The Ghostscript Interpreter Application Programming Interface (API)

Table of contents

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 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>
and link with:
cc -o gsc dxmainc.c -framework Ghostscript
To get a useable executable. Ghostscript.framework must be properly installed in the search path for this to work.

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.

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.

The address passed in pexit_code will be used to return the exit code for the interpreter in case of a quit or fatal error. The user_errors argument is normally set to zero to indicate that errors should be handled through the normal mechanisms within the interpreted code. If set to a negative value, the functions will return an error code directly to the caller, bypassing the interpreted language. A positive user_errors is treated the same as zero.

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

Some overly simple examples of using the Ghostscript interpreter library are below. Some more examples including other programming languages may be found at http://www.cs.wisc.edu/~ghost/doc/gsapi.htm.

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"

void *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;
}

void *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

Replace main() in either of the above with the following code, showing how you can feed Ghostscript piecemeal:
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;
}

Example 4

When feeding Ghostscript piecemeal buffers, one can use the normal operators to configure things and invoke library routines. For example, to parse a PDF file one could say:
    code = gsapi_run_string(minst, "(example.pdf) .runlibfile", 0, &exit_code);
and Ghostscript would open and process the file named "example.pdf" as if it had been passed as an argument to gsapi_init_with_args().

Multiple threads

The Ghostscript library should have been compiled with a thread safe run time library. Synchronisation of threads is entirely up to the caller. The exported gsapi_*() functions must be called from one thread only.

Standard input and output

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

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
Options include The format values are described in gdevdsp.h. The format is flexible enough to support common Windows, OS/2, Linux and Mac raster formats. To select the display device with a Windows 24-bit RGB raster:
    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).

On some platforms, the calling convention for the display device callbacks in gdevdsp.h is not the same as the exported gsapi_*() functions in iapi.h.


Copyright © 2000-2006 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 that license. Refer to licensing information at http://www.artifex.com/ or contact Artifex Software, Inc., 7 Mt. Lassen Drive - Suite A-134, San Rafael, CA 94903, U.S.A., +1(415)492-9861, for further information.

Ghostscript version 8.54, 17 May 2006