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<title>How to use the Ghostscript Dynamic Link Library (DLL)</title>
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How to use the Ghostscript Dynamic Link Library (DLL)
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<h2>Table of contents</h2>
<blockquote><ul>
<li><a href="#DLL">What is the Ghostscript DLL?</a>
<li><a href="#Common_functions">Platform-independent DLL functions</a>
<ul>
<li><a href="#revision"><b><tt>gsdll_revision()</tt></b></a>
<li><a href="#init"><b><tt>gsdll_init()</tt></b></a>
<li><a href="#execute_begin"><b><tt>gsdll_execute_begin()</tt></b></a>
<li><a href="#execute_cont"><b><tt>gsdll_execute_cont()</tt></b></a>
<li><a href="#execute_end"><b><tt>gsdll_execute_end()</tt></b></a>
<li><a href="#exit"><b><tt>gsdll_exit()</tt></b></a>
<li><a href="#lock_device"><b><tt>gsdll_lock_device()</tt></b></a>
</ul>
<li><a href="#Callback">Callback function</a>
<li><a href="#OS2_device">Ghostscript DLL device for OS/2</a>
<ul>
<li><a href="#OS2_bmp"><b><tt>gsdll_get_bitmap()</tt></b></a>
<li><a href="#OS2_example">Example DLL usage for OS/2</a>
</ul>
<li><a href="#Win_device">Ghostscript DLL device for MS Windows</a>
<ul>
<li><a href="#Win_copydib"><b><tt>gsdll_copy_dib()</tt></b></a>
<li><a href="#Win_copypalette"><b><tt>gsdll_copy_palette()</tt></b></a>
<li><a href="#Win_draw"><b><tt>gsdll_draw()</tt></b></a>
<li><a href="#Win_get_row"><b><tt>gsdll_get_bitmap_row()</tt></b></a>
</ul>
<li><a href="#Win16">Ghostscript DLL Device for 16-bit MS Windows</a>
</ul></blockquote>
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<p>For other information, see the <a href="Readme.htm">Ghostscript
overview</a>.
<p>
<b>WARNING:</b> The API described in this document is subject to changes in
future releases, possibly ones that are not backward compatible with what
is described here.
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<h1><a name="DLL"></a>What is the Ghostscript DLL?</h1>
<p>
For the OS/2, Win16 and Win32 platforms, Ghostscript is built as a dynamic
link library (DLL), and to provide the interface described in the <a
href="Use.htm">usage documentation</a>, a smaller independent executable
(<b><tt>.EXE</tt></b>) loads this DLL, which provides all the interaction
with the windowing system, including image windows and, if necessary, a
text window. This document describes the DLL interface, which consists of
eight main functions, <a href="#functions">seven platform-independent
ones</a> provided by the DLL and one, <a href="#Callback">the callback
function</a>, provided by the caller. The DLL provides some other
platform-specific functions for display devices.
<p>
The DLL's name and characteristics differ among the three platforms:
<ul>
<li>The <a href="#OS2_device">OS/2 DLL <b><tt>GSDLL2.DLL</tt></b></a> has
MULTIPLE NONSHARED data segments and can be called by multiple programs
simultaneously.
<li>The <a href="#Win_device">Win32 DLL <b><tt>GSDLL32.DLL</tt></b></a> has
MULTIPLE NONSHARED data segments. Under Win32s it can be used by only one
program at a time, but under Windows 95 or Windows NT it can be called by
multiple programs simultaneously.
<li>The <a href="#Win16">Win16 DLL <b><tt>GSDLL16.DLL</tt></b></a> is a
large-memory model DLL with far static data. Due to the limitations of
16-bit MS Windows -- Windows 3.n -- the DLL can be used by only one program
at a time.
</ul>
<p>
The source for the executable is in <b><tt>dp</tt></b>*.* (OS/2) and
<b><tt>dw</tt></b>*.* (Windows). See these source files for examples of
how to use the DLL.
<hr>
<h1><a name="Common_functions"></a>Platform-independent DLL functions</h1>
<p>
The seven functions provided by the DLL are
<ul>
<li><b><tt>int GSDLLAPI <a href="#revision">gsdll_revision</a>(char **product, char **copyright, long *gs_revision, long *gs_revisiondate)</tt></b>
<li><b><tt>int GSDLLAPI <a href="#init">gsdll_init</a>(GSDLL_CALLBACK callback, HWND hwnd, int argc, char *argv[]);</tt></b>
<li><b><tt>int GSDLLAPI <a href="#execute_begin">gsdll_execute_begin</a>(void);</tt></b>
<li><b><tt>int GSDLLAPI <a href="#execute_cont">gsdll_execute_cont</a>(const char *str, int len);</tt></b>
<li><b><tt>int GSDLLAPI <a href="#execute_end">gsdll_execute_end</a>(void);</tt></b>
<li><b><tt>int GSDLLAPI <a href="#exit">gsdll_exit</a>(void);</tt></b>
<li><b><tt>int GSDLLAPI <a href="#lock_device">gsdll_lock_device</a>(unsigned char *device, int flag);</tt></b>
<p>
where <b><tt>GSDLLAPI</tt></b> is defined under OS/2 as
<blockquote><b><tt>
#define GSDLLAPI
</tt></b></blockquote>
<p>
and under MS Windows as
<blockquote><b><tt>
#define GSDLLAPI CALLBACK _export
</tt></b></blockquote>
</ul>
<h2><a name="revision"></a><b><tt>gsdll_revision()</tt></b></h2>
<blockquote>
This function returns the revision numbers and strings of the Ghostscript
DLL; you should call it before <b><tt>gsdll_init()</tt></b> to make sure
that the correct version of the Ghostscript DLL has been loaded. For
example
<blockquote>
<pre>char *product;
char *copyright;
long revision;
long revisiondate;
gsdll_revision(&product, &copyright, &revision, &revisiondate);
</pre></blockquote>
<p>
You may use <b><tt>NULL</tt></b> pointers if you do not want a particular
value.
</blockquote>
<h2><a name="init"></a><b><tt>gsdll_init()</tt></b></h2>
<blockquote>
<b><tt>gsdll_init()</tt></b> must be called after loading the DLL and
before executing any Ghostscript commands. The arguments are the address
of the callback function, a parent window handle, the count of arguments
and an array of pointers to the arguments. For example
<blockquote>
<pre>char *argv[5];
argv[0] = "gswin.exe";
argv[1] = "-Ic:\\gs;c:\gs\\fonts";
argv[2] = "-dNOPAUSE",
argv[3] = "-sDEVICE=djet500",
argv[4] = NULL;
argc = 4;
code = gsdll_init(gsdll_callback, hwnd, argc, argv);
</pre></blockquote>
<p>
<b><tt>hwnd</tt></b> is used as the parent window handle for any windows
created by Ghostscript. <b><tt>hwnd</tt></b> may be <b><tt>NULL</tt></b>
if the caller has no windows, but if it is <b><tt>NULL</tt></b>, you should
avoid using devices which may open windows.
<p>
A return code of 0 (zero) indicates no errors, and the code may now call
<b><tt>gsdll_execute_begin()</tt></b> or <b><tt>gsdll_exit()</tt></b>. If
the return value is non-zero then <b><tt>gsdll_exit()</tt></b> must not be
called.
<p>
A return value of <b><tt>GSDLL_INIT_QUIT</tt></b> indicates that one of the
command line files or arguments called "<b><tt>quit</tt></b>", or that
Ghostscript was reading stdin and reached end-of-file. This is not an
error. <b><tt>gsdll_exit()</tt></b> must not be called.
<p>
A return value of <b><tt>GSDLL_INIT_IN_USE</tt></b> indicates that the DLL
is in use by another application (Windows 3.1 only). The DLL should be
immediately unloaded (or the caller terminated).
<b><tt>gsdll_exit()</tt></b> must not be called.
</blockquote>
<h2><a name="execute_begin"></a><b><tt>gsdll_execute_begin()</tt></b></h2>
<blockquote>
This must be called after <b><tt>gsdll_init()</tt></b> and before
<b><tt>gsdll_execute_cont()</tt></b>.
</blockquote>
<h2><a name="execute_cont"></a><b><tt>gsdll_execute_cont()</tt></b></h2>
<blockquote>
After successfully calling <b><tt>gsdll_init()</tt></b> and
<b><tt>gsdll_execute_begin()</tt></b>, commands may be given to Ghostscript
with <b><tt>gsdll_execute_cont()</tt></b>. Examples are:
<blockquote>
<pre>char *command = "1 2 add == flush\n";
code = gsdll_execute_cont(command, strlen(command));
command = "qu"
code = gsdll_execute_cont(command, strlen(command));
command = "it\n"
code = gsdll_execute_cont(command, strlen(command));
</pre>
<a name="cont_returns"></a>
<table cellpadding=0 cellspacing=0>
<tr><th colspan=3 bgcolor="#CCCC00"><hr><font size="+1">Return codes from <b><tt>gsdll_execute_cont()</tt></b></font><hr>
<tr valign=bottom>
<th align=left>Code
<td>
<th align=left>Status
<tr> <td colspan=3><hr>
<tr valign=top> <td align=left>0
<td>
<td>No errors
<tr valign=top> <td align=left>< 0
<td>
<td>Error
<tr valign=top> <td align=left><= -100
<td>
<td>"<b><tt>quit</tt></b>" has been executed, or fatal error.
<b><tt>gsdll_exit()</tt></b> must then be called: do not call
<b><tt>gsdll_execute_end()</tt></b>.
</table>
</blockquote>
<b><tt>gsdll_execute_cont()</tt></b>
does not flush stdio, so if you want to see output from Ghostscript you
must do this explicitly as shown in the example above.
<p>
When executing a string with <b><tt>gsdll_execute_cont()</tt></b>,
<b><tt>currentfile</tt></b> is the input from
<b><tt>gsdll_execute_cont()</tt></b>. Reading from <b><tt>%stdin</tt></b>
uses the callback.
</blockquote>
<h2><a name="execute_end"></a><b><tt>gsdll_execute_end()</tt></b></h2>
<blockquote>
If <b><tt>gsdll_execute_cont()</tt></b> did not return an error, then
<b><tt>gsdll_execute_end()</tt></b> must be called after
<b><tt>gsdll_execute_cont()</tt></b> and before
<b><tt>gsdll_exit()</tt></b>.
</blockquote>
<h2><a name="exit"></a><b><tt>gsdll_exit()</tt></b></h2>
<blockquote>
Call gsdll_exit() to terminate the Ghostscript DLL. It must be called if a
fatal error has occurred; see the <a href="#cont_returns">return value
of <b><tt>gsdll_execute_cont()</tt></b></a>. After calling
<b><tt>gsdll_exit()</tt></b>, there are two options:
<ul>
<li>Unload the DLL, either by terminating the application or by
calling <b><tt>DosFreeModule</tt></b> (OS/2) or <b><tt>FreeLibrary</tt></b> (MS Windows).
<li>Call <b><tt>gsdll_init()</tt></b> again to restart Ghostscript.
</ul>
</blockquote>
<h2><a name="lock_device"></a><b><tt>gsdll_lock_device()</tt></b></h2>
<blockquote>
Since the caller may be multithreaded, a lock is needed to control access
to the display device, and <b><tt>gsdll_lock_device()</tt></b> provides that
locking.
<blockquote>
<pre>int gsdll_lock_device(unsigned char *device, int flag);
/* Lock the device if flag = TRUE */
/* Unlock the device if flag = FALSE */
/* device is a pointer to Ghostscript os2dll or mswindll device */
/* from GSDLL_DEVICE message. */
/* Return value is the lock count. */
</pre>
<table cellpadding=0 cellspacing=0>
<tr><th colspan=3 bgcolor="#CCCC00"><hr><font size="+1">Locking and unlocking devices</font><hr>
<tr valign=top> <td>To lock a device
<td>
<td><b><tt>gsdll_lock_device(device, 1)</tt></b>;
<tr valign=top> <td>To unlock a device
<td>
<td><b><tt>gsdll_lock_device(device, 0)</tt></b>;
</table>
</blockquote>
<p>
This function is typically used to lock the device while repainting a
window or copying the device bitmap to the clipboard. Ghostscript may draw
into the device bitmap or update the palette entries while the device is
locked by the caller, but locking the device prevents the Ghostscript DLL
from closing the device or changing its size or depth.
<p>
Under OS/2, Windows 95 and Windows NT, this lock is implemented using a
mutual exclusion semaphore (mutex). The return value is the lock count,
which is either 0 ("unlocked") or 1 ("locked"). The function blocks until
the device is locked by the caller.
<p>
Under Win16 or Win32s, <b><tt>gsdll_lock_device()</tt></b> always returns
immediately with a lock count as its return value. A lock count of 2 or
more is definite indication of an error, probably calling the function
twice. Access to the device should be controlled by checking the Windows
message queue only when the bitmap is not being accessed.
</blockquote>
<hr>
<h1><a name="Callback"></a>Callback function</h1>
<blockquote>
As an argument to <a href="#init"><b><tt>gsdll_init()</tt></b></a> the
caller must provide a callback function which the DLL invokes for stdio and
to notify the caller about device events. The function provided by the
caller has this prototype:
<blockquote><b><tt>
int gsdll_callback(int message, char *str, unsigned long count);
</tt></b></blockquote>
<p>
The Pascal calling convention is not used. An example callback function
is:
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