SYNOPSIS

clang [-c|-S|-E] -std=standard -g
  [-O0|-O1|-O2|-O3|-Ofast|-Os|-Oz|-O|-O4]
  -Wwarnings... -pedantic
  -Idir... -Ldir...
  -Dmacro[=defn]
  -ffeature-option...
  -mmachine-option...
  -o output-file
  -stdlib=library 
  input-filenames

DESCRIPTION

clang is a C, C++, and Objective-C compiler which encompasses preprocessing, parsing, optimization, code generation, assembly, and linking. Depending on which high-level mode setting is passed, Clang will stop before doing a full link. While Clang is highly integrated, it is important to understand the stages of compilation, to understand how to invoke it. These stages are:

Driver

The clang executable is actually a small driver which controls the overall execution of other tools such as the compiler, assembler and linker. Typically you do not need to interact with the driver, but you transparently use it to run the other tools.

Preprocessing

This stage handles tokenization of the input source file, macro expansion, #include expansion and handling of other preprocessor directives. The output of this stage is typically called a ``.i'' (for C), ``.ii'' (for C++), ``.mi'' (for Objective-C) , or ``.mii'' (for Objective-C++) file.

Parsing and Semantic Analysis

This stage parses the input file, translating preprocessor tokens into a parse tree. Once in the form of a parser tree, it applies semantic analysis to compute types for expressions as well and determine whether the code is well formed. This stage is responsible for generating most of the compiler warnings as well as parse errors. The output of this stage is an ``Abstract Syntax Tree'' (AST).

Code Generation and Optimization

This stage translates an AST into low-level intermediate code (known as ``LLVM IR'') and ultimately to machine code. This phase is responsible for optimizing the generated code and handling target-specific code generation. The output of this stage is typically called a ``.s'' file or ``assembly'' file.

Clang also supports the use of an integrated assembler, in which the code generator produces object files directly. This avoids the overhead of generating the ``.s'' file and of calling the target assembler.

Assembler

This stage runs the target assembler to translate the output of the compiler into a target object file. The output of this stage is typically called a ``.o'' file or ``object'' file.

Linker

This stage runs the target linker to merge multiple object files into an executable or dynamic library. The output of this stage is typically called an ``a.out'', ``.dylib'' or ``.so'' file.

The Clang compiler supports a large number of options to control each of these stages. In addition to compilation of code, Clang also supports other tools:

Clang Static Analyzer

The Clang Static Analyzer is a tool that scans source code to try to find bugs through code analysis. This tool uses many parts of Clang and is built into the same driver. Please see <http://clang-analyzer.llvm.org> for more details on how to use the static analyzer.

OPTIONS

Stage Selection Options

-E

Run the preprocessor stage.

-fsyntax-only

Run the preprocessor, parser and type checking stages.

-S

Run the previous stages as well as LLVM generation and optimization stages and target-specific code generation, producing an assembly file.

-c

Run all of the above, plus the assembler, generating a target ``.o'' object file.

no stage selection option

If no stage selection option is specified, all stages above are run, and the linker is run to combine the results into an executable or shared library.

Language Selection and Mode Options

-x language

Treat subsequent input files as having type language.

-std=language

Specify the language standard to compile for.

-stdlib=library

Specify the C++ standard library to use; supported options are libstdc++ and libc++.

-ansi

Same as -std=c89.

-ObjC++

Treat source input files as Objective-C++ inputs.

-ObjC

Treat source input files as Objective-C inputs.

-trigraphs

Enable trigraphs.

-ffreestanding

Indicate that the file should be compiled for a freestanding, not a hosted, environment.

-fno-builtin

Disable special handling and optimizations of builtin functions like strlen and malloc.

-fmath-errno

Indicate that math functions should be treated as updating errno.

-fpascal-strings

Enable support for Pascal-style strings with ``\pfoo''.

-fms-extensions

Enable support for Microsoft extensions.

-fmsc-version=

Set _MSC_VER. Defaults to 1300 on Windows. Not set otherwise.

-fborland-extensions

Enable support for Borland extensions.

-fwritable-strings

Make all string literals default to writable. This disables uniquing of strings and other optimizations.

-flax-vector-conversions

Allow loose type checking rules for implicit vector conversions.

-fblocks

Enable the ``Blocks'' language feature.

-fobjc-gc-only

Indicate that Objective-C code should be compiled in GC-only mode, which only works when Objective-C Garbage Collection is enabled.

-fobjc-gc

Indicate that Objective-C code should be compiled in hybrid-GC mode, which works with both GC and non-GC mode.

-fobjc-abi-version=version

Select the Objective-C ABI version to use. Available versions are 1 (legacy ``fragile'' ABI), 2 (non-fragile ABI 1), and 3 (non-fragile ABI 2).

-fobjc-nonfragile-abi-version=version

Select the Objective-C non-fragile ABI version to use by default. This will only be used as the Objective-C ABI when the non-fragile ABI is enabled (either via -fobjc-nonfragile-abi, or because it is the platform default).

-fobjc-nonfragile-abi

Enable use of the Objective-C non-fragile ABI. On platforms for which this is the default ABI, it can be disabled with -fno-objc-nonfragile-abi.

Target Selection Options

Clang fully supports cross compilation as an inherent part of its design. Depending on how your version of Clang is configured, it may have support for a number of cross compilers, or may only support a native target.

-arch architecture

Specify the architecture to build for.

-mmacosx-version-min=version

When building for Mac OS X, specify the minimum version supported by your application.

-miphoneos-version-min

When building for iPhone OS, specify the minimum version supported by your application.

-march=cpu

Specify that Clang should generate code for a specific processor family member and later. For example, if you specify -march=i486, the compiler is allowed to generate instructions that are valid on i486 and later processors, but which may not exist on earlier ones.

Code Generation Options

-O0 -O1 -O2 -O3 -Ofast -Os -Oz -O -O4

Specify which optimization level to use:

-O0

Means ``no optimization'': this level compiles the fastest and generates the most debuggable code.

-O1

Somewhere between -O0 and -O2.

-O2

Moderate level of optimization which enables most optimizations.

-O3

Like -O2, except that it enables optimizations that take longer to perform or that may generate larger code (in an attempt to make the program run faster).

-Ofast

Enables all the optimizations from -O3 along with other aggressive optimizations that may violate strict compliance with language standards.

-Os

Like -O2 with extra optimizations to reduce code size.

-Oz

Like -Os (and thus -O2), but reduces code size further.

-O

Equivalent to -O2.

-O4 and higher

Currently equivalent to -O3

-g

Generate debug information. Note that Clang debug information works best at -O0.

-fstandalone-debug -fno-standalone-debug

Clang supports a number of optimizations to reduce the size of debug information in the binary. They work based on the assumption that the debug type information can be spread out over multiple compilation units. For instance, Clang will not emit type definitions for types that are not needed by a module and could be replaced with a forward declaration. Further, Clang will only emit type info for a dynamic C++ class in the module that contains the vtable for the class.

The -fstandalone-debug option turns off these optimizations. This is useful when working with 3rd-party libraries that don't come with debug information. This is the default on Darwin. Note that Clang will never emit type information for types that are not referenced at all by the program.

-fexceptions

Enable generation of unwind information, this allows exceptions to be thrown through Clang compiled stack frames. This is on by default in x86-64.

-ftrapv

Generate code to catch integer overflow errors. Signed integer overflow is undefined in C, with this flag, extra code is generated to detect this and abort when it happens.

-fvisibility

This flag sets the default visibility level.

-fcommon

This flag specifies that variables without initializers get common linkage. It can be disabled with -fno-common.

-ftls-model

Set the default thread-local storage (TLS) model to use for thread-local variables. Valid values are: ``global-dynamic'', ``local-dynamic'', ``initial-exec'' and ``local-exec''. The default is ``global-dynamic''. The default model can be overridden with the tls_model attribute. The compiler will try to choose a more efficient model if possible.

-flto -emit-llvm

Generate output files in LLVM formats, suitable for link time optimization. When used with -S this generates LLVM intermediate language assembly files, otherwise this generates LLVM bitcode format object files (which may be passed to the linker depending on the stage selection options).

Driver Options

-###

Print (but do not run) the commands to run for this compilation.

--help

Display available options.

-Qunused-arguments

Don't emit warning for unused driver arguments.

-Wa,args

Pass the comma separated arguments in args to the assembler.

-Wl,args

Pass the comma separated arguments in args to the linker.

-Wp,args

Pass the comma separated arguments in args to the preprocessor.

-Xanalyzer arg

Pass arg to the static analyzer.

-Xassembler arg

Pass arg to the assembler.

-Xlinker arg

Pass arg to the linker.

-Xpreprocessor arg

Pass arg to the preprocessor.

-o file

Write output to file.

-print-file-name=file

Print the full library path of file.

-print-libgcc-file-name

Print the library path for ``libgcc.a''.

-print-prog-name=name

Print the full program path of name.

-print-search-dirs

Print the paths used for finding libraries and programs.

-save-temps

Save intermediate compilation results.

-integrated-as -no-integrated-as

Used to enable and disable, respectively, the use of the integrated assembler. Whether the integrated assembler is on by default is target dependent.

-time

Time individual commands.

-ftime-report

Print timing summary of each stage of compilation.

-v

Show commands to run and use verbose output.

Diagnostics Options

-fshow-column -fshow-source-location -fcaret-diagnostics -fdiagnostics-fixit-info -fdiagnostics-parseable-fixits -fdiagnostics-print-source-range-info -fprint-source-range-info -fdiagnostics-show-option -fmessage-length

These options control how Clang prints out information about diagnostics (errors and warnings). Please see the Clang User's Manual for more information.

Preprocessor Options

-Dmacroname=value

Adds an implicit #define into the predefines buffer which is read before the source file is preprocessed.

-Umacroname

Adds an implicit #undef into the predefines buffer which is read before the source file is preprocessed.

-include filename

Adds an implicit #include into the predefines buffer which is read before the source file is preprocessed.

-Idirectory

Add the specified directory to the search path for include files.

-Fdirectory

Add the specified directory to the search path for framework include files.

-nostdinc

Do not search the standard system directories or compiler builtin directories for include files.

-nostdlibinc

Do not search the standard system directories for include files, but do search compiler builtin include directories.

-nobuiltininc

Do not search clang's builtin directory for include files.

AUTHOR

Maintained by the Clang / LLVM Team (<http://clang.llvm.org>).