CLANG(1) BSD General Commands Manual CLANG(1)
clang -- the Clang C, C++, and Objective-C compiler
clang [-c|-S|-E] std=standard -g
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 parser 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.
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.
The Clang compiler supports a large number of options to control each of these stages. In addition to compilation of code, Clang also sup-
ports other tools:
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
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
-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
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++.
-ansi Same as -std=c89.
Treat source input files as Objective-C++ inputs.
-ObjC Treat source input files as Objective-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 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.
Enable the "Blocks" language feature.
Indicate that Objective-C code should be compiled in GC-only mode, which only works when Objective-C Garbage Collection is enabled.
Indicate that Objective-C code should be compiled in hybrid-GC mode, which works with both GC and non-GC mode.
Select the Objective-C ABI version to use. Available versions are 1 (legacy "fragile" ABI), 2 (non-fragile ABI 1), and 3 (non-fragile
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 Mac OS/X, specify the minimum version supported by your application.
When building for iPhone OS, specify the minimum version supported 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
Code Generation Options
-O0 -O1 -O2 -O3 -Ofast -Os -Oz -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 lan-
-Os Like -O2 with extra optimizations to reduce code size.
-Oz Like -Os (and thus -O2), but reduces code size further.
-O Equivalent to -O2.
-O4 and higher
Currently equivalent to -O3
-g Generate debug information. Note that Clang debug information works best at -O0. At higher optimization levels, only line number
information is currently available.
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.
Generate output files in LLVM formats, suitable for link time optimization. When used with -S this generates LLVM intermediate lan-
guage assembly files, otherwise this generates LLVM bitcode format object files (which may be passed to the linker depending on the
stage selection options).
-### Print the commands to run for this compilation.
--help Display available options.
Don't emit warning 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 "libgcc.a".
Print the full program path of name.
Print the paths used for finding libraries and programs.
Save intermediate compilation results.
Used to enable and disable, respectively, the use of the integrated assembler. Whether the integrated assembler is on by default is
-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 -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.
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 include files.
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 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 delimitor, 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.
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).
November 18, 2013 BSD