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++. If not specified, platform default will be used.

-rtlib=<library>¶

Specify the compiler runtime library to use; supported options are libgcc and compiler-rt. If not specified, platform default will be used.

-ansi¶

Same as -std=c89.

-ObjC, -ObjC++¶

Treat source input files as Objective-C and Object-C++ inputs respectively.

-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-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, -fno-objc-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, -Og, -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.

-Og Like -O1. In future versions, this option might disable different optimizations in order to improve debuggability.

-O Equivalent to -O2.

-O4 and higher

Currently equivalent to -O3

-g, -gline-tables-only, -gmodules¶

Control debug information output. Note that Clang debug information works best at -O0. When more than one option starting with -g is specified, the last one wins:

-g Generate debug information.

-gline-tables-only Generate only line table debug information. This allows for symbolicated backtraces with inlining information, but does not include any information about variables, their locations or types.

-gmodules Generate debug information that contains external references to types defined in Clang modules or precompiled headers instead of emitting redundant debug type information into every object file. This option transparently switches the Clang module format to object file containers that hold the Clang module together with the debug information. When compiling a program that uses Clang modules or precompiled headers, this option produces complete debug information with faster compile times and much smaller object files.

This option should not be used when building static libraries for distribution to other machines because the debug info will contain references to the module cache on the machine the object files in the library were built on.

-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, -fno-common¶

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

-ftls-model=<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, -flto=full, -flto=thin, -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).

The default for -flto is “full”, in which the LLVM bitcode is suitable for monolithic Link Time Optimization (LTO), where the linker merges all such modules into a single combined module for optimization. With “thin”, ThinLTO compilation is invoked instead.

Driver Options¶

-###¶

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

--help¶

Display available options.

-Qunused-arguments¶

Do not emit any warnings 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 the currently used compiler runtime library (“libgcc.a” or “libclang_rt.builtins.*.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.

-save-stats, -save-stats=cwd, -save-stats=obj¶

Save internal code generation (LLVM) statistics to a file in the current directory (-save-stats/”-save-stats=cwd”) or the directory of the output file (“-save-state=obj”).

-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¶

-D<macroname>=<value>¶

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

-U<macroname>¶

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.

-I<directory>¶

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

-F<directory>¶

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.