Provided by: clang-3.5_3.5.2-3ubuntu1_amd64 
NAME
clang - the Clang C, C++, and Objective-C compiler
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.
ENVIRONMENT
TMPDIR, TEMP, TMP
These environment variables are checked, in order, for the location to write temporary files used
during the compilation process.
CPATH
If this environment variable is present, it is treated as a delimited list of paths to be added to
the default system include path list. The delimiter is the platform dependent delimitor, as used in
the PATH environment variable.
Empty components in the environment variable are ignored.
C_INCLUDE_PATH, OBJC_INCLUDE_PATH, CPLUS_INCLUDE_PATH, OBJCPLUS_INCLUDE_PATH
These environment variables specify additional paths, as for CPATH, which are only used when
processing the appropriate language.
MACOSX_DEPLOYMENT_TARGET
If -mmacosx-version-min is unspecified, the default deployment target is read from this environment
variable. This option only affects darwin targets.
BUGS
To report bugs, please visit <http://llvm.org/bugs/>. Most bug reports should include preprocessed source files (use the -E option) and the full output of the compiler, along with information to reproduce.
SEE ALSO
as(1), ld(1)
AUTHOR
Maintained by the Clang / LLVM Team (<http://clang.llvm.org>).