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authorSam James <sam@gentoo.org>2022-09-21 14:18:08 +0100
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parentImport Ghostscript 9.56.1 (diff)
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- <title>The GhostPDL Interpreter Framework</title>
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-</head>
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-<body>
- <header><div class="title"><a href="index.html"><h1 aria-label="title">Ghostscript documentation</h1><h2 aria-label="version"></h2></a></div><a href="Search.htm" aria-label="Search" id="searchSite"><div class="search"></div></a></header>
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- <article>
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-<!--START EDITING HERE-->
-
-<h1>The GhostPDL Interpreter Framework</h1>
-
-<h2><a name="toc"></a>Table of contents</h2>
-<ul class="toc">
- <li><a href="#What_Is_This">What is the GhostPDL Interpreter Framework?</a></li>
- <li><a href="#API">The API</a></li>
- <li>
- <ul>
- <li><a href="#run">The run_string functions</a></li>
- <li><a href="#string_vs_file">String vs File functions</a></li>
- </ul>
- </li>
- <li><a href="#executable">The GPDL executable</a></li>
- <li><a href="#different_switches">Differences in switches from Ghostscript</a></li>
- <li><a href="#languages">Supported languages</a></li>
- <li>
- <ul>
- <li><a href="#PJL">PJL</a></li>
- <li><a href="#PCL">PCL</a></li>
- <li><a href="#PCLXL">PCLXL</a></li>
- <li><a href="#XPS">XPS</a></li>
- <li><a href="#POSTSCRIPT">POSTSCRIPT</a></li>
- <li><a href="#URF">URF</a></li>
- <li><a href="#JPG">JPG</a></li>
- <li><a href="#PWG">PWG</a></li>
- <li><a href="#TIFF">TIFF</a></li>
- <li><a href="#JBIG2">JBIG2</a></li>
- <li><a href="#JP2K">JP2K</a></li>
- <li><a href="#PNG">PNG</a></li>
- </ul>
- </li>
- <li><a href="#new_language">Adding a new language</a></li>
- <li>
- <ul>
- <li><a href="proc_characteristics">proc_characteristics</a></li>
- <li><a href="proc_allocate_interp_instance">proc_allocate_interp_instance</a></li>
- <li><a href="proc_get_device_memory">proc_get_device_memory</a></li>
- <li><a href="proc_set_param">proc_set_param</a></li>
- <li><a href="proc_add_path">proc_add_path</a></li>
- <li><a href="proc_post_args_init">proc_post_args_init</a></li>
- <li><a href="proc_init_job">proc_init_job</a></li>
- <li><a href="proc_run_prefix_commands">proc_run_prefix_commands</a></li>
- <li><a href="proc_process_file">proc_process_file</a></li>
- <li><a href="proc_process_begin">proc_process_begin</a></li>
- <li><a href="proc_process">proc_process</a></li>
- <li><a href="proc_process_end">proc_process_end</a></li>
- <li><a href="proc_flush_to_eoj">proc_flush_to_eoj</a></li>
- <li><a href="proc_process_eof">proc_process_eof</a></li>
- <li><a href="proc_report_errors">proc_report_errors</a></li>
- <li><a href="proc_dnit_job">proc_dnit_job</a></li>
- <li><a href="proc_deallocate_interp_instance">proc_deallocate_interp_instance</a></li>
- </ul>
- </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>.</p>
-
-<!-- [1.3 end hint] ======================================================== -->
-
-<hr>
-
-<!-- [1.0 end visible header] ============================================== -->
-
-<!-- [2.0 begin contents] ================================================== -->
-
-
-<h2><a name="What_Is_This"></a>What is the GhostPDL Interpreter Framework?</h2>
-
-<p>
-The GhostPDL interpreter framework (henceforth, just GhostPDL) is
-a framework into which multiple interpreters can be fitted, together
-with a set of interpreters for different input &quot;languages&quot;.</p>
-<p>The intent is that a build of GPDL will behave as much as possible
-like a build of Ghostscript, but should be able to transparently
-cope with as many different input formats as it has interpreters built
-into it.</p>
-<p>It can be built as a dynamic link library
-(DLL) on Microsoft Windows, as a shared object on the
-Linux, Unix and MacOS X platforms. With some changes, it could be built
-as a static library.</p>
-<p>The reason we dub it a &quot;Framework&quot; is that it is
-designed to be modular, and to readily allow new interpreters to be
-swapped in and out to extend and customise its capabilities.</p>
-
-<p>Jobs must be separated by <code>UEL</code> (Universal End of Language)
-code sequences (or PJL fragments, which implicitly start with a
-<code>UEL</code> marker).</p>
-
-<h2><a name="API"></a>The API</h2>
-
-<p>The API for GPDL (found in <a href="../pcl/pl/plapi.h">plapi.h</a>)
-is deliberately designed to be nearly identical
-to that for Ghostscript itself (found in <a href="../psi/iapi.h">iapi.h</a>).
-Details of Ghostscript's API can be
-found <a href="API.htm">here</a>, and we refer you to that. Only
-differences from that API will be documented here. In particular
-the differences in the handling of switches within the
-<code>gsapi_init_with_args</code> are documented
-<a href="#different_switches">here</a>.</p>
-<p>Our intent is that existing users of the Ghostscript API should be
-able to drop a GPDL DLL in as a replacement with little to no code
-changes.</p>
-
-<h3><a name="run"></a>The run_string APIs</h3>
-
-<p>The <code>run_string</code> APIs have parameters for
-<code>int user_errors</code> and <code>int *error_code</code>.
-Within Ghostscript, these allow a caller to bypass the language's
-standard error handling, and to trap errors externally.
-This makes no sense within a printer implementation (and, in
-particular, no sense for a postscript interpreter running
-jobs within a JobServer loop as GPDL does). Thus these
-parameters are kept for ABI compatibility, but are largely
-ignored within the GPDL implementation of gsapi.
-For sanity, pass 0 for <code>user_errors</code>, and expect
-<code>*error_code</code> to be set to 0 on exit.</p>
-
-<h3><a name="string_vs_file"></a>String vs File functions</h3>
-
-<p>Some file types, such as Postscript and PCL, are designed to be
-'streamable'; that is to say that the files can be fed in and
-consumed as they are interpreted. Other file types, such as PDF or
-XPS require the whole file to be available so that the interpreters
-can seek back and forth within it to extract the data they require.</p>
-
-<p>Traditionally, Ghostscript has relied upon the caller knowing
-which type of data is which; streamable data can be fed in to the
-system using the <code>gsapi_run_string</code> APIs, and complete files
-of data can be fed in using <code>gsapi_run_file</code> or
-<code>gsapi_init_with_args</code> APIs. Streamable data contained
-with a file is simple to deal with, as it's trivial for an interpreter
-to read bytes from a file, but doing random access on a stream is
-much harder.</p>
-
-<p>In systems where data is being streamed in, but it is required
-to be available for random access, the entire stream must be buffered
-for presentation to the language interpreters. With Ghostscript,
-the responsibility for doing this fell to the caller. Unfortunately,
-this means that the caller also has to be responsible for scanning
-the incoming data to spot when it is in a format that requires
-buffering. With the explosion of formats that GPDL supports this
-quickly becomes unpalatable.</p>
-
-<p>While the caller is still free to do such buffering of data
-itself, GPDL will do it automatically if required. If a language
-spots that data being fed to it via the <code>gsapi_run_string</code>
-APIs is in a format that requires buffering, the entire string of
-data will be automatically collected together, and then be
-represented internally to the <code>gsapi_run_file</code> API.</p>
-
-<p>The current implementation buffers the data in memory. Alternative
-implementations could use a backing store if such a thing were
-available. For server based applications the use of memory is not
-likely to be a problem (assuming reasonably sized input files at least).
-For printer integrations, it's entirely possible that no backing
-store will be available, so RAM may be the only option. Integrators
-may wish to place a limit on the size of files that can be buffered
-in this way.</p>
-
-<h2><a name="executable"></a>The GPDL executable</h2>
-
-<p>
-The GPDL executable is a small executable that loads the DLL/shared object,
-and implements, pretty much, the interface described in the
-<a href="Use.htm">usage documentation</a> for Ghostscript, but capable of
-reading any of the configured languages. This executable provides all
-the interaction with the windowing system, including image windows.</p>
-
-<p>Areas where the GPDL executable differs from the Ghostscript
-executable are described <a href="#different_switches">below</a>. These
-are primarily to do with (slightly) esoteric requirements when
-running Postscript in an interactive environment. Whereas Ghostscript
-is a generic Postscript interpreter, GPDL is unashamedly targeted
-as a print processor.</p>
-
-<p>
-The GPDL framework's library name and characteristics differ
-for each platform:</p>
-
-<ul>
-<li>The Win32 DLL <code>gpdldll32.dll</code>
-can be used by multiple programs simultaneously, but only once
-within each process.</li>
-
-<li>The Linux shared object <code>libgs.so</code>
-can be used by multiple programs simultaneously.</li>
-</ul>
-<p>
-The source for the executable is in <code>plw</code>*.* (Windows)
-and <code>plx</code>*.* (Linux/Unix).
-See these source files for examples of how to use the DLL.</p>
-
-<p>
-At this stage, GhostPDL does not support multiple instances
-of the interpreter within a single process.</p>
-
-<h2><a name="different_switches"></a>Differences in switches from Ghostscript</h3>
-
-<p>The <code>gsapi_init_with_args</code> API entrypoint takes a
-set of arguments, which can include various switches. We document
-the differences between Ghostscript's handling of switches and
-GhostPDLs here. The GhostPDL executable directly maps parameters
-given on the command-line into this list, so this list of differences
-applies both to API calls and uses of the executable.</p>
-
-<p>GhostPDL does not support the <code>-_</code>, <code>-+</code>,
-<code>-@</code>, <code>-B</code>, <code>-F</code>, <code>-M</code>,
-<code>-N</code>, <code>-P</code>, <code>-u</code>, and <code>-X</code>
-switches that Ghostscript does.
-
-<p>GhostPDL supports a few switches that the Ghostscript executable
-does not.</p>
-
-<ul>
- <li><b><code>-E #</code>:</b> Sets the &quot;error reporting mode&quot;
-for PCL/PXL.</li>
- <li><b><code>-H #x#x#x#</code>:</b> Sets the hardware margins to the
-given left/bottom/right/top values (in points).</li>
- <li><b><code>-j &lt;string&gt;</code>:</b> Passes the string to the
- PJL level. The string is split on ';'.</li>
- <li><b><code>-J</code>:</b> Same as <code>-j</code>.</li>
- <li><b><code>-l {RTL,PCL5E,PCL5C}</code>:</b> Sets the
- &quot;personality&quot; of the PCL/PXL interpreter.</li>
- <li><b><code>-L &lt;language&gt;</code>:</b> Sets the language
- to be used. Run with <code>-L</code> and no string to see a list
- of languages supported in your build.</li>
- <li><b><code>-m #x#</code>:</b> Sets the margin values to the
- left/bottom values (in points).</li>
-</ul>
-
-<h2><a name="languages"></a>Supported languages</h2>
-
-<p>The following is a (possibly non-exhaustive) list of languages
-for which implementations exist for GhostPDL. We use the term
-&quot;language&quot; to include both full page description languages
-(such as PCL, Postscript and XPS), and handlers for simpler formats
-(such as JPEG, PNG and TIFF).</p>
-<p>It is inherent in the design that new handlers can be included,
-and unwanted formats can be dropped from any given build/integration.</p>
-
-<h3><a name="PJL"></a>PJL</h3>
-<p>PJL is special, in that it is the central language implementation
-to which we return between all jobs. As such it always appears first
-in the list, and must be present in all builds.</p>
-<h3><a name="PCL"></a>PCL</h3>
-<p>The PCL language implementation supports PCL5C/PXL5E and HPGL/2
-with RTL.</p>
-<h3><a name="PCLXL"></a>PCLXL</h3>
-<p>The PCLXL language implementation supports PCLXL.</p>
-<h3><a name="XPS"></a>XPS</h3>
-<p>The XPS language implementation supports the XML Paper Specification
-format, as used in modern Windows systems printing pipelines.</p>
-<h3><a name="POSTSCRIPT"></a>POSTSCRIPT</h3>
-<p>The POSTSCRIPT language implementation is the Ghostscript
- Postscript interpreter, as described in the accompanying
- documentation.</p>
-<h3><a name="URF"></a>URF</h3>
-<p>Currently available to commercial customers only, the URF language
-implementation implements support for the URF file format, as used
-by Apple Airprint.</p>
-<h3><a name="JPG"></a>JPG</h3>
-<p>The JPG language implementation supports JPEG files
-(in both JFIF and EXIF formats).</p>
-<h3><a name="PWG"></a>PWG</h3>
-<p>The PWG language implementation supports PWG
-(Printer Working Group) format files.</p>
-<h3><a name="TIFF"></a>TIFF</h3>
-<p>The TIFF language implementation supports TIFF format files in
-a variety of compression formats, depending on the exact configuration
-of libtiff used in the build.</p>
-<h3><a name="JBIG2"></a>JBIG2</h3>
-<p>The JBIG2 language implementation supports JBIG2 format images.</p>
-<h3><a name="JP2K"></a>JP2K</h3>
-<p>The JP2K language implementation supports JPEG2000 and JPX format
-images.</p>
-<h3><a name="PNG"></a>PNG</h3>
-<p>The PNG language implementation supports PNG format images.</p>
-
-<h2><a name="new_language"></a>Adding a new language</h2>
-
-<p>Each language implementation in the system appears as an instance
-of a <code>pl_interp_implementation_t</code> structure.
-
-<pre>
-typedef struct pl_interp_implementation_s
-{
- /* Procedure vector */
- pl_interp_proc_characteristics_t proc_characteristics;
- pl_interp_proc_allocate_interp_instance_t proc_allocate_interp_instance;
- pl_interp_proc_get_device_memory_t proc_get_device_memory;
- pl_interp_proc_set_param_t proc_set_param;
- pl_interp_proc_add_path_t proc_add_path;
- pl_interp_proc_post_args_init_t proc_post_args_init;
- pl_interp_proc_init_job_t proc_init_job;
- pl_interp_proc_run_prefix_commands_t proc_run_prefix_commands;
- pl_interp_proc_process_file_t proc_process_file;
- pl_interp_proc_process_begin_t proc_process_begin;
- pl_interp_proc_process_t proc_process;
- pl_interp_proc_process_end_t proc_process_end;
- pl_interp_proc_flush_to_eoj_t proc_flush_to_eoj;
- pl_interp_proc_process_eof_t proc_process_eof;
- pl_interp_proc_report_errors_t proc_report_errors;
- pl_interp_proc_dnit_job_t proc_dnit_job;
- pl_interp_proc_deallocate_interp_instance_t
- proc_deallocate_interp_instance;
- void *interp_client_data;
-} pl_interp_implementation_t;
-</pre>
-
-<p>This structure consists of series of function pointers, each
-of which performs some portion of the processing required to handle
-detection of language type and processing of data. These function
-pointers are described in detail below.</p>
-<p>In addition, the <code>interp_client_data</code> field is
-used to hold the running state of a given interpreter.</p>
-
-<p>All the languages to be supported in the build are listed
-in the <code>pdl_implementations</code> array in
-<a href="../pcl/pl/plimpl.c">plimpl.c</a>. To add a new implementation
-the name of the appropriate <code>pl_interp_implementation_t</code>
-should be added here.</a>
-
-<p>The existing range of language implementations may prove useful
-as references when implementing new ones. They can be found as
-<code>gpdl/*top.c</code>. In particular,
-<code><a href="../gpdl/pngtop.c">pngtop.c</a></code> is a simple
-implementation of a well-defined, relatively simple file format
-(PNG), based upon a well-known and well-documented library (libpng),
-so is probably a good place to start.</p>
-
-<h3><a name="proc_characteristics"></a>proc_characteristics</h3>
-<pre>
-typedef const pl_interp_characteristics_t
- *(*pl_interp_proc_characteristics_t) (const pl_interp_implementation_t *);
-</pre>
-<p>This entrypoint is called to request details of the characteristics
-of the language. This must be implemented in all instances.</p>
-<p>This returns a pointer to a <code>pl_interp_characteristics_t</code>
- structure:</p>
-<pre>
-typedef struct pl_interp_characteristics_s
-{
- const char *language; /* generic language should correspond with
- HP documented PJL name */
- int (*auto_sense)(const char *string, int length); /* routine used to detect language - returns a score: 0 is definitely not, 100 is definitely yes. */
- const char *manufacturer; /* manuf str */
- const char *version; /* version str */
- const char *build_date; /* build date str */
-} pl_interp_characteristics_t;
-</pre>
-<p>The <code>language</code> entry contains a simple NULL terminated
- string that names the interpreter. Similarly, the <code>manufacturer</code>,
- <code>version</code>, and <code>build_date</code> fields are for
- informational purposes only.</p>
-<p>The <code>auto_sense</code> function is called with a prefix of data
- from each new source. Each language is passed the data in turn, and
- &quot;scores;&quot; according to how sure it is the file is that format.</p>
-<p>For many file formats this means looking for known in the first
-few bytes (such as PNG or TIFF looking for their signature bytes). For
-others, such as XPS, the best that can be done is to spot that it's a
- zip file. For still others, such as PCL, heuristics have to be used.</p>
-<p>A 'definite' match is returned as 100, a definite rejection as 0, with
-intermediate values used appropriately.</p>
-<h3><a name="proc_allocate_interp_instance"></a>proc_allocate_interp_instance</h3>
-<pre>
-typedef int (*pl_interp_proc_allocate_interp_instance_t)
- (pl_interp_implementation_t *,
- gs_memory_t *);
-</pre>
-<p>On startup, the GPDL library calls around all the languages
-via this function, getting them to allocate themselves an instance.
- What this means will vary between languages, but typically it involves
- allocating a state structure, and storing the pointer to that in the
- <code>interp_client_data</code> pointer of the
- <code>pl_interp_implementation_t *</code>. Part of this state structure
- will typically be a <code>gstate</code> to use to drive the graphics
- library.</p>
-<h3><a name="proc_get_device_memory"></a>proc_get_device_memory</h3>
-<pre>
-typedef gs_memory_t *(*pl_interp_proc_get_device_memory_t)
- (pl_interp_implementation_t *);
-</pre>
-<p>On shutdown, the GPDL library calls around all the languages
-via this function, getting them to release their resources and
-deallocate any memory.</p>
-<h3><a name="proc_set_param"></a>proc_set_param</h3>
-<pre>
-typedef int (*pl_interp_proc_set_param_t) (pl_interp_implementation_t *,
- pl_set_param_type,
- const char *,
- const void *);
-</pre>
-<p>Some languages (particularly Postscript) can have their behaviour
-changed by the use of parameters. This function provides a generic
-method for the GPDL library to pass parameters into a language. Each
-language is free to ignore the parameters that do not apply to it. For
-most languages, this can safely be passed as <code>NULL</code>.</p>
-<h3><a name="proc_add_path"></a>proc_add_path</h3>
-<pre>
-typedef int (*pl_interp_proc_add_path_t) (pl_interp_implementation_t *,
- const char *);
-</pre>
-<p>Some languages (particularly Postscript) have the ability to
- open files from the local storage. These files can be found
- in a variety of different locations within the local storage.
- As such this call allows the GPDL library to add paths to
- the current list of locations that will be searched. For
- most languages, this can safely be passed as <code>NULL</code>.</p>
-<h3><a name="proc_post_args_init"></a>proc_post_args_init</h3>
-<pre>
-typedef int (*pl_interp_proc_post_args_init_t) (pl_interp_implementation_t *);
-</pre>
-<p></p>
-<h3><a name="proc_init_job"></a>proc_init_job</h3>
-<pre>
-typedef int (*pl_interp_proc_init_job_t) (pl_interp_implementation_t *,
- gx_device *);
-</pre>
-<p>Once the GPDL library has identified which language should be used
-for an incoming job, it will call this entrypoint to initialise the
-language for the job. What this means will vary between languages,
-but at the very minimum the job will need to take note of the device
-to be used.</p>
-<h3><a name="proc_run_prefix_commands"></a>proc_run_prefix_commands</h3>
-<pre>
-typedef int (*pl_interp_proc_run_prefix_commands_t)
- (pl_interp_implementation_t *,
- const char *prefix);
-</pre>
-<p>The GPDL library (and executable) allow language commands to be
-sent in the argument parameters using the <code>-c</code> switch. These
-are collected into a buffer, and forwarded to a language to be run
-as part of the same job as any following file.</p>
-<p>Currently, only the Postscript language handler implements this
-function, all others should pass <code>NULL</code>.</p>
-<h3><a name="proc_process_file"></a>proc_process_file</h3>
-<pre>
-typedef int (*pl_interp_proc_process_file_t) (pl_interp_implementation_t *,
- const char *);
-</pre>
-<p>If the GPDL library is given a filename to process, and this function
-is non-NULL, it will call this to run the file. For file formats such as
-PDF (which need to be buffered so they can be read out of order), this
-can avoid the need to feed in all the data via <code>proc_process</code>,
-buffer it somewhere, and then process it at the end.</p>
-<p>For many languages this can be <code>NULL</code>.</p>
-<h3><a name="proc_process_begin"></a>proc_process_begin</h3>
-<pre>
-typedef int (*pl_interp_proc_process_begin_t) (pl_interp_implementation_t *);
-</pre>
-<p>Once the GPDL library has data to process (that it cannot process
-with <code>proc_process_file</code>, it will call this function to
-setup the transfer of data.</p>
-<h3><a name="proc_process"></a>proc_process</h3>
-<pre>
-typedef int (*pl_interp_proc_process_t) (pl_interp_implementation_t *,
- stream_cursor_read *);
-</pre>
-<p>After the GPDL library has called <code>proc_process_begin</code>
-this function may be called multiple times to actually transfer
-the data in. The implementation is expected to consume as many bytes
-as it can (but not necessarily all of them) before returning with
-an updated read pointer. If this function cannot advance without more
-data, it should return with <code>gs_error_NeedInput</code>.</p>
-<h3><a name="proc_process_end"></a>proc_process_end</h3>
-<pre>
-typedef int (*pl_interp_proc_process_end_t) (pl_interp_implementation_t *);
-</pre>
-<p>After the GPDL library has called <code>proc_process_begin</code>
-(and possibly made a number of calls to <code>proc_process</code>) it
-will call <code>proc_process_end</code> to signify the end of the
-data. This does not necessarily signal the end of the job.</p>
-<h3><a name="proc_flush_to_eoj"></a>proc_flush_to_eoj</h3>
-<pre>
-typedef int (*pl_interp_proc_flush_to_eoj_t) (pl_interp_implementation_t *,
- stream_cursor_read *);
-</pre>
-<p>In the event of a problem while processing data, GPDL may seek to
-abandon processing of a transfer in progress by calling
-<code>proc_flush_to_eoj</code>. If possible, the language should
-continue to process data until a reasonable stopping point, or until
-<code>UEL</code> is reached.</p>
-<h3><a name="proc_process_eof"></a>proc_process_eof</h3>
-<pre>
-typedef int (*pl_interp_proc_process_eof_t) (pl_interp_implementation_t *);
-</pre>
-<p>Called when GPDL reaches EOF in processing a job. A language
-implementation should assume no more data is forthcoming.</p>
-<h3><a name="proc_report_errors"></a>proc_report_errors</h3>
-<pre>
-typedef int (*pl_interp_proc_report_errors_t) (pl_interp_implementation_t *,
- int,
- long,
- bool);
-</pre>
-<p>Called after running a job to give the language implementation the
-chance to report any errors it may have detected as it ran.</p>
-<h3><a name="proc_dnit_job"></a>proc_dnit_job</h3>
-<pre>
-typedef int (*pl_interp_proc_dnit_job_t) (pl_interp_implementation_t *);
-</pre>
-<p>Called after a job is complete so that the language implementation
-may clean up. The interpreter is kept open so that more jobs can be
-fed to it, but no state should be kept from this job to the next.</p>
-<h3><a name="proc_deallocate_interp_instance"></a>proc_deallocate_interp_instance</h3>
-<pre>
-typedef int (*pl_interp_proc_deallocate_interp_instance_t)
- (pl_interp_implementation_t *);
-</pre>
-<p>Called on shutdown of the GPDL library to close down the language
-instance and free all the resources.</p>
-
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-This software is distributed under license and may not be copied, modified
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