## Introduction

The CUPS filter and backend APIs define standard exit codes and provide access to the backchannel data stream. They are only used when writing backends, filters, and port monitors.

## General Usage

The <cups/backend.h> and <cups/cups.h> header files must be included to use the `CUPS_BACKEND_` constants and `cupsBackChannel` functions, respectively.

Programs using these functions must be linked to the CUPS library: libcups.a, libcups.so.2, libcups.2.dylib, libcups_s.a, or libcups2.lib depending on the platform. The following command compiles myprogram.c using GCC and the CUPS library:

    gcc -o myprogram myprogram.c -lcups

## Compatibility

All of these functions require CUPS 1.2 or higher.