Ubuntu Manpage: clang - the Clang C, C++, and Objective-C compiler

NAME

       clang - the Clang C, C++, and Objective-C compiler

SYNOPSIS

       clang [options] filename …

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 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.

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 <‐
https://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=<standard>
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=<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.

-target <architecture>
Specify the architecture to build for.

-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.

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 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_INCLUDE_PATH
              These environment variables specify additional paths, as for  CPATH,  which  are  only  used  when
              processing the appropriate language.

BUGS

       To  report  bugs,  please  visit  <https://bugs.llvm.org/>.  Most bug reports should include preprocessed
       source files (use the -E option) and  the  full  output  of  the  compiler,  along  with  information  to
       reproduce.

AUTHOR

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

COPYRIGHT

       2007-2020, The Clang Team