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 semantic analysis 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 gnu17, 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++20

ISO C++ 2020 with amendments
gnu++20

ISO C++ 2020 with amendments and GNU extensions
c++23

ISO C++ 2023 with amendments
gnu++23

ISO C++ 2023 with amendments and GNU extensions
c++2c

Working draft for C++2c
gnu++2c

Working draft for C++2c with GNU extensions

The default C++ language standard is gnu++17.

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. Note that it is assumed that a freestanding environment will
additionally provide memcpy, memmove, memset and memcmp implementations, as these
are needed for efficient codegen for many programs.

-fno-builtin
Disable special handling and optimizations of well-known library functions, like
strlen() and malloc().

-fno-builtin-<function>
Disable special handling and optimizations for the specific library function. For
example, -fno-builtin-strlen removes any special handling for the strlen() library
function.

-fno-builtin-std-<function>
Disable special handling and optimizations for the specific C++ standard library
function in namespace std. For example, -fno-builtin-std-move_if_noexcept removes
any special handling for the std::move_if_noexcept() library function.

For C standard library functions that the C++ standard library also provides in
namespace std, use -fno-builtin-<function> instead.

-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. When on Windows, this defaults to either the same value as the
currently installed version of cl.exe, or 1933. 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, -flax-vector-conversions=<kind>, -fno-lax-vector-conversions
Allow loose type checking rules for implicit vector conversions. Possible values
of <kind>:

• none: allow no implicit conversions between vectors

• integer: allow implicit bitcasts between integer vectors of the same overall
bit-width

• all: allow implicit bitcasts between any vectors of the same overall bit-width

<kind> defaults to integer if unspecified.

-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 (all platforms).

--print-supported-cpus
Print out a list of supported processors for the given target (specified through
--target=<architecture> or -arch <architecture>). If no target is specified, the
system default target will be used.

-mcpu=?, -mtune=?
Acts as an alias for --print-supported-cpus.

-mcpu=help, -mtune=help
Acts as an alias for --print-supported-cpus.

-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 -O1.

-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.

-feliminate-unused-debug-types
By default, Clang does not emit type information for types that are defined but not
used in a program. To retain the debug info for these unused types, the negation
-fno-eliminate-unused-debug-types can be used.

-fexceptions
Allow exceptions to be thrown through Clang compiled stack frames (on many targets,
this will enable unwind information for functions that might have an exception
thrown through them). For most targets, this is enabled by default for C++.

-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.

NOTE:
On Darwin, when using -flto along with -g and compiling and linking in separate
steps, you also need to pass -Wl,-object_path_lto,<lto-filename>.o at the
linking step to instruct the ld64 linker not to delete the temporary object file
generated during Link Time Optimization (this flag is automatically passed to
the linker by Clang if compilation and linking are done in a single step). This
allows debugging the executable as well as generating the .dSYM bundle using
dsymutil(1).

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”).

You can also use environment variables to control the statistics reporting.
Setting CC_PRINT_INTERNAL_STAT to 1 enables the feature, the report goes to stdout
in JSON format.

Setting CC_PRINT_INTERNAL_STAT_FILE to a file path makes it report statistics to
the given file in the JSON format.

Note that -save-stats take precedence over CC_PRINT_INTERNAL_STAT and
CC_PRINT_INTERNAL_STAT_FILE.

-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.

-fkeep-system-includes
Usable only with -E. Do not copy the preprocessed content of “system” headers to
the output; instead, preserve the #include directive. This can greatly reduce the
volume of text produced by -E which can be helpful when trying to produce a “small”
reproduceable test case.

This option does not guarantee reproduceability, however. If the including source
defines preprocessor symbols that influence the behavior of system headers (for
example, _XOPEN_SOURCE) the operation of -E will remove that definition and thus
can change the semantics of the included header. Also, using a different version of
the system headers (especially a different version of the STL) may result in
different behavior. Always verify the preprocessed file by compiling it separately.

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://github.com/llvm/llvm-project/issues/>.  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

       2007-2024, The Clang Team