This tutorial describes how to create Clang plugin and covers the next things:
- environment setup
- basic plugin setup
- setup Xcode project for plugin development
- warnings reporting
- errors reporting
- Xcode integration
- interactive hints for errors/warnings riddance
tl;dr
Clang Rocks!!!
You can find the plugin here.
Intro
While working on BloodMagic, I realised that it’d be nice to have a tool for checking semantic errors related to BM usage. For example: in the interface property marked as lazy, but not defined as @dynamic in the implementation, or property marked as lazy, but class container doesn’t support injections.
I concluded that I need to work with and I need a full-featured parser.
I’ve tried different approaches: flex+bison, libclang, but ultimately I decided to write a Clang plugin.
Just for testing purposes I’ve started a simple plugin with the following goals:
- use Xcode for development
- integrate ready plugin into Xcode and use it workaday
- plugin should report warnings, errors and propose interactive hints for fixes (via Xcode UI)
Features of the test plugin:
- report warning in case of class’ name starts with lowercase letter
- report error in case of class’ name contains underscore
_ - propose hints for fixes
Environment setup
For plugin development we need llvm/clang, built from source
1 2 3 4 5 | cd /opt sudo mkdir llvm sudo chown `whoami` llvm cd llvm export LLVM_HOME=`pwd` |
Current clang version on my system – 3.3.1, so let’s build respective version:
1 2 3 4 5 6 7 8 9 | git clone -b release_33 https://github.com/llvm-mirror/llvm.git llvm git clone -b release_33 https://github.com/llvm-mirror/clang.git llvm/tools/clang git clone -b release_33 https://github.com/llvm-mirror/clang-tools-extra.git llvm/tools/clang/tools/extra git clone -b release_33 https://github.com/llvm-mirror/compiler-rt.git llvm/projects/compiler-rt mkdir llvm_build cd llvm_build cmake ../llvm -DCMAKE_BUILD_TYPE:STRING=Release make -j`sysctl -n hw.logicalcpu` |
Basic plugin setup
Create directory for plugin
1 2 | cd $LLVM_HOME mkdir toy_clang_plugin; cd toy_clang_plugin |
Our plugin based on example from Clang repo and here is it’s structure:
1 2 3 | ToyClangPlugin.exports CMakeLists.txt ToyClangPlugin.cpp |
We’ll use one source file for simplification
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 | #include "clang/Frontend/FrontendPluginRegistry.h" #include "clang/AST/AST.h" #include "clang/AST/ASTConsumer.h" #include "clang/Frontend/CompilerInstance.h" using namespace clang; namespace { class ToyConsumer : public ASTConsumer { }; class ToyASTAction : public PluginASTAction { public: virtual clang::ASTConsumer *CreateASTConsumer(CompilerInstance &Compiler, llvm::StringRef InFile) { return new ToyConsumer; } bool ParseArgs(const CompilerInstance &CI, const std::vector<std::string>& args) { return true; } }; } static clang::FrontendPluginRegistry::Add<ToyASTAction> X("ToyClangPlugin", "Toy Clang Plugin"); |
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 | cmake_minimum_required (VERSION 2.6) project (ToyClangPlugin) set( CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin ) set( CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib ) set( CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib ) set( LLVM_HOME /opt/llvm ) set( LLVM_SRC_DIR ${LLVM_HOME}/llvm ) set( CLANG_SRC_DIR ${LLVM_HOME}/llvm/tools/clang ) set( LLVM_BUILD_DIR ${LLVM_HOME}/llvm_build ) set( CLANG_BUILD_DIR ${LLVM_HOME}/llvm_build/tools/clang) add_definitions (-D__STDC_LIMIT_MACROS -D__STDC_CONSTANT_MACROS) add_definitions (-D_GNU_SOURCE -DHAVE_CLANG_CONFIG_H) set (CMAKE_CXX_COMPILER "${LLVM_BUILD_DIR}/bin/clang++") set (CMAKE_CC_COMPILER "${LLVM_BUILD_DIR}/bin/clang") set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fPIC -fno-common -Woverloaded-virtual -Wcast-qual -fno-strict-aliasing -pedantic -Wno-long-long -Wall -Wno-unused-parameter -Wwrite-strings -fno-exceptions -fno-rtti") set (CMAKE_MODULE_LINKER_FLAGS "-Wl,-flat_namespace -Wl,-undefined -Wl,suppress") set (LLVM_LIBS LLVMJIT LLVMX86CodeGen LLVMX86AsmParser LLVMX86Disassembler LLVMExecutionEngine LLVMAsmPrinter LLVMSelectionDAG LLVMX86AsmPrinter LLVMX86Info LLVMMCParser LLVMCodeGen LLVMX86Utils LLVMScalarOpts LLVMInstCombine LLVMTransformUtils LLVMipa LLVMAnalysis LLVMTarget LLVMCore LLVMMC LLVMSupport LLVMBitReader LLVMOption ) macro(add_clang_plugin name) set (srcs ${ARGN}) include_directories( "${LLVM_SRC_DIR}/include" "${CLANG_SRC_DIR}/include" "${LLVM_BUILD_DIR}/include" "${CLANG_BUILD_DIR}/include" ) link_directories( "${LLVM_BUILD_DIR}/lib" ) add_library( ${name} SHARED ${srcs} ) if (SYMBOL_FILE) set_target_properties( ${name} PROPERTIES LINK_FlAGS "-exported_symbols_list ${SYMBOL_FILE}") endif() foreach (clang_lib ${CLANG_LIBS}) target_link_libraries( ${name} ${clang_lib} ) endforeach() foreach (llvm_lib ${LLVM_LIBS}) target_link_libraries( ${name} ${llvm_lib} ) endforeach() foreach (user_lib ${USER_LIBS}) target_link_libraries( ${name} ${user_lib} ) endforeach() endmacro(add_clang_plugin) set(SYMBOL_FILE ToyClangPlugin.exports) set (CLANG_LIBS clang clangFrontend clangAST clangAnalysis clangBasic clangCodeGen clangDriver clangFrontendTool clangLex clangParse clangSema clangEdit clangSerialization clangStaticAnalyzerCheckers clangStaticAnalyzerCore clangStaticAnalyzerFrontend ) set (USER_LIBS pthread curses ) add_clang_plugin(ToyClangPlugin ToyClangPlugin.cpp ) set_target_properties(ToyClangPlugin PROPERTIES LINKER_LANGUAGE CXX PREFIX "") |
1 | __ZN4llvm8Registry* |
Now we’re able to generate Xcode-project, based on CMakeLists.txt
1 2 3 | mkdir build; cd build cmake -G Xcode .. open ToyClangPlugin.xcodeproj |
Run ALL_BUILD target, and you’ll see dynamic library at lib/Debug/ToyCLangPlugin.dylib.
RecursiveASTVisitor
Clang AST module provides RecursiveASTVisitor, which allows us to traverse via AST.
We just need to create a subclass and implement interesting methods.
For test we’ll print all the found class names.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 | class ToyClassVisitor : public RecursiveASTVisitor<ToyClassVisitor> { public: bool VisitObjCInterfaceDecl(ObjCInterfaceDecl *declaration) { printf("ObjClass: %s\n", declaration->getNameAsString().c_str()); return true; } }; class ToyConsumer : public ASTConsumer { public: void HandleTranslationUnit(ASTContext &context) { visitor.TraverseDecl(context.getTranslationUnitDecl()); } private: ToyClassVisitor visitor; }; |
Let’s create test source file and check how plugin works.
1 2 3 4 5 6 7 8 9 | #import <Foundation/Foundation.h> @interface ToyObject : NSObject @end @implementation ToyObject @end |
Rebuild plugin and run
1 2 3 4 5 | /opt/llvm/toy_clang_plugin/build $ $LLVM_HOME/llvm_build/bin/clang ../test.m \ -Xclang -load \ -Xclang lib/Debug/ToyClangPlugin.dylib \ -Xclang -plugin \ -Xclang ToyClangPlugin |
We’ll see a huge list of classes.
Report warnings
Let’s report warning in case if class’ name starts with lowercase letter
Add ASTContext to ToyClassVisitor
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 | class ToyClassVisitor : public RecursiveASTVisitor<ToyClassVisitor> { private: ASTContext *context; public: void setContext(ASTContext &context) { this->context = &context; } // ... }; // ... void HandleTranslationUnit(ASTContext &context) { visitor.setContext(context); visitor.TraverseDecl(context.getTranslationUnitDecl()); } // ... |
Add check
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 | bool VisitObjCInterfaceDecl(ObjCInterfaceDecl *declaration) { checkForLowercasedName(declaration); return true; } // ... void checkForLowercasedName(ObjCInterfaceDecl *declaration) { StringRef name = declaration->getName(); char c = name[0]; if (isLowercase(c)) { DiagnosticsEngine &diagEngine = context->getDiagnostics(); unsigned diagID = diagEngine.getCustomDiagID(DiagnosticsEngine::Warning, "Class name should not start with lowercase letter"); SourceLocation location = declaration->getLocation(); diagEngine.Report(location, diagID); } } |
Add some class with “bad” name
1 2 3 4 5 6 7 | @interface bad_ToyObject : NSObject @end @implementation bad_ToyObject @end |
rebuild and run
1 2 3 4 5 6 7 8 9 10 | /opt/llvm/toy_clang_plugin/build $ $LLVM_HOME/llvm_build/bin/clang ../test.m \ -Xclang -load \ -Xclang lib/Debug/ToyClangPlugin.dylib \ -Xclang -plugin \ -Xclang ToyClangPlugin ../test.m:11:12: warning: Class name should not start with lowercase letter @interface bad_ToyObject : NSObject ^ 1 warning generated. |
Report error
Let’s generate error in case of class’ name contains _
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 | void checkForUnderscoreInName(ObjCInterfaceDecl *declaration) { size_t underscorePos = declaration->getName().find('_'); if (underscorePos != StringRef::npos) { DiagnosticsEngine &diagEngine = context->getDiagnostics(); unsigned diagID = diagEngine.getCustomDiagID(DiagnosticsEngine::Error, "Class name with `_` forbidden"); SourceLocation location = declaration->getLocation().getLocWithOffset(underscorePos); diagEngine.Report(location, diagID); } } bool VisitObjCInterfaceDecl(ObjCInterfaceDecl *declaration) { // disable this check temporary // checkForLowercasedName(declaration); checkForUnderscoreInName(declaration); return true; } |
Output after running
1 2 3 4 5 6 7 8 9 10 | /opt/llvm/toy_clang_plugin/build $ $LLVM_HOME/llvm_build/bin/clang ../test.m \ -Xclang -load \ -Xclang lib/Debug/ToyClangPlugin.dylib \ -Xclang -plugin \ -Xclang ToyClangPlugin ../test.m:11:15: error: Class name with `_` forbidden @interface bad_ToyObject : NSObject ^ 1 error generated. |
Uncomment first check checkForLowercasedName and you’ll see both error and warning in the output
1 2 3 4 5 6 7 8 9 10 11 12 13 | /opt/llvm/toy_clang_plugin/build $ $LLVM_HOME/llvm_build/bin/clang ../test.m \ -Xclang -load \ -Xclang lib/Debug/ToyClangPlugin.dylib \ -Xclang -plugin \ -Xclang ToyClangPlugin ../test.m:11:12: warning: Class name should not start with lowercase letter @interface bad_ToyObject : NSObject ^ ../test.m:11:15: error: Class name with `_` forbidden @interface bad_ToyObject : NSObject ^ 1 warning and 1 error generated. |
Xcode integration
Unfortunately, system (under ‘system’ I mean Xcode’s clang) clang doesn’t support plugins, so we need to hack Xcode a bit, to allow using of custom compiler.
Unzip this archive and run following commands:
1 2 | sudo mv HackedClang.xcplugin `xcode-select -print-path`/../PlugIns/Xcode3Core.ideplugin/Contents/SharedSupport/Developer/Library/Xcode/Plug-ins sudo mv HackedBuildSystem.xcspec `xcode-select -print-path`/Platforms/iPhoneSimulator.platform/Developer/Library/Xcode/Specifications |
This will enable custom compiler for Xcode.
Reopen Xcode and you’ll see new compiler:
Create new project and select newly added custom clang in Build settings
To enable plugin add following parameters to the OTHER_CFLAGS section
1 | -Xclang -load -Xclang /opt/llvm/toy_clang_plugin/build/lib/Debug/ToyClangPlugin.dylib -Xclang -add-plugin -Xclang ToyClangPlugin |
Note, that we use -add-plugin here, because we want to add our ASTAction, not to replace the existing
Also, you should disable modules for this target/build
Add test.m to this project, or create new one, with class names that match plugin criteria
After build you’ll see error and warnings in a more familiar form
Interactive hints
It’s time to add FixItHints for both warning and error
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 | void checkForLowercasedName(ObjCInterfaceDecl *declaration) { StringRef name = declaration->getName(); char c = name[0]; if (isLowercase(c)) { std::string tempName = name; tempName[0] = toUppercase(c); StringRef replacement(tempName); SourceLocation nameStart = declaration->getLocation(); SourceLocation nameEnd = nameStart.getLocWithOffset(name.size()); FixItHint fixItHint = FixItHint::CreateReplacement(SourceRange(nameStart, nameEnd), replacement); DiagnosticsEngine &diagEngine = context->getDiagnostics(); unsigned diagID = diagEngine.getCustomDiagID(DiagnosticsEngine::Warning, "Class name should not start with lowercase letter"); SourceLocation location = declaration->getLocation(); diagEngine.Report(location, diagID).AddFixItHint(fixItHint); } } void checkForUnderscoreInName(ObjCInterfaceDecl *declaration) { StringRef name = declaration->getName(); size_t underscorePos = name.find('_'); if (underscorePos != StringRef::npos) { std::string tempName = name; std::string::iterator end_pos = std::remove(tempName.begin(), tempName.end(), '_'); tempName.erase(end_pos, tempName.end()); StringRef replacement(tempName); SourceLocation nameStart = declaration->getLocation(); SourceLocation nameEnd = nameStart.getLocWithOffset(name.size()); FixItHint fixItHint = FixItHint::CreateReplacement(SourceRange(nameStart, nameEnd), replacement); DiagnosticsEngine &diagEngine = context->getDiagnostics(); unsigned diagID = diagEngine.getCustomDiagID(DiagnosticsEngine::Error, "Class name with `_` forbidden"); SourceLocation location = declaration->getLocation().getLocWithOffset(underscorePos); diagEngine.Report(location, diagID).AddFixItHint(fixItHint); } } |
Rebuild plugin and try to build test project
Conclusion
As you can see, creating a clang plugin is relatively simple, but it needs some dirty hacks with Xcode, and you should use custom built clang, so I’d not recommend you to use this clang for building apps for production usage. Apple provides patched version, and we can’t know the difference between them. Also, it needs a lot of efforts to make it work, which doesn’t make it widely usable.
Another issue you might face is unstable API, cause it uses internal API which changes continuously.
You still can use it on your system for different diagnostic purposes, but please do not force other people to depend on such heavyweight things.
If you have any comments, questions or suggestions feel free to ask me on twitter, open issue on GitHub, or just leave a comment here.
Happy hacking!
