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 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
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
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 tar-
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
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 optimiza-
tion stages and target-specific code generation, producing an
-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
Treat subsequent input files as having type language.
Specify the language standard to compile for.
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 used.
-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.
Disable special handling and optimizations of builtin functions
like strlen() and malloc().
Indicate that math functions should be treated as updating
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.
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
Specify the architecture to build for.
When building for Mac OS X, specify the minimum version sup-
ported by your application.
When building for iPhone OS, specify the minimum version sup-
ported by your application.
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-
-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 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.
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.
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 gener-
ated to detect this and abort when it happens.
This flag sets the default visibility level.
This flag specifies that variables without initializers get com-
mon 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 intermedi-
ate language assembly files, otherwise this generates LLVM bit-
code 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
-### 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 direc-
tory of the output file ("-save-state=obj").
Used to enable and disable, respectively, the use of the inte-
grated 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, -fdiagnos-
tics-fixit-info, -fdiagnostics-parseable-fixits, -fdiagnos-
tics-print-source-range-info, -fprint-source-range-info, -fdiagnos-
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
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 loca-
tion 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, OBJC-
These environment variables specify additional paths, as for
CPATH, which are only used when processing the appropriate lan-
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 <http://llvm.org/bugs/>. 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>)
2007-2018, The Clang Team
4 May 23, 2018 CLANG(1)