# Using the qpdf Library

## Using qpdf from C++

The source tree for the qpdf package has an `examples` directory that contains a few example programs. The `libqpdf/QPDFJob.cc` source file also serves as a useful example since it exercises almost all of the qpdf library’s public interface. The best source of documentation on the library itself is reading comments in `include/qpdf/QPDF.hh`, `include/qpdf/QPDFWriter.hh`, and `include/qpdf/QPDFObjectHandle.hh`.

All header files are installed in the `include/qpdf` directory. It is recommend that you use `#include <qpdf/QPDF.hh>` rather than adding `include/qpdf` to your include path.

qpdf installs a `pkg-config` configuration with package name `libqpdf` and a `cmake` configuration with package name `qpdf`. The `libqpdf` target is exported in the `qpdf::` namespace. The following is an example of a `CMakeLists.txt` file for a single-file executable that links with qpdf:

    cmake_minimum_required(VERSION 3.16)
    project(some-application LANGUAGES CXX)
    find_package(qpdf)
    add_executable(some-application some-application.cc)
    target_link_libraries(some-application qpdf::libqpdf)

The qpdf library is safe to use in a multithreaded program, but no individual `qpdf` object instance (including `QPDF`, `QPDFObjectHandle`, or `QPDFWriter`) can be used in more than one thread at a time. Multiple threads may simultaneously work with different instances of these and all other qpdf objects.

## Using qpdf from other languages

The qpdf library is implemented in C++, which makes it hard to use directly in other languages. There are a few things that can help.

“C”

  The qpdf library includes a “C” language interface that provides a subset of the overall capabilities. The header file `qpdf/qpdf-c.h` includes information about its use. As long as you use a C++ linker, you can link C programs with qpdf and use the C API. For languages that can directly load methods from a shared library, the C API can also be useful. People have reported success using the C API from other languages on Windows by directly calling functions in the DLL.

Python

  A Python module called pikepdf provides a clean and highly functional set of Python bindings to the qpdf library. Using pikepdf, you can work with PDF files in a natural way and combine qpdf’s capabilities with other functionality provided by Python’s rich standard library and available modules.

Other Languages

  Starting with version 11.0.0, the qpdf command-line tool can produce an unambiguous JSON representation of a PDF file and can also create or update PDF files using this JSON representation. qpdf versions from 8.3.0 through 10.6.3 had a more limited JSON output format. The qpdf JSON format makes it possible to inspect and modify the structure of a PDF file down to the object level from the command-line or from any language that can handle JSON data. Please see qpdf JSON for details.

Wrappers

  The qpdf Wiki contains a list of Wrappers around qpdf. These may have varying degrees of functionality or completeness. If you know of (or have written) a wrapper that you’d like include, open an issue at https://github.com/qpdf/qpdf/issues/new and ask for it to be added to the list.

## A Note About Unicode File Names

When strings are passed to qpdf library routines either as `char*` or as `std::string`, they are treated as byte arrays except where otherwise noted. When Unicode is desired, qpdf wants UTF-8 unless otherwise noted in comments in header files. In modern UNIX/Linux environments, this generally does the right thing. In Windows, it’s a bit more complicated. Starting in qpdf 8.4.0, passwords that contain Unicode characters are handled much better, and starting in qpdf 8.4.1, the library attempts to properly handle Unicode characters in filenames. In particular, in Windows, if a UTF-8 encoded string is used as a filename in either `QPDF` or `QPDFWriter`, it is internally converted to `wchar_t*`, and Unicode-aware Windows APIs are used. As such, qpdf will generally operate properly on files with non-ASCII characters in their names as long as the filenames are UTF-8 encoded for passing into the qpdf library API, but there are still some rough edges, such as the encoding of the filenames in error messages or CLI output messages. Patches or bug reports are welcome for any continuing issues with Unicode file names in Windows.