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clang - the Clang C, C++, and Objective-C compiler
:program:`clang` [*options*] *filename ...*
:program:`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:
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.
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 parse 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.
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.
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.
:program:`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 <> for more details
on how to use the static analyzer.
Stage Selection Options
.. option:: -E
Run the preprocessor stage.
.. option:: -fsyntax-only
Run the preprocessor, parser and type checking stages.
.. option:: -S
Run the previous stages as well as LLVM generation and optimization stages
and target-specific code generation, producing an assembly file.
.. option:: -c
Run all of the above, plus the assembler, generating a target ".o" object file.
.. option:: 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
.. option:: -x <language>
Treat subsequent input files as having type language.
.. option:: -std=<language>
Specify the language standard to compile for.
.. option:: -stdlib=<library>
Specify the C++ standard library to use; supported options are libstdc++ and
libc++. If not specified, platform default will be used.
.. option:: -rtlib=<library>
Specify the compiler runtime library to use; supported options are libgcc and
compiler-rt. If not specified, platform default will be used.
.. option:: -ansi
Same as -std=c89.
.. option:: -ObjC, -ObjC++
Treat source input files as Objective-C and Object-C++ inputs respectively.
.. option:: -trigraphs
Enable trigraphs.
.. option:: -ffreestanding
Indicate that the file should be compiled for a freestanding, not a hosted,
.. option:: -fno-builtin
Disable special handling and optimizations of builtin functions like
:c:func:`strlen` and :c:func:`malloc`.
.. option:: -fmath-errno
Indicate that math functions should be treated as updating :c:data:`errno`.
.. option:: -fpascal-strings
Enable support for Pascal-style strings with "\\pfoo".
.. option:: -fms-extensions
Enable support for Microsoft extensions.
.. option:: -fmsc-version=
Set _MSC_VER. Defaults to 1300 on Windows. Not set otherwise.
.. option:: -fborland-extensions
Enable support for Borland extensions.
.. option:: -fwritable-strings
Make all string literals default to writable. This disables uniquing of
strings and other optimizations.
.. option:: -flax-vector-conversions
Allow loose type checking rules for implicit vector conversions.
.. option:: -fblocks
Enable the "Blocks" language feature.
.. option:: -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).
.. option:: -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 :option:`-fobjc-nonfragile-abi`, or because it is the platform
.. option:: -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 :option:`-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.
.. option:: -arch <architecture>
Specify the architecture to build for.
.. option:: -mmacosx-version-min=<version>
When building for Mac OS X, specify the minimum version supported by your
.. option:: -miphoneos-version-min
When building for iPhone OS, specify the minimum version supported by your
.. option:: -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
.. option:: -O0, -O1, -O2, -O3, -Ofast, -Os, -Oz, -Og, -O, -O4
Specify which optimization level to use:
:option:`-O0` Means "no optimization": this level compiles the fastest and
generates the most debuggable code.
:option:`-O1` Somewhere between :option:`-O0` and :option:`-O2`.
:option:`-O2` Moderate level of optimization which enables most
:option:`-O3` Like :option:`-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).
:option:`-Ofast` Enables all the optimizations from :option:`-O3` along
with other aggressive optimizations that may violate strict compliance with
language standards.
:option:`-Os` Like :option:`-O2` with extra optimizations to reduce code
:option:`-Oz` Like :option:`-Os` (and thus :option:`-O2`), but reduces code
size further.
:option:`-Og` Like :option:`-O1`. In future versions, this option might
disable different optimizations in order to improve debuggability.
:option:`-O` Equivalent to :option:`-O2`.
:option:`-O4` and higher
Currently equivalent to :option:`-O3`
.. option:: -g, -gline-tables-only, -gmodules
Control debug information output. Note that Clang debug information works
best at :option:`-O0`. When more than one option starting with `-g` is
specified, the last one wins:
:option:`-g` Generate debug information.
:option:`-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.
:option:`-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.
.. option:: -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 :option:`-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
.. option:: -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.
.. option:: -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.
.. option:: -fvisibility
This flag sets the default visibility level.
.. option:: -fcommon, -fno-common
This flag specifies that variables without initializers get common linkage.
It can be disabled with :option:`-fno-common`.
.. option:: -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.
.. option:: -flto, -flto=full, -flto=thin, -emit-llvm
Generate output files in LLVM formats, suitable for link time optimization.
When used with :option:`-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 :option:`-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", :doc:`ThinLTO <../ThinLTO>`
compilation is invoked instead.
Driver Options
.. option:: -###
Print (but do not run) the commands to run for this compilation.
.. option:: --help
Display available options.
.. option:: -Qunused-arguments
Do not emit any warnings for unused driver arguments.
.. option:: -Wa,<args>
Pass the comma separated arguments in args to the assembler.
.. option:: -Wl,<args>
Pass the comma separated arguments in args to the linker.
.. option:: -Wp,<args>
Pass the comma separated arguments in args to the preprocessor.
.. option:: -Xanalyzer <arg>
Pass arg to the static analyzer.
.. option:: -Xassembler <arg>
Pass arg to the assembler.
.. option:: -Xlinker <arg>
Pass arg to the linker.
.. option:: -Xpreprocessor <arg>
Pass arg to the preprocessor.
.. option:: -o <file>
Write output to file.
.. option:: -print-file-name=<file>
Print the full library path of file.
.. option:: -print-libgcc-file-name
Print the library path for the currently used compiler runtime library
("libgcc.a" or "libclang_rt.builtins.*.a").
.. option:: -print-prog-name=<name>
Print the full program path of name.
.. option:: -print-search-dirs
Print the paths used for finding libraries and programs.
.. option:: -save-temps
Save intermediate compilation results.
.. option:: -save-stats, -save-stats=cwd, -save-stats=obj
Save internal code generation (LLVM) statistics to a file in the current
directory (:option:`-save-stats`/"-save-stats=cwd") or the directory
of the output file ("-save-state=obj").
.. option:: -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
.. option:: -time
Time individual commands.
.. option:: -ftime-report
Print timing summary of each stage of compilation.
.. option:: -v
Show commands to run and use verbose output.
Diagnostics Options
.. option:: -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
.. option:: -D<macroname>=<value>
Adds an implicit #define into the predefines buffer which is read before the
source file is preprocessed.
.. option:: -U<macroname>
Adds an implicit #undef into the predefines buffer which is read before the
source file is preprocessed.
.. option:: -include <filename>
Adds an implicit #include into the predefines buffer which is read before the
source file is preprocessed.
.. option:: -I<directory>
Add the specified directory to the search path for include files.
.. option:: -F<directory>
Add the specified directory to the search path for framework include files.
.. option:: -nostdinc
Do not search the standard system directories or compiler builtin directories
for include files.
.. option:: -nostdlibinc
Do not search the standard system directories for include files, but do
search compiler builtin include directories.
.. option:: -nobuiltininc
Do not search clang's builtin directory for include files.
.. envvar:: TMPDIR, TEMP, TMP
These environment variables are checked, in order, for the location to write
temporary files used during the compilation process.
.. envvar:: 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 delimiter, as used in the PATH environment variable.
Empty components in the environment variable are ignored.
These environment variables specify additional paths, as for :envvar:`CPATH`, which are
only used when processing the appropriate language.
If :option:`-mmacosx-version-min` is unspecified, the default deployment
target is read from this environment variable. This option only affects
Darwin targets.
To report bugs, please visit <>. Most bug reports should
include preprocessed source files (use the :option:`-E` option) and the full
output of the compiler, along with information to reproduce.
:manpage:`as(1)`, :manpage:`ld(1)`