CLANG(1) Clang CLANG(1)
clang - the Clang C, C++, and Objective-C compiler
clang [options] filename ...
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.
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
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
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
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.
Stage Selection Options
-E Run the preprocessor stage.
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"
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
Treat subsequent input files as having type language.
Specify the language standard to compile for.
Supported values for the C language are:
ISO C 1990
ISO C 1990 with amendment 1
ISO C 1990 with GNU extensions
ISO C 1999
ISO C 1999 with GNU extensions
ISO C 2011
ISO C 2011 with GNU extensions
ISO C 2017
ISO C 2017 with GNU extensions
The default C language standard is gnu17, except on PS4, where it
Supported values for the C++ language are:
ISO C++ 1998 with amendments
ISO C++ 1998 with amendments and GNU extensions
ISO C++ 2011 with amendments
ISO C++ 2011 with amendments and GNU extensions
ISO C++ 2014 with amendments
ISO C++ 2014 with amendments and GNU extensions
ISO C++ 2017 with amendments
ISO C++ 2017 with amendments and GNU extensions
Working draft for ISO C++ 2020
Working draft for ISO C++ 2020 with GNU extensions
The default C++ language standard is gnu++14.
Supported values for the OpenCL language are:
The default OpenCL language standard is cl1.0.
Supported values for the CUDA language are:
Specify the C++ standard library to use; supported options are
libstdc++ and libc++. If not specified, platform default will be
Specify the compiler runtime library to use; supported options are
libgcc and compiler-rt. If not specified, platform default will be
-ansi Same as -std=c89.
Treat source input files as Objective-C and Object-C++ inputs
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.
Disable special handling and optimizations of builtin functions
like strlen() and malloc().
Indicate that math functions should be treated as updating errno.
Enable support for Pascal-style strings with "\pfoo".
Enable support for Microsoft extensions.
Set _MSC_VER. Defaults to 1300 on Windows. Not set otherwise.
Enable support for Borland extensions.
Make all string literals default to writable. This disables
uniquing of strings and other optimizations.
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
<kind> defaults to integer if unspecified.
Enable the "Blocks" language feature.
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).
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).
Enable use of the Objective-C non-fragile ABI. On platforms for
which this is the default ABI, it can be disabled with
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.
Specify the architecture to build for.
When building for macOS, specify the minimum version supported by
When building for iPhone OS, specify the minimum version supported
by your application.
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.
Acts as an alias for --print-supported-cpus.
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
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
-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.
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.
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.
Enable generation of unwind information. This allows exceptions to
be thrown through Clang compiled stack frames. This is on by
default in x86-64.
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.
This flag sets the default visibility level.
This flag specifies that variables without initializers get common
linkage. It can be disabled with -fno-common.
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.
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
-### Print (but do not run) the commands to run for this compilation.
--help Display available options.
Do not emit any warnings for unused driver arguments.
Pass the comma separated arguments in args to the assembler.
Pass the comma separated arguments in args to the linker.
Pass the comma separated arguments in args to the preprocessor.
Pass arg to the static analyzer.
Pass arg to the assembler.
Pass arg to the linker.
Pass arg to the preprocessor.
Write output to file.
Print the full library path of file.
Print the library path for the currently used compiler runtime
library ("libgcc.a" or "libclang_rt.builtins.*.a").
Print the full program path of name.
Print the paths used for finding libraries and programs.
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").
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.
Print timing summary of each stage of compilation.
-v Show commands to run and use verbose output.
-fshow-column, -fshow-source-location, -fcaret-diagnostics,
These options control how Clang prints out information about
diagnostics (errors and warnings). Please see the Clang User's
Manual for more information.
Adds an implicit #define into the predefines buffer which is read
before the source file is preprocessed.
Adds an implicit #undef into the predefines buffer which is read
before the source file is preprocessed.
Adds an implicit #include into the predefines buffer which is read
before the source file is preprocessed.
Add the specified directory to the search path for include files.
Add the specified directory to the search path for framework
Do not search the standard system directories or compiler builtin
directories for include files.
Do not search the standard system directories for include files,
but do search compiler builtin include directories.
Do not search clang's builtin directory for include files.
TMPDIR, TEMP, TMP
These environment variables are checked, in order, for the
location to write temporary files used during the compilation
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,
These environment variables specify additional paths, as for
CPATH, which are only used when processing the appropriate
If -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 <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.
Maintained by the Clang / LLVM Team (<http://clang.llvm.org>)
2007-2021, The Clang Team
Apple Clang 1300.0.29.30 October 4, 2021 CLANG(1)