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The Ghostscript Interpreter Application Programming Interface (API)

Table of contents

• What is the Ghostscript Interpreter API?
• Exported functions
  • gsapi_revision
  • gsapi_new_instance
  • gsapi_delete_instance
  • gsapi_set_stdio
  • gsapi_set_poll
  • gsapi_set_display_callback
  • gsapi_run_string_begin
  • gsapi_run_string_continue
  • gsapi_run_string_end
  • gsapi_run_string_with_length
  • gsapi_run_string
  • gsapi_init_with_args
  • gsapi_run_*
  • gsapi_exit
  • gsapi_set_visual_tracer
  • Return codes
• Example usage
• Standard input and output
• Display device

For other information, see the Ghostscript overview.

WARNING: 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.

What is the Ghostscript Interpreter API?

The Ghostscript interpreter can be built as a dynamic link library (DLL) on the Windows or OS/2 platforms, as a shared object on the Linux platform and as a framework on MacOS X. With some changes, it could be built as a static library. This document describes the Application Programming Interface (API) for the Ghostscript interpreter library. This should not be confused with the Ghostscript library which provides a graphics library but not the interpreter.

This supercedes the old DLL interface.

To provide the interface described in the usage documentation, a smaller independent executable loads the DLL/shared object. This executable must provide all the interaction with the windowing system, including image windows and, if necessary, a text window.

The Ghostscript interpreter library's name and characteristics differ for each platform:

• The Win32 DLL gsdll32.dll has MULTIPLE NONSHARED data segments. Under Win32s it can be used by only one program at a time, but under Windows 95/98 or Windows NT it can be called by multiple programs simultaneously.
• The OS/2 DLL gsdll2.dll has MULTIPLE NONSHARED data segments and can be called by multiple programs simultaneously.
• The Linux shared object libgs.so can be used by multiple programs simultaneously.
• The MacOS X Ghostscript.framework can also be used by multiple applications at once.

The source for the executable is in dw*.* (Windows), dp*.* (OS/2) and dx*.* (Linux). See these source files for examples of how to use the DLL. A similar example for MacOS using the Carbon API is provided in dm*.*.

The source file dxmainc.c can also serve as an example of how to use the framework shared component on MacOS X, providing the same command-line tool it does on any linux, bsd or similar operating system. Just change the header includes to use the Ghostscript namespace:

#include <Ghostscript/ierrors.h>
#include <Ghostscript/iapi.h>

cc -o gsc dxmainc.c -framework Ghostscript

At this stage, Ghostscript does not support multiple instances of the interpreter within a single process.

Exported functions

The functions exported by the DLL/shared object are described in the header file iapi.h and are summarised below. Omitted from the summary are the calling convention (e.g. __stdcall), details of return values and error handling.

• int gsapi_revision (gsapi_revision_t *pr, int len);
• int gsapi_new_instance (gs_main_instance **pinstance, void *caller_handle);
• void gsapi_delete_instance (gs_main_instance *instance);
• int gsapi_set_stdio (gs_main_instance *instance, int(*stdin_fn)(void *caller_handle, char *buf, int len), int(*stdout_fn)(void *caller_handle, const char *str, int len), int(*stderr_fn)(void *caller_handle, const char *str, int len));
• int gsapi_set_poll (gs_main_instance *instance, int(*poll_fn)(void *caller_handle));
• int gsapi_set_display_callback (gs_main_instance *instance, display_callback *callback);
• int gsapi_init_with_args (gs_main_instance *instance, int argc, char **argv);
• int gsapi_run_string_begin (gs_main_instance *instance, int user_errors, int *pexit_code);
• int gsapi_run_string_continue (gs_main_instance *instance, const char *str, unsigned int length, int user_errors, int *pexit_code);
• int gsapi_run_string_end (gs_main_instance *instance, int user_errors, int *pexit_code);
• int gsapi_run_string_with_length (gs_main_instance *instance, const char *str, unsigned int length, int user_errors, int *pexit_code);
• int gsapi_run_string (gs_main_instance *instance, const char *str, int user_errors, int *pexit_code);
• int gsapi_run_file (gs_main_instance *instance, const char *file_name, int user_errors, int *pexit_code);
• int gsapi_exit (gs_main_instance *instance);
• int gsapi_set_visual_tracer (gstruct vd_trace_interface_s *I);

gsapi_revision()

This function returns the revision numbers and strings of the Ghostscript interpreter library; you should call it before any other interpreter library functions to make sure that the correct version of the Ghostscript interpreter has been loaded.

typedef struct gsapi_revision_s {
    const char *product;
    const char *copyright;
    long revision;
    long revisiondate;
} gsapi_revision_t;
gsapi_revision_t r;

if (gsapi_revision(&r, sizeof(r)) == 0) {
    if (r.revision < 650)
       printf("Need at least Ghostscript 6.50");
}
else {
    printf("revision structure size is incorrect");
}

gsapi_new_instance()

Create a new instance of Ghostscript. This instance is passed to most other gsapi functions. The caller_handle will be provided to callback functions. At this stage, Ghostscript supports only one instance.

gsapi_delete_instance()

Destroy an instance of Ghostscript. Before you call this, Ghostscript must have finished. If Ghostscript has been initialised, you must call gsapi_exit before gsapi_delete_instance.

gsapi_set_stdio()

Set the callback functions for stdio The stdin callback function should return the number of characters read, 0 for EOF, or -1 for error. The stdout and stderr callback functions should return the number of characters written.

gsapi_set_poll()

Set the callback function for polling. This is used for handling window events or cooperative multitasking. This function will only be called if the Ghostscript interpreter was compiled with CHECK_INTERRUPTS as described in gpcheck.h. The polling function should return zero if all is well, and return negative if it wants ghostscript to abort. The polling function must be fast.

gsapi_set_display_callback()

Set the callback structure for the display device. If the display device is used, this must be called after gsapi_new_instance() and before gsapi_init_with_args(). See gdevdsp.h for more details.

gsapi_init_with_args()

Initialise the interpreter. This calls gs_main_init_with_args() in imainarg.c. See below for return codes. The arguments are the same as the "C" main function: argv[0] is ignored and the user supplied arguments are argv[1] to argv[argc-1].

gsapi_run_*()

The gsapi_run_* functions are like gs_main_run_* except that the error_object is omitted. If these functions return <= -100, either quit or a fatal error has occured. You must call gsapi_exit() next. The only exception is gsapi_run_string_continue() which will return e_NeedInput if all is well. See below for return codes.

There is a 64 KB length limit on any buffer submitted to a gsapi_run_* function for processing. If you have more than 65535 bytes of input then you must split it into smaller pieces and submit each in a separate gsapi_run_string_continue() call.

gsapi_exit()

Exit the interpreter. This must be called on shutdown if gsapi_init_with_args() has been called, and just before gsapi_delete_instance().

gsapi_set_visual_tracer()

Pass visual tracer interface block to Ghostscript. See Lib.htm for reference about the interface block structure. This function is useful for debug clients only. Release clients must not call it.

Return codes

The gsapi_init_with_args, gsapi_run_* and gsapi_exit functions return an integer code.

Return codes from gsapi_*()
Code		Status
0		No errors
e_Quit		"quit" has been executed. This is not an error. gsapi_exit() must be called next.
e_NeedInput		More input is needed by gsapi_run_string_continue(). This is not an error.
e_Info		"gs -h" has been executed. This is not an error. gsapi_exit() must be called next.
< 0		Error
<= -100		Fatal error. gsapi_exit() must be called next.

The gsapi_run_*() functions do not flush stdio. If you want to see output from Ghostscript you must do this explicitly as shown in the example below.

When executing a string with gsapi_run_string_*(), currentfile is the input from the string. Reading from %stdin uses the stdin callback.

Example Usage

To use try out these examples in a development environment like Microsoft's developer tools or Metrowerks Codewarrior, create a new project, save the example source code as a .c file and add it, along with the Ghostscript dll or shared library. You will also need to make sure the Ghostscript headers are available, either by adding their location (the src directory in the Ghostscript source distribution) to the project's search path, or by copying ierrors.h and iapi.h into the same directory as the example source.

Example 1

/* Example of using GS DLL as a ps2pdf converter.  */

#if defined(_WIN32) && !defined(_Windows)
# define _Windows
#endif
#ifdef _Windows
/* add this source to a project with gsdll32.dll, or compile it directly with:
 *   cl -D_Windows -Isrc -Febin\ps2pdf.exe ps2pdf.c bin\gsdll32.lib
 */
# include <windows.h>
# define GSDLLEXPORT __declspec(dllimport)
#endif

#include "ierrors.h"
#include "iapi.h"

gs_main_instance *minst;

int main(int argc, char *argv[])
{
    int code, code1;
    const char * gsargv[10];
    int gsargc;
    gsargv[0] = "ps2pdf";	/* actual value doesn't matter */
    gsargv[1] = "-dNOPAUSE";
    gsargv[2] = "-dBATCH";
    gsargv[3] = "-dSAFER";
    gsargv[4] = "-sDEVICE=pdfwrite";
    gsargv[5] = "-sOutputFile=out.pdf";
    gsargv[6] = "-c";
    gsargv[7] = ".setpdfwrite";
    gsargv[8] = "-f";
    gsargv[9] = "input.ps";
    gsargc=10;

    code = gsapi_new_instance(&minst, NULL);
    if (code < 0)
	return 1;
    code = gsapi_init_with_args(minst, gsargc, gsargv);
    code1 = gsapi_exit(minst);
    if ((code == 0) || (code == e_Quit))
	code = code1;

    gsapi_delete_instance(minst);

    if ((code == 0) || (code == e_Quit))
	return 0;
    return 1;
}

Example 2

/* Similar to command line gs */

#if defined(_WIN32) && !defined(_Windows)
# define _Windows
#endif
#ifdef _Windows
/* Compile directly with:
 *   cl -D_Windows -Isrc -Febin\gstest.exe gstest.c bin\gsdll32.lib
 */
# include <windows.h>
# define GSDLLEXPORT __declspec(dllimport)
#endif
#include <stdio.h>
#include "ierrors.h"
#include "iapi.h"

/* stdio functions */
static int GSDLLCALL
gsdll_stdin(void *instance, char *buf, int len)
{
    int ch;
    int count = 0;
    while (count < len) {
	ch = fgetc(stdin);
	if (ch == EOF)
	    return 0;
	*buf++ = ch;
	count++;
	if (ch == '\n')
	    break;
    }
    return count;
}

static int GSDLLCALL
gsdll_stdout(void *instance, const char *str, int len)
{
    fwrite(str, 1, len, stdout);
    fflush(stdout);
    return len;
}

static int GSDLLCALL
gsdll_stderr(void *instance, const char *str, int len)
{
    fwrite(str, 1, len, stderr);
    fflush(stderr);
    return len;
}

gs_main_instance *minst;
const char start_string[] = "systemdict /start get exec\n";

int main(int argc, char *argv[])
{
    int code, code1;
    int exit_code;

    code = gsapi_new_instance(&minst, NULL);
    if (code < 0)
	return 1;
    gsapi_set_stdio(minst, gsdll_stdin, gsdll_stdout, gsdll_stderr);
    code = gsapi_init_with_args(minst, argc, argv);
    if (code == 0)
	code = gsapi_run_string(minst, start_string, 0, &exit_code);
    code1 = gsapi_exit(minst);
    if ((code == 0) || (code == e_Quit))
	code = code1;

    gsapi_delete_instance(minst);

    if ((code == 0) || (code == e_Quit))
	return 0;
    return 1;
}

Example 3

const char *command = "1 2 add == flush\n";

int main(int argc, char *argv[])
{
    int code, code1;
    int exit_code;

    code = gsapi_new_instance(&minst, NULL);
    if (code < 0)
	return 1;
    code = gsapi_init_with_args(minst, argc, argv);

    if (code == 0) {
	gsapi_run_string_begin(minst, 0, &exit_code);
	gsapi_run_string_continue(minst, command, strlen(command), 0, &exit_code);
	gsapi_run_string_continue(minst, "qu", 2, 0, &exit_code);
	gsapi_run_string_continue(minst, "it", 2, 0, &exit_code);
	gsapi_run_string_end(minst, 0, &exit_code);
    }

    code1 = gsapi_exit(minst);
    if ((code == 0) || (code == e_Quit))
	code = code1;

    gsapi_delete_instance(minst);

    if ((code == 0) || (code == e_Quit))
	return 0;
    return 1;
}

Multiple threads

Standard input and output

When using the Ghostscript interpreter library interface, you have a choice of two standard input/output methods.

• If you do nothing, the "C" stdio will be used.
• If you use gsapi_set_stdio(), all stdio will be redirected to the callback functions you provide. This would be used in a graphical user interface environment where stdio is not available, or where you wish to process Ghostscript input or output.

The callback functions are described in iapi.h.

Display device

The display device is available for use with the Ghostscript interpreter library. This is described in the file gdevdsp.h. This device provides you with access to the raster output of Ghostscript. It is your responsibility to copy this raster to a display window or printer.

To use this device, you must provide a callback structure with addresses of a number of callback functions. The address of the callback structure is provided using gsapi_set_display_callback(). This must be called after gsapi_new_instance() and before gsapi_init_with_args().

The callbacks are for device open, close, resize, sync, page, memory allocation and updating. Each callback function contains a handle can be set using

-dDisplayHandle=1234

The device raster format can be configured using

-dDisplayFormat=NNNN

• native, gray, RGB or CMYK color spaces.
• alpha byte (ignored).
• 1 to 16 bits/component.
• bigendian (RGB) or littleendian (BGR) order.
• top first or bottom first raster.
• 16 bits/pixel with 555 or 565 bitfields.

    char **nargv;
    char arg1[64];
    char arg2[64];
    char arg3[64];
    code = gsapi_new_instance(&minst, NULL);
    gsapi_set_stdio(minst, gsdll_stdin, gsdll_stdout, gsdll_stderr);
    code = gsapi_set_display_callback(minst, &display_callback);
    sprintf(arg1, "-sDEVICE=display");
    sprintf(arg2, "-dDisplayHandle=%d", 0);
    sprintf(arg3, "-dDisplayFormat=%d", 
        DISPLAY_COLORS_RGB | DISPLAY_ALPHA_NONE | DISPLAY_DEPTH_8 | 
        DISPLAY_LITTLEENDIAN | DISPLAY_BOTTOMFIRST);
    nargv = (char **)malloc((argc + 4) * sizeof(char *));
    nargv[0] = argv[0];
    nargv[1] = arg1;
    nargv[2] = arg2;
    nargv[3] = arg3;
    memcpy(nargv + 4, argv + 1, argc * sizeof(char *));
    argc += 3;
    code = gsapi_init_with_args(minst, argc, nargv);

The display device provides you with the address and size of the raster using the display_size() callback. You are then responsible for displaying this raster. Some examples are in dwmain.c (Windows), dpmain.c (OS/2) and dxmain.c (X11/Linux), and dmmain.c (MacOS Classic or Carbon).

Copyright © 2003 artofcode LLC. All rights reserved.

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 the license contained in the file LICENSE in this distribution. For more information about licensing, please refer to http://www.ghostscript.com/licensing/. For information on commercial licensing, go to http://www.artifex.com/licensing/ or contact Artifex Software, Inc., 101 Lucas Valley Road #110, San Rafael, CA 94903, U.S.A., +1(415)492-9861.

Ghostscript version 8.15, 22 September 2004