# 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 `qpdf/qpdf.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.

When linking against the qpdf static library, you may also need to specify `-lz -ljpeg` on your link command. If your system understands how to read libtool `.la` files, this may not be necessary.

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 8.3.0, the qpdf command-line tool can produce a JSON representation of the PDF file’s non-content data. This can facilitate interacting programmatically with PDF files through qpdf’s command line interface. For more information, please see QPDF JSON.

## 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 our CLI output messages. Patches or bug reports are welcome for any continuing issues with Unicode file names in Windows.