===============

pycparser v2.21

===============

.. image:: https://github.com/eliben/pycparser/workflows/pycparser-tests/badge.svg

:align: center

:target: https://github.com/eliben/pycparser/actions

----

.. contents::

:backlinks: none

.. sectnum::

Introduction

============

What is pycparser?

------------------

**pycparser** is a parser for the C language, written in pure Python. It is a

module designed to be easily integrated into applications that need to parse

C source code.

What is it good for?

--------------------

Anything that needs C code to be parsed. The following are some uses for

**pycparser**, taken from real user reports:

* C code obfuscator

* Front-end for various specialized C compilers

* Static code checker

* Automatic unit-test discovery

* Adding specialized extensions to the C language

One of the most popular uses of **pycparser** is in the `cffi

<https://cffi.readthedocs.io/en/latest/>`_ library, which uses it to parse the

declarations of C functions and types in order to auto-generate FFIs.

**pycparser** is unique in the sense that it's written in pure Python - a very

high level language that's easy to experiment with and tweak. To people familiar

with Lex and Yacc, **pycparser**'s code will be simple to understand. It also

has no external dependencies (except for a Python interpreter), making it very

simple to install and deploy.

Which version of C does pycparser support?

------------------------------------------

**pycparser** aims to support the full C99 language (according to the standard

ISO/IEC 9899). Some features from C11 are also supported, and patches to support

more are welcome.

**pycparser** supports very few GCC extensions, but it's fairly easy to set

things up so that it parses code with a lot of GCC-isms successfully. See the

`FAQ <https://github.com/eliben/pycparser/wiki/FAQ>`_ for more details.

What grammar does pycparser follow?

-----------------------------------

**pycparser** very closely follows the C grammar provided in Annex A of the C99

standard (ISO/IEC 9899).

How is pycparser licensed?

--------------------------

`BSD license <https://github.com/eliben/pycparser/blob/master/LICENSE>`_.

Contact details

---------------

For reporting problems with **pycparser** or submitting feature requests, please

open an `issue <https://github.com/eliben/pycparser/issues>`_, or submit a

pull request.

Installing

==========

Prerequisites

-------------

* **pycparser** was tested on Python 2.7, 3.4-3.6, on both Linux and

Windows. It should work on any later version (in both the 2.x and 3.x lines)

as well.

* **pycparser** has no external dependencies. The only non-stdlib library it

uses is PLY, which is bundled in ``pycparser/ply``. The current PLY version is

3.10, retrieved from `<http://www.dabeaz.com/ply/>`_

Note that **pycparser** (and PLY) uses docstrings for grammar specifications.

Python installations that strip docstrings (such as when using the Python

``-OO`` option) will fail to instantiate and use **pycparser**. You can try to

work around this problem by making sure the PLY parsing tables are pre-generated

in normal mode; this isn't an officially supported/tested mode of operation,

though.

Installation process

--------------------

Installing **pycparser** is very simple. Once you download and unzip the

package, you just have to execute the standard ``python setup.py install``. The

setup script will then place the ``pycparser`` module into ``site-packages`` in

your Python's installation library.

Alternatively, since **pycparser** is listed in the `Python Package Index

<https://pypi.org/project/pycparser/>`_ (PyPI), you can install it using your

favorite Python packaging/distribution tool, for example with::

> pip install pycparser

Known problems

--------------

* Some users who've installed a new version of **pycparser** over an existing

version ran into a problem using the newly installed library. This has to do

with parse tables staying around as ``.pyc`` files from the older version. If

you see unexplained errors from **pycparser** after an upgrade, remove it (by

deleting the ``pycparser`` directory in your Python's ``site-packages``, or

wherever you installed it) and install again.

Using

=====

Interaction with the C preprocessor

-----------------------------------

In order to be compilable, C code must be preprocessed by the C preprocessor -

``cpp``. ``cpp`` handles preprocessing directives like ``#include`` and

``#define``, removes comments, and performs other minor tasks that prepare the C

code for compilation.

For all but the most trivial snippets of C code **pycparser**, like a C

compiler, must receive preprocessed C code in order to function correctly. If

you import the top-level ``parse_file`` function from the **pycparser** package,

it will interact with ``cpp`` for you, as long as it's in your PATH, or you

provide a path to it.

Note also that you can use ``gcc -E`` or ``clang -E`` instead of ``cpp``. See

the ``using_gcc_E_libc.py`` example for more details. Windows users can download

and install a binary build of Clang for Windows `from this website

<http://llvm.org/releases/download.html>`_.

What about the standard C library headers?

------------------------------------------

C code almost always ``#include``\s various header files from the standard C

library, like ``stdio.h``. While (with some effort) **pycparser** can be made to

parse the standard headers from any C compiler, it's much simpler to use the

provided "fake" standard includes for C11 in ``utils/fake_libc_include``. These

are standard C header files that contain only the bare necessities to allow

valid parsing of the files that use them. As a bonus, since they're minimal, it

can significantly improve the performance of parsing large C files.

The key point to understand here is that **pycparser** doesn't really care about

the semantics of types. It only needs to know whether some token encountered in

the source is a previously defined type. This is essential in order to be able

to parse C correctly.

See `this blog post

<https://eli.thegreenplace.net/2015/on-parsing-c-type-declarations-and-fake-headers>`_

for more details.

Note that the fake headers are not included in the ``pip`` package nor installed

via ``setup.py`` (`#224 <https://github.com/eliben/pycparser/issues/224>`_).

Basic usage

-----------

Take a look at the |examples|_ directory of the distribution for a few examples

of using **pycparser**. These should be enough to get you started. Please note

that most realistic C code samples would require running the C preprocessor

before passing the code to **pycparser**; see the previous sections for more

details.

.. |examples| replace:: ``examples``

.. _examples: examples

Advanced usage

--------------

The public interface of **pycparser** is well documented with comments in

``pycparser/c_parser.py``. For a detailed overview of the various AST nodes

created by the parser, see ``pycparser/_c_ast.cfg``.

There's also a `FAQ available here <https://github.com/eliben/pycparser/wiki/FAQ>`_.

In any case, you can always drop me an `email <eliben@gmail.com>`_ for help.

Modifying

=========

There are a few points to keep in mind when modifying **pycparser**:

* The code for **pycparser**'s AST nodes is automatically generated from a

configuration file - ``_c_ast.cfg``, by ``_ast_gen.py``. If you modify the AST

configuration, make sure to re-generate the code. This can be done by running

the ``_build_tables.py`` script from the ``pycparser`` directory.

* Make sure you understand the optimized mode of **pycparser** - for that you

must read the docstring in the constructor of the ``CParser`` class. For

development you should create the parser without optimizations, so that it

will regenerate the Yacc and Lex tables when you change the grammar.

Package contents

================

Once you unzip the ``pycparser`` package, you'll see the following files and

directories:

README.rst:

This README file.

LICENSE:

The pycparser license

setup.py:

Installation script

examples/:

A directory with some examples of using **pycparser**

pycparser/:

The **pycparser** module source code.

tests/:

Unit tests.

utils/fake_libc_include:

Minimal standard C library include files that should allow to parse any C code.

Note that these headers now include C11 code, so they may not work when the

preprocessor is configured to an earlier C standard (like ``-std=c99``).

utils/internal/:

Internal utilities for my own use. You probably don't need them.

Contributors

============

Some people have contributed to **pycparser** by opening issues on bugs they've

found and/or submitting patches. The list of contributors is in the CONTRIBUTORS

file in the source distribution. After **pycparser** moved to Github I stopped

updating this list because Github does a much better job at tracking

contributions.