clang/docs/CommandGuide/clang.md

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

## SYNOPSIS

{program}`clang` \[*options*\] *filename ...*

## DESCRIPTION

{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:

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.

{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 <https://clang-analyzer.llvm.org> for more details
on how to use the static analyzer.

## OPTIONS

### Stage Selection Options

:::{option} -E
Run the preprocessor stage.
:::

:::{option} -fsyntax-only
Run the preprocessor, parser and semantic analysis 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=<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
>
> `c23`
>
> `iso9899:2024`
>
> > ISO C 2023
>
> `gnu23`
>
> > ISO C 2023 with GNU extensions
>
> `c2y`
>
> > ISO C 202y
>
> `gnu2y`
>
> > ISO C 202y 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
>
> `c++2d`
>
> > Working draft for C++2d
>
> `gnu++2d`
>
> > Working draft for C++2d 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)
:::

:::{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,
environment. Note that a freestanding build still requires linking against a C
Standard Library which supports the freestanding interfaces for the specified
language mode and target environment. This includes functions like `memcpy`,
`memmove`, and `memset`.
:::

:::{option} -fno-builtin
Disable special handling and optimizations of well-known library functions,
like {c:func}`strlen` and {c:func}`malloc`.
:::

:::{option} -fno-builtin-<function>
Disable special handling and optimizations for the specific library function.
For example, `-fno-builtin-strlen` removes any special handling for the
{c:func}`strlen` library function.
:::

:::{option} -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
{cpp:func}`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.
:::

:::{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`. When on Windows, this defaults to either the same value as
the currently installed version of cl.exe, or `1933`. 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, -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.
:::

:::{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
default).
:::

:::{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 (Mac OS X specific).
:::

:::{option} -target <architecture>
Specify the architecture to build for (all platforms).
:::

:::{option} -mmacos-version-min=<version>
When building for macOS, specify the minimum version supported by your
application.
:::

:::{option} -miphoneos-version-min
When building for iPhone OS, specify the minimum version supported by your
application.
:::

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

:::{option} -mcpu=?, -mtune=?
Acts as an alias for {option}`--print-supported-cpus`.
:::

:::{option} -mcpu=help, -mtune=help
Acts as an alias for {option}`--print-supported-cpus`.
:::

:::{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.
:::

:::{option} --print-enabled-extensions
Prints the list of extensions that are enabled for the target specified by the
combination of `--target`, `-march`, and `-mcpu` values. Currently, this
option is only supported on AArch64 and RISC-V. On RISC-V, this option also
prints out the ISA string of enabled extensions.
:::

:::{option} --print-supported-extensions
Prints the list of all extensions that are supported for every CPU target
for an architecture (specified through `--target=<architecture>` or
{option}`-arch` `<architecture>`). If no target is specified, the system
default target will be used. Currently, this option is only supported on
AArch64 and RISC-V.
:::

### 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
> optimizations.
>
> {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. This has been deprecated since Clang 19. There is no
> timeline yet for removal; the aim is to discourage use of {option}`-Ofast`
> due to the surprising behavior of an optimization flag changing the
> observable behavior of correct code.
>
> If {option}`-Ofast` has been specified and is the effective optimization
> level (i.e. there is no later `-O` option specified), then the option can be
> replaced in the option string with `-O3 -ffast-math -fstrict-aliasing`.
> ({option}`-fstrict-aliasing` is the default on non-Windows, non-UEFI
> platforms). If {option}`-Ofast` has been specified but is not the effective
> optimization level, then it can be removed or replaced with {option}`-O3`.
>
> {option}`-Os` Like {option}`-O2` with extra optimizations to reduce code
> size.
>
> {option}`-Oz` Like {option}`-Os` (and thus {option}`-O2`), but reduces code
> size further.
>
> {option}`-Og` Similar to {option}`-O1`, but with slightly reduced
> optimization and better variable visibility. The same optimizations are run
> as at {option}`-O1`, but the `-fextend-variable-liveness` flag is
> also set, which tries to prevent optimizations from reducing the liveness of
> user variables, improving their availability when debugging.
>
> {option}`-O` Equivalent to {option}`-O1`.
>
> {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
program.
:::

:::{option} -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.
:::

:::{option} -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++.
:::

:::{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.

:::{note}
On Darwin, when using {option}`-flto` along with {option}`-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 {manpage}`dsymutil(1)`.
:::
::::

### 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-stats=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`.
:::

:::{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
dependent.
:::

:::{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.
:::

:::{option} -nostdinc++
Do not search the system C++ standard library directory for include files.
:::

:::{option} -fkeep-system-includes
Usable only with {option}`-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 {option}`-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 {option}`-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

:::{envvar} TMPDIR, TEMP, TMP
These environment variables are checked, in order, for the location to write
temporary files used during the compilation process.
:::

```{eval-rst}
.. envvar:: CPATH

  This environment variable specifies additional (non-system) header search
  paths to be used to find included header files. These paths are searched after
  paths specified with the :option:`-I\<directory\>` option, but before any
  system header search paths. Paths are delimited by the platform dependent
  delimiter as used in the ``PATH`` environment variable. Empty entries in the
  delimited path list, including those at the beginning or end of the list, are
  treated as specifying the compiler's current working directory.
```

:::{envvar} C_INCLUDE_PATH, OBJC_INCLUDE_PATH, CPLUS_INCLUDE_PATH, OBJCPLUS_INCLUDE_PATH
These environment variables specify additional system header file search
paths to be used when processing the corresponding language. Search paths are
delimited as for the {envvar}`CPATH` environment variable.
:::

:::{envvar} MACOSX_DEPLOYMENT_TARGET
If {option}`-mmacos-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 <https://github.com/llvm/llvm-project/issues/>. 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.

## SEE ALSO

{manpage}`as(1)`, {manpage}`ld(1)`