clang is a C, C++, and Objective-C compiler which encompasses prepro‐
cessing, 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 im‐
portant 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, as‐
sembler 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 to‐
kens 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 tar‐
get 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.
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.
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 optimiza‐
tion 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 exe‐
cutable or shared library.
Language Selection and Mode Options
-x
Treat subsequent input files as having type language.
-std=
Specify the language standard to compile for.
Supported values for the C language are:
c89
c90
iso9899:1990
ISO C 1990
iso9899:199409
ISO C 1990 with amendment 1
gnu89
gnu90
ISO C 1990 with GNU extensions
c99
iso9899:1999
ISO C 1999
gnu99
ISO C 1999 with GNU extensions
c11
iso9899:2011
ISO C 2011
gnu11
ISO C 2011 with GNU extensions
c17
iso9899:2017
ISO C 2017
gnu17
ISO C 2017 with GNU extensions
The default C language standard is gnu11, except on PS4, where
it is gnu99.
Supported values for the C++ language are:
c++98
c++03
ISO C++ 1998 with amendments
gnu++98
gnu++03
ISO C++ 1998 with amendments and GNU extensions
c++11
ISO C++ 2011 with amendments
gnu++11
ISO C++ 2011 with amendments and GNU extensions
c++14
ISO C++ 2014 with amendments
gnu++14
ISO C++ 2014 with amendments and GNU extensions
c++17
ISO C++ 2017 with amendments
gnu++17
ISO C++ 2017 with amendments and GNU extensions
c++2a
Working draft for ISO C++ 2020
gnu++2a
Working draft for ISO C++ 2020 with GNU extensions
The default C++ language standard is gnu++14.
Supported values for the OpenCL language are:
cl1.0
OpenCL 1.0
cl1.1
OpenCL 1.1
cl1.2
OpenCL 1.2
cl2.0
OpenCL 2.0
The default OpenCL language standard is cl1.0.
Supported values for the CUDA language are:
cuda
NVIDIA CUDA(tm)
-stdlib=
Specify the C++ standard library to use; supported options are
libstdc++ and libc++. If not specified, platform default will be
used.
-rtlib=
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 er‐
rno.
-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=
Select the Objective-C non-fragile ABI version to use by de‐
fault. 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 de‐
sign. Depending on how your version of Clang is configured, it may
have support for a number of cross compilers, or may only support a na‐
tive target.
-target
Specify the architecture to build for.
-march=
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 opti‐
mizations.
-O3 Like -O2, except that it enables optimizations that take
longer to perform or that may generate larger code (in an at‐
tempt to make the program run faster).
-Ofast Enables all the optimizations from -O3 along with
other aggressive optimizations that may violate strict com‐
pliance 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 infor‐
mation 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 informa‐
tion. This allows for symbolicated backtraces with inlining
information, but does not include any information about vari‐
ables, 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 assump‐
tion that the debug type information can be spread out over mul‐
tiple 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 gener‐
ated 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 com‐
mon 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”, “lo‐
cal-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 ef‐
ficient 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 intermedi‐
ate language assembly files, otherwise this generates LLVM bit‐
code format object files (which may be passed to the linker de‐
pending 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 in‐
stead.
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,
Pass the comma separated arguments in args to the assembler.
-Wl,
Pass the comma separated arguments in args to the linker.
-Wp,
Pass the comma separated arguments in args to the preprocessor.
-Xanalyzer
Pass arg to the static analyzer.
-Xassembler
Pass arg to the assembler.
-Xlinker
Pass arg to the linker.
-Xpreprocessor
Pass arg to the preprocessor.
-o
Write output to file.
-print-file-name=
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=
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 direc‐
tory of the output file (“-save-state=obj”).
-integrated-as, -no-integrated-as
Used to enable and disable, respectively, the use of the inte‐
grated assembler. Whether the integrated assembler is on by de‐
fault 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, -fdiagnos‐
tics-fixit-info, -fdiagnostics-parseable-fixits, -fdiagnos‐
tics-print-source-range-info, -fprint-source-range-info, -fdiagnos‐
tics-show-option, -fmessage-length
These options control how Clang prints out information about di‐
agnostics (errors and warnings). Please see the Clang User’s
Manual for more information.
Preprocessor Options
-D=
Adds an implicit #define into the predefines buffer which is
read before the source file is preprocessed.
-U
Adds an implicit #undef into the predefines buffer which is read
before the source file is preprocessed.
-include
Adds an implicit #include into the predefines buffer which is
read before the source file is preprocessed.
-I
Add the specified directory to the search path for include
files.
-F
Add the specified directory to the search path for framework in‐
clude 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.
TMPDIR, TEMP, TMP
These environment variables are checked, in order, for the loca‐
tion to write temporary files used during the compilation
process.
CPATH If this environment variable is present, it is treated as a de‐
limited 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.
C_INCLUDE_PATH, OBJC_INCLUDE_PATH, CPLUS_INCLUDE_PATH, OBJCPLUS_IN‐
CLUDE_PATH
These environment variables specify additional paths, as for
CPATH, which are only used when processing the appropriate lan‐
guage.
To report bugs, please visit . Most bug reports
should include preprocessed source files (use the -E option) and the
full output of the compiler, along with information to reproduce.
