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<h1>Guide to Ghostscript Source Code</h1>
<h2><a name="toc"></a>Table of contents</h2>
<ul class="toc">
<li><a href="#Overview">Conceptual overview</a></li>
<li><a href="#PostScript_interpreter">PostScript Interpreter</a></li>
<li><a href="#PDF_interpreter">PDF interpreter</a></li>
<li><a href="#Graphics_library">Graphics library</a></li>
<li>
<ul>
<li><a href="#Drivers">Device drivers</a></li>
<li><a href="#Platform_specific_code">Platform-specific code</a></li>
</ul>
</li>
<li><a href="#Makefiles">Makefiles</a></li>
</ul>
<!-- [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 documents on <a href="Make.htm">how to build
Ghostscript</a> from source, <a href="C-style.htm">Ghostscript C coding
guidelines</a>, <a href="Drivers.htm">drivers</a>, the
<a href="Lib.htm">Ghostscript library</a> and <a href="Install.htm">how to
install Ghostscript</a>.
<!-- [1.3 end hint] ======================================================== -->
<hr>
<!-- [1.0 end visible header] ============================================== -->
<!-- [2.0 begin contents] ================================================== -->
<h2><a name="Overview"></a>Conceptual overview</h2>
<p>
The Ghostscript source code is divided conceptually as follows:
<blockquote><table>
<tr>
<th align="left" colspan="2">PostScript interpreter:</th>
</tr>
<tr>
<td>PostScript operators</td>
<td><code>z</code>*<code>.h</code> and <code>z</code>*<code>.c</code></td>
</tr>
<tr>
<td>Other interpreter code</td>
<td><code>i</code>*<code>.h</code> and <code>i</code>*<code>.c</code></td>
</tr>
<tr>
<td>PostScript code</td>
<td><code>gs_</code>*<code>.ps</code></td>
</tr>
<tr>
<th align="left" colspan="2">PDF interpreter:</th>
</tr>
<tr>
<td>PostScript code</td>
<td><code>pdf_</code>*<code>.ps</code></td>
</tr>
<tr>
<th align="left" colspan="2">Graphics library:</th>
</tr>
<tr>
<td>Main library code</td>
<td><code>g</code>*<code>.h</code> and <code>g</code>*<code>.c</code></td>
</tr>
<tr>
<td>Streams</td>
<td><code>s</code>*<code>.h</code> and <code>s</code>*<code>.c</code></td>
</tr>
<tr>
<td><a href="#Drivers">Device drivers</a></td>
<td><code>gdev</code>*<code>.h</code> and <code>gdev</code>*<code>.c</code></td>
</tr>
<tr>
<td><a href="#Platform_specific_code">Platform-specific code</a></td>
<td><code>gp</code>*<code>.h</code> and <code>gp</code>*<code>.c</code></td>
</tr>
</table></blockquote>
<hr>
<h2><a name="PostScript_interpreter"></a>PostScript Interpreter</h2>
<p>
<code>gs.c</code> is the main program for the interactive language
interpreter; <code>gserver.c</code> is an alternative main program that
is a rudimentary server. If you configure Ghostscript as a server rather
than an interactive program, you will use <code>gserver.c</code> instead
of <code>gs.c</code>.
<p>
Files named <code>z</code>*<code>.c</code> are Ghostscript operator
files. The names of the files generally follow the section headings of the
operator summary in section 6.2 (Second Edition) or 8.2 (Third Edition) of
the PostScript Language Reference Manual. Each operator XXX is implemented
by a procedure named <code>z</code>XXX, for example,
<code>zfill</code> and <code>zarray</code>.
<p>
Files named <code>i</code>*<code>.c</code>, and *<code>.h</code>
other than <code>g</code>*<code>.h</code>, are the rest of the
interpreter. See the makefile for a little more information on how the
files are divided functionally.
<p>
The main loop of the PostScript interpreter is the <code>interp</code>
procedure in <code>interp.c</code>. When the interpreter is reading
from an input file, it calls the token scanner in
<code>iscan</code>*<code>.c</code>.
<p>
<code>idebug.c</code> contains a lot of debugger-callable routines
useful for printing PostScript objects when debugging.
<hr>
<h2><a name="PDF_interpreter"></a>PDF interpreter</h2>
<p>
The PDF interpreter is written entirely in PostScript. Its main loop is
the <code>.pdfrun</code> procedure in <code>pdf_base.ps</code>. When
the PDF interpreter is configured into the build, it redefines the
"<code>run</code>" operator to test whether the file is a PDF file.
This redefinition is near the beginning of <code>pdf_main.ps</code>.
<hr>
<h2><a name="Graphics_library"></a>Graphics library</h2>
<p>
Files beginning with <code>gs</code>, <code>gx</code>, or
<code>gz</code> (both <code>.c</code> and <code>.h</code>), other
than <code>gs.c</code> and <code>gserver.c</code>, are the
Ghostscript library. Files beginning with <code>gdev</code> are device
drivers or related code, also part of the library. Other files beginning
with <code>g</code> are library files that don't fall neatly into either
the kernel or the driver category.
<p>
Files named <code>s</code>*<code>.c</code> and
<code>s</code>*<code>.h</code> are a flexible stream package,
including the Level 2 PostScript "filters" supported by Ghostscript. See
<code>stream.h</code>, <code>scommon.h</code>, and
<code>strimpl.h</code> for all the details.
<h3><a name="Drivers"></a>Device drivers</h3>
<p>
The interface between the graphics library and device drivers is the only
really well documented one in all of Ghostscript: see the
<a href="Drivers.htm">documentation on drivers</a>.
<p>
In addition to many real device and file format drivers listed in
<code>devs.mak</code> and <code>contrib.mak</code>, a number of
drivers are used for internal purposes. You can search
<code>lib.mak</code> for files named
<code>gdev</code>*<code>.c</code> to find almost all of them.
<p>
Drivers are divided into "printer" drivers, which support banding, and
non-printer drivers, which don't. The decision whether banding is
required is made (by default on the basis of how much memory is available)
in the procedure <code>gdev_prn_alloc</code> in
<code>gdevprn.c</code>: it implements this decision by filling the
virtual procedure table for the printer device in one of two different
ways.
<p>
A good simple "printer" (bandable) driver to read is
<code>gdevmiff.c</code>: it's less than 100 lines, of which much is
boilerplate. There are no simple non-printer drivers that actually drive
devices: probably the simplest non-printer driver for reading is
<code>gdevm8.c</code>, which implements 8-bit-deep devices that only
store the bits in memory.
<h3><a name="Platform_specific_code"></a>Platform-specific code</h3>
<p>
There are very few platform dependencies in Ghostscript. Ghostscript deals
with them in three ways:
<ul>
<li>Files named *<code>_.h</code> substitute for the corresponding
<code><</code>*<code>.h></code> file by adding conditionals
that provide a uniform set of system interfaces on all platforms.
<li>The file <code>arch.h</code> contains a set of
mechanically-discovered platform properties like byte order, size of
<code>int</code>, etc. These properties, <b>not</b> the names of
specific platforms, are used to select between different algorithms or
parameters at compile time.
<li>Files named <code>gp</code>*<code>.h</code> define interfaces
that are intended to be implemented differently on each platform, but whose
specification is common to all platforms.
</ul>
<p>
The platform-specific implementations of the
<code>gp</code>*<code>.h</code> interfaces have names of the form
"<code>gp_</code><em>{platform}</em><code>.c</code>, specifically
(this list may be out of date):
<blockquote><table>
<tr>
<th colspan="2">Platform-specific interfaces</th>
</tr>
<tr>
<th align="left">Routine</th>
<th align="left">Platform</th>
</tr>
<tr>
<td><code>gp_dosfb.c</code></td>
<td>DOS</td>
</tr>
<tr>
<td><code>gp_dosfs.c</code></td>
<td>DOS and MS Windows</td>
</tr>
<tr>
<td><code>gp_itbc.c</code></td>
<td>DOS, Borland compilers</td>
</tr>
<tr>
<td><code>gp_iwatc.c</code></td>
<td>DOS, Watcom or Microsoft compiler</td>
</tr>
<tr>
<td><code>gp_msdos.c</code></td>
<td>DOS and MS Windows</td>
</tr>
<tr>
<td><code>gp_ntfs.c</code></td>
<td>MS Windows NT</td>
</tr>
<tr>
<td><code>gp_os2.c</code></td>
<td>OS/2</td>
</tr>
<tr>
<td><code>gp_os9.c</code></td>
<td>OS-9</td>
</tr>
<tr>
<td><code>gp_unifs.c</code></td>
<td>Unix, OS-9, and QNX</td>
</tr>
<tr>
<td><code>gp_unix.c</code></td>
<td>Unix and QNX</td>
</tr>
<tr>
<td><code>gp_vms.c</code></td>
<td>VMS</td>
</tr>
<tr>
<td><code>gp_win32.c</code></td>
<td>MS Windows NT</td>
</tr>
</table></blockquote>
<p>
If you are going to extend Ghostscript to new machines or operating
systems, check the *<code>_.h</code> files for <code>ifdef</code> on
things other than <code>DEBUG</code>. You should probably plan to make
a new makefile and a new <code>gp_</code>XXX<code>.c</code> file.
<hr>
<h2><a name="Makefiles"></a>Makefiles</h2>
<p>
This section is only for advanced developers who need to integrate
Ghostscript into a larger program at build time.
<p>
NOTE: THIS SECTION IS INCOMPLETE. IT WILL BE IMPROVED IN A LATER REVISION.
<p>
The Ghostscript makefiles are meant to be organized according to the
following two principles:
<ul>
<li>All the parameters that vary from platform to platform appear in the
top-level makefile for a given platform. ("Platform" = OS + compiler +
choice of interpreter vs. library)
<li>All the rules and definitions that can meaningfully be shared among more
than 1 platform appear in a makefile that is "included" by a makefile
(normally the top-level makefile) for those platforms.
</ul>
<p>
Thus, for example:
<ul>
<li>Rules and definitions shared by all builds are in
<code>gs.mak</code>.
<li>Rules and definitions specific to the library (on all platforms) are in
<code>lib.mak</code>. In principle this could be merged with
<code>gs.mak</code>, but we wanted to leave open the possibility that
<code>gs.mak</code> might be useful with hypothetical interpreter-only
products.
<li>Stuff specific to interpreters (on all platforms) is in
<code>int.mak</code>.
<li>Stuff specific to all Unix platforms should be in a single
<code>unix.mak</code> file, but because <code>make</code> sometimes
cares about the order of definitions, and because some of it is shared with
DV/X, it got split between <code>unix-aux.mak</code>,
<code>unix-end.mak</code>, <code>unixhead.mak</code>,
<code>unixinst.mak</code>, and <code>unixlink.mak</code>.
</ul>
<p>
For MS-DOS and MS Windows builds, there should be:
<ul>
<li>A makefile for all MS OS (DOS or Windows) builds, for all
compilers and products.
<li>Perhaps a makefile for all MS-DOS builds, for all compilers and
products, although since Watcom is the only such compiler we're likely to
support this may be overkill.
<li>A makefile for all MS Windows builds, for all compilers and products.
<li>A makefile for all Watcom builds (DOS or Windows), for all products.
<li>A top-level makefile for the Watcom DOS interpreter product.
<li>A top-level makefile for the Watcom Windows interpreter product.
<li>A top-level makefile for the Watcom DOS library "product".
<li>A top-level makefile for the Watcom Windows library "product".
<li>A makefile for all Borland builds (DOS or Windows), for all
products.
</ul>
<p>
and so on.
<!-- [2.0 end contents] ==================================================== -->
<!-- [3.0 begin visible trailer] =========================================== -->
<hr>
<p>
<small>Copyright © 2000-2022 Artifex Software, Inc. All rights reserved.</small>
<p>
This software is provided AS-IS with no warranty, either express or
implied.
This software is distributed under license and may not be copied, modified
or distributed except as expressly authorized under the terms of that
license. Refer to licensing information at <a href="https://www.artifex.com">https://www.artifex.com</a>
or contact Artifex Software, Inc., 1305 Grant Avenue - Suite 200,
Novato, CA 94945, U.S.A., +1(415)492-9861, for further information.
<p>
<small>Ghostscript version 9.56.1, 4 April 2022
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