.. role:: raw-html(raw)
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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 relatively stable 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
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