CLANG(1)

📦 version 12.0.1, in ubuntu jammy

✨ NAME ✨

Okay so bestie, meet clang — the Clang C, C++, and Objective-C compiler. That's it, that's the name, she's iconic 💅

🫶 SYNOPSIS 🫶

clang [options] filename …

🌈 DESCRIPTION 🌈

So like, clang is a C, C++, and Objective-C compiler that handles preprocessing, parsing, optimization, code generation, assembly, and linking. Depending on which high-level mode setting you pass in, Clang will stop before doing a full link. And listen — even though Clang is super highly integrated, it's genuinely important to understand the stages of compilation so you actually know how to invoke it. The stages are:

  • Driver — the clang executable is actually just a small driver that controls the overall execution of the other tools, like the compiler, assembler, and linker. Typically you don't need to interact with the driver directly, but you transparently use it to run all the other tools. She's the manager, not the star 💅
  • 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), .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 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). 🌳
  • 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. No cap, that's a whole shortcut 🔥

  • 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.

🧚 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. She's out here finding your bugs before you do 🙈

💖 OPTIONS 💖

🌸 Stage Selection Options 🌸

  • -E — run the preprocessor stage.
  • -fsyntax-only — 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 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. Full send 🚀

🦋 Language Selection and Mode Options 🦋

  • -x <language> — treat subsequent input files as having type language.
  • -std=<standard> — specify the language standard to compile for. Okay this one's a whole buffet, stay with me 🍓

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.

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++2a — Working draft for ISO C++ 2020
  • gnu++2a — Working draft for ISO C++ 2020 with GNU extensions

The default C++ language standard is gnu++14.

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.
  • -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.
  • -fno-builtin — disable special handling and optimizations of builtin functions like strlen() and malloc().
  • -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. Defaults to 1300 on Windows. 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.
  • -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. Either way she's flexible 💅

  • -target <architecture> — specify the architecture to build for.
  • --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.
  • -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. So like, backwards compatibility who? 👑

🔥 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 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. She's a little unhinged and does not care about the rules, bestie — proceed with awareness.
    • -Os — like -O2 with extra optimizations to reduce code size.
    • -Oz — like -Os (and thus -O2), but reduces code size further.
  • -Og — lowkey the same as -O1. Heads up bestie: in future versions this one might turn off different optimizations to make debugging easier. 🐛
  • -O — literally just -O1. Same vibe.
  • -O4 and higher — rn they're all equivalent to -O3. No extra slay, sorry. 💅

-g, -gline-tables-only, -gmodules 👑

These control debug info output. And okay, important: Clang debug info works best at -O0, no cap. If you stack up more than one option starting with -g, the last one wins. Iconic behaviour, honestly.

  • -g — generate debug information. The whole buffet. ✨
  • -gline-tables-only — generate only line table debug info. So you still get symbolicated backtraces with inlining info, but zero info about variables, their locations, or their types. It's giving "bare minimum but make it functional". 💀
  • -gmodules — generate debug info that holds external references to types defined in Clang modules or precompiled headers, instead of dumping redundant debug type info into every single object file. This option transparently switches the Clang module format to object file containers that hold the Clang module together with the debug information. So when you're compiling a program that uses Clang modules or precompiled headers, you get complete debug info, faster compile times, and much smaller object files. Ate. 🫶
🚨 Okay listen: do not use this when building static libraries for distribution to other machines. Because the debug info will contain references to the module cache on the machine where the library's object files were built. That's not portable, bestie, that's a whole mess. 🙈

-fstandalone-debug / -fno-standalone-debug

Clang has a bunch of optimizations that shrink debug info in the binary, and they work on the assumption that debug type information can be spread out across multiple compilation units. Like, for instance, Clang won't emit type definitions for types that aren't needed by a module and could just be replaced with a forward declaration. And further — Clang will only emit type info for a dynamic C++ class in the module that contains the vtable for that class.

-fstandalone-debug turns all of those optimizations off. Super useful when you're working with 3rd-party libraries that don't come with debug info. And fun fact: this is the default on Darwin. 🍎

But note — Clang will never emit type information for types that aren't referenced at all by the program. That part's non-negotiable. Periodt.

-feliminate-unused-debug-types

By default, Clang does not emit type information for types that are defined but not used in a program. And if you want to keep the debug info for those unused types anyway? Just use the negation: -fno-eliminate-unused-debug-types. Reverse uno card. 🃏

-fexceptions

Enable generation of unwind information. This lets exceptions be thrown through Clang compiled stack frames. And it's on by default in x86-64, so half of y'all already have it. 🔥

-ftrapv

Generate code to catch integer overflow errors. Signed integer overflow is undefined in C (yes, actually undefined, not just "probably bad"). With this flag, extra code gets generated to detect it and abort when it happens. Safety net vibes. 🪤

-fvisibility

This flag sets the default visibility level. That's it, that's the whole thing. Short and sweet. 💅

-fcommon, -fno-common

This flag specifies that variables without initializers get common linkage. You can turn it off with -fno-common. Flip a coin, pick a lane. 🪙

-ftls-model=<model>

Set the default thread-local storage (TLS) model to use for thread-local variables. The valid values are: "global-dynamic", "local-dynamic", "initial-exec" and "local-exec". The default is "global-dynamic".

And the default model can be overridden with the tls_model attribute. Also the compiler will try to choose a more efficient model if it can — overachiever behaviour, we love to see it. ✨

-flto, -flto=full, -flto=thin, -emit-llvm

Generate output files in LLVM formats, suitable for link time optimization. When you pair it with -S this generates LLVM intermediate language assembly files; otherwise it 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", where the LLVM bitcode is suitable for monolithic Link Time Optimization (LTO) — the linker merges all such modules into a single combined module for optimization. With "thin", ThinLTO compilation is invoked instead. Skinny legend. 🧚

NOTE: On Darwin, when you're 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. Why? To instruct the ld64 linker not to delete the temporary object file generated during Link Time Optimization. (Clang automatically passes this flag to the linker if compilation and linking are done in a single step, so you're only on your own for the two-step version.) This lets you debug the executable and generate the .dSYM bundle using dsymutil(1). 🎀

🚗 Driver Options 🚗

  • -### — print (but do not run) the commands to run for this compilation. Ghost-running, basically. 👻
  • --help — display available options. Your little menu of possibilities.
  • -Qunused-arguments — do not emit any warnings for unused driver arguments. Silence the hater energy. 🤫
  • -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. Keepsake mode. 🎁
  • -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").
  • -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. Stopwatch era. ⏱️
  • -ftime-report — print timing summary of each stage of compilation.
  • -v — show commands to run and use verbose output. Yap mode activated. 🗣️

🩺 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.

🌍 ENVIRONMENT 🌍

  • 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 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, OBJCPLUS_INCLUDE_PATH — these environment variables specify additional paths, as for CPATH, which are only used when processing the appropriate language. Language-specific besties. 💜

💀 BUGS 💀

To report bugs, please visit https://bugs.llvm.org/. And fr fr, most bug reports should include preprocessed source files (use the -E option) and the full output of the compiler, along with information to reproduce. Give them the whole story, not just the plot twist. 😭

👀 SEE ALSO 👀

as(1), ld(1)

👑 AUTHOR 👑

Maintained by the Clang / LLVM Team (http://clang.llvm.org)

2007-2022, The Clang Team

And that's a wrap on our little saga, bestie — this page is CLANG(1), dated Feb 04, 2022, and it came to you courtesy of the Clang / LLVM Team. Go forth and compile something iconic. 💖✨🦋

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