# 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_init_with_args`
  - `gsapi_run_*`
  - `gsapi_exit`
  - Return codes
- Typical 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 provides 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. The source file

`dxmainc.c` can serve as an example of how to use the framework shared component on MacOS X. Just change the header includes to use the Ghostscript namespace:

>     #include <Ghostscript/errors.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.

- `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);`

### `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`.

### `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.

### `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.

### `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()`.

### 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.

## Typical Usage

A really short example of using the Ghostscript interpreter library is:

    #include <stdio.h>
    #include "errors.h"
    #include "iapi.h"

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

    static int
    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;
        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);
        gsapi_exit(minst);

        gsapi_delete_instance(minst);

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

Another partial code example showing how you can feed Ghostscript piecemeal:

        char *command = "1 2 add == flush\n";
        int exit_code;
        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);

## 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.

## 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

Options include

- 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.

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).

Copyright © 2001 artofcode LLC. All rights reserved.

This file is part of AFPL Ghostscript. See the Aladdin Free Public License (the "License") for full details of the terms of using, copying, modifying, and redistributing AFPL Ghostscript.

Ghostscript version 7.04, 31 January 2002