clang/docs/LibClang.rst

.. role:: raw-html(raw)
    :format: html

Libclang tutorial
=================
The C Interface to Clang provides a relatively small API that exposes facilities for parsing source code into an abstract syntax tree (AST), loading already-parsed ASTs, traversing the AST, associating physical source locations with elements within the AST, and other facilities that support Clang-based development tools.
This C interface to Clang will never provide all of the information representation stored in Clang's C++ AST, nor should it: the intent is to maintain an API that is :ref:`relatively stable <Stability>` from one release to the next, providing only the basic functionality needed to support development tools.
The entire C interface of libclang is available in the file `Index.h`_

Essential types overview
-------------------------

All types of libclang are prefixed with ``CX``

CXIndex
~~~~~~~
An Index that consists of a set of translation units that would typically be linked together into an executable or library.

CXTranslationUnit
~~~~~~~~~~~~~~~~~
A single translation unit, which resides in an index.

CXCursor
~~~~~~~~
A cursor representing a pointer to some element in the abstract syntax tree of a translation unit.


Code example
""""""""""""

.. code-block:: cpp

  // file.cpp
  struct foo{
    int bar;
    int* bar_pointer;
  };

.. code-block:: cpp

  #include <clang-c/Index.h>
  #include <iostream>

  int main(){
    CXIndex index = clang_createIndex(0, 0); //Create index
    CXTranslationUnit unit = clang_parseTranslationUnit(
      index,
      "file.cpp", nullptr, 0,
      nullptr, 0,
      CXTranslationUnit_None); //Parse "file.cpp"


    if (unit == nullptr){
      std::cerr << "Unable to parse translation unit. Quitting.\n";
      return 0;
    }
    CXCursor cursor = clang_getTranslationUnitCursor(unit); //Obtain a cursor at the root of the translation unit
  }

Visiting elements of an AST
~~~~~~~~~~~~~~~~~~~~~~~~~~~
The elements of an AST can be recursively visited with pre-order traversal with ``clang_visitChildren``.

.. code-block:: cpp

  clang_visitChildren(
    cursor, //Root cursor
    [](CXCursor current_cursor, CXCursor parent, CXClientData client_data){

      CXString current_display_name = clang_getCursorDisplayName(current_cursor);
      //Allocate a CXString representing the name of the current cursor

      std::cout << "Visiting element " << clang_getCString(current_display_name) << "\n";
      //Print the char* value of current_display_name

      clang_disposeString(current_display_name);
      //Since clang_getCursorDisplayName allocates a new CXString, it must be freed. This applies
      //to all functions returning a CXString

      return CXChildVisit_Recurse;


    }, //CXCursorVisitor: a function pointer
    nullptr //client_data
    );

The return value of ``CXCursorVisitor``, the callable argument of ``clang_visitChildren``, can return one of the three:

#. ``CXChildVisit_Break``: Terminates the cursor traversal

#. ``CXChildVisit_Continue``: Continues the cursor traversal with the next sibling of the cursor just visited, without visiting its children.

#. ``CXChildVisit_Recurse``: Recursively traverse the children of this cursor, using the same visitor and client data

The expected output of that program is

.. code-block::

  Visiting element foo
  Visiting element bar
  Visiting element bar_pointer


Extracting information from a Cursor
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. The following functions take a ``CXCursor`` as an argument and return associated information.



Extracting the Cursor kind
""""""""""""""""""""""""""

``CXCursorKind clang_getCursorKind(CXCursor)`` Describes the kind of entity that a cursor refers to. Example values:

- ``CXCursor_StructDecl``: A C or C++ struct.
- ``CXCursor_FieldDecl``: A field in a struct, union, or C++ class.
- ``CXCursor_CallExpr``: An expression that calls a function.


Extracting the Cursor type
""""""""""""""""""""""""""
``CXType clang_getCursorType(CXCursor)``: Retrieve the type of a CXCursor (if any).

A ``CXType`` represents a complete C++ type, including qualifiers and pointers. It has a member field ``CXTypeKind kind`` and additional opaque data.

Example values for ``CXTypeKind kind``

- ``CXType_Invalid``: Represents an invalid type (e.g., where no type is available)
- ``CXType_Pointer``: A pointer to another type
- ``CXType_Int``: Regular ``int``
- ``CXType_Elaborated``: Represents a type that was referred to using an elaborated type keyword e.g. struct S, or via a qualified name, e.g., N::M::type, or both.

Any ``CXTypeKind`` can be converted to a ``CXString`` using ``clang_getTypeKindSpelling(CXTypeKind)``.

A ``CXType`` holds additional necessary opaque type info, such as:

- Which struct was referred to?
- What type is the pointer pointing to?
- Qualifiers (e.g. ``const``, ``volatile``)?

Qualifiers of a ``CXType`` can be queried with:

- ``clang_isConstQualifiedType(CXType)`` to check for ``const``
- ``clang_isRestrictQualifiedType(CXType)`` to check for ``restrict``
- ``clang_isVolatileQualifiedType(CXType)`` to check for ``volatile``

Code example
""""""""""""
.. code-block:: cpp

  //structs.cpp
  struct A{
    int value;
  };
  struct B{
    int value;
    A struct_value;
  };

.. code-block:: cpp

  #include <clang-c/Index.h>
  #include <iostream>

  int main(){
    CXIndex index = clang_createIndex(0, 0); //Create index
    CXTranslationUnit unit = clang_parseTranslationUnit(
      index,
      "structs.cpp", nullptr, 0,
      nullptr, 0,
      CXTranslationUnit_None); //Parse "structs.cpp"

    if (unit == nullptr){
      std::cerr << "Unable to parse translation unit. Quitting.\n";
      return 0;
    }
    CXCursor cursor = clang_getTranslationUnitCursor(unit); //Obtain a cursor at the root of the translation unit

    clang_visitChildren(
    cursor,
    [](CXCursor current_cursor, CXCursor parent, CXClientData client_data){
      CXType cursor_type = clang_getCursorType(current_cursor);

      CXString type_kind_spelling = clang_getTypeKindSpelling(cursor_type.kind);
      std::cout << "Type Kind: " << clang_getCString(type_kind_spelling);
      clang_disposeString(type_kind_spelling);

      if(cursor_type.kind == CXType_Pointer ||                     // If cursor_type is a pointer
        cursor_type.kind == CXType_LValueReference ||              // or an LValue Reference (&)
        cursor_type.kind == CXType_RValueReference){               // or an RValue Reference (&&),
        CXType pointed_to_type = clang_getPointeeType(cursor_type);// retrieve the pointed-to type

        CXString pointed_to_type_spelling = clang_getTypeSpelling(pointed_to_type);     // Spell out the entire
        std::cout << "pointing to type: " << clang_getCString(pointed_to_type_spelling);// pointed-to type
        clang_disposeString(pointed_to_type_spelling);
      }
      else if(cursor_type.kind == CXType_Record){
        CXString type_spelling = clang_getTypeSpelling(cursor_type);
        std::cout <<  ", namely " << clang_getCString(type_spelling);
        clang_disposeString(type_spelling);
      }
      std::cout << "\n";
      return CXChildVisit_Recurse;
    },
    nullptr
    );

The expected output of program is:

.. code-block::

  Type Kind: Record, namely A
  Type Kind: Int
  Type Kind: Record, namely B
  Type Kind: Int
  Type Kind: Record, namely A
  Type Kind: Record, namely A


Reiterating the difference between ``CXType`` and ``CXTypeKind``: For an example

.. code-block:: cpp

   const char* __restrict__ variable;

- Type Kind will be: ``CXType_Pointer`` spelled ``"Pointer"``
- Type will be a complex ``CXType`` structure, spelled ``"const char* __restrict__``

Retrieving source locations
"""""""""""""""""""""""""""

``CXSourceRange clang_getCursorExtent(CXCursor)`` returns a ``CXSourceRange``, representing a half-open range in the source code.

Use ``clang_getRangeStart(CXSourceRange)`` and ``clang_getRangeEnd(CXSourceRange)`` to retrieve the starting and end ``CXSourceLocation`` from a source range, respectively.

Given a ``CXSourceLocation``, use ``clang_getExpansionLocation`` to retrieve file, line and column of a source location.

Code example
""""""""""""
.. code-block:: cpp

  // Again, file.cpp
  struct foo{
    int bar;
    int* bar_pointer;
  };
.. code-block:: cpp

  clang_visitChildren(
    cursor,
    [](CXCursor current_cursor, CXCursor parent, CXClientData client_data){

      CXType cursor_type = clang_getCursorType(current_cursor);
      CXString cursor_spelling = clang_getCursorSpelling(current_cursor);
      CXSourceRange cursor_range = clang_getCursorExtent(current_cursor);
      std::cout << "Cursor " << clang_getCString(cursor_spelling);

      CXFile file;
      unsigned start_line, start_column, start_offset;
      unsigned end_line, end_column, end_offset;

      clang_getExpansionLocation(clang_getRangeStart(cursor_range), &file, &start_line, &start_column, &start_offset);
      clang_getExpansionLocation(clang_getRangeEnd  (cursor_range), &file, &end_line  , &end_column  , &end_offset);
      std::cout << " spanning lines " << start_line << " to " << end_line;
      clang_disposeString(cursor_spelling);

      std::cout << "\n";
      return CXChildVisit_Recurse;
    },
    nullptr
  );

The expected output of this program is:

.. code-block::

  Cursor foo spanning lines 2 to 5
  Cursor bar spanning lines 3 to 3
  Cursor bar_pointer spanning lines 4 to 4

Complete example code
~~~~~~~~~~~~~~~~~~~~~

.. code-block:: cpp

  #include <clang-c/Index.h>
  #include <iostream>

  int main(){
    CXIndex index = clang_createIndex(0, 0); //Create index
    CXTranslationUnit unit = clang_parseTranslationUnit(
      index,
      "file.cpp", nullptr, 0,
      nullptr, 0,
      CXTranslationUnit_None); //Parse "file.cpp"

    if (unit == nullptr){
      std::cerr << "Unable to parse translation unit. Quitting.\n";
      return 0;
    }
    CXCursor cursor = clang_getTranslationUnitCursor(unit); //Obtain a cursor at the root of the translation unit


    clang_visitChildren(
    cursor,
    [](CXCursor current_cursor, CXCursor parent, CXClientData client_data){
      CXType cursor_type = clang_getCursorType(current_cursor);

      CXString type_kind_spelling = clang_getTypeKindSpelling(cursor_type.kind);
      std::cout << "TypeKind: " << clang_getCString(type_kind_spelling);
      clang_disposeString(type_kind_spelling);

      if(cursor_type.kind == CXType_Pointer ||                     // If cursor_type is a pointer
        cursor_type.kind == CXType_LValueReference ||              // or an LValue Reference (&)
        cursor_type.kind == CXType_RValueReference){               // or an RValue Reference (&&),
        CXType pointed_to_type = clang_getPointeeType(cursor_type);// retrieve the pointed-to type

        CXString pointed_to_type_spelling = clang_getTypeSpelling(pointed_to_type);     // Spell out the entire
        std::cout << "pointing to type: " << clang_getCString(pointed_to_type_spelling);// pointed-to type
        clang_disposeString(pointed_to_type_spelling);
      }
      else if(cursor_type.kind == CXType_Record){
        CXString type_spelling = clang_getTypeSpelling(cursor_type);
        std::cout <<  ", namely " << clang_getCString(type_spelling);
        clang_disposeString(type_spelling);
      }
      std::cout << "\n";
      return CXChildVisit_Recurse;
    },
    nullptr
    );


    clang_visitChildren(
    cursor,
    [](CXCursor current_cursor, CXCursor parent, CXClientData client_data){

      CXType cursor_type = clang_getCursorType(current_cursor);
      CXString cursor_spelling = clang_getCursorSpelling(current_cursor);
      CXSourceRange cursor_range = clang_getCursorExtent(current_cursor);
      std::cout << "Cursor " << clang_getCString(cursor_spelling);

      CXFile file;
      unsigned start_line, start_column, start_offset;
      unsigned end_line, end_column, end_offset;

      clang_getExpansionLocation(clang_getRangeStart(cursor_range), &file, &start_line, &start_column, &start_offset);
      clang_getExpansionLocation(clang_getRangeEnd  (cursor_range), &file, &end_line  , &end_column  , &end_offset);
      std::cout << " spanning lines " << start_line << " to " << end_line;
      clang_disposeString(cursor_spelling);

      std::cout << "\n";
      return CXChildVisit_Recurse;
    },
    nullptr
    );
  }


.. _Index.h: https://github.com/llvm/llvm-project/blob/main/clang/include/clang-c/Index.h

.. _Stability:

ABI and API Stability
---------------------

The C interfaces in libclang are intended to be relatively stable. This allows
a programmer to use libclang without having to worry as much about Clang
upgrades breaking existing code. However, the library is not unchanging. For
example, the library will gain new interfaces over time as needs arise,
existing APIs may be deprecated for eventual removal, etc. Also, the underlying
implementation of the facilities by Clang may change behavior as bugs are
fixed, features get implemented, etc.

The library should be ABI and API stable over time, but ABI- and API-breaking
changes can happen in the following (non-exhaustive) situations:

* Adding new enumerator to an enumeration (can be ABI-breaking in C++).
* Removing an explicitly deprecated API after a suitably long deprecation
  period.
* Using implementation details, such as names or comments that say something
  is "private", "reserved", "internal", etc.
* Bug fixes and changes to Clang's internal implementation happen routinely and
  will change the behavior of callers.
* Rarely, bug fixes to libclang itself.

The library has version macros (``CINDEX_VERSION_MAJOR``,
``CINDEX_VERSION_MINOR``, and ``CINDEX_VERSION``) which can be used to test for
specific library versions at compile time. The ``CINDEX_VERSION_MAJOR`` macro
is only incremented if there are major source- or ABI-breaking changes. Except
for removing an explicitly deprecated API, the changes listed above are not
considered major source- or ABI-breaking changes. Historically, the value this
macro expands to has not changed, but may be incremented in the future should
the need arise. The ``CINDEX_VERSION_MINOR`` macro is incremented as new APIs
are added. The ``CINDEX_VERSION`` macro expands to a value based on the major
and minor version macros.

In an effort to allow the library to be modified as new needs arise, the
following situations are explicitly unsupported:

* Loading different library versions into the same executable and passing
  objects between the libraries; despite general ABI stability, different
  versions of the library may use different implementation details that are not
  compatible across library versions.
* For the same reason as above, serializing objects from one version of the
  library and deserializing with a different version is also not supported.

Note: because libclang is a wrapper around the compiler frontend, it is not a
`security-sensitive component`_ of the LLVM Project. Consider using a sandbox
or some other mitigation approach if processing untrusted input.

.. _security-sensitive component: https://llvm.org/docs/Security.html#what-is-considered-a-security-issue