0.h</a>,
<a href="../src/gxfont1.h">src/gxfont1.h</a>,
<a href="../src/gxfont42.h">src/gxfont42.h</a>,
<a href="../src/gxftype.h">src/gxftype.h</a>,
<a href="../src/gxttf.h">src/gxttf.h</a>.

<dt>
Character rendering + font cache, generic:
<dd>
<a href="../src/gsccode.h">src/gsccode.h</a>,
<a href="../src/gschar.c">src/gschar.c</a>,
<a href="../src/gschar.h">src/gschar.h</a>,
<a href="../src/gscpm.h">src/gscpm.h</a>,
<a href="../src/gstext.c">src/gstext.c</a>,
<a href="../src/gstext.h">src/gstext.h</a>,
<a href="../src/gxbcache.c">src/gxbcache.c</a>,
<a href="../src/gxbcache.h">src/gxbcache.h</a>,
<a href="../src/gxccache.c">src/gxccache.c</a>,
<a href="../src/gxccman.c">src/gxccman.c</a>,
<a href="../src/gxchar.c">src/gxchar.c</a>,
<a href="../src/gxchar.h">src/gxchar.h</a>,
<a href="../src/gxfcache.h">src/gxfcache.h</a>,
<a href="../src/gxtext.h">src/gxtext.h</a>.

<dt>
Character rendering, specific FontTypes:
<dd>
<a href="../src/gschar0.c">src/gschar0.c</a>,
<a href="../src/gscrypt1.c">src/gscrypt1.c</a>,
<a href="../src/gscrypt1.h">src/gscrypt1.h</a>,
<a href="../src/gstype1.c">src/gstype1.c</a>,
<a href="../src/gstype1.h">src/gstype1.h</a>,
<a href="../src/gstype2.c">src/gstype2.c</a>,
<a href="../src/gstype42.c">src/gstype42.c</a>,
<a href="../src/gxchrout.c">src/gxchrout.c</a>,
<a href="../src/gxchrout.h">src/gxchrout.h</a>,
<a href="../src/gxhint1.c">src/gxhint1.c</a>,
<a href="../src/gxhint2.c">src/gxhint2.c</a>,
<a href="../src/gxhint3.c">src/gxhint3.c</a>,
<a href="../src/gxop1.h">src/gxop1.h</a>,
<a href="../src/gxtype1.c">src/gxtype1.c</a>,
<a href="../src/gxtype1.h">src/gxtype1.h</a>.

</dl>

<h4><a name="Images"></a>Images</h4>

<dl>

<dt>
Buffered API (mostly for PostScript interpreter):
<dd>
<a href="../src/gsimage.c">src/gsimage.c</a>,
<a href="../src/gsimage.h">src/gsimage.h</a>.

<dt>
Generic support:
<dd>
<a href="../src/gsiparam.h">src/gsiparam.h</a>,
<a href="../src/gxiclass.h">src/gxiclass.h</a>,
<a href="../src/gximage.c">src/gximage.c</a>,
<a href="../src/gximage.h">src/gximage.h</a>,
<a href="../src/gxiparam.h">src/gxiparam.h</a>.

<dt>
Type 1 and 4 images:
<dd>

<dl>

<dt>
Setup:
<dd>
<a href="../src/gsiparm4.h">src/gsiparm4.h</a>,
<a href="../src/gximage1.c">src/gximage1.c</a>,
<a href="../src/gximage4.c">src/gximage4.c</a>.

<dt>
Rendering:
<dd>
<a href="../src/gxi12bit.c">src/gxi12bit.c</a>,
<a href="../src/gxicolor.c">src/gxicolor.c</a>,
<a href="../src/gxidata.c">src/gxidata.c</a>,
<a href="../src/gxifast.c">src/gxifast.c</a>,
<a href="../src/gximono.c">src/gximono.c</a>,
<a href="../src/gxino12b.c">src/gxino12b.c</a>,
<a href="../src/gxipixel.c">src/gxipixel.c</a>,
<a href="../src/gxiscale.c">src/gxiscale.c</a>.

</dl>

<dt>
Type 2 images (Display PostScript):
<dd>
<a href="../src/gsiparm2.h">src/gsiparm2.h</a>,
<a href="../src/gximage2.c">src/gximage2.c</a>.

<dt>
Type 3 images:
<dd>
<a href="../src/gsipar3x.h">src/gsipar3x.h</a>,
<a href="../src/gsiparm3.h">src/gsiparm3.h</a>,
<a href="../src/gximag3x.c">src/gximag3x.c</a>,
<a href="../src/gximag3x.h">src/gximag3x.h</a>,
<a href="../src/gximage3.c">src/gximage3.c</a>,
<a href="../src/gximage3.h">src/gximage3.h</a>.

<dt>
Other:
<dd>
<a href="../src/gsimpath.c">src/gsimpath.c</a>.

</dl>

<h4><a name="Paint"></a>Paint</h4>

<p>
Ghostscript uses 4 internal representations of color.  We list them here in
the order in which they occur in the rendering pipeline.

<ol>

<li>Clients of the graphics library normally specify colors using the
<em>client color</em> structure (<b><tt>gs_client_color</tt></b>, defined in
<a href="../src/gsccolor.h">src/gsccolor.h</a>), consisting of one or more
numeric values and/or a pointer to a Pattern instance.  This corresponds
directly to the values that would be passed to the PostScript
<b><tt>setcolor</tt></b> operator: one or more (floating-point) numeric
components and/or a Pattern.  Client colors are interpreted relative to a
color space (<b><tt>gs_color_space</tt></b>, defined in <a
href="../src/gscspace.h">src/gscspace.h</a> and <a
href="../src/gxcspace.h">src/gxcspace.h</a>, with specific color spaces
defined in other files).  Client colors do not explicitly reference the
color space in which they are are interpreted: <b><tt>setcolor</tt></b> uses
the color space in the graphics state, while images and shadings explicitly
specify the color space to be used.

<li>For ordinary non-Pattern colors, the first step in color rendering
reduces a client color to a <em>concrete</em> color -- a set of values in a
color space that corresponds to the device's color model (except for
possible conversions between DeviceGray, DeviceRGB, and DeviceCMYK),
together with an identification of the associated color space.  (The
confusion here between color spaces and color models will have to be cleaned
up when we implement native Separation/DeviceN colors.)  Concrete colors are
like the numeric values in a client color, except that they are represented
by arrays of <b><tt>frac</tt></b> values (defined in <a
href="../src/gxfrac.h">src/gxfrac.h</a>) rather than floats.  The procedure
for this step is the virtual <b><tt>concretize_color</tt></b> and
<b><tt>concrete_space</tt></b> procedures in the (original) color space.
This step reduces Indexed colors, CIEBased colors, and Separation and
DeviceN colors that use the alternate space.

<li>The final step requires mapping a concrete color to the device's color
model, done by procedures in <a href="../src/gxcmap.c">src/gxcmap.c</a>.
These procedures combine the following three conceptual sub-steps:

<ul>

<li>A possible mapping between Device color spaces, possibly involving black
generation and undercolor removal.  The non-trivial cases are implemented in
<a href="../src/gxdcconv.c">src/gxdcconv.c</a>.

<li>Application of the transfer function(s) (done in-line).

<li>Halftoning if necessary: see below.

</ul>

The result is called (inappropriately) a <em>device color</em>
(<b><tt>gx_device_color</tt></b>, defined in <a
href="../src/gsdcolor.h">src/gsdcolor.h</a> and <a
href="../src/gxdcolor.h">src/gxdcolor.h</a>).  For ordinary non-Pattern
colors, a device color is either a pure color, or a halftone.  The device
and color model associated with a device color are implicit.  The procedure
for this step is the virtual <b><tt>remap_concrete_color</tt></b> procedure
in the color space.

<li>The pure colors that underlie a device color are opaque <em>pixel
values</em> defined by the device (misnamed <b><tt>gx_color_index</tt></b>,
defined in <a href="../src/gscindex.h">src/gscindex.h</a>).  The device with
which they are associated is implicit.  Although the format and
interpretation of a pixel value are known only to the device, the device's
color model and color representation capabilities are public, defined by a
<b><tt>gx_color_info</tt></b> structure stored in the device (defined in <a
href="../src/gxdevcli.h">src/gxdevcli.h</a>).  Virtual procedures of the
device driver map between pixel values and RGB or CMYK.  (This area is
untidy and will need to be cleaned up when we implement native
Separation/DeviceN colors.)

</ol>

<p>
Steps 2 and 3 are normally combined into a single step for efficiency, as
the <b><tt>remap_color</tt></b> virtual procedure in a color space.

<p>
Using a device color to actually paint pixels requires a further step called
<em>color loading</em>, implemented by the <b><tt>load</tt></b> virtual
procedure in the device color.  This does nothing for pure colors, but loads
the caches for halftones and Patterns.

<p>
All of the above steps -- concretizing, mapping to a device color, and color
loading -- are done as late as possible, normally not until the color is
actually needed for painting.

<p>
All painting operations (fill, stroke, imagemask/show) eventually call a
virtual procedure in the device color, either <b><tt>fill_rectangle</tt></b>
or <b><tt>fill_mask</tt></b> to actually paint pixels.  For rectangle fills,
pure colors call the device's <b><tt>fill_rectangle</tt></b> procedure;
halftones and tiled Patterns call the device's
<b><tt>tile_rectangle</tt></b>; shaded Patterns, and painting operations
that involve a RasterOp, do something more complicated.

<dl>

<dt>
Color specification:
<dd>
<a href="../src/gsccolor.h">src/gsccolor.h</a>,
<a href="../src/gscolor.c">src/gscolor.c</a>,
<a href="../src/gscolor.h">src/gscolor.h</a>,
<a href="../src/gscolor1.c">src/gscolor1.c</a>,
<a href="../src/gscolor1.h">src/gscolor1.h</a>,
<a href="../src/gscolor2.c">src/gscolor2.c</a>,
<a href="../src/gscolor2.h">src/gscolor2.h</a>,
<a href="../src/gscolor3.c">src/gscolor3.c</a>,
<a href="../src/gscolor3.h">src/gscolor3.h</a>,
<a href="../src/gshsb.c">src/gshsb.c</a>,
<a href="../src/gshsb.h">src/gshsb.h</a>,
<a href="../src/gxcolor2.h">src/gxcolor2.h</a>,
<a href="../src/gxcvalue.h">src/gxcvalue.h</a>.

<dt>
Color spaces:
<dd>
<a href="../src/gscdevn.c">src/gscdevn.c</a>,
<a href="../src/gscdevn.h">src/gscdevn.h</a>,
<a href="../src/gscie.c">src/gscie.c</a>,
<a href="../src/gscie.h">src/gscie.h</a>,
<a href="../src/gscpixel.c">src/gscpixel.c</a>,
<a href="../src/gscpixel.h">src/gscpixel.h</a>,
<a href="../src/gscscie.c">src/gscscie.c</a>,
<a href="../src/gscsepnm.h">src/gscsepnm.h</a>,
<a href="../src/gscsepr.c">src/gscsepr.c</a>,
<a href="../src/gscsepr.h">src/gscsepr.h</a>,
<a href="../src/gscspace.c">src/gscspace.c</a>,
<a href="../src/gscspace.h">src/gscspace.h</a>,
<a href="../src/gscssub.c">src/gscssub.c</a>,
<a href="../src/gscssub.h">src/gscssub.h</a>,
<a href="../src/gxcdevn.h">src/gxcdevn.h</a>,
<a href="../src/gxcie.h">src/gxcie.h</a>,
<a href="../src/gxcspace.h">src/gxcspace.h</a>.

<dt>
Color mapping:
<dd>
<a href="../src/gsciemap.c">src/gsciemap.c</a>,
<a href="../src/gscindex.h">src/gscindex.h</a>,
<a href="../src/gscrd.c">src/gscrd.c</a>,
<a href="../src/gscrd.h">src/gscrd.h</a>,
<a href="../src/gscrdp.c">src/gscrdp.c</a>,
<a href="../src/gscrdp.h">src/gscrdp.h</a>,
<a href="../src/gscsel.h">src/gscsel.h</a>,
<a href="../src/gsdcolor.h">src/gsdcolor.h</a>,
<a href="../src/gxcindex.h">src/gxcindex.h</a>,
<a href="../src/gxcmap.c">src/gxcmap.c</a>,
<a href="../src/gxcmap.h">src/gxcmap.h</a>,
<a href="../src/gxctable.c">src/gxctable.c</a>,
<a href="../src/gxctable.h">src/gxctable.h</a>,
<a href="../src/gxdcconv.c">src/gxdcconv.c</a>,
<a href="../src/gxdcconv.h">src/gxdcconv.h</a>,
<a href="../src/gxdcolor.c">src/gxdcolor.c</a>,
<a href="../src/gxdcolor.h">src/gxdcolor.h</a>,
<a href="../src/gxdither.c">src/gxdither.c</a>,
<a href="../src/gxdither.h">src/gxdither.h</a>,
<a href="../src/gxfmap.h">src/gxfmap.h</a>,
<a href="../src/gxlum.h">src/gxlum.h</a>,
<a href="../src/gxtmap.h">src/gxtmap.h</a>.

<p>
ICC profiles are in some ways a special case of color mapping, but are
not standard in PostScript.

<dt>
Color mapping:
<dd>
<a href="../src/gsicc.c">src/gsicc.c</a>,
<a href="../src/gsicc.h">src/gsicc.h</a>,

</dl>

<p>
Ghostscript represents halftones internally by "whitening orders" --
essentially, arrays of arrays of bit coordinates within a halftone cell,
specifying which bits are inverted to get from halftone level K to level
K+1.  The code does support all of the PostScript halftone types, but they
are all ultimately reduced to whitening orders.

<p>
Threshold arrays, the more conventional representation of halftones, can be
mapped to whitening orders straightforwardly; however, whitening orders can
represent non-monotonic halftones (halftones where the bits turned on for
level K+1 don't necessarily include all the bits turned on for level K),
while threshold arrays cannot.  On the other hand, threshold arrays allow
rapid conversion of images (using a threshold comparison for each pixel)
with no additional space, while whitening orders do not: they require
storing the rendered halftone cell for each possible level as a bitmap.

<p>
Ghostscript uses two distinct types of rendered halftones -- that is, the
bitmap(s) that represent a particular level.

<ul>

<li>Binary halftones.  The rendered halftone is a single bit plane; each bit
selects one of two pure colors.  These are fast but limited: they are used
for monochrome output devices, or for color devices in those cases where
only two distinct colors are involved in a halftone (e.g., a pure cyan shade
on a CMYK device).  The device color for a binary halftone stores a pointer
to the halftone bitmap, and the two pure colors.

<li>Multi-plane halftones.  Internally, each plane is rendered individually.
Since there isn't enough room to store all 2^N pure colors, multi-plane
halftones only store the scaled values for the individual components; the
halftone renderer maps these to the pure colors on the fly, then combines
the planes to assemble an N-bit index into the list of colors for each
pixel, and stores the color into the fully rendered halftone.

</ul>

<p>
The halftone level for rendering a color is computed in <a
href="../src/gxdither.c">src/gxdither.c</a>; the actual halftone mask or
tile is computed either in <a href="../src/gxcht.c">src/gxcht.c</a> (for
multi-plane halftones), or in <a href="../src/gxht.c">src/gxht.c</a> and <a
href="../src/gxhtbit.c">src/gxhtbit.c</a> (for binary halftones).

<dl>

<dt>
Halftoning:
<dd>
<a href="../src/gsht.c">src/gsht.c</a>,
<a href="../src/gsht.h">src/gsht.h</a>,
<a href="../src/gsht1.c">src/gsht1.c</a>,
<a href="../src/gsht1.h">src/gsht1.h</a>,
<a href="../src/gshtscr.c">src/gshtscr.c</a>,
<a href="../src/gshtx.c">src/gshtx.c</a>,
<a href="../src/gshtx.h">src/gshtx.h</a>,
<a href="../src/gxcht.c">src/gxcht.c</a>,
<a href="../src/gxdht.h">src/gxdht.h</a>,
<a href="../src/gxdhtres.h">src/gxdhtres.h</a>,
<a href="../src/gxht.c">src/gxht.c</a>,
<a href="../src/gxht.h">src/gxht.h</a>,
<a href="../src/gxhtbit.c">src/gxhtbit.c</a>,
<a href="../src/gxhttile.h">src/gxhttile.h</a>,
<a href="../src/gxhttype.h">src/gxhttype.h</a>,
<a href="../src/gzht.h">src/gzht.h</a>.

</dl>

<p>
Pattern colors (tiled patterns and shadings) each use a slightly different
approach from solid colors.

<p>
The device color for a tiled (PatternType 1) pattern contains a pointer to a
pattern instance, plus (for uncolored patterns) the device color to be
masked.  The pattern instance includes a procedure that actually paints the
pattern if the pattern is not in the cache.  For the PostScript interpreter,
this procedure returns an <b><tt>e_RemapColor</tt></b> exception code: this
eventually causes the interpreter to run the pattern's PaintProc, loading
the rendering into the cache, and then re-execute the original drawing
operator.

<dl>

<dt>
Patterns:
<dd>
<a href="../src/gspcolor.c">src/gspcolor.c</a>,
<a href="../src/gspcolor.h">src/gspcolor.h</a>,
<a href="../src/gsptype1.c">src/gsptype1.c</a>,
<a href="../src/gsptype1.h">src/gsptype1.h</a>,
<a href="../src/gxp1fill.c">src/gxp1fill.c</a>,
<a href="../src/gxp1impl.h">src/gxp1impl.h</a>,
<a href="../src/gxpcache.h">src/gxpcache.h</a>,
<a href="../src/gxpcmap.c">src/gxpcmap.c</a>,
<a href="../src/gxpcolor.h">src/gxpcolor.h</a>.

</dl>

<p>
The device color for a shading (PatternType 2) pattern also contains a
pointer to a pattern instance.  Shadings are not cached: painting with a
shading runs the shading algorithm every time.

<dl>

<dt>
Shading:
<dd>
<a href="../src/gsptype2.c">src/gsptype2.c</a>,
<a href="../src/gsptype2.h">src/gsptype2.h</a>,
<a href="../src/gsshade.c">src/gsshade.c</a>,
<a href="../src/gsshade.h">src/gsshade.h</a>,
<a href="../src/gxshade.c">src/gxshade.c</a>,
<a href="../src/gxshade.h">src/gxshade.h</a>,
<a href="../src/gxshade1.c">src/gxshade1.c</a>,
<a href="../src/gxshade4.c">src/gxshade4.c</a>,
<a href="../src/gxshade4.h">src/gxshade4.h</a>,
<a href="../src/gxshade6.c">src/gxshade6.c</a>.

</dl>

<p>
In addition to the PostScript graphics model, Ghostscript supports RasterOp,
a weak form of alpha channel, and eventually the full PDF 1.4 transparency
model.  The implemention of these facilities is quite slipshod and
scattered: only RasterOp is really implemented fully.  There is a general
compositing architecture, but it is hardly used at all, and in particular is
not used for RasterOp.  ****** TO BE COMPLETED ******

<dl>

<dt>
Compositing architecture:
<dd>
<a href="../src/gscompt.h">src/gscompt.h</a>,
<a href="../src/gxcomp.h">src/gxcomp.h</a>.

<dt>
RasterOp:
<dd>
<a href="../src/gdevdrop.c">src/gdevdrop.c</a>,
<a href="../src/gdevrops.c">src/gdevrops.c</a>,
<a href="../src/gsnorop.c">src/gsnorop.c</a>,
<a href="../src/gsrop.c">src/gsrop.c</a>,
<a href="../src/gsrop.h">src/gsrop.h</a>,
<a href="../src/gsropc.c">src/gsropc.c</a>,
<a href="../src/gsropc.h">src/gsropc.h</a>,
<a href="../src/gsropt.h">src/gsropt.h</a>,
<a href="../src/gsroptab.c">src/gsroptab.c</a>,
<a href="../src/gxdevrop.h">src/gxdevrop.h</a>,
<a href="../src/gxropc.h">src/gxropc.h</a>.

<dt>
Alpha channel and compositing:
<dd>
<a href="../src/gsalpha.c">src/gsalpha.c</a>,
<a href="../src/gsalpha.h">src/gsalpha.h</a>,
<a href="../src/gsalphac.c">src/gsalphac.c</a>,
<a href="../src/gsalphac.h">src/gsalphac.h</a>,
<a href="../src/gsdpnext.h">src/gsdpnext.h</a>,
<a href="../src/gxalpha.h">src/gxalpha.h</a>.

<dt>
Advanced transparency:
<dd>
<a href="../src/gstparam.h">src/gstparam.h</a>,
<a href="../src/gstrans.c">src/gstrans.c</a>,
<a href="../src/gstrans.h">src/gstrans.h</a>,
<a href="../src/gxblend.c">src/gxblend.c</a>,
<a href="../src/gxblend.h">src/gxblend.h</a>,
<a href="../src/gdevp14.c">src/gdevp14.c</a>.
<a href="../src/gdevp14.h">src/gdevp14.h</a>.
<a href="../src/gdevpnga.c">src/gdevpnga.c</a>.

</dl>

<h4><a name="Clipping"></a>Clipping</h4>

<p>
The Ghostscript graphics library implements clipping by inserting a clipping
device in the device pipeline.  The clipping device modifies all drawing
operations to confine them to the clipping region.

<p>
The library supports three different kinds of clipping:

<dl>

<dt>
Region/path clipping
<dd>
This corresponds to the PostScript concept of a clipping path.  The clipping
region is specified either by a list of rectangles (subject to the
constraints documented in <a href="../src/gxcpath.h">src/gxcpath.h</a>), or
by a path that is converted to such a list of rectangles.

<dt>
Stationary mask clipping
<dd>
This corresponds to the mask operand of a PostScript ImageType 3 image.  The
clipping region is specified by a bitmap and an (X,Y) offset in the
coordinate space.

<dt>
Tiled mask clipping
<dd>
This corresponds to the region painted by a PostScript Pattern, for the case
where the Pattern does not completely cover its bounding box but the
combined transformation matrix has no skew or non-orthogonal rotation (i.e.,
XStep and YStep map respectively to (X,0) and (0,Y) or vice versa).  The
clipping region is specified by a bitmap and an (X,Y) offset in the
coordinate space, and is replicated indefinitely in both X and Y.

</dl>

<p>
Note that simply scan-converting a clipping path in the usual way does not
produce a succession of rectangles that can simply be stored as the list for
region-based clipping: in general, the rectangles do not satisfy the
constraint for rectangle lists specified in <a
href="../src/gxcpath.h">src/gxcpath.h</a>, since they may overlap in X, Y,
or both.  A non-trivial "clipping list accumulator" device is needed to
produce a rectangle list that does satisfy the constraint.

<dl>

<dt>
Clipping support:
<dd>
<a href="../src/gxclip.c">src/gxclip.c</a>,
<a href="../src/gxclip.h">src/gxclip.h</a>.

<dt>
Region/path clipping:
<dd>
<a href="../src/gxcpath.c">src/gxcpath.c</a>,
<a href="../src/gxcpath.h">src/gxcpath.h</a>,
<a href="../src/gzcpath.h">src/gzcpath.h</a>.

<dt>
Clipping list accumulator:
<dd>
<a href="../src/gxacpath.c">src/gxacpath.c</a>,
<a href="../src/gzacpath.h">src/gzacpath.h</a>.

<dt>
Mask clipping support:
<dd>
<a href="../src/gxmclip.c">src/gxmclip.c</a>,
<a href="../src/gxmclip.h">src/gxmclip.h</a>.

<dt>
Stationary mask clipping:
<dd>
<a href="../src/gxclipm.c">src/gxclipm.c</a>,
<a href="../src/gxclipm.h">src/gxclipm.h</a>.

<dt>
Tiled mask clipping:
<dd>
<a href="../src/gxclip2.c">src/gxclip2.c</a>,
<a href="../src/gxclip2.h">src/gxclip2.h</a>.

</dl>

<h4><a name="Other_graphics"></a>Other graphics</h4>

<dl>

<dt>
Miscellaneous graphics state:
<dd>
<a href="../src/gsclipsr.c">src/gsclipsr.c</a>,
<a href="../src/gsclipsr.h">src/gsclipsr.h</a>,
<a href="../src/gsdps.c">src/gsdps.c</a>,
<a href="../src/gsdps.h">src/gsdps.h</a>,
<a href="../src/gsdps1.c">src/gsdps1.c</a>,
<a href="../src/gsistate.c">src/gsistate.c</a>,
<a href="../src/gsline.c">src/gsline.c</a>,
<a href="../src/gsline.h">src/gsline.h</a>,
<a href="../src/gslparam.h">src/gslparam.h</a>,
<a href="../src/gsstate.c">src/gsstate.c</a>,
<a href="../src/gsstate.h">src/gsstate.h</a>,
<a href="../src/gstrap.c">src/gstrap.c</a>,
<a href="../src/gstrap.h">src/gstrap.h</a>,
<a href="../src/gxclipsr.h">src/gxclipsr.h</a>,
<a href="../src/gxistate.h">src/gxistate.h</a>,
<a href="../src/gxline.h">src/gxline.h</a>,
<a href="../src/gxstate.h">src/gxstate.h</a>,
<a href="../src/gzline.h">src/gzline.h</a>,
<a href="../src/gzstate.h">src/gzstate.h</a>.

</dl>

<h4><a name="Driver_support"></a>Driver support</h4>

<dl>

<dt>
Generic driver support:
<dd>
<a href="../src/gdevdcrd.c">src/gdevdcrd.c</a>,
<a href="../src/gdevdcrd.h">src/gdevdcrd.h</a>,
<a href="../src/gdevemap.c">src/gdevemap.c</a>,
<a href="../src/gsdevice.c">src/gsdevice.c</a>,
<a href="../src/gsdevice.h">src/gsdevice.h</a>,
<a href="../src/gsdparam.c">src/gsdparam.c</a>,
<a href="../src/gsxfont.h">src/gsxfont.h</a>,
<a href="../src/gxdevbuf.h">src/gxdevbuf.h</a>,
<a href="../src/gxdevcli.h">src/gxdevcli.h</a>,
<a href="../src/gxdevice.h">src/gxdevice.h</a>,
<a href="../src/gxrplane.h">src/gxrplane.h</a>,
<a href="../src/gxxfont.h">src/gxxfont.h</a>.

<dt>
Accessing rendered bits:
<dd>
<a href="../src/gdevdbit.c">src/gdevdbit.c</a>,
<a href="../src/gdevdgbr.c">src/gdevdgbr.c</a>,
<a href="../src/gxbitfmt.h">src/gxbitfmt.h</a>,
<a href="../src/gxgetbit.h">src/gxgetbit.h</a>.

<dt>
"Printer" driver support:
<dd>
<a href="../src/gdevmeds.c">src/gdevmeds.c</a>,
<a href="../src/gdevmeds.h">src/gdevmeds.h</a>,
<a href="../src/gdevppla.c">src/gdevppla.c</a>,
<a href="../src/gdevppla.h">src/gdevppla.h</a>,
<a href="../src/gdevprn.c">src/gdevprn.c</a>,
<a href="../src/gdevprn.h">src/gdevprn.h</a>,
<a href="../src/gdevprna.c">src/gdevprna.c</a>,
<a href="../src/gdevprna.h">src/gdevprna.h</a>,
<a href="../src/gxband.h">src/gxband.h</a>,
<a href="../src/gxpageq.c">src/gxpageq.c</a>,
<a href="../src/gxpageq.h">src/gxpageq.h</a>.

<dt>
High-level device support:
<dd>
<a href="../src/gdevvec.c">src/gdevvec.c</a>,
<a href="../src/gdevvec.h">src/gdevvec.h</a>.

<dt>
Banding:
<dd>
<a href="../src/gxclbits.c">src/gxclbits.c</a>,
<a href="../src/gxcldev.h">src/gxcldev.h</a>,
<a href="../src/gxclfile.c">src/gxclfile.c</a>,
<a href="../src/gxclimag.c">src/gxclimag.c</a>,
<a href="../src/gxclio.h">src/gxclio.h</a>,
<a href="../src/gxclist.c">src/gxclist.c</a>,
<a href="../src/gxclist.h">src/gxclist.h</a>,
<a href="../src/gxcllzw.c">src/gxcllzw.c</a>,
<a href="../src/gxclmem.c">src/gxclmem.c</a>,
<a href="../src/gxclmem.h">src/gxclmem.h</a>,
<a href="../src/gxclpage.c">src/gxclpage.c</a>,
<a href="../src/gxclpage.h">src/gxclpage.h</a>,
<a href="../src/gxclpath.c">src/gxclpath.c</a>,
<a href="../src/gxclpath.h">src/gxclpath.h</a>,
<a href="../src/gxclrast.c">src/gxclrast.c</a>,
<a href="../src/gxclread.c">src/gxclread.c</a>,
<a href="../src/gxclrect.c">src/gxclrect.c</a>,
<a href="../src/gxclutil.c">src/gxclutil.c</a>,
<a href="../src/gxclzlib.c">src/gxclzlib.c</a>.

</dl>

<h3><a name="Device_drivers"></a>Device drivers</h3>

<p>
See <a href="Drivers.htm">doc/Drivers.htm</a> for extensive documentation on
the interface between the core code and drivers.

<p>
The driver API includes high-level (path / image / text), mid-level
(polygon), and low-level (rectangle / raster) operations.  Most devices
implement only the low-level operations, and let generic code break down the
high-level operations.  However, some devices produce high-level output, and
therefore must implement the high-level operations.

<h4><a name="Internal_devices"></a>Internal devices</h4>

<p>
There are a number of "devices" that serve internal purposes.  Some of these
are meant to be real rendering targets; others are intended for use in
device pipelines.  The rendering targets are:

<dl>

<dt>
Memory devices, depth-independent:
<dd>
<a href="../src/gdevmem.c">src/gdevmem.c</a>,
<a href="../src/gdevmem.h">src/gdevmem.h</a>,
<a href="../src/gdevmpla.c">src/gdevmpla.c</a>,
<a href="../src/gdevmpla.h">src/gdevmpla.h</a>,
<a href="../src/gdevmrop.h">src/gdevmrop.h</a>,
<a href="../src/gsdevmem.c">src/gsdevmem.c</a>,
<a href="../src/gxdevmem.h">src/gxdevmem.h</a>.

<dt>
Memory devices, specific depths:
<dd>
<a href="../src/gdevm1.c">src/gdevm1.c</a>,
<a href="../src/gdevm2.c">src/gdevm2.c</a>,
<a href="../src/gdevm4.c">src/gdevm4.c</a>,
<a href="../src/gdevm8.c">src/gdevm8.c</a>,
<a href="../src/gdevm16.c">src/gdevm16.c</a>,
<a href="../src/gdevm24.c">src/gdevm24.c</a>,
<a href="../src/gdevm32.c">src/gdevm32.c</a>,
<a href="../src/gdevmr1.c">src/gdevmr1.c</a>,
<a href="../src/gdevmr2n.c">src/gdevmr2n.c</a>,
<a href="../src/gdevmr8n.c">src/gdevmr8n.c</a>.

<dt>
Alpha-related devices:
<dd>
<a href="../src/gdevabuf.c">src/gdevabuf.c</a>,
<a href="../src/gdevalph.c">src/gdevalph.c</a>.

<dt>
Other devices:
<dd>
<a href="../src/gdevdflt.c">src/gdevdflt.c</a>,
<a href="../src/gdevhit.c">src/gdevhit.c</a>,
<a href="../src/gdevht.c">src/gdevht.c</a>,
<a href="../src/gdevht.h">src/gdevht.h</a>,
<a href="../src/gdevmrun.c">src/gdevmrun.c</a>,
<a href="../src/gdevmrun.h">src/gdevmrun.h</a>,
<a href="../src/gdevplnx.c">src/gdevplnx.c</a>,
<a href="../src/gdevplnx.h">src/gdevplnx.h</a>,

</dl>

<p>
The forwarding devices meant for use in pipelines are:

<dl>

<dt>
The bounding box device:
<dd>
<a href="../src/gdevbbox.h">src/gdevbbox.h</a>,
<a href="../src/gdevbbox.c">src/gdevbbox.c</a>.

<dt>
Clipping devices:
<dd>
See under <a href="#Clipping">Clipping</a> above.

<dt>
Device filter stack:
<dd>
<a href="../src/gsdfilt.c">src/gsdfilt.c</a>,
<a href="../src/gsdfilt.h">src/gsdfilt.h</a>,

<dt>
Other devices:
<dd>
<a href="../src/gdevcmap.c">src/gdevcmap.c</a>,
<a href="../src/gdevcmap.h">src/gdevcmap.h</a>,
<a href="../src/gdevnfwd.c">src/gdevnfwd.c</a>.

</dl>

<h4><a name="PS_and_PDF_writers"></a>PostScript and PDF writers</h4>

<p>
Because PostScript and PDF have the same graphics model, lexical syntax, and
stack-based execution model, the drivers that produce PostScript and PDF
output share a significant amount of support code.  In the future, the
PostScript output driver should be replaced with a slightly modified version
of the PDF driver, since the latter is far more sophisticated (in
particular, it has extensive facilities for image compression and for
handling text and fonts).

<dl>

<dt>
Shared support:
<dd>

<dl>
Writing fonts:
<dd>
<a href="../src/gdevpsf.h">src/gdevpsf.h</a>,
<a href="../src/gdevpsf1.c">src/gdevpsf1.c</a>,
<a href="../src/gdevpsf2.c">src/gdevpsf2.c</a>,
<a href="../src/gdevpsfm.c">src/gdevpsfm.c</a>,
<a href="../src/gdevpsft.c">src/gdevpsft.c</a>,
<a href="../src/gdevpsfu.c">src/gdevpsfu.c</a>,
<a href="../src/gdevpsfx.c">src/gdevpsfx.c</a>.

<dt>
Other:
<dd>
<a href="../src/gdevpsdf.h">src/gdevpsdf.h</a>,
<a href="../src/gdevpsdi.c">src/gdevpsdi.c</a>,
<a href="../src/gdevpsdp.c">src/gdevpsdp.c</a>,
<a href="../src/gdevpsds.c">src/gdevpsds.c</a>,
<a href="../src/gdevpsds.h">src/gdevpsds.h</a>,
<a href="../src/gdevpsdu.c">src/gdevpsdu.c</a>.

</dl>

<dt>
PostScript output driver ([e]pswrite):
<dd>
<a href="../src/gdevps.c">src/gdevps.c</a>,
<a href="../src/gdevpsu.c">src/gdevpsu.c</a>,
<a href="../src/gdevpsu.h">src/gdevpsu.h</a>.

<dt>
PDF output driver (pdfwrite):
<dd>
<dl>

<dt>
Substrate:
<dd>
<a href="../src/gdevpdfo.c">src/gdevpdfo.c</a>,
<a href="../src/gdevpdfo.h">src/gdevpdfo.h</a>,
<a href="../src/gdevpdfr.c">src/gdevpdfr.c</a>,
<a href="../src/gdevpdfu.c">src/gdevpdfu.c</a>.

<dt>
Text and fonts:
<dd>
<a href="../src/gdevpdfe.c">src/gdevpdfe.c</a>,
<a href="../src/gdevpdff.c">src/gdevpdff.c</a>,
<a href="../src/gdevpdff.h">src/gdevpdff.h</a>,
<a href="../src/gdevpdfs.c">src/gdevpdfs.c</a>,
<a href="../src/gdevpdft.c">src/gdevpdft.c</a>,
<a href="../src/gdevpdfw.c">src/gdevpdfw.c</a>.

<dt>
Graphics:
<dd>
<a href="../src/gdevpdfc.c">src/gdevpdfc.c</a>,
<a href="../src/gdevpdfd.c">src/gdevpdfd.c</a>,
<a href="../src/gdevpdfg.c">src/gdevpdfg.c</a>,
<a href="../src/gdevpdfg.h">src/gdevpdfg.h</a>,
<a href="../src/gdevpdfv.c">src/gdevpdfv.c</a>.

<dt>
Images:
<dd>
<a href="../src/gdevpdfb.c">src/gdevpdfb.c</a>,
<a href="../src/gdevpdfi.c">src/gdevpdfi.c</a>,
<a href="../src/gdevpdfj.c">src/gdevpdfj.c</a>.

<dt>
Other:
<dd>
<a href="../src/gdevpdf.c">src/gdevpdf.c</a>,
<a href="../src/gdevpdfm.c">src/gdevpdfm.c</a>,
<a href="../src/gdevpdfp.c">src/gdevpdfp.c</a>,
<a href="../src/gdevpdfx.h">src/gdevpdfx.h</a>.

</dl>

</dl>

<h4><a name="High_level_devices"></a>Other high-level devices</h4>

<p>
Currently, the CGM driver is raster-only.  If anyone cares seriously about
CGM in the future, this driver should be upgraded to a higher level.

<dl>

<dt>
PCL XL output device (pxlmono, pxlcolor):
<dd>
<a href="../src/gdevpx.c">src/gdevpx.c</a>,
<a href="../src/gdevpxat.h">src/gdevpxat.h</a>,
<a href="../src/gdevpxen.h">src/gdevpxen.h</a>,
<a href="../src/gdevpxop.h">src/gdevpxop.h</a>,
<a href="../src/gdevpxut.c">src/gdevpxut.c</a>,
<a href="../src/gdevpxut.h">src/gdevpxut.h</a>.

<dt>
Other high-level devices:
<dd>
<a href="../src/gdevtrac.c">src/gdevtrac.c</a>.

</dl>

<h4><a name="Other_maintained_drivers"></a>Other maintained drivers</h4>

<p>
The standard Ghostscript distribution includes a collection of drivers,
mostly written by Aladdin Enterprises, that are "maintained" in the same
sense as the Ghostscript core code.

<dl>

<dt>
Display drivers:
<dd>
<a href="../src/gdev8bcm.c">src/gdev8bcm.c</a>,
<a href="../src/gdev8bcm.h">src/gdev8bcm.h</a>,
<a href="../src/gdevegaa.asm">src/gdevegaa.asm</a>,
<a href="../src/gdevevga.c">src/gdevevga.c</a>,
<a href="../src/gdevl256.c">src/gdevl256.c</a>,
<a href="../src/gdevpccm.c">src/gdevpccm.c</a>,
<a href="../src/gdevpccm.h">src/gdevpccm.h</a>,
<a href="../src/gdevpcfb.c">src/gdevpcfb.c</a>,
<a href="../src/gdevpcfb.h">src/gdevpcfb.h</a>,
<a href="../src/gdevs3ga.c">src/gdevs3ga.c</a>,
<a href="../src/gdevsco.c">src/gdevsco.c</a>,
<a href="../src/gdevsvga.c">src/gdevsvga.c</a>,
<a href="../src/gdevsvga.h">src/gdevsvga.h</a>,
<a href="../src/gdevvglb.c">src/gdevvglb.c</a>.

<dt>
Window system drivers:
<dd>

<dl>

<dt>
X Windows:
<dd>
<a href="../src/gdevx.c">src/gdevx.c</a>,
<a href="../src/gdevx.h">src/gdevx.h</a>,
<a href="../src/gdevxalt.c">src/gdevxalt.c</a>,
<a href="../src/gdevxcmp.c">src/gdevxcmp.c</a>,
<a href="../src/gdevxcmp.h">src/gdevxcmp.h</a>,
<a href="../src/gdevxini.c">src/gdevxini.c</a>,
<a href="../src/gdevxres.c">src/gdevxres.c</a>,
<a href="../src/gdevxxf.c">src/gdevxxf.c</a>.

<dt>
Microsoft Windows:
<dd>
<a href="../src/gdevmswn.c">src/gdevmswn.c</a>,
<a href="../src/gdevmswn.h">src/gdevmswn.h</a>,
<a href="../src/gdevmsxf.c">src/gdevmsxf.c</a>,
<a href="../src/gdevwddb.c">src/gdevwddb.c</a>,
<a href="../src/gdevwdib.c">src/gdevwdib.c</a>.

<dt>
OS/2 Presentation Manager:
<dd>
<a href="../src/gdevpm.c">src/gdevpm.c</a>,
<a href="../src/gdevpm.h">src/gdevpm.h</a>,
<a href="../src/gspmdrv.c">src/gspmdrv.c</a>,
<a href="../src/gspmdrv.h">src/gspmdrv.h</a>.

</dl>

<dt>
Raster file output drivers:
<dd>

<dl>

<dt>
Fax and TIFF:
<dd>
<a href="../src/gdevfax.c">src/gdevfax.c</a>,
<a href="../src/gdevfax.h">src/gdevfax.h</a>,
<a href="../src/gdevtfax.c">src/gdevtfax.c</a>,
<a href="../src/gdevtfax.h">src/gdevtfax.h</a>,
<a href="../src/gdevtifs.c">src/gdevtifs.c</a>,
<a href="../src/gdevtifs.h">src/gdevtifs.h</a>,
<a href="../src/gdevtfnx.c">src/gdevtfnx.c</a>.

<dt>
(Low-level) CGM:
<dd>
<a href="../src/gdevcgm.c">src/gdevcgm.c</a>,
<a href="../src/gdevcgml.c">src/gdevcgml.c</a>,
<a href="../src/gdevcgml.h">src/gdevcgml.h</a>,
<a href="../src/gdevcgmx.h">src/gdevcgmx.h</a>.

<dt>
Other raster file formats:
<dd>
<a href="../src/gdevbit.c">src/gdevbit.c</a>,
<a href="../src/gdevbmp.c">src/gdevbmp.c</a>,
<a href="../src/gdevbmp.h">src/gdevbmp.h</a>,
<a href="../src/gdevbmpa.c">src/gdevbmpa.c</a>,
<a href="../src/gdevbmpc.c">src/gdevbmpc.c</a>,
<a href="../src/gdevjpeg.c">src/gdevjpeg.c</a>,
<a href="../src/gdevmiff.c">src/gdevmiff.c</a>,
<a href="../src/gdevp2up.c">src/gdevp2up.c</a>,
<a href="../src/gdevpcx.c">src/gdevpcx.c</a>,
<a href="../src/gdevpbm.c">src/gdevpbm.c</a>,
<a href="../src/gdevpng.c">src/gdevpng.c</a>,
<a href="../src/gdevpsim.c">src/gdevpsim.c</a>.

</dl>

<dt>
Printer drivers:
<dd>

<dl>

<dt>
Operating system printer services:
<dd>
<a href="../src/gdevos2p.c">src/gdevos2p.c</a>,
<a href="../src/gdevwpr2.c">src/gdevwpr2.c</a>,
<a href="../src/gdevwprn.c">src/gdevwprn.c</a>.

<dt>
H-P monochrome printers:
<dd>
<a href="../src/gdevdljm.c">src/gdevdljm.c</a>,
<a href="../src/gdevdljm.h">src/gdevdljm.h</a>,
<a href="../src/gdevdjet.c">src/gdevdjet.c</a>,
<a href="../src/gdevlj56.c">src/gdevlj56.c</a>.

<dt>
Other printers:
<dd>
<a href="../src/gdevatx.c">src/gdevatx.c</a>.

</dl>

</dl>

<h4><a name="Contributed_drivers"></a>Contributed drivers</h4>

<p>
This list is likely to be incomplete and inaccurate: see <a
href="../src/contrib.mak">src/contrib.mak</a> for the real one.

<dl>

<dt>
Display and window system drivers:
<dd>
<a href="../src/gdev3b1.c">src/gdev3b1.c</a>,
<a href="../src/gdevherc.c">src/gdevherc.c</a>,
<a href="../src/gdevpe.c">src/gdevpe.c</a>,
<a href="../src/gdevsnfb.c">src/gdevsnfb.c</a>,
<a href="../src/gdevsun.c">src/gdevsun.c</a>.

<dt>
Raster file output drivers:
<dd>
<a href="../src/gdevcfax.c">src/gdevcfax.c</a>,
<a href="../src/gdevcif.c">src/gdevcif.c</a>,
<a href="../src/gdevdfax.c">src/gdevdfax.c</a>,
<a href="../src/gdevifno.c">src/gdevifno.c</a>,
<a href="../src/gdevmgr.c">src/gdevmgr.c</a>,
<a href="../src/gdevmgr.h">src/gdevmgr.h</a>,
<a href="../src/gdevsgi.c">src/gdevsgi.c</a>,
<a href="../src/gdevsgi.h">src/gdevsgi.h</a>,
<a href="../src/gdevsunr.c">src/gdevsunr.c</a>.

<dt>
Printer drivers:
<dd>
<a href="../lib/bj8.rpd">lib/bj8.rpd</a>,
<a href="../lib/cbjc600.ppd">lib/cbjc600.ppd</a>,
<a href="../lib/cbjc800.ppd">lib/cbjc800.ppd</a>,
<a href="../src/gdev3852.c">src/gdev3852.c</a>,
<a href="../src/gdev4081.c">src/gdev4081.c</a>,
<a href="../src/gdev4693.c">src/gdev4693.c</a>,
<a href="../src/gdev8510.c">src/gdev8510.c</a>,
<a href="../src/gdevadmp.c">src/gdevadmp.c</a>,
<a href="../src/gdevbj10.c">src/gdevbj10.c</a>,
<a href="../src/gdevbjc.h">src/gdevbjc.h</a>,
<a href="../src/gdevbjcl.c">src/gdevbjcl.c</a>,
<a href="../src/gdevbjcl.h">src/gdevbjcl.h</a>,
<a href="../src/gdevccr.c">src/gdevccr.c</a>,
<a href="../src/gdevcdj.c">src/gdevcdj.c</a>,
<a href="../src/gdevclj.c">src/gdevclj.c</a>,
<a href="../src/gdevcljc.c">src/gdevcljc.c</a>,
<a href="../src/gdevcp50.c">src/gdevcp50.c</a>,
<a href="../src/gdevcslw.c">src/gdevcslw.c</a>,
<a href="../src/gdevdjtc.c">src/gdevdjtc.c</a>,
<a href="../src/gdevdm24.c">src/gdevdm24.c</a>,
<a href="../src/gdevepsc.c">src/gdevepsc.c</a>,
<a href="../src/gdevepsn.c">src/gdevepsn.c</a>,
<a href="../src/gdevescp.c">src/gdevescp.c</a>,
<a href="../src/gdevhl7x.c">src/gdevhl7x.c</a>,
<a href="../src/gdevhpij.c">src/gdevhpij.c</a>,
<a href="../src/gdevhpij.h">src/gdevhpij.h</a>,
<a href="../src/gdevijs.c">src/gdevijs.c</a>,
<a href="../src/gdevimgn.c">src/gdevimgn.c</a>,
<a href="../src/gdevl31s.c">src/gdevl31s.c</a>,
<a href="../src/gdevlbp8.c">src/gdevlbp8.c</a>,
<a href="../src/gdevlp8k.c">src/gdevlp8k.c</a>,
<a href="../src/gdevlxm.c">src/gdevlxm.c</a>,
<a href="../src/gdevn533.c">src/gdevn533.c</a>,
<a href="../src/gdevo182.c">src/gdevo182.c</a>,
<a href="../src/gdevokii.c">src/gdevokii.c</a>,
<a href="../src/gdevpcl.c">src/gdevpcl.c</a>,
<a href="../src/gdevpcl.h">src/gdevpcl.h</a>,
<a href="../src/gdevphex.c">src/gdevphex.c</a>,
<a href="../src/gdevpjet.c">src/gdevpjet.c</a>,
<a href="../src/gdevsj48.c">src/gdevsj48.c</a>,
<a href="../src/gdevsppr.c">src/gdevsppr.c</a>,
<a href="../src/gdevstc.c">src/gdevstc.c</a>,
<a href="../src/gdevstc.h">src/gdevstc.h</a>,
<a href="../src/gdevstc1.c">src/gdevstc1.c</a>,
<a href="../src/gdevstc2.c">src/gdevstc2.c</a>,
<a href="../src/gdevstc3.c">src/gdevstc3.c</a>,
<a href="../src/gdevstc4.c">src/gdevstc4.c</a>,
<a href="../src/gdevtknk.c">src/gdevtknk.c</a>,
<a href="../src/gdevupd.c">src/gdevupd.c</a>.

</dl>

<h3><a name="PostScript_interpreter"></a>PostScript interpreter</h3>

<p>
The PostScript interpreter is conceptually simple: in fact, an interpreter
that could execute "3 4 add =" and print "7" was running 3 weeks after the
first line of Ghostscript code was written.  However, a number of
considerations make the code large and complex.

<p>
The interpreter is designed to run in environments with very limited memory.
The main consequence of this is that it cannot allocate its stacks
(dictionary, execution, operand) as ordinary arrays, since the
user-specified stack size limit may be very large.  Instead, it allocates
them as a linked list of blocks.  See below for more details.

<p>
The interpreter must never cause a C runtime error that it cannot trap.
Unfortunately, C implementations almost never provide the ability to trap
stack overflow.  In order to put a fixed bound on the C stack size, the
interpreter never implements PostScript recursion by C recursion.  This
means that any C code that logically needs to call the interpreter must
instead push a continuation (including all necessary state information) on
the PostScript execution stack, followed by the PostScript object to be
executed, and then <em>return</em> to the interpreter.  Unfortunately, since
PostScript Level 2 introduces streams whose data source can be a PostScript
procedure, any code that reads or writes stream data must be prepared to
suspend itself, storing all necessary state in a continuation.  There are
some places where this is extremely awkward, such as the scanner/parser.

<p>
The use of continuations affects many places in the interpreter, and even
some places in the graphics library.  For example, when processing an image,
one may need to call a PostScript procedure as part of mapping a CIE color
to a device color.  Ghostscript uses a variety of dodges to handle this: for
example, in the case of CIE color mapping, all of the PostScript procedures
are pre-sampled and the results cached.  The Adobe implementation limits
this kind of recursion to a fixed number of levels (5?): this would be
another acceptable approach, but at this point it would require far more
code restructuring than it would be worth.

<p>
A significant amount of the PostScript language implementation is in fact
written in PostScript.  Writing in PostScript leverages the C code for
multi-threading, garbage collection, error handling, continuations for
streams, etc., etc.; also, we have found PostScript in general more concise
and easier to debug than C, mostly because of memory management issues.  So
given the choice, we tended to implement a feature in PostScript if it
worked primarily with PostScript data structures, wasn't heavily used
(example: font loading), or if it interacted with the stream or other
callback machinery (examples: ReusableFileDecode streams, resourceforall).
Often we would add non-standard PostScript operators for functions that had
to run faster or that did more C-like things, such as the media matching
algorithm for setpagedevice.

</ul>

<h4><a name="Main_program"></a>Main program</h4>

<p>
The main program of the interpreter is normally invoked from the command
line, but it has an API as well.  In fact, it has two APIs: one that
recognizes the existence of multiple "interpreter instances" (although it
currently provides a default instance, which almost all clients use), and a
completely different one designed for Windows DLLs.  These should be unified
as soon as possible, since there are two steadily growing incompatible
bodies of client code.

<dl>

<dt>
Files:
<dd>
<a href="../src/gs.c">src/gs.c</a>,
<a href="../src/gserver.c">src/gserver.c</a>,
<a href="../src/iccinit0.c">src/iccinit0.c</a>,
<a href="../src/iinit.c">src/iinit.c</a>,
<a href="../src/iinit.h">src/iinit.h</a>,
<a href="../src/imain.c">src/imain.c</a>,
<a href="../src/imain.h">src/imain.h</a>,
<a href="../src/imainarg.c">src/imainarg.c</a>,
<a href="../src/imainarg.h">src/imainarg.h</a>,
<a href="../src/iminst.h">src/iminst.h</a>,
<a href="../src/main.h">src/main.h</a>.

</dl>

<h4><a name="Data_structures"></a>Data structures</h4>

<p>
The main data structures visible to the PostScript programmers are arrays,
contexts, dictionaries, names, and stacks.

<p>
Arrays have no unusual properties.  See under <a href="#Refs">Refs</a> below
for more information about how array elements are stored.

<p>
Contexts are used to hold the interpreter state even in configurations that
don't include the Display PostScript multiple context extension.  Context
switching is implemented by a complex cooperation of C and PostScript code.

<p>
Dictionaries have two special properties worth noting:

<ul>

<li>They use an optimized storage representation if all the keys are names,
which is almost always the case.

<li>They interact with a caching scheme used to accelerate name lookup in
the interpreter.

</ul>

<p>
Names are allocated in blocks.  The characters and hash chains are stored
separately from the lookup cache information, so that in the future, most of
the former can be compiled into the executable and shared or put in ROM.
(This is not actually done yet.)

<p>
As mentioned above, each stack is allocated as a linked list of blocks.
However, for reasonable performance, operators must normally be able to
access their operands and produce their results using indexing rather than
an access procedure.  This is implemented by ensuring that all the operands
of an operator are in the topmost block of the stack, using guard entries
that cause an internal error if the condition isn't met.  See <a
href="../src/iostack.h">src/iostack.h</a> for more details.

<dl>

<dt>
General data structures:
<dd>

<dl>

<dt>
Contexts:
<dd>
<a href="../src/icontext.c">src/icontext.c</a>,
<a href="../src/icontext.h">src/icontext.h</a>,
<a href="../src/icstate.h">src/icstate.h</a>.

<dt>
Dictionaries:
<dd>
<a href="../src/iddict.h">src/iddict.h</a>,
<a href="../src/idict.h">src/idict.h</a>,
<a href="../src/idict.c">src/idict.c</a>,
<a href="../src/idictdef.h">src/idictdef.h</a>.

<dt>
Names:
<dd>
<a href="../src/iname.c">src/iname.c</a>,
<a href="../src/iname.h">src/iname.h</a>,
<a href="../src/inamedef.h">src/inamedef.h</a>,
<a href="../src/inameidx.h">src/inameidx.h</a>,
<a href="../src/inames.h">src/inames.h</a>,
<a href="../src/inamestr.h">src/inamestr.h</a>.

<dt>
Stacks:
<dd>
<a href="../src/isdata.h">src/isdata.h</a>,
<a href="../src/istack.c">src/istack.c</a>,
<a href="../src/istack.h">src/istack.h</a>,
<a href="../src/istkparm.h">src/istkparm.h</a>.

</dl>

<dt>
Specific stacks:
<dd>

<dl>

<dt>
Dictionary stack:
<dd>
<a href="../src/dstack.h">src/dstack.h</a>,
<a href="../src/estack.h">src/estack.h</a>,
<a href="../src/iddstack.h">src/iddstack.h</a>,
<a href="../src/idsdata.h">src/idsdata.h</a>,
<a href="../src/idstack.c">src/idstack.c</a>,
<a href="../src/idstack.h">src/idstack.h</a>.

<dt>
Execution stack:
<dd>
<a href="../src/iesdata.h">src/iesdata.h</a>,
<a href="../src/iestack.h">src/iestack.h</a>.

<dt>
Operand stack:
<dd>
<a href="../src/iosdata.h">src/iosdata.h</a>,
<a href="../src/iostack.h">src/iostack.h</a>,
<a href="../src/ostack.h">src/ostack.h</a>.

</dl>

</dl>

<h4><a name="Interpreter_loop"></a>Interpreter loop</h4>

<dl>

<dt>
Files:
<dd>
<a href="../src/interp.c">src/interp.c</a>,
<a href="../src/interp.h">src/interp.h</a>.

</dl>

<h4><a name="Scanning_parsing"></a>Scanning/parsing</h4>

<p>
PostScript parsing consists essentially of token scanning, and is simple in
principle.  The scanner is complex because it must be able to suspend its
operation at any time (i.e., between any two input characters) to allow an
interpreter callout, if its input is coming from a procedure-based stream
and the procedure must be called to provide more input data.

<dl>

<dt>
Main scanner:
<dd>
<a href="../src/iscan.c">src/iscan.c</a>,
<a href="../src/iscan.h">src/iscan.h</a>,
<a href="../src/iscannum.c">src/iscannum.c</a>,
<a href="../src/iscannum.h">src/iscannum.h</a>,
<a href="../src/scanchar.h">src/scanchar.h</a>,
<a href="../src/scantab.c">src/scantab.c</a>.

<dt>
Binary tokens:
<dd>
<a href="../src/btoken.h">src/btoken.h</a>,
<a href="../src/ibnum.c">src/ibnum.c</a>,
<a href="../src/ibnum.h">src/ibnum.h</a>,
<a href="../src/inobtokn.c">src/inobtokn.c</a>,
<a href="../src/iscanbin.c">src/iscanbin.c</a>,
<a href="../src/iscanbin.h">src/iscanbin.h</a>.

<dt>
DSC parsing:
<dd>
<a href="../src/dscparse.c">src/dscparse.c</a>,
<a href="../src/dscparse.h">src/dscparse.h</a>.

</dl>

<h4><a name="Standard_operators"></a>Standard operators</h4>

<dl>

<dt>
Non-output-related:
<dd>

<dl>

<dt>
Filters:
<dd>
<a href="../src/ifilter.h">src/ifilter.h</a>,
<a href="../src/ifilter2.h">src/ifilter2.h</a>,
<a href="../src/ifrpred.h">src/ifrpred.h</a>,
<a href="../src/ifwpred.h">src/ifwpred.h</a>,
<a href="../src/istream.h">src/istream.h</a>,
<a href="../src/zfbcp.c">src/zfbcp.c</a>,
<a href="../src/zfdctd.c">src/zfdctd.c</a>,
<a href="../src/zfdcte.c">src/zfdcte.c</a>,
<a href="../src/zfdecode.c">src/zfdecode.c</a>,
<a href="../src/zfilter.c">src/zfilter.c</a>,
<a href="../src/zfilter2.c">src/zfilter2.c</a>,
<a href="../src/zfilterx.c">src/zfilterx.c</a>,
<a href="../src/zfmd5.c">src/zfmd5.c</a>,
<a href="../src/zfarc4.c">src/zfarc4.c</a>,
<a href="../src/zfproc.c">src/zfproc.c</a>,
<a href="../src/zfrsd.c">src/zfrsd.c</a>,
<a href="../src/zfzlib.c">src/zfzlib.c</a>.

<dt>
File and stream I/O:
<dd>
<a href="../src/files.h">src/files.h</a>,
<a href="../src/itoken.h">src/itoken.h</a>,
<a href="../src/zbseq.c">src/zbseq.c</a>,
<a href="../src/zdscpars.c">src/zdscpars.c</a>,
<a href="../src/zfile.c">src/zfile.c</a>,
<a href="../src/zfileio.c">src/zfileio.c</a>,
<a href="../src/ztoken.c">src/ztoken.c</a>.

<dt>
Data structures:
<dd>
<a href="../src/zarray.c">src/zarray.c</a>,
<a href="../src/zdict.c">src/zdict.c</a>,
<a href="../src/zgeneric.c">src/zgeneric.c</a>,
<a href="../src/zpacked.c">src/zpacked.c</a>,
<a href="../src/zstring.c">src/zstring.c</a>.

<dt>
Functions:
<dd>
<a href="../src/ifunc.h">src/ifunc.h</a>,
<a href="../src/zfunc.c">src/zfunc.c</a>,
<a href="../src/zfunc0.c">src/zfunc0.c</a>,
<a href="../src/zfunc3.c">src/zfunc3.c</a>,
<a href="../src/zfunc4.c">src/zfunc4.c</a>,

<dt>
Other:
<dd>
<a href="../src/ivmem2.h">src/ivmem2.h</a>,
<a href="../src/zarith.c">src/zarith.c</a>,
<a href="../src/zcontext.c">src/zcontext.c</a>,
<a href="../src/zcontrol.c">src/zcontrol.c</a>,
<a href="../src/zmath.c">src/zmath.c</a>,
<a href="../src/zmatrix.c">src/zmatrix.c</a>,
<a href="../src/zmisc.c">src/zmisc.c</a>,
<a href="../src/zmisc1.c">src/zmisc1.c</a>,
<a href="../src/zmisc2.c">src/zmisc2.c</a>,
<a href="../src/zmisc3.c">src/zmisc3.c</a>,
<a href="../src/zrelbit.c">src/zrelbit.c</a>,
<a href="../src/zstack.c">src/zstack.c</a>,
<a href="../src/ztype.c">src/ztype.c</a>,
<a href="../src/zusparam.c">src/zusparam.c</a>,
<a href="../src/zvmem.c">src/zvmem.c</a>,
<a href="../src/zvmem2.c">src/zvmem2.c</a>.

</dl>

<dt>
Output-related:
<dd>

<dl>

<dt>
Device management:
<dd>
<a href="../src/zdevcal.c">src/zdevcal.c</a>,
<a href="../src/zdevice.c">src/zdevice.c</a>,
<a href="../src/zdevice2.c">src/zdevice2.c</a>,
<a href="../src/ziodev.c">src/ziodev.c</a>,
<a href="../src/ziodev2.c">src/ziodev2.c</a>,
<a href="../src/ziodevs.c">src/ziodevs.c</a>,
<a href="../src/ziodevsc.c">src/ziodevsc.c</a>,
<a href="../src/zmedia2.c">src/zmedia2.c</a>,
<a href="../src/zdfilter.c">src/zdfilter.c</a>.

<dt>
Fonts and text:
<dd>
<a href="../src/bfont.h">src/bfont.h</a>,
<a href="../src/ccfont.h">src/ccfont.h</a>,
<a href="../src/iccfont.c">src/iccfont.c</a>,
<a href="../src/icfontab.c">src/icfontab.c</a>,
<a href="../src/ichar.h">src/ichar.h</a>,
<a href="../src/ichar1.h">src/ichar1.h</a>,
<a href="../src/icharout.h">src/icharout.h</a>,
<a href="../src/icid.h">src/icid.h</a>,
<a href="../src/ifcid.h">src/ifcid.h</a>,
<a href="../src/ifont.h">src/ifont.h</a>,
<a href="../src/ifont1.h">src/ifont1.h</a>,
<a href="../src/ifont2.h">src/ifont2.h</a>,
<a href="../src/ifont42.h">src/ifont42.h</a>,
<a href="../src/zbfont.c">src/zbfont.c</a>,
<a href="../src/zcfont.c">src/zcfont.c</a>,
<a href="../src/zchar.c">src/zchar.c</a>,
<a href="../src/zchar1.c">src/zchar1.c</a>,
<a href="../src/zchar2.c">src/zchar2.c</a>,
<a href="../src/zchar32.c">src/zchar32.c</a>,
<a href="../src/zchar42.c">src/zchar42.c</a>,
<a href="../src/zcharout.c">src/zcharout.c</a>,
<a href="../src/zcharx.c">src/zcharx.c</a>,
<a href="../src/zcid.c">src/zcid.c</a>,
<a href="../src/zfcid.c">src/zfcid.c</a>,
<a href="../src/zfcid0.c">src/zfcid0.c</a>,
<a href="../src/zfcid1.c">src/zfcid1.c</a>,
<a href="../src/zfcmap.c">src/zfcmap.c</a>,
<a href="../src/zfont.c">src/zfont.c</a>,
<a href="../src/zfont0.c">src/zfont0.c</a>,
<a href="../src/zfont1.c">src/zfont1.c</a>,
<a href="../src/zfont2.c">src/zfont2.c</a>,
<a href="../src/zfont32.c">src/zfont32.c</a>,
<a href="../src/zfont42.c">src/zfont42.c</a>.

<dt>
Color, pattern, and halftone:
<dd>
<a href="../src/icie.h">src/icie.h</a>,
<a href="../src/icolor.h">src/icolor.h</a>,
<a href="../src/icremap.h">src/icremap.h</a>,
<a href="../src/icsmap.h">src/icsmap.h</a>,
<a href="../src/iht.h">src/iht.h</a>,
<a href="../src/ipcolor.h">src/ipcolor.h</a>,
<a href="../src/zcie.c">src/zcie.c</a>,
<a href="../src/zcolor.c">src/zcolor.c</a>,
<a href="../src/zcolor1.c">src/zcolor1.c</a>,
<a href="../src/zcolor2.c">src/zcolor2.c</a>,
<a href="../src/zcrd.c">src/zcrd.c</a>,
<a href="../src/zcsdevn.c">src/zcsdevn.c</a>,
<a href="../src/zcsindex.c">src/zcsindex.c</a>,
<a href="../src/zcspixel.c">src/zcspixel.c</a>,
<a href="../src/zcssepr.c">src/zcssepr.c</a>,
<a href="../src/zicc.c">src/zicc.c</a>,
<a href="../src/zhsb.c">src/zhsb.c</a>,
<a href="../src/zht.c">src/zht.c</a>,
<a href="../src/zht1.c">src/zht1.c</a>,
<a href="../src/zht2.c">src/zht2.c</a>,
<a href="../src/zpcolor.c">src/zpcolor.c</a>,
<a href="../src/zshade.c">src/zshade.c</a>,
<a href="../src/ztrans.c">src/ztrans.c</a>.

<dt>
Images:
<dd>
<a href="../src/iimage.h">src/iimage.h</a>,
<a href="../src/iimage2.h">src/iimage2.h</a>,
<a href="../src/zimage.c">src/zimage.c</a>,
<a href="../src/zimage2.c">src/zimage2.c</a>,
<a href="../src/zimage3.c">src/zimage3.c</a>.

<dt>
Other graphics:
<dd>
<a href="../src/igstate.h">src/igstate.h</a>,
<a href="../src/zdpnext.c">src/zdpnext.c</a>,
<a href="../src/zdps.c">src/zdps.c</a>,
<a href="../src/zdps1.c">src/zdps1.c</a>,
<a href="../src/zgstate.c">src/zgstate.c</a>,
<a href="../src/zpaint.c">src/zpaint.c</a>,
<a href="../src/zpath.c">src/zpath.c</a>,
<a href="../src/zpath1.c">src/zpath1.c</a>,
<a href="../src/zrop.c">src/zrop.c</a>,
<a href="../src/ztrap.c">src/ztrap.c</a>,
<a href="../src/zupath.c">src/zupath.c</a>.

</dl>

<dt>
Operator support:
<dd>
<a href="../src/oparc.h">src/oparc.h</a>,
<a href="../src/opcheck.h">src/opcheck.h</a>,
<a href="../src/opdef.h">src/opdef.h</a>,
<a href="../src/oper.h">src/oper.h</a>,
<a href="../src/opextern.h">src/opextern.h</a>.

</dl>

<h4><a name="Non_standard_operators"></a>Non-standard operators</h4>

<p>
The interpreter includes many non-standard operators.  Most of these provide
some part of the function of a standard operator, so that the standard
operator itself can be implemented in PostScript: these are not of interest
to users, and their function is usually obvious from the way they are used.
However, some non-standard operators provide access to additional,
non-standard facilities that users might want to know about, such as
transparency, RasterOp, and in-memory rendering.  These are documented at <a
href="Language.htm#Additional_operators">Language.htm#Additional_operators</a>.

<p>
We don't document the complete set of non-standard operators here, because
the set changes frequently.  However, all non-standard operators are
supposed to have names that begin with '.', so you can find them all by
executing the following (Unix) command:

<blockquote><pre>
grep '{".[.]' src/[zi]*.c
</pre></blockquote>

<p>
In addition to individual non-standard operators implemented in the same
files as standard ones, there are several independent optional packages of
non-standard operators.  As with other non-standard operators, the names of
all the operators in these packages begin with '.'.  We list those packages
here.

<dl>

<dt>
<a href="../src/zdosio.c">src/zdosio.c</a>
<dd>
Provides access to PC hardware I/O through MS-DOS system calls.  Probably no
longer useful.

<dt>
<a href="../src/zdouble.c">src/zdouble.c</a>
<dd>
Provides "double" floating point arithmetic, using 8-byte strings to hold
values.  Developed under a contract; probably used only by the group that
funded the development.

<dt>
<a href="../src/zsysvm.c">src/zsysvm.c</a>
<dd>
Provides operators for allocating objects in specific VM spaces,
disregarding the current VM mode.

<dt>
<a href="../src/zccube.c">src/zccube.c</a>
<dd>
Provides an operator for building a color cube from an arbitrary tint
transform, so that tint transforms can be applied from inside the
graphics library without calling back to the PostScript interpreter.

</dl>

<h4><a name="Interpreter_support"></a>Interpreter support</h4>

<p>
Memory management (refs, GC, save/restore) -- see <a
href="#PostScript_interpreter_extensions">below</a>.

<dl>

<dt>
Miscellaneous support:
<dd>
<a href="../src/errors.h">src/errors.h</a>,
<a href="../src/ghost.h">src/ghost.h</a>,
<a href="../src/iconf.c">src/iconf.c</a>,
<a href="../src/iconf.h">src/iconf.h</a>,
<a href="../src/idparam.c">src/idparam.c</a>,
<a href="../src/idparam.h">src/idparam.h</a>,
<a href="../src/ilevel.h">src/ilevel.h</a>,
<a href="../src/inouparm.c">src/inouparm.c</a>,
<a href="../src/iparam.c">src/iparam.c</a>,
<a href="../src/iparam.h">src/iparam.h</a>,
<a href="../src/iparray.h">src/iparray.h</a>,
<a href="../src/iutil.c">src/iutil.c</a>,
<a href="../src/iutil.h">src/iutil.h</a>,
<a href="../src/iutil2.c">src/iutil2.c</a>,
<a href="../src/iutil2.h">src/iutil2.h</a>,
<a href="../src/iutilasm.asm">src/iutilasm.asm</a>.

</dl>

<h4><a name="PostScript_code"></a>PostScript code</h4>

<dl>

<dt>
Initialization and language support:
<dd>

<dl>

<dt>
All configurations:
<dd>
<a href="../lib/gs_init.ps">lib/gs_init.ps</a>,
<a href="../lib/gs_statd.ps">lib/gs_statd.ps</a>.

<dt>
Level 2:
<dd>
<a href="../lib/gs_btokn.ps">lib/gs_btokn.ps</a>,
<a href="../lib/gs_dps1.ps">lib/gs_dps1.ps</a>,
<a href="../lib/gs_dps2.ps">lib/gs_dps2.ps</a>,
<a href="../lib/gs_lev2.ps">lib/gs_lev2.ps</a>,
<a href="../lib/gs_res.ps">lib/gs_res.ps</a>,
<a href="../lib/gs_setpd.ps">lib/gs_setpd.ps</a>.

<dt>
LanguageLevel 3:
<dd>
<a href="../lib/gs_frsd.ps">lib/gs_frsd.ps</a>,
<a href="../lib/gs_ll3.ps">lib/gs_ll3.ps</a>,
<a href="../lib/gs_trap.ps">lib/gs_trap.ps</a>.

<dt>
Display PostScript:
<dd>
<a href="../lib/gs_dpnxt.ps">lib/gs_dpnxt.ps</a>,
<a href="../lib/gs_dps.ps">lib/gs_dps.ps</a>.

<dt>
ICC color profiles:
<dd>
<a href="../lib/gs_icc.ps">lib/gs_icc.ps</a>.

</dl>

<dt>
Font loading and support:
<dd>

<dl>

<dt>
Font name mapping:
<dd>
<a href="../lib/Fontmap">lib/Fontmap</a>,
<a href="../lib/Fontmap.ATB">lib/Fontmap.ATB</a>,
<a href="../lib/Fontmap.ATM">lib/Fontmap.ATM</a>,
<a href="../lib/Fontmap.GS">lib/Fontmap.GS</a>,
<a href="../lib/Fontmap.OS2">lib/Fontmap.OS2</a>,
<a href="../lib/Fontmap.OSF">lib/Fontmap.OSF</a>,
<a href="../lib/Fontmap.SGI">lib/Fontmap.SGI</a>,
<a href="../lib/Fontmap.Sol">lib/Fontmap.Sol</a>,
<a href="../lib/Fontmap.Ult">lib/Fontmap.Ult</a>,
<a href="../lib/Fontmap.VMS">lib/Fontmap.VMS</a>.

<dt>
Generic:
<dd>
<a href="../lib/gs_ccfnt.ps">lib/gs_ccfnt.ps</a>,
<a href="../lib/gs_fonts.ps">lib/gs_fonts.ps</a>.

<dt>
Type 1 and CFF:
<dd>
<a href="../lib/gs_cff.ps">lib/gs_cff.ps</a>,
<a href="../lib/gs_diskf.ps">lib/gs_diskf.ps</a>,
<a href="../lib/gs_type1.ps">lib/gs_type1.ps</a>.

<dt>
TrueType:
<dd>
<a href="../lib/gs_ttf.ps">lib/gs_ttf.ps</a>,
<a href="../lib/gs_typ42.ps">lib/gs_typ42.ps</a>.

<dt>
CID-keyed:
<dd>
<a href="../lib/gs_cidcm.ps">lib/gs_cidcm.ps</a>,
<a href="../lib/gs_cidfn.ps">lib/gs_cidfn.ps</a>,
<a href="../lib/gs_cmap.ps">lib/gs_cmap.ps</a>.

<dt>
Other:
<dd>
<a href="../lib/gs_kanji.ps">lib/gs_kanji.ps</a>,
<a href="../lib/gs_pfile.ps">lib/gs_pfile.ps</a>,
<a href="../lib/gs_typ32.ps">lib/gs_typ32.ps</a>.

</dl>

<dt>
Encodings:
<dd>

<dl>

<dt>
Adobe-specified:
<dd>
<a href="../lib/gs_ce_e.ps">lib/gs_ce_e.ps</a>,
<a href="../lib/gs_dbt_e.ps">lib/gs_dbt_e.ps</a>,
<a href="../lib/gs_il1_e.ps">lib/gs_il1_e.ps</a>,
<a href="../lib/gs_mex_e.ps">lib/gs_mex_e.ps</a>,
<a href="../lib/gs_mro_e.ps">lib/gs_mro_e.ps</a>,
<a href="../lib/gs_pdf_e.ps">lib/gs_pdf_e.ps</a>,
<a href="../lib/gs_std_e.ps">lib/gs_std_e.ps</a>,
<a href="../lib/gs_sym_e.ps">lib/gs_sym_e.ps</a>,
<a href="../lib/gs_wan_e.ps">lib/gs_wan_e.ps</a>.

<dt>
Additional:
<dd>
<a href="../lib/gs_il2_e.ps">lib/gs_il2_e.ps</a>,
<a href="../lib/gs_ksb_e.ps">lib/gs_ksb_e.ps</a>,
<a href="../lib/gs_wl1_e.ps">lib/gs_wl1_e.ps</a>,
<a href="../lib/gs_wl2_e.ps">lib/gs_wl2_e.ps</a>,
<a href="../lib/gs_wl5_e.ps">lib/gs_wl5_e.ps</a>.

<dt>
Pseudo-encodings for internal use:
<dd>
<a href="../lib/gs_css_e.ps">lib/gs_css_e.ps</a>,
<a href="../lib/gs_lgo_e.ps">lib/gs_lgo_e.ps</a>,
<a href="../lib/gs_lgx_e.ps">lib/gs_lgx_e.ps</a>,
<a href="../lib/gs_mgl_e.ps">lib/gs_mgl_e.ps</a>.

</dl>

<dt>
Miscellaneous:
<dd>

<dl>

<dt>
Other support:
<dd>
<a href="../lib/gs_agl.ps">lib/gs_agl.ps</a>,
<a href="../lib/gs_dscp.ps">lib/gs_dscp.ps</a>,
<a href="../lib/gs_epsf.ps">lib/gs_epsf.ps</a>,
<a href="../lib/gs_pdfwr.ps">lib/gs_pdfwr.ps</a>,
<a href="../lib/gs_rdlin.ps">lib/gs_rdlin.ps</a>.

<dt>
X Windows icon bitmaps:
<dd>
<a href="../lib/gs_l.xbm">lib/gs_l.xbm</a>,
<a href="../lib/gs_l.xpm">lib/gs_l.xpm</a>,
<a href="../lib/gs_l_m.xbm">lib/gs_l_m.xbm</a>,
<a href="../lib/gs_m.xbm">lib/gs_m.xbm</a>,
<a href="../lib/gs_m.xpm">lib/gs_m.xpm</a>,
<a href="../lib/gs_m_m.xbm">lib/gs_m_m.xbm</a>,
<a href="../lib/gs_s.xbm">lib/gs_s.xbm</a>,
<a href="../lib/gs_s.xpm">lib/gs_s.xpm</a>,
<a href="../lib/gs_s_m.xbm">lib/gs_s_m.xbm</a>,
<a href="../lib/gs_t.xbm">lib/gs_t.xbm</a>,
<a href="../lib/gs_t.xpm">lib/gs_t.xpm</a>,
<a href="../lib/gs_t_m.xbm">lib/gs_t_m.xbm</a>.

<dt>
Not currently used:
<dd>
<a href="../lib/gs_cmdl.ps">lib/gs_cmdl.ps</a>,
<a href="../lib/gs_fform.ps">lib/gs_fform.ps</a>,
<a href="../lib/gs_l2img.ps">lib/gs_l2img.ps</a>.

</dl>

</dl>

<h3><a name="PDF_interpreter"></a>PDF interpreter</h3>

<p>
Ghostscript's PDF interpreter is written entirely in PostScript, because its
data structures are the same as PostScript's, and it is much more convenient
to manipulate PostScript-like data structures in PostScript than in C.
There is definitely a performance cost, but apparently not a substantial
one: we considered moving the main interpreter loop (read a token using
slightly different syntax than PostScript, push it on the stack if literal,
look it up in a special dictionary for execution if not) into C, but we did
some profiling and discovered that this wasn't accounting for enough of the
time to be worthwhile.

<p>
Until recently, there was essentially no C code specifically for the purpose
of supporting PDF interpretation.  The one major exception is the PDF 1.4
transparency features, which we (but not Adobe) have made available to
PostScript code.

<p>
In addition to patching the <b><tt>run</tt></b> operator to detect PDF
files, the interpreter provides some procedures in <a
href="../lib/pdf_main.ps">lib/pdf_main.ps</a> that are meant to be called
from applications such as previewers.

<dl>

<dt>
Files:
<dd>
<a href="../lib/pdf_base.ps">lib/pdf_base.ps</a>,
<a href="../lib/pdf_draw.ps">lib/pdf_draw.ps</a>,
<a href="../lib/pdf_font.ps">lib/pdf_font.ps</a>,
<a href="../lib/pdf_main.ps">lib/pdf_main.ps</a>,
<a href="../lib/pdf_ops.ps">lib/pdf_ops.ps</a>,
<a href="../lib/pdf_sec.ps">lib/pdf_sec.ps</a>.

</dl>

<h3><a name="Build_process"></a>Build process</h3>

<h4><a name="Makefile_structure"></a>Makefile structure</h4>

<p>
Ghostscript's makefiles embody a number of design decisions and assumptions
that may not be obvious from a casual reading of the code.

<ul>

<li>All files are stored in subdirectories.  The names of all subdirectories
used in the build process are defined in the top-level makefiles for the
various platforms: there are no "hard wired" directory names in any makefile
rule.  Subdirectory names in the makefiles are relative to the directory
that is current at build time: normally this directory is the parent of the
various subdirectories, and holds only a <b><tt>makefile</tt></b>, which in
turn simply references the real top-level makefile in the source
subdirectory.

<li>All compiler and linker switches are likewise defined by macros, again
preferably in the top-level platform makefile.

<li>There is an absolute distinction between "source-like" subdirectories,
which are read-only during the build process, and "object-like"
subdirectories, all of whose contents are generated by the build process and
which can be emptied (<b><tt>rm&nbsp;*</tt></b>) at any time with no bad
effects.  The source subdirectories are defined by macros named
<b><tt>xxxSRCDIR</tt></b>.

<li>Object subdirectories may distinguish further between those that hold
the results of the build process that are needed at run time (i.e., that
should be included in a run-time distribution), defined by
<b><tt>BINDIR</tt></b>, and those that are not needed at run time, defined
by <b><tt>xxxGENDIR</tt></b> and <b><tt>xxxOBJDIR</tt></b>.  (The
distinction between these is historical and probably no longer relevant.)

<li>There may be multiple object subdirectories for different build
configurations.  On Unix, the <b><tt>obj</tt></b> and <b><tt>bin</tt></b>
directories are used for normal production builds, the
<b><tt>debugobj</tt></b> directory for debugging builds, and the
<b><tt>pgobj</tt></b> directory for profiling builds; other platforms may
use different conventions.  The Unix makefiles support targets named
<b><tt>debug</tt></b> and <b><tt>pg</tt></b> for debugging and profiling
builds respectively; other platforms generally do not.

<li>Makefiles will be maintained by hand.  This requires editing the
following makefile elements whenever the relevant source files changes:

<ul>

<li>Every header (.h) file must have a corresponding macro definition in a
makefile.  If <b><tt>abc.h</tt></b> #includes <b><tt>def.h</tt></b> and
<b><tt>xyz.h</tt></b>, the definition must have the form

<blockquote><pre>
xyz_h=$(xxxSRCD)xyz.h $(def_h) $(xyz_h)
</pre></blockquote>

where <b><tt>xxxSRCD</tt></b> is the macro defining the relevant source
directory (including a trailing '/').  Note that the '.' in the file name
has been replaced by an underscore.  Note also that the definition must
follow all definitions it references, since some <b><tt>make</tt></b>
programs expand macros in definitions at the time of definition rather than
at the time of use.

<li>Every .c file must have a corresponding rule in a makefile.  If
<b><tt>abc.c</tt></b> #includes <b><tt>def.h</tt></b> and
<b><tt>lmn.h</tt></b>, the rule must have approximately the form

<blockquote><pre>
$(xxxOBJD)abc.$(OBJ) : $(xxxSRCD)abc.c $(def_h) $(lmn_h)
    $(xxCC) $(xxO_)abc.$(OBJ) $(C_) $(xxxSRCD)abc.c
</pre></blockquote>

where <b><tt>xxxSRCD</tt></b> is as before; <b><tt>xxxOBJD</tt></b> is the
relevant object directory; <b><tt>xxCC</tt></b> defines the name of the C
compiler plus the relevant compilation switches; and <b><tt>xxO_</tt></b>
and <b><tt>C_</tt></b> are macros used to bridge syntactic differences
between different <b><tt>make</tt></b> programs.

</ul>

</ul>

<p>
The requirement to keep makefiles up to date by hand has been controversial.
Two alternatives are generally proposed.

<ul>

<li>Programs like <b><tt>makedeps</tt></b>, which generate build rules
automatically from the #include lists in C files.  We have found such
programs useless: they "wire in" specific, concrete directory names, not
only for our own code but even for the system header files; they have to be
run manually whenever code files are added, renamed, or deleted, or whenever
the list of #includes in any file changes; and they cannot deal with
different source files requiring different compilation switches.

<li>MSVC-style "project" files.  These are equally useless: they are not
portable across different vendors' tools -- in fact, there may not even be a
usable import/export facility to convert their data to or from text form --
and they cannot combine configuration-independent data with
configuration-specific data.

</ul>

<p>
We have seriously considered writing our own build program in Tcl or Python
that would eliminate these problems, or perhaps porting the tools developed
by Digital's Vesta research project (if we can get access to them); however,
either of these approaches would create potential portability problems of
its own, not to mention difficulties in integrating with others' build
systems.

<p>
In the meantime, to ease the pain for mortal users and developers on
Unix platforms, we have adopted the standard autoconf tool to
systematically generate an appropriate top-level makefile for the
target system. As currently implemented, its use makes no deep changes
to the Ghostscript build system, but rather generates a makefile, more
or less the same as would be achieved by starting from a
platform-specific toplevel makefile and hand-editing. The relevant
autoconf sources are in <a
href="../src/Makefile.in">src/Makefile.in</a>, and <a
href="../src/configure.ac">configure.ac</a>. These sources are used in
the release process to create a <a href="../configure">configure</a>
script in the toplevel directory of releases. Also, there is an <a
href="../autogen.sh">autogen.sh</a> script to automate the autoconf
process somewhat. The names and locations of these files were chosen
to be clear to those familiar with autoconf, even though they represent
a slight departure from Ghostscript tradition.

<p>
For more information about makefiles:

<ul>

<li>For a detailed list of makefiles, and a discussion of makefile issues
related to portability, see the <a href="#Makefiles">Makefiles</a> section
below.

<li>For more detailed information about editing configuration information in
makefiles, see <a
href="Make.htm#Makefile_overview">doc/Make.htm#Makefile_overview</a>.

<li>For a complete example of adding a new driver to a makefile, see <a
href="Drivers.htm#Adding_drivers">doc/Drivers.htm#Adding_drivers</a>.

<li>For a few more notes on makefile coding conventions, see <a
href="C-style.htm#Makefiles">doc/C-style.htm#Makefiles</a>.

</ul>

<h4><a name="dev_files"></a>.dev files</h4>

<p>
On top of the general conventions just described, Ghostscript's makefiles
add a further layer of structure in order to support an open-ended set of
fine-grained, flexible configuration options.  Selecting an option (usually
called a "module") for inclusion in the build may affect the build in many
ways:

<ul>

<li>Almost always, it requires linking in some compiled code files.

<li>It may require running some additional initialization procedures at
startup.

<li>It may require reading in some additional PostScript files at startup.
For example, a Level 2 PostScript build requires support for PostScript
resources and for setpagedevice, which are implemented in PostScript code.

<li>It may require adding entries to a variety of internal tables that
catalogue such things as drivers, IODevices, Function types, etc.

<li>It may require that other particular modules be included.  For example,
the "PostScript Level 2" module requires the modules for various filters,
color spaces, etc.

<li>It may require <em>removing</em> some other (default) module from the
build.  For example, the core (Level 1) PostScript build has a "stub" for
binary tokens, which are a Level 2 feature but are referenced by the core
scanner: a Level 2 build must remove the stub.  For more information about
this, look for the string <b><tt>-replace</tt></b> in the makefiles and in
<a href="../src/genconf.c">src/genconf.c</a>.

</ul>

<p>
Each module is defined in the makefiles by rules that create a file named
<b><em>xxx</em><tt>.dev</tt></b>.  The dependencies of the rule include all
the files that make up the module (compiled code files, PostScript files,
etc.); the body of the rule creates the .dev file by writing the description
of the module into it.  A program called <b><tt>genconf</tt></b>, described
in the next section, merges all the relevant .dev files together.  For
examples of .dev rules, see any of the Ghostscript makefiles.

<p>
Ultimately, a person must specify the root set of modules to include in a
build (which of course may require other modules, recursively).
Ghostscript's makefiles do this with a set of macros called
<b><tt>FEATURE_DEVS</tt></b> and <b><tt>DEVICE_DEVS</tt>n</b>, defined in
each top-level makefile, but nothing in the module machinery depends on
this.

<h4><a name="Generators"></a>Generators</h4>

<p>
Ghostscript's build procedure is somewhat unusual in that it compiles and
then executes some support programs during the build process.  These
programs then generate data or source code that is used later on in the
build.

<p>
The most important and complex of the generator programs is
<b><tt>genconf</tt></b>.  <b><tt>genconf</tt></b> merges all the .dev files
that make up the build, and creates three or more output files used in later
stages:

<ul>

<li><b><tt>gconfig.h</tt></b>, consisting mainly of macro calls, one call
per "resource" making up the build, other than "resources" listed in the
other output files.

<li><b><tt>gconfigf.h</tt></b>, produced only for PostScript builds with
compiled-in fonts, consisting of one macro call per font.

<li>A linker control file whose name varies from one platform to another,
containing the list of compiled code files to be linked.

<li>If necessary, another linker control file, also varying between
platforms, that contains other information for the linker such as the list
of system libraries to be searched.  (On Unix platforms, the two linker
control functions are combined in a single file.)

</ul>

<dl>

<dt>
Source generators:
<dd>

<dl>

<dt>
<a href="../src/ansi2knr.c">src/ansi2knr.c</a>
<dd>
Converts ANSI C source to traditional (also known as "K&amp;R") C.  Used if
and only if compiling with a pre-ANSI C compiler.

<dt>
<a href="../src/genarch.c">src/genarch.c</a>
<dd>
Creates a header file containing a variety of information about the hardware
and compiler that isn't provided in any standard system header file.  Always
used.

<dt>
<a href="../src/genconf.c">src/genconf.c</a> (also generates non-source)
<dd>
Constructs header files and linker control files from the collection of
options and modules that make up the build.  See above.  Always used.

<dt>
<a href="../src/genht.c">src/genht.c</a>
<dd>
Converts a PostScript halftone (in a particular constrained format) to a C
data structure that can be compiled into an executable.  Only used if any
such halftones are included in the build.

<dt>
<a href="../src/geninit.c">src/geninit.c</a>
<dd>
Converts PostScript initialization files to C data structures that can be
compiled into an executable.  Only used when building a PostScript
interpreter, and only if <b><tt>COMPILE_INITS</tt></b> was set to 1 in the
makefile.

</dl>

<dt>
Other generators:
<dd>

<dl>

<dt>
<a href="../src/echogs.c">src/echogs.c</a>
<dd>
Implements a variety of shell-like functions to get around quirks,
limitations, and omissions in the shells on various platforms.  Always used.

<dt>
<a href="../src/genconf.c">src/genconf.c</a> (also generates source)
<dd>
See above.

<dt>
<a href="../src/gendev.c">src/gendev.c</a> (not used)
<dd>
Was intended as a replacement for <b><tt>genconf</tt></b>, but was never
completed.

</dl>

</dl>

<h4><a name="Build_support"></a>Support</h4>

<p>
There are a number of programs, scripts, and configuration files that exist
only for the sake of the build process.

<dl>

<dt>
Files for PC environments:
<dd>
<a href="../src/bcc32.cfg">src/bcc32.cfg</a>,
<a href="../src/cp.bat">src/cp.bat</a>,
<a href="../src/cp.cmd">src/cp.cmd</a>,
<a href="../src/dw32c.def">src/dw32c.def</a>,
<a href="../src/dwmain.rc">src/dwmain.rc</a>,
<a href="../src/dwmain16.def">src/dwmain16.def</a>,
<a href="../src/dwmain32.def">src/dwmain32.def</a>,
<a href="../src/dwsetup.def">src/dwsetup.def</a>,
<a href="../src/dwsetup.rc">src/dwsetup.rc</a>,
<a href="../src/dwuninst.def">src/dwuninst.def</a>,
<a href="../src/dwuninst.rc">src/dwuninst.rc</a>,
<a href="../src/gs16spl.def">src/gs16spl.def</a>,
<a href="../src/gs16spl.rc">src/gs16spl.rc</a>,
<a href="../src/gsdll2.def">src/gsdll2.def</a>,
<a href="../src/gsdll2.rc">src/gsdll2.rc</a>,
<a href="../src/gsdll32.def">src/gsdll32.def</a>,
<a href="../src/gsdll32.rc">src/gsdll32.rc</a>,
<a href="../src/gsdll32w.lnk">src/gsdll32w.lnk</a>,
<a href="../src/gsgraph.icx">src/gsgraph.icx</a>,
<a href="../src/gsos2.def">src/gsos2.def</a>,
<a href="../src/gsos2.icx">src/gsos2.icx</a>,
<a href="../src/gsos2.rc">src/gsos2.rc</a>,
<a href="../src/gspmdrv.def">src/gspmdrv.def</a>,
<a href="../src/gspmdrv.icx">src/gspmdrv.icx</a>,
<a href="../src/gspmdrv.rc">src/gspmdrv.rc</a>,
<a href="../src/gstext.icx">src/gstext.icx</a>,
<a href="../src/gswin.rc">src/gswin.rc</a>,
<a href="../src/gswin32.rc">src/gswin32.rc</a>,
<a href="../src/gswin386.rc">src/gswin386.rc</a>,
<a href="../src/mv.bat">src/mv.bat</a>,
<a href="../src/mv.cmd">src/mv.cmd</a>,
<a href="../src/rm.bat">src/rm.bat</a>,
<a href="../src/rm.cmd">src/rm.cmd</a>,
<a href="../src/turboc.cfg">src/turboc.cfg</a>.

<dt>
Files for OpenVMS:
<dd>
<a href="../src/append_l.com">src/append_l.com</a>,
<a href="../src/copy_one.com">src/copy_one.com</a>,
<a href="../src/rm_all.com">src/rm_all.com</a>,
<a href="../src/rm_one.com">src/rm_one.com</a>.

<dt>
Other files:
<dd>
<a href="../src/bench.c">src/bench.c</a>,
<a href="../src/catmake">src/catmake</a>,
<a href="../src/ccgs">src/ccgs</a>,
<a href="../src/instcopy">src/instcopy</a>.

</dl>

<h3><a name="Utilities"></a>Utilities</h3>

<p>
Ghostscript comes with many utilities for doing things like viewing bitmap
files and converting between file formats.  Some of these are written in
PostScript, some as scripts, and some as scripts that invoke special
PostScript code.

<h4><a name="Utilities_in_PostScript"></a>Utilities in PostScript</h4>

<p>
These are all documented in <a href="Psfiles.htm">doc/Psfiles.htm</a>, q.v.

<h4><a name="Utility_scripts"></a>Utility scripts</h4>

<p>
Many of these scripts come in both Unix and MS-DOS (<b><tt>.bat</tt></b>
versions; some also have an OS/2 (<b><tt>.cmd</tt></b>) version.  The choice
of which versions are provided is historical and irregular.  These scripts
should all be documented somewhere, but currently, many of them have man
pages, a few have their own documentation in the doc directory, and some
aren't documented at all.

<dl>

<dt>
Script files without PC versions:
<dd>
<a href="../lib/afmdiff.awk">lib/afmdiff.awk</a>,
<a href="../lib/dvipdf">lib/dvipdf</a>,
<a href="../lib/fixmswrd.pl">lib/fixmswrd.pl</a>,
<a href="../lib/lprsetup.sh">lib/lprsetup.sh</a>,
<a href="../lib/pfbtopfa">lib/pfbtopfa</a>,
<a href="../lib/pj-gs.sh">lib/pj-gs.sh</a>,
<a href="../lib/pphs">lib/pphs</a>,
<a href="../lib/printafm">lib/printafm</a>,
<a href="../lib/pv.sh">lib/pv.sh</a>,
<a href="../lib/sysvlp.sh">lib/sysvlp.sh</a>,
<a href="../lib/unix-lpr.sh">lib/unix-lpr.sh</a>,
<a href="../lib/wftopfa">lib/wftopfa</a>.

<dt>
Script files with PC versions:
<dd>
<a href="../lib/bdftops">lib/bdftops</a>,
<a href="../lib/bdftops.bat">lib/bdftops.bat</a>,
<a href="../lib/bdftops.cmd">lib/bdftops.cmd</a>,
<a href="../lib/eps2eps">lib/eps2eps</a>,
<a href="../lib/eps2eps.bat">lib/eps2eps.bat</a>,
<a href="../lib/eps2eps.cmd">lib/eps2eps.cmd</a>,
<a href="../lib/font2c">lib/font2c</a>,
<a href="../lib/font2c.bat">lib/font2c.bat</a>,
<a href="../lib/font2c.cmd">lib/font2c.cmd</a>,
<a href="../lib/gsbj">lib/gsbj</a>,
<a href="../lib/gsbj.bat">lib/gsbj.bat</a>,
<a href="../lib/gsdj">lib/gsdj</a>,
<a href="../lib/gsdj.bat">lib/gsdj.bat</a>,
<a href="../lib/gsdj500">lib/gsdj500</a>,
<a href="../lib/gsdj500.bat">lib/gsdj500.bat</a>,
<a href="../lib/gslj">lib/gslj</a>,
<a href="../lib/gslj.bat">lib/gslj.bat</a>,
<a href="../lib/gslp">lib/gslp</a>,
<a href="../lib/gslp.bat">lib/gslp.bat</a>,
<a href="../lib/gsnd">lib/gsnd</a>,
<a href="../lib/gsnd.bat">lib/gsnd.bat</a>,
<a href="../lib/pdf2dsc">lib/pdf2dsc</a>,
<a href="../lib/pdf2dsc.bat">lib/pdf2dsc.bat</a>,
<a href="../lib/pdf2ps">lib/pdf2ps</a>,
<a href="../lib/pdf2ps.bat">lib/pdf2ps.bat</a>,
<a href="../lib/pdf2ps.cmd">lib/pdf2ps.cmd</a>,
<a href="../lib/pdfopt">lib/pdfopt</a>,
<a href="../lib/pdfopt.bat">lib/pdfopt.bat</a>,
<a href="../lib/pf2afm">lib/pf2afm</a>,
<a href="../lib/pf2afm.bat">lib/pf2afm.bat</a>,
<a href="../lib/pf2afm.cmd">lib/pf2afm.cmd</a>,
<a href="../lib/ps2ascii">lib/ps2ascii</a>,
<a href="../lib/ps2ascii.bat">lib/ps2ascii.bat</a>,
<a href="../lib/ps2ascii.cmd">lib/ps2ascii.cmd</a>,
<a href="../lib/ps2epsi">lib/ps2epsi</a>,
<a href="../lib/ps2epsi.bat">lib/ps2epsi.bat</a>,
<a href="../lib/ps2epsi.cmd">lib/ps2epsi.cmd</a>,
<a href="../lib/ps2pdf">lib/ps2pdf</a>,
<a href="../lib/ps2pdf.bat">lib/ps2pdf.bat</a>,
<a href="../lib/ps2pdf.cmd">lib/ps2pdf.cmd</a>,
<a href="../lib/ps2pdf12">lib/ps2pdf12</a>,
<a href="../lib/ps2pdf12.bat">lib/ps2pdf12.bat</a>,
<a href="../lib/ps2pdf12.cmd">lib/ps2pdf12.cmd</a>,
<a href="../lib/ps2pdf13">lib/ps2pdf13</a>,
<a href="../lib/ps2pdf13.bat">lib/ps2pdf13.bat</a>,
<a href="../lib/ps2pdf13.cmd">lib/ps2pdf13.cmd</a>,
<a href="../lib/ps2pdf14">lib/ps2pdf14</a>,
<a href="../lib/ps2pdf14.bat">lib/ps2pdf14.bat</a>,
<a href="../lib/ps2pdf14.cmd">lib/ps2pdf14.cmd</a>,
<a href="../lib/ps2pdfwr">lib/ps2pdfwr</a>,
<a href="../lib/ps2pdfxx.bat">lib/ps2pdfxx.bat</a>,
<a href="../lib/ps2ps">lib/ps2ps</a>,
<a href="../lib/ps2ps.bat">lib/ps2ps.bat</a>,
<a href="../lib/ps2ps.cmd">lib/ps2ps.cmd</a>.

<dt>
Script files with only PC versions:
<dd>
<a href="../lib/gsndt.bat">lib/gsndt.bat</a>,
<a href="../lib/gssetgs.bat">lib/gssetgs.bat</a>,
<a href="../lib/gst.bat">lib/gst.bat</a>,
<a href="../lib/gstt.bat">lib/gstt.bat</a>,
<a href="../lib/lp386.bat">lib/lp386.bat</a>,
<a href="../lib/lp386r2.bat">lib/lp386r2.bat</a>,
<a href="../lib/lpgs.bat">lib/lpgs.bat</a>,
<a href="../lib/lpr2.bat">lib/lpr2.bat</a>,
<a href="../lib/pftogsf.bat">lib/pftogsf.bat</a>,
<a href="../lib/wmakebat.bat">lib/wmakebat.bat</a>.

</dl>

<hr>

<h2><a name="Memory_management"></a>Memory management</h2>

<h3><a name="Memory_manager_architecture"></a>Memory manager architecture</h3>

<p>
In many environments, the memory manager is a set of library facilities that
implicitly manage the entire address space in a homogenous manner.
Ghostscript's memory manager architecture has none of these properties:

<ul>

<li>Rather than a single library accessed as procedures, Ghostscript
includes multiple allocator types, each of which in turn may have multiple
instances (allocators).  Allocators are 'objects' with a substantial set of
virtual functions.

<li>Rather than managing the entire address space, each allocator manages a
storage pool, which it may or may not be able to expand or reduce by calling
on a 'parent' allocator.

<li>Rather than a single genus of untyped storage blocks, Ghostscript's
allocators provide two genera -- type-tagged 'objects', and 'strings' --
with substantially different properties.

</ul>

<h4><a name="Objects_vs_strings"></a>Objects vs strings</h4>

<p>
As noted above, allocators provide two different storage genera.

<p>
Objects:

<ul>
<li>Are aligned in storage to satisfy the most stringent alignment
requirement imposed by the CPU or compiler;
<li>Can be referenced only by pointers to their start, not to any internal
location, unless special arrangements are made;
<li>May contain pointers to other objects, or to strings;
<li>Have an associated <em>structure descriptor</em> that specifies their
size (usually) and the location of any pointers contained within them.
</ul>

<p>
Given a pointer to an object, the allocator that allocated it must be able
to return the object's size and the pointer to its structure descriptor.
(It is up to the client to know what allocator allocated an object.)

<p>
Strings:

<ul>
<li>Are not aligned in storage;
<li>Can be referenced by pointers (consisting of a starting address and a
length) to any substring, starting anywhere within the string;
<li>May not contain pointers;
<li>Do not have a structure descriptor.
</ul>

<p>
The object/string distinction reflects a space/capability tradeoff.  The
per-object space overhead of the standard type of allocator is typically 12
bytes; this is too much to impose on every string of a few bytes.  On the
other hand, restricting object pointers to reference the start of the object
currently makes object garbage collection and compaction more
space-efficient.  If we were to redesign the standard allocator, we would
probably opt for a different design in which strings were allocated within
container objects of a few hundred bytes, and pointers into the middle of
all objects were allowed.

<h4><a name="Structure_descriptors"></a>Structure descriptors</h4>

<p>
Every object allocated by a Ghostscript allocator has an associated
structure descriptor, which the caller provides when allocating the object.
The structure descriptor serves several purposes:

<ul>
<li>Specifying the size of the object for allocation;
<li>Providing pointer-enumeration and pointer-relocation procedures for
the garbage collector;
<li>Providing an optional finalization procedure to be called when the
object is freed (either explicitly or automatically).
</ul>

<p>
Structure descriptors are read-only, and are normally defined statically
using one of the large set of <b><tt>gs_private_st_</tt></b> or
<b><tt>gs_public_st_</tt></b> macros in <a
href="../src/gsstruct.h">src/gsstruct.h</a>.

<p>
While the structure descriptor normally specifies the size of the object,
one can also allocate an array of bytes or objects, whose size is a multiple
of the size in the descriptor.  For this reason, every object stores its
size as well as a reference to its descriptor.

<p>
Because the standard Ghostscript garbage collector is conservative and can
move objects, every object allocated by a Ghostscript allocator must have an
accurate structure descriptor.  If you define a new type of object
(structure) that will be allocated in storage managed by Ghostscript, you
<em>must</em> create an accurate descriptor for it, and use that descriptor
to allocate it.  The process of creating accurate descriptors for all
structures was long and painful, and accounted for many hard-to-diagnose
bugs.

<p>
By convention, the structure descriptor for structure type
<b><tt>xxx_t</tt></b> is named <b><tt>st_xxx</tt></b> (this is preferred),
or occasionally <b><tt>st_xxx_t</tt></b>.

<p>
Note that a structure descriptor is only required for objects allocated by
the Ghostscript allocator.  A structure type <b><tt>xxx_t</tt></b> does not
require a structure descriptor if instances of that type are used
<em>only</em> in the following ways:

<ul>

<li>Instances are allocated only on the C stack, e.g., as
<b><tt>xxx_t&nbsp;xxx1,&nbsp;xxx2;</tt></b>, or on the C heap, with
<b><tt>malloc</tt></b> or through the Ghostscript "wrapper" defined in <a
href="../src/gsmalloc.h">src/gsmalloc.h</a>.

<li>Pointers to instances are not stored in places where the garbage
collector will try to trace the pointer.

<li>Code never attempts to get the structure type or size of an instance
through the allocator API.

</ul>

<p>
In general, structures without descriptors are problem-prone, and are
deprecated; in new code, they should only be used if the structure is
confined to a single .c file and its instances are only allocated on the C
stack.

<dl>
<dt>
Files:
<dd>
<a href="../src/gsstruct.h">src/gsstruct.h</a>,
<a href="../src/gsstype.h">src/gsstype.h</a>.
</dl>

<h4><a name="Garbage_collection"></a>Garbage collection</h4>

<p>
The allocator architecture is designed to support compacting garbage
collection.  Every object must be able to enumerate all the pointers it
contains, both for tracing and for relocation.  As noted just above, the
structure descriptor provides procedures that do this.

<p>
Whether or not a particular allocator type actually provides a garbage
collector is up to the allocator: garbage collection is invoked through a
virtual procedure.  In practice, however, there are only two useful garbage
collectors for Ghostscript's own allocator:

<ul>
<li>The "real" garbage collector associated with the PostScript interpreter,
described <a href="#Interpreter_GC">below</a>;
<li>A "non" garbage collector that only merges adjacent free blocks.
</ul>

<dl>
<dt>
Files:
<dd>
<a href="../src/gsgc.h">src/gsgc.h</a>,
<a href="../src/gsnogc.c">src/gsnogc.c</a>,
<a href="../src/gsnogc.h">src/gsnogc.h</a>.
</dl>

<h4><a name="Movability"></a>Movability</h4>

<p>
As just noted, objects are normally movable by the garbage collector.
However, some objects must be immovable, usually because some other piece of
software must retain pointers to them.  The allocator API includes
procedures for allocating both movable (default) and immovable objects.
Note, however, that even immovable objects must be traceable (have a
structure descriptor), and may be freed, by the garbage collector.

<h4><a name="Parent_hierarchy"></a>Parent hierarchy</h4>

<p>
When an allocator needs to add memory to the pool that it manages, it
requests the memory from its <em>parent</em> allocator.  Every allocator has
a pointer to its parent; multiple allocators may share a single parent.  The
ultimate ancestor of all allocators that can expand their pool dynamically
is an allocator that calls <b><tt>malloc</tt></b>, described <a
href="#malloc">below</a>.  However, especially in embedded environments, an
allocator may be limited to a fixed-size pool assigned to it when it is
created.

<h4><a name="Allocator_API"></a>Allocator API</h4>

In summary, the allocator API provides the following principal operations:

<ul>
<li>Allocate and free movable (default) or immovable objects and strings.
<li>Return the structure type and size of an object.
<li>Resize (shrink or grow) movable objects and strings, preserving
the contents insofar as possible.
<li>Report the size of the managed pool, and how much of it is in use.
<li>Register and unregister root pointers for the garbage collector.
<li>Free the allocator itself.
<li>Consolidate adjacent free blocks to reduce fragmentation.
</ul>

<p>
For details, see <a href="../src/gsmemory.h">src/gsmemory.h</a>.

<p>
The allocator API also includes one special hook for the PostScript
interpreter: the concept of stable allocators.  See the section on <a
href="#save_forgetsave_restore"><b><tt>save</tt></b> and
<b><tt>restore</tt></b></a> below for details.

<dl>
<dt>
Files:
<dd>
<a href="../src/gsmemraw.h">src/gsmemraw.h</a>,
<a href="../src/gsmemory.c">src/gsmemory.c</a>,
<a href="../src/gsmemory.h">src/gsmemory.h</a>,
<a href="../src/gsstruct.h">src/gsstruct.h</a>,
<a href="../src/gsstype.h">src/gsstype.h</a>.
</dl>

<h3><a name="Freeing_storage"></a>Freeing storage</h3>

<p>
Ghostscript's memory management architecture provides three different ways
to free objects: explicitly, by reference counting, or by garbage
collection.  They provide different safety / performance / convenience
tradeoffs; we believe that all three are necessary.

<p>
Objects are always freed as a whole; strings may be freed piecemeal.

<p>
An object may have an associated finalization procedure, defined in the
structure descriptor.  This procedure is called just before the object is
freed, independent of which method is being used to free the object.  A few
types of objects have a virtual finalization procedure as well: the
finalization procedure defined in the descriptor simply calls the one in the
object.

<h4><a name="Explicit_freeing"></a>Explicit freeing</h4>

<p>
Objects and strings may be freed explicitly, using the
<b><tt>gs_free_</tt></b> virtual procedures in the allocator API.  It is up
to the client to ensure that all allocated objects are freed at most once,
and that there are no dangling pointers.

<p>
Explicit freeing is the fastest method, but is the least convenient and
least safe.  It is most appropriate when storage is freed in the same
procedure where it is allocated, or for storage that is known to be
referenced by only one pointer.

<h4><a name="Reference_counting"></a>Reference counting</h4>

<p>
Objects may be managed by reference counting.  When an object is allocated,
its reference count may be set to 0 or 1.  Subsequently, when the reference
count is decremented to 0, the object is freed.

<p>
The reference counting machinery provides its own virtual finalization
procedure for all reference-counted objects.  The machinery calls this
procedure when it is about to free the object (but not when the object is
freed in any other way, which is probably a design bug).  This is in
addition to (and called before) any finalization procedure associated with
the object type.

<p>
Reference counting is as fast as explicit freeing, but takes more space in
the object.  It is most appropriate for relatively large objects which are
referenced only from a small set of pointers.  Note that reference counting
cannot free objects that are involved in a pointer cycle (e.g., A -> B -> C
-> A).

<dl>
<dt>
Files:
<dd>
<a href="../src/gsrefct.h">src/gsrefct.h</a>.
</dl>

<h4><a name="Real_garbage_collection"></a>(Real) garbage collection</h4>

<p>
Objects and strings may be freed automatically by a garbage collector.  See
<a href="#Interpreter_GC">below</a>.

<h3><a name="Special_implementations"></a>Special implementations</h3>

<h4><a name="malloc"></a>malloc</h4>

<p>
As mentioned <a href="#Parent_hierarchy">above</a>, the ultimate ancestor of
all allocators with an expandable pool is one that calls
<b><tt>malloc</tt></b>.

<dl>
<dt>
Files:
<dd>
<a href="../src/gsmalloc.h">src/gsmalloc.h</a>,
<a href="../src/gsmalloc.c">src/gsmalloc.c</a>.
</dl>

<h4><a name="Locking"></a>Locking</h4>

<p>
In a multi-threaded environment, if an allocator must be callable from
multiple threads (for example, if it is used to allocate structures in one
thread that are passed to, and freed by, another thread), the allocator must
provide mutex protection.  Ghostscript provides this capability in the form
of a <em>wrapper</em> allocator, that simply forwards all calls to a
<em>target</em> allocator under protection of a mutex.  Using the wrapper
technique, any allocator can be made thread-safe.

<dl>
<dt>
Files:
<dd>
<a href="../src/gsmemlok.h">src/gsmemlok.h</a>,
<a href="../src/gsmemlok.c">src/gsmemlok.c</a>.
</dl>

<h4><a name="Retrying"></a>Retrying</h4>

<p>
In an embedded environment, job failure due to memory exhaustion is very
undesirable.  Ghostscript provides a wrapper allocator that, when an
allocation attempt fails, calls a client-provided procedure that can attempt
to free memory, then ask for the original allocation to be retried.  For
example, such a procedure can wait for a queue to empty, or can free memory
occupied by caches.

<dl>
<dt>
Files:
<dd>
<a href="../src/gsmemret.h">src/gsmemret.h</a>,
<a href="../src/gsmemret.c">src/gsmemret.c</a>.
</dl>

<h3><a name="Standard_implementation"></a>Standard implementation</h3>

<p>
The standard Ghostscript allocator gets storage from its parent (normally
the <b><tt>malloc</tt></b> allocator) in large blocks called
<em>chunks</em>, and then allocates objects up from the low end and strings
down from the high end.  Large objects or strings are allocated in their own
chunk.

<p>
The standard allocator maintains a set of free-block lists for small object
sizes, one list per size (rounded up to the word size), plus a free-block
list for large objects (but not for objects so large that they get their own
chunk: when such an object is freed, its chunk is returned to the parent).
The lists are not sorted; adjacent blocks are only merged if needed.

<p>
While the standard allocator implements the generic allocator API, and is
usable with the library alone, it includes a special hook for the PostScript
interpreter to aid in the efficient allocation of PostScript composite
objects (arrays and dictionaries).  See the section on <a
href="#Refs">Refs</a> below for details.

<dl>
<dt>
Files:
<dd>
<a href="../src/gsalloc.c">src/gsalloc.c</a>,
<a href="../src/gsalloc.h">src/gsalloc.h</a>,
<a href="../src/gxalloc.h">src/gxalloc.h</a>,
<a href="../src/gxobj.h">src/gxobj.h</a>.
</dl>

<h3><a name="PostScript_interpreter_extensions"></a>PostScript interpreter extensions</h3>

<p>
The PostScript interpreter uses an allocator that extends the graphic
library's standard allocator to handle PostScript objects,
<b><tt>save</tt></b> and <b><tt>restore</tt></b>, and real garbage
collection.

<h4><a name="Refs"></a>Refs (PostScript "objects")</h4>

<p>
Ghostscript represents what the PLRM calls PostScript "objects" using a
structure called a <b><tt>ref</tt></b>, defined in <a
href="../src/iref.h">src/iref.h</a>; packed refs, used for the elements of
packed arrays, are defined in <a href="../src/ipacked.h">src/ipacked.h</a>.
See those files for detailed information.

<dl>
<dt>
Files:
<dd>
<a href="../src/ipacked.h">src/ipacked.h</a>,
<a href="../src/iref.h">src/iref.h</a>.
</dl>

<p>
The PLRM calls for two types of "virtual memory" (VM) space: global and
local.  Ghostscript adds a third space, <em>system</em> VM, whose lifetime
is an entire session -- i.e., it is effectively "permanent".  All three
spaces are subject to garbage collection.  There is a separate allocator
instance for each VM space (actually, two instances each for global and
local spaces; see <a href="#save_forgetsave_restore">below</a>).  In a
system with multiple contexts and multiple global or local VMs, each global
or local VM has its own allocator instance(s).

<p>
Refs that represent PostScript composite objects, and therefore include
pointers to stored data, include a 2-bit VM space tag to indicate in which
VM the object data are stored.  In addition to system, global, and local VM,
there is a tag for "foreign" VM, which means that the memory is not managed
by a Ghostscript allocator at all.  Every store into a composite object must
check for <b><tt>invalidaccess</tt></b>: the VM space tag values are chosen
to help make this check efficient.  See <a
href="../src/ivmspace.h">src/ivmspace.h</a>, <a
href="../src/iref.h">src/iref.h</a>, and <a
href="../src/store.h">src/store.h</a> for details.

<dl>
<dt>
Files:
<dd>
<a href="../src/ivmspace.h">src/ivmspace.h</a>.
</dl>

<p>
PostScript composite objects (arrays and dictionaries) are usually small.
Using a separate memory manager object for each composite object would waste
a lot of space for object headers.  Therefore, the interpreter's memory
manager packs multiple composite objects (also called "ref-containing
objects") into a single memory manager object, similar to the way the memory
manager packs multiple objects into a chunk (see <a
href="#Standard_implementation">above</a>).  See <a
href="../src/gxalloc.h">src/gxalloc.h</a> for details.  This memory manager
object has a structure descriptor, like all other memory manager objects.

<p>
Note that the <b><tt>value.pdict</tt></b>, <b><tt>value.refs</tt></b>, or
<b><tt>value.packed</tt></b> member of a ref must point to a PostScript
composite object, and therefore can point into the middle of a memory
manager object.  This requires special handling by the garbage collector (<a
href="#Interpreter_GC">q.v.</a>).

<dl>
<dt>
Files:
<dd>
<a href="../src/ialloc.c">src/ialloc.c</a>,
<a href="../src/ialloc.h">src/ialloc.h</a>,
<a href="../src/iastate.h">src/iastate.h</a>,
<a href="../src/iastruct.h">src/iastruct.h</a>,
<a href="../src/ilocate.c">src/ilocate.c</a>,
<a href="../src/imemory.h">src/imemory.h</a>,
<a href="../src/istruct.h">src/istruct.h</a>.
</dl>

<h4><a name="save_forgetsave_restore"></a>save/.forgetsave/restore</h4>

<p>
In addition to <b><tt>save</tt></b> and <b><tt>restore</tt></b>, Ghostscript
provides a <b><tt>.forgetsave</tt></b> operator that makes things as though
a given <b><tt>save</tt></b> had never happened.  (In data base terminology,
<b><tt>save</tt></b> is "begin transaction", <b><tt>restore</tt></b> is
"abort transaction", and <b><tt>.forgetsave</tt></b> is "end/commit
transaction").  <b><tt>.forgetsave</tt></b> was implemented for a specific
commercial customer (who may no longer even be using it): it was a pain to
make work, but it's in the code now, and should be maintained.  See the
extensive comments in <a href="../src/isave.c">src/isave.c</a> for more
information about how these operations work.

<dl>
<dt>
Files:
<dd>
<a href="../src/idosave.h">src/idosave.h</a>,
<a href="../src/isave.c">src/isave.c</a>,
<a href="../src/isave.h">src/isave.h</a>,
<a href="../src/isstate.h">src/isstate.h</a>,
<a href="../src/store.h">src/store.h</a>.
</dl>

<h4><a name="Stable_allocators"></a>Stable allocators</h4>

<p>
Even though <b><tt>save</tt></b> and <b><tt>restore</tt></b> are concepts
from the PostScript interpreter, the generic allocator architecture and API
include a feature to support them, called <em>stable</em> allocators.  Every
allocator has an associated stable allocator, which tags pointers with the
same VM space number but which is not subject to <b><tt>save</tt></b> and
<b><tt>restore</tt></b>.  System VM is intrinsically stable (its associated
stable allocator is the same allocator), so there are only 5 allocators in
ordinary single-context usage: system VM, stable global VM, ordinary global
VM, stable local VM, ordinary local VM.

<p>
The reason that we cannot simply allocate all stable objects in system VM is
that their refs must still be tagged with the correct VM space number, so
that the check against storing pointers from global VM to local VM can be
enforced properly.

<p>
All PostScript objects are normally allocated with the non-stable
allocators.  The stable allocators should be used with care, since using
them can easily create dangling pointers: if storage allocated with a stable
allocator contains any references to PostScript objects, the client is
responsible for ensuring that the references don't outlive the referenced
objects, normally by ensuring that any such referenced objects are allocated
at the outermost <b><tt>save</tt></b> level.  (The original reason for
wanting stable allocators was the PostScript stacks, which are essentially
PostScript arrays but are not subject to <b><tt>save</tt></b> and
<b><tt>restore</tt></b>.)  For more examples, search the sources for
references to <b><tt>gs_memory_stable</tt></b>.

<h4><a name="Interpreter_GC"></a>Garbage collection</h4>

<p>
The interpreter's garbage collector is a compacting, non-conservative,
mark-and-sweep collector.

<ul>
<li>It compacts storage because that is the only way to avoid permanent
sandbars, which is essential in limited-memory environments.
<li>It is non-conservative because conservative collectors (which attempt
to determine whether arbitrary bit patterns are pointers) cannot compact.
<li>It uses mark-and-sweep, rather than a more modern copying approach,
because it cannot afford the extra memory required for copying.
</ul>

<p>
Roots for tracing must be registered with the allocator.  Most roots are
registered during initialization.

<p>
"Mark-and-sweep" is a bit of a misnomer.  The garbage collector actually has
5 main phases:

<ul>
<li>Sweep to clear marks;
<li>Trace and mark;
<li>Sweep to compute relocation;
<li>Sweep to relocate pointers;
<li>Sweep and compact.
</ul>

<p>
There is some extra complexity to handle collecting local VM only.  In this
case, all pointers in global VM are treated as roots, and global VM is not
compacted.

<p>
As noted above, PostScript arrays and strings can have refs that point
within them (because of <b><tt>getinterval</tt></b>).  Thus the garbage
collector must mark each element of an array, and even each byte of a
string, individually.  Specifically, it marks objects, refs, and strings
using 3 different mechanisms:

<ul>

<li>Objects have a mark bit in their header: see
<a href="../src/gxobj.h">src/gxobj.h</a>,

<li>Refs and packed refs have a reserved mark bit: see <a
href="../src/iref.h">src/iref.h</a> and <a
href="../src/ipacked.h">src/ipacked.h</a>.

<li>Strings use a separate bit table, with one bit per string byte: see
<a href="../src/gxalloc.h">src/gxalloc.h</a>,

</ul>

<p>
Similarly, it records the relocation information for objects, refs, and
strings differently:

<ul>

<li>Objects record relocation in the object header.

<li>Refs record relocation in unused fields of refs such as nulls that
don't use their <b><tt>value</tt></b> field.  Every memory manager object
that stores ref-containing objects as described above has an extra, unused
ref at the end for this purpose.

<li>Strings use a separate relocation table.

</ul>

<dl>
<dt>
Files:
<dd>
<a href="../src/igc.c">src/igc.c</a>,
<a href="../src/igc.h">src/igc.h</a>,
<a href="../src/igcref.c">src/igcref.c</a>,
<a href="../src/igcstr.c">src/igcstr.c</a>,
<a href="../src/igcstr.h">src/igcstr.h</a>,
<a href="../src/ireclaim.c">src/ireclaim.c</a>.
</dl>

<hr>

<h2><a name="Portability"></a>Portability</h2>

<p>
One of Ghostscript's most important features is its great portability across
platforms (CPUs, operating systems, compilers, and build tools).  The code
supports portability through two mechanisms:

<ul>

<li><a href="#Structural">Structural mechanisms</a> -- segregating
platform-dependent information into files in a particular way.

<li><a href="#Coding">Coding standards</a> -- avoiding relying on byte
order, scalar size, and platform-specific compiler or library features.

</ul>

<h3><a name="Structural"></a>Structural</h3>

<h4><a name="CPU_and_compiler"></a>CPU and compiler</h4>

<p>
Ghostscript attempts to discover characteristics of the CPU and compiler
automatically during the build process, by compiling and then executing a
program called <b><tt>genarch</tt></b>.  <b><tt>genarch</tt></b> generates a
file <b><tt>obj/arch.h</tt></b>, which almost all Ghostscript files then
include.  This works well for things like word size, byte order, and
floating point representation, but it can't determine whether or not a
compiler supports a particular feature, because if a feature is absent, the
compilation may fail.

<dl>
<dt>
Files:
<dd>
<a href="../src/genarch.c">src/genarch.c</a>,
<a href="../obj/arch.h">obj/arch.h</a>.
</dl>

<h4><a name="Library_headers"></a>Library headers</h4>

<p>
Despite the supposed standardization of ANSI C, platforms vary considerably
in where (and whether) they provide various standard library facilities.
Currently, Ghostscript's build process doesn't attempt to sort this out
automatically.  Instead, for each library header file
<b><tt>&lt;</tt></b><em>xxx</em><b><tt>.h&gt;</tt></b> there is a
corresponding Ghostscript source file
<b><tt>src/</tt></b><em>xxx</em><b><tt>_.h</tt></b>, containing a set of
compile-time conditionals that attempt to select the correct platform header
file, or in some cases substitute Ghostscript's own code for a missing
facility.  You may need to edit these files when moving to platforms with
unusually non-standard libraries.

<dl>
<dt>
Files:
<dd>
<a href="../src/ctype_.h">src/ctype_.h</a>,
<a href="../src/dirent_.h">src/dirent_.h</a>,
<a href="../src/dos_.h">src/dos_.h</a>,
<a href="../src/errno_.h">src/errno_.h</a>,
<a href="../src/fcntl_.h">src/fcntl_.h</a>,
<a href="../src/jerror_.h">src/jerror_.h</a>,
<a href="../src/malloc_.h">src/malloc_.h</a>,
<a href="../src/math_.h">src/math_.h</a>,
<a href="../src/memory_.h">src/memory_.h</a>,
<a href="../src/pipe_.h">src/pipe_.h</a>,
<a href="../src/png_.h">src/png_.h</a>,
<a href="../src/stat_.h">src/stat_.h</a>,
<a href="../src/stdio_.h">src/stdio_.h</a>,
<a href="../src/string_.h">src/string_.h</a>,
<a href="../src/time_.h">src/time_.h</a>,
<a href="../src/unistd_.h">src/unistd_.h</a>,
<a href="../src/vmsmath.h">src/vmsmath.h</a>,
<a href="../src/windows_.h">src/windows_.h</a>,
<a href="../src/x_.h">src/x_.h</a>.
</dl>

<p>
It has been suggested that the GNU <b><tt>configure</tt></b> scripts do the
above better, for Unix systems, than Ghostscript's current methods.  While
this may be true, we have found <b><tt>configure</tt></b> scripts difficult
to write, understand, and maintain; and the <b><tt>autoconf</tt></b> tool
for generating <b><tt>configure</tt></b> scripts, which we found easy to
use, doesn't cover much of the ground that Ghostscript requires.

<h4><a name="Cross_platform_APIs"></a>Cross-platform APIs</h4>

<p>
For a few library facilities that are available on all platforms but are not
well standardized, or that may need to be changed for special environments,
Ghostscript defines its own APIs.  It is an architectural property of
Ghostscript that the implementations of these APIs are the only .c files for
which the choice of platform (as opposed to choices of drivers or optional
features) determines whether they are compiled and linked into an
executable.

<dl>

<dt>
API:
<dd>
<a href="../src/gp.h">src/gp.h</a>,
<a href="../src/gpcheck.h">src/gpcheck.h</a>,
<a href="../src/gpgetenv.h">src/gpgetenv.h</a>,
<a href="../src/gpmisc.h">src/gpmisc.h</a>,
<a href="../src/gpsync.h">src/gpsync.h</a>.

<dt>
Implementation files shared among multiple platforms:
<dd>
<a href="../src/gp_getnv.c">src/gp_getnv.c</a>,
<a href="../src/gp_mktmp.c">src/gp_mktmp.c</a>,
<a href="../src/gp_nsync.c">src/gp_nsync.c</a>,
<a href="../src/gp_psync.c">src/gp_psync.c</a>,
<a href="../src/gp_strdl.c">src/gp_strdl.c</a>,
<a href="../src/gpmisc.c">src/gpmisc.c</a>.

<dt>
Platform-specific implementation files:
<dd>
<a href="../src/gp_dosfe.c">src/gp_dosfe.c</a>,
<a href="../src/gp_dosfs.c">src/gp_dosfs.c</a>,
<a href="../src/gp_dvx.c">src/gp_dvx.c</a>,
<a href="../src/gp_iwatc.c">src/gp_iwatc.c</a>,
<a href="../src/gp_msdos.c">src/gp_msdos.c</a>,
<a href="../src/gp_mshdl.c">src/gp_mshdl.c</a>,
<a href="../src/gp_msio.c">src/gp_msio.c</a>,
<a href="../src/gp_mslib.c">src/gp_mslib.c</a>,
<a href="../src/gp_mswin.c">src/gp_mswin.c</a>,
<a href="../src/gp_mswin.h">src/gp_mswin.h</a>,
<a href="../src/gp_ntfs.c">src/gp_ntfs.c</a>,
<a href="../src/gp_os2.c">src/gp_os2.c</a>,
<a href="../src/gp_os9.c">src/gp_os9.c</a>,
<a href="../src/gp_stdia.c">src/gp_stdia.c</a>,
<a href="../src/gp_stdin.c">src/gp_stdin.c</a>,
<a href="../src/gp_sysv.c">src/gp_sysv.c</a>,
<a href="../src/gp_unifn.c">src/gp_unifn.c</a>,
<a href="../src/gp_unifs.c">src/gp_unifs.c</a>,
<a href="../src/gp_unix.c">src/gp_unix.c</a>,
<a href="../src/gp_vms.c">src/gp_vms.c</a>,
<a href="../src/gp_wgetv.c">src/gp_wgetv.c</a>,
<a href="../src/gp_win32.c">src/gp_win32.c</a>,
<a href="../src/gp_wsync.c">src/gp_wsync.c</a>.

</dl>

<h4><a name="Makefiles"></a>Makefiles</h4>

<p>
For information on the structure and conventions used within makefiles, see
the <a href="#Makefile_structure">Makefile structure</a> section above.

<p>
Ghostscript's makefiles are structured very similarly to the cross-platform
library files.  The great majority of the makefiles are portable across all
platforms and all versions of <b><tt>make</tt></b>.  To achieve this, the
platform-independent makefiles must obey two constraints beyond those of the
POSIX <b><tt>make</tt></b> program:

<ul>

<li>No conditionals or <b><tt>include</tt></b>s are allowed.  While most
<b><tt>make</tt></b> programs now provide some form of conditional execution
and some form of inclusion, there is no agreement on the syntax.
(Conditionals and includes are allowed in platform-dependent makefiles; in
fact, an inclusion facility is required.)

<li>There must be a space on both sides of the : that separates the target
of a rule from its dependencies.  This is required for compatibility with
the OpenVMS <b><tt>MMS</tt></b> and <b><tt>MMK</tt></b> programs.

</ul>

<p>
The top-level makefile for each platform (where "platform" includes the OS,
the compiler, and the flavor of <b><tt>make</tt></b>) contains all the build
options, plus <b><tt>include</tt></b>s for the generic makefiles and any
platform-dependent makefiles that are shared among multiple platforms.

<p>
While most of the top-level makefiles build a PostScript and/or PDF
interpreter configuration, there are also a few makefiles that build a test
program that only uses the graphics library without any language
interpreter.  Among other things, this can be helpful in verifying that no
accidental dependencies on the interpreter have crept into the library or
drivers.

<p>
For families of similar platforms, the question arises whether to use
multiple top-level makefiles, or whether to use a single top-level makefile
that may require minor editing for some (or all) platforms.  Ghostscript
currently uses the following top-level makefiles for building interpreter
configurations:

<ul>

<li>Unix (including Linux):
<ul>
<li><a href="../src/unix-gcc.mak">src/unix-gcc.mak</a>,
for Unix with gcc.
<li><a href="../src/unixansi.mak">src/unixansi.mak</a>,
for Unix with an ANSI C compiler other than gcc.
<li><a href="../src/unixtrad.mak">src/unixtrad.mak</a>,
for Unix with a "traditional" or "K&amp;R" compiler.
</ul>

<li>PC:
<ul>
<li><a href="../src/bcwin32.mak">src/bcwin32.mak</a>,
for MS Windows with Borland C++ Builder.
<li><a href="../src/msvc32.mak">src/msvc32.mak</a>,
for MS Windows with Microsoft Visual C (MSVC).
<li><a href="../src/os2.mak">src/os2.mak</a>,
for MS-DOS or OS/2 GCC/EMX environment.
<li><a href="../src/watc.mak">src/watc.mak</a>,
for extended MS-DOS with Watcom C.
<li><a href="../src/watcw32.mak">src/watcw32.mak</a>,
for MS Windows with Watcom C.
</ul>

<li>Macintosh:
<ul>
<li><a href="../src/macos-mcp.mak">src/macos-mcp.mak</a>,
for Macintosh MCP.
</ul>


<li>Other:
<ul>
<li><a href="../src/all-arch.mak">src/all-arch.mak</a>,
for building on many Unix systems in a networked test environment.
<li><a href="../src/dvx-gcc.mak">src/dvx-gcc.mak</a>,
for DesqView/X with gcc.
<li><a href="../src/openvms.mak">src/openvms.mak</a>,
for OpenVMS with Digital's CC compiler and the MMS build program.
<li><a href="../src/openvms.mmk">src/openvms.mmk</a>,
for OpenVMS with Digital's CC compiler and the MMK build program.
</ul>

</ul>

<p>
The following top-level makefiles build the library test program:

<ul>
<li><a href="../src/ugcclib.mak">src/ugcclib.mak</a>,
on Unix with gcc.
<li><a href="../src/msvclib.mak">src/msvclib.mak</a>,
on MS Windows with MSVC.
<li><a href="../src/watclib.mak">src/watclib.mak</a>,
on extended MS-DOS with Watcom C.
</ul>

<p>
The MSVC makefiles may require editing to select between different versions
of MSVC, since different versions may have slightly incompatible command
line switches or customary installation path names.  The Unix makefiles
often require editing to deal with differing library path names and/or
library names.  For details, see <a href="Make.htm#Unix_build">the Unix
section</a> of the documentation for building Ghostscript.

<dl>

<dt>
Library test program:
<dd>
<a href="../src/gslib.c">src/gslib.c</a>.

<dt>
Platform-independent makefiles:
<dd>

<dl>

<dt>
Graphics library and support:
<dd>
<a href="../src/contrib.mak">src/contrib.mak</a>,
<a href="../src/devs.mak">src/devs.mak</a>,
<a href="../src/gs.mak">src/gs.mak</a>,
<a href="../src/lib.mak">src/lib.mak</a>,
<a href="../src/version.mak">src/version.mak</a>.

<dt>
PostScript interpreter and fonts:
<dd>
<a href="../src/cfonts.mak">src/cfonts.mak</a>,
<a href="../src/int.mak">src/int.mak</a>,
<a href="../src/wmin.mak">src/wmin.mak</a>.

<dt>
Third-party libraries:
<dd>
<a href="../src/icclib.mak">src/icclib.mak</a>,
<a href="../src/ijs.mak">src/ijs.mak</a>,
<a href="../src/jpeg.mak">src/jpeg.mak</a>,
<a href="../src/libpng.mak">src/libpng.mak</a>,
<a href="../src/zlib.mak">src/zlib.mak</a>.

</dl>

<dt>
Shared platform-dependent makefiles:
<dd>

<dl>

<dt>
Unix:
<dd>
<a href="../src/unix-aux.mak">src/unix-aux.mak</a>,
<a href="../src/unix-dll.mak">src/unix-dll.mak</a>,
<a href="../src/unix-end.mak">src/unix-end.mak</a>,
<a href="../src/unixhead.mak">src/unixhead.mak</a>,
<a href="../src/unixinst.mak">src/unixinst.mak</a>,
<a href="../src/unixlink.mak">src/unixlink.mak</a>.

<dt>
Microsoft Windows and MS-DOS:
<dd>
<a href="../src/msvccmd.mak">src/msvccmd.mak</a>,
<a href="../src/msvctail.mak">src/msvctail.mak</a>,
<a href="../src/pcwin.mak">src/pcwin.mak</a>,
<a href="../src/wccommon.mak">src/wccommon.mak</a>,
<a href="../src/wctail.mak">src/wctail.mak</a>,
<a href="../src/winint.mak">src/winint.mak</a>,
<a href="../src/winlib.mak">src/winlib.mak</a>,
<a href="../src/winplat.mak">src/winplat.mak</a>.

<dt>
Other:
<dd>
<a href="../src/dvx-head.mak">src/dvx-head.mak</a>,
<a href="../src/dvx-tail.mak">src/dvx-tail.mak</a>.

</dl>

</dl>

<h3><a name="Coding"></a>Coding</h3>

<p>
Coding for portability requires avoiding both <em>explicit</em>
dependencies, such as platform-dependent <b><tt>#ifdef</tt></b>s, and
<em>implicit</em> dependencies, such as dependencies on byte order or the
size of the integral types.

<h4><a name="Explicit_dependencies"></a>Explicit dependencies</h4>

<p>
The platform-independent .c files never, ever, use <b><tt>#ifdef</tt></b> or
<b><tt>#if</tt></b> to select code for specific platforms.  Instead, we
always try to characterize some abstract property that is being tested.  For
example, rather than checking for macros that are defined on those specific
platforms that have 64-bit <b><tt>long</tt></b> values, we define a macro
<b><tt>ARCH_SIZEOF_LONG</tt></b> that can then be tested.  Such macros are
always defined in a .h file, either automatically in <b><tt>arch.h</tt></b>,
or explicitly in a <em>xxx</em><b><tt>_.h</tt></b> file, as described in
earlier sections.

<dl>
<dt>
Files:
<dd>
<a href="../src/std.h">src/std.h</a>,
<a href="../src/stdpre.h">src/stdpre.h</a>.
</dl>

<h4><a name="Implicit_dependencies"></a>Implicit dependencies</h4>

<p>
The most common source of byte ordering dependencies is casting between
types (T1 *) and (T2 *) where T1 and T2 are numeric types that aren't merely
signed/unsigned variants of each other.  To avoid this, the only casts
allowed in the code are between numeric types, from a pointer type to a long
integral type, and between pointer types.

<p>
Ghostscript's code assumes the following about the sizes of various types:

<dl>
<dt>char<dd>8 bits
<dt>short<dd>16 bits
<dt>int<dd>32 or 64 bits
<dt>long<dd>32 or 64 bits
<dt>float<dd>32 bits (may work with 64 bits)
<dt>double<dd>64 bits (may work with 128 bits)
</dl>

<p>
The code does not assume that the <b><tt>char</tt></b> type is signed (or
unsigned); except for places where the value is always a literal string, or
for interfacing to library procedures, the code uses <b><tt>byte</tt></b> (a
Ghostscript synonym for <b><tt>unsigned char</tt></b>) almost everywhere.

<p>
Pointers are signed on some platforms and unsigned on others.  In the few
places in the memory manager where it's necessary to reliably order-compare
(as opposed to equality-compare) pointers that aren't known to point to the
same allocated block of memory, the code uses the
<b><tt>PTR_</tt></b><em>relation</em> macros rather than direct comparisons.

<p>
See the files listed above for other situations where a macro provides
platform-independence or a workaround for bugs in specific compilers or
libraries (of which there are a distressing number).

<h4><a name="Platform_specific_code"></a>Platform-specific code</h4>

<p>
There are some features that are inherently platform-specific:

<ul>

<li>Microsoft Windows requires a lot of special top-level code, and also has
an installer and uninstaller.

<li>OS/2 requires a little special code.

<li>MacOS also requires special top-level code (now distributed with the
standard Ghostscript package).

<li>All platforms supporting DLLs (currently all three of the above) share
some special top-level code.

</ul>

<dl>

<dt>
MS Windows files:
<dd>
<a href="../src/dpmain.c">src/dpmain.c</a>,
<a href="../src/dwdll.c">src/dwdll.c</a>,
<a href="../src/dwdll.h">src/dwdll.h</a>,
<a href="../src/dwimg.c">src/dwimg.c</a>,
<a href="../src/dwimg.h">src/dwimg.h</a>,
<a href="../src/dwinst.cpp">src/dwinst.cpp</a>,
<a href="../src/dwinst.h">src/dwinst.h</a>,
<a href="../src/dwmain.c">src/dwmain.c</a>,
<a href="../src/dwmain.h">src/dwmain.h</a>,
<a href="../src/dwmainc.c">src/dwmainc.c</a>,
<a href="../src/dwnodll.c">src/dwnodll.c</a>,
<a href="../src/dwreg.c">src/dwreg.c</a>,
<a href="../src/dwreg.h">src/dwreg.h</a>,
<a href="../src/dwsetup.cpp">src/dwsetup.cpp</a>,
<a href="../src/dwsetup.h">src/dwsetup.h</a>,
<a href="../src/dwtext.c">src/dwtext.c</a>,
<a href="../src/dwtext.h">src/dwtext.h</a>,
<a href="../src/dwuninst.cpp">src/dwuninst.cpp</a>,
<a href="../src/dwuninst.h">src/dwuninst.h</a>,
<a href="../src/gp_msdll.c">src/gp_msdll.c</a>,
<a href="../src/gp_mspol.c">src/gp_mspol.c</a>,
<a href="../src/gp_msprn.c">src/gp_msprn.c</a>,
<a href="../src/gs16spl.c">src/gs16spl.c</a>,
<a href="../src/gsdllwin.h">src/gsdllwin.h</a>.

<dt>
OS/2 files:
<dd>
<a href="../src/gsdllos2.h">src/gsdllos2.h</a>.

<dt>
Unix files:
<dd>
<a href="../src/dxmain.c">src/dxmain.c</a>.
<a href="../src/dxmainc.c">src/dxmainc.c</a>.

<dt>
Macintosh files:
<dd>
<a href="../src/gdevmac.c">src/gdevmac.c</a>.
<a href="../src/gdevmac.h">src/gdevmac.h</a>.
<a href="../src/gdevmacpictop.h">src/gdevmacpictop.h</a>.
<a href="../src/gdevmacttf.h">src/gdevmacttf.h</a>.
<a href="../src/gdevmacxf.c">src/gdevmacxf.c</a>.
<a href="../src/gp_mac.c">src/gp_mac.c</a>.
<a href="../src/gp_mac.h">src/gp_mac.h</a>.
<a href="../src/gp_macio.c">src/gp_macio.c</a>.
<a href="../src/macgenmcpxml.sh">src/macgenmcpxml.sh</a>.
<a href="../src/macsysstat.h">src/macsysstat.h</a>.
<a href="../src/macsystypes.h">src/macsystypes.h</a>.

<dt>
VMS files:
<dd>
<a href="../src/vms_x_fix.h">src/vms_x_fix.h</a>.

<dt>
DLL files:
<dd>
<a href="../src/gsdll.c">src/gsdll.c</a>,
<a href="../src/gsdll.h">src/gsdll.h</a>,
<a href="../src/gdevdsp.c">src/gdevdsp.c</a>,
<a href="../src/gdevdsp.h">src/gdevdsp.h</a>,
<a href="../src/gdevdsp2.h">src/gdevdsp2.h</a>,
<a href="../src/iapi.c">src/iapi.c</a>,
<a href="../src/iapi.h">src/iapi.h</a>,
<a href="../src/idisp.c">src/idisp.c</a>,
<a href="../src/idisp.h">src/idisp.h</a>.
<p>
The new DLL interface (new as of 7.0) is especially useful with the
new display device, so it is included here. Both are due to Russell
Lang.

</dl>


<hr>

<h2><a name="Adding_features_and_options"></a>Adding features and options</h2>

<p>
[Ray, please supply more information about what you want here]

<h2><a name="Troubleshooting"></a>Troubleshooting</h2>

<p>
The Ghostscript code has many tracing and debugging features that can be
enabled at run time using the <b><tt>-Z</tt></b> command line switch, if the
executable was compiled with <b><tt>DEBUG</tt></b> defined.  One
particularly useful combination is <b><tt>-Z@\?</tt></b>, which fills free
memory blocks with a pattern and also turns on run-time memory consistency
checking.  For more information, see <a
href="Use.htm#Debugging">doc/Use.htm#Debugging</a>; you can also search for
occurrences of <b><tt>if_debug</tt></b> or <b><tt>gs_debug_c</tt></b> in the
source code.  Note that many of these features are in the graphics library
and do not require a PostScript interpreter.

<p>
The code also contains many run-time procedures whose only purpose is to be
called from the debugger to print out various data structures, including all
the procedures in <a href="../src/idebug.c">src/idebug.c</a> (for the
PostScript interpreter) and the <b><tt>debug_dump_</tt></b> procedures in <a
href="../src/gsmisc.c">src/gsmisc.c</a>.

<dl>
<dt>
Files:
<dd>
<a href="Use.htm#Debugging">doc/Use.htm#Debugging</a>,
<a href="../src/gdebug.h">src/gdebug.h</a>,
<a href="../src/gsmdebug.h">src/gsmdebug.h</a>,
<a href="../src/idebug.h">src/idebug.h</a>,
<a href="../src/idebug.c">src/idebug.c</a>.
</dl>

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<hr>

<p>
<small>Copyright &copy; 2001 artofcode LLC.
All rights reserved.</small>

<p>
<small>This file is part of AFPL Ghostscript.  See the <a
href="Public.htm">Aladdin Free Public License</a> (the "License") for full
details of the terms of using, copying, modifying, and redistributing
AFPL Ghostscript.</small>

<p>
<small>Ghostscript version 7.04, 31 January 2002

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</html>