CLANG(1)

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💅 CLANG(1) — the Clang C, C++, and Objective-C compiler 👑

okay bestie, so this is the manual for Clang, and it's literally the compiler that eats your .c, .cpp and Objective-C files and turns them into something the machine actually runs. it's giving main character energy in the LLVM universe. stay with me bc there's a LOT here and it's all important 👑

🎀 NAME 🎀

clang — the Clang C, C++, and Objective-C compiler. that's the whole vibe. no frills, just slay.

✨ SYNOPSIS ✨

clang [options] filename …

so basically: you give it some options, you give it some filenames (filename …, and yeah you can give it multiple, she's flexible 💅), and she goes to work.

🦋 DESCRIPTION 🦋

clang is a C, C++, and Objective-C compiler that does EVERYTHING: preprocessing, parsing, optimization, code generation, assembly, and linking. like, she does not need a committee. she does it all herself.

Depending on which high-level mode setting you pass, Clang will stop before doing a full link — so you can tap out of the pipeline early if you want. and while Clang is highly integrated (she's a unit, fr fr), it's super important to understand the stages of compilation so you actually know how to invoke her. no cap. here are the stages:

🌸 Driver

The clang executable is literally just a small driver that controls the overall execution of the other tools — the compiler, the assembler, and the linker. Typically you don't need to interact with the driver directly, you just transparently use it to run the other tools. she's behind the scenes, running the group chat. 👑

🎀 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 file (for C), a .ii file (for C++), a .mi file (for Objective-C), or a .mii file (for Objective-C++). yes the file extensions are chaotic, we accept that. 💅

🦋 Parsing and Semantic Analysis

This stage parses the input file, translating preprocessor tokens into a parse tree. once it's in the form of a parse tree, it applies semantic analysis to compute types for expressions as well as 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). so if you're getting yelled at by warnings, this is the girl doing the yelling. 😭

🔥 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. skipping a step? we love efficiency. ⭐

👑 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. and that's the final form, bestie. 🫶

🧚 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. for more details on how to use the static analyzer, check out https://clang-analyzer.llvm.org. she's looking for bugs before they even happen. psychic behaviour, honestly. 🔮

💅 OPTIONS 💅

🌟 Stage Selection Options

these decide how far down the pipeline she goes before she stops. pick your stopping point, queen.

  • -E — Run the preprocessor stage. and then she's out. 💨
  • -fsyntax-only — Run the preprocessor, parser and semantic analysis stages. so she checks your vibes but produces nothing. 💀
  • -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. the full send, no skips. 🔥

🌸 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. (noted, PlayStation girlies 🎮)

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. (yes, those. she supports them. periodt.)
  • -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. so uh, if you sign up for freestanding, those four are on YOU bestie 💀
  • -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. (important lil distinction, don't mix them up 🫶)

  • -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. (she's giving you the power but taking away the optimizations, balanced as all things should be ⚖️)
  • -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. she contains multitudes. 🌍

  • -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> — okay bestie, this one says 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. it's giving optimized for your CPU 💅

✨ Code Generation Options ✨

  • -O0, -O1, -O2, -O3, -Ofast, -Os, -Oz, -Og, -O, -O4 — bestie, pick your optimization level. Here's the menu:
    • -O0 — Means “no optimization”: this level compiles the fastest and generates the most debuggable code. debugging queen 👑
    • -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. ⚠️ standards? never heard of her
    • -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. 🚫 no cap, don't do it
  • -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. 😱 undefined behavior? we stay safe
  • -fvisibility — This flag sets the default visibility level. mysterious 💜
  • -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.

...and okay, plot twist bestie, because there's a but here and it's a big one: this option does not guarantee reproduceability. 😭 Nope, no cap. If the source you're including defines preprocessor symbols that influence the behavior of system headers (think stuff like _XOPEN_SOURCE), then running -E will remove that definition and can literally change the semantics of the included header. 💀 Also — and this is the sneaky part — using a different version of the system headers (especially a different version of the STL) may result in different behavior. So always verify the preprocessed file by compiling it separately. Always. Every time. Periodt. 💅

🌸 ENVIRONMENT 🌸

Bestie, these are the env vars that clang is lowkey peeking at while it does its thing. 👀

  • TMPDIR, TEMP, TMP — these three get checked in order 🫶 to figure out where to write the temporary files used during the compilation process.
  • CPATH — if this one is present, it's treated as a delimited list of paths that get added to the default system include path list. The delimiter is the platform dependent delimiter — the same one you'd use in the PATH environment variable. And FYI: empty components in the environment variable are ignored. 🚫
  • C_INCLUDE_PATH, OBJC_INCLUDE_PATH, CPLUS_INCLUDE_PATH, OBJCPLUS_INCLUDE_PATH — these cuties specify additional paths, exactly as for CPATH, except they're only used when processing the appropriate language. So C_INCLUDE_PATH is doing its C thing, CPLUS_INCLUDE_PATH is serving C++ realness, and so on. It's giving language-specific. ✨

🐛 BUGS 🐛

Found a bug? Not me gatekeeping the bug tracker — to report bugs, please visit:

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

And bestie, most bug reports should include preprocessed source files (that's your cue to use the -E option 🫡) plus the full output of the compiler, along with the information needed to reproduce. Give the maintainers everything they need, they're doing the lord's work. 💖

👀 SEE ALSO 👀

Go read these too, they're iconic:

👑 Author 👑

Maintained by the Clang / LLVM Team (<http://clang.llvm.org>). Main character energy, honestly. 💜

2007-2026, The Clang Team

And that's the whole page, bestie!! 🦋 This was CLANG(1), page 15 of the manual, dated Sep 09, 2026 — go forth and compile responsibly, -E your bugs before you report them, and keep those headers verified. 🌈💅🫶

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