GDC(1)

📦 version 16.2.0+git9497, in opensuse-leap 16.0

🌸 NAME 🌸

gdc — a GCC-based compiler for the D language, bestie. This is the GNU GDC(1) manual page, and it's literally the main character for compiling D with GCC energy.

✨ SYNOPSIS ✨

gdc [-c|-S] [-g] [-pg]
    [-Olevel] [-Wwarn...]
    [-Idir...] [-Ldir...]
    [-foption...] [-mmachine-option...]
    [-o outfile] [@file] infile...

Only the most useful options are listed here — see below for the rest.

💖 DESCRIPTION 💖

The gdc command is the GNU compiler for the D language and supports many of the same options as gcc. This manual only documents the options specific to gdc, so no cap: it's the <pink>gdc-specific</pink> lore.

👑 OPTIONS 👑

📁 Input and Output files 📁

For any given input file, the file name suffix determines what kind of compilation is done. The following kinds of input file names are supported:

  • file.d — D source files. Classic D energy.
  • file.dd — Ddoc source files. Documentation mode activated.
  • file.di — D interface files. The little import-friendly besties.

You can specify more than one input file on the gdc command line, each being compiled separately in the compilation process. If you specify a -o file option, all the input files are compiled together, producing a single output file, named file. This is allowed even when using -S or -c — yes, even then. 💅

A D interface file contains only what an import of the module needs, rather than the whole implementation of that module. They can be created by gdc from a D source file by using the -H option. When the compiler resolves an import declaration, it searches for matching .di files first, then for .d. So .di gets first dibs, periodt.

A Ddoc source file contains code in the D macro processor language. It is primarily designed for use in producing user documentation from embedded comments, with a slight affinity towards HTML generation. If a .d source file starts with the string Ddoc then it is treated as general purpose documentation, not as a D source file. Cute, iconic, slightly chaotic. 🎀

🏃 Runtime Options 🏃

These options affect the runtime behavior of programs compiled with gdc. Runtime vibes only.

  • -fall-instantiations — Generate code for all template instantiations. The default template emission strategy is to not generate code for declarations that were either instantiated speculatively, such as from __traits(compiles, ...), or that come from an imported module not being compiled. So this option is like "okay, compile everything, no holding back." 🔥
  • -fno-assert — Turn off code generation for assert contracts. Bye-bye assert codegen.
  • -fno-bounds-check — Turns off array bounds checking for all functions, which can improve performance for code that uses arrays extensively. Note that this can result in unpredictable behavior if the code in question actually does violate array bounds constraints. It is safe to use this option if you are sure that your code never throws a RangeError. > ⚠️ That "unpredictable behavior" part is not a vibe, bestie — only use this if you're truly sure.
  • -fbounds-check=value — An alternative to -fbounds-check that allows more control as to where bounds checking is turned on or off. The following values are supported:
    • on — Turns on array bounds checking for all functions.
    • safeonly — Turns on array bounds checking only for @safe functions.
    • off — Turns off array bounds checking completely.
  • -fno-builtin — Don't recognize built-in functions unless they begin with the prefix __builtin_. By default, the compiler will recognize when a function in the core.stdc package is a built-in function. So this is the "no mysterious builtins unless they're wearing the __builtin_ name tag" option.
  • -fcheckaction=value — This option controls what code is generated on an assertion, bounds check, or final switch failure. The following values are supported:
    • context — Throw an AssertError with extra context information.
    • halt — Halt the program execution.
    • throw — Throw an AssertError (the default).
  • -fdebug, -fdebug=value — Turn on compilation of conditional debug code into the program. The -fdebug option itself sets the debug level to 1, while -fdebug= enables debug code that are identified by any of the following values:
    • ident — Turns on compilation of any debug code identified by ident.
  • -fno-druntime — Implements https://dlang.org/spec/betterc.html. Assumes that compilation targets an environment without a D runtime library.

This is equivalent to compiling with the following options:

gdc -nophoboslib -fno-exceptions -fno-moduleinfo -fno-rtti
  • -fextern-std=standard — Sets the C++ name mangling compatibility to the version identified by standard. The following values are supported:
    • c++98, c++03 — Sets __traits(getTargetInfo, "cppStd") to 199711.
    • c++11 — Sets __traits(getTargetInfo, "cppStd") to 201103.
    • c++14 — Sets __traits(getTargetInfo, "cppStd") to 201402.
    • c++17 — Sets __traits(getTargetInfo, "cppStd") to 201703. This is the default.
    • c++20 — Sets __traits(getTargetInfo, "cppStd") to 202002.
    • c++23 — Sets __traits(getTargetInfo, "cppStd") to 202302.
  • -finclude-imports — Include imported modules in the compilation, as if they were given on the command line. When this option is enabled, all imported modules are compiled except those that are part of libphobos.
  • -fno-invariants — Turns off code generation for class invariant contracts.
  • -fmain — Generates a default main() function when compiling. This is useful when unittesting a library, as it enables running the unittests in a library without having to manually define an entry-point function. This option does nothing when main is already defined in user code.
  • -fno-moduleinfo — Turns off generation of the ModuleInfo and related functions that would become unreferenced without it, which may allow linking to programs not written in D. Functions that are not be generated include module constructors and destructors (static this and static ~this), unittest code, and DSO registry functions for dynamically linked code.
  • -fonly=filename — Tells the compiler to parse and run semantic analysis on all modules on the command line, but only generate code for the module specified by filename. Main character module only. ✨
  • -fno-postconditions — Turns off code generation for postcondition out contracts.
  • -fno-preconditions — Turns off code generation for precondition in contracts.
  • -fpreview=id — Turns on an upcoming D language change identified by id. The following values are supported:
    • all — Turns on all upcoming D language features.
    • bitfields — Implements bit-fields in D.
    • dip1000 — Implements https://github.com/dlang/DIPs/blob/master/DIPs/other/DIP1000.md (Scoped pointers).
    • dip1008 — Implements https://github.com/dlang/DIPs/blob/master/DIPs/other/DIP1008.md (Allow exceptions in @nogc code).
    • dip1021 — Implements https://github.com/dlang/DIPs/blob/master/DIPs/accepted/DIP1021.md (Mutable function arguments).
    • dip25 — Implements https://github.com/dlang/DIPs/blob/master/DIPs/archive/DIP25.md (Sealed references).
    • dtorfields — Turns on generation for destructing fields of partially constructed objects.
    • fieldwise — Turns on generation of struct equality to use field-wise comparisons.
    • fixaliasthis — Implements new lookup rules that check the current scope for alias this before searching in upper scopes.
    • fiximmutableconv — Disallows unsound immutable conversions that were formerly incorrectly permitted.
    • in — Implements in parameters to mean scope const [ref] and accepts rvalues.
    • inclusiveincontracts — Implements in contracts of overridden methods to be a superset of parent contract.
    • nosharedaccess — Turns off and disallows all access to shared memory objects.
    • rvaluerefparam — Implements rvalue arguments to ref parameters.
    • systemvariables — Disables access to variables marked @system from @safe code.
  • -frelease — Turns on compiling in release mode, which means not emitting runtime checks for contracts and asserts. Array bounds checking is not done for @system and @trusted functions, and assertion failures are undefined behavior. > ⚠️ Undefined behavior is giving "we're in the danger zone" — release mode is not for the faint of heart.

This is equivalent to compiling with the following options:

gdc -fno-assert -fbounds-check=safe -fno-invariants \
    -fno-postconditions -fno-preconditions -fno-switch-errors
  • -frevert= — Turns off a D language feature identified by id. The following values are supported:
  • -fno-rtti — Turns off generation of run-time type information for all user defined types. Any code that uses features of the language that require access to this information will result in an error. So if your code needs RTTI, it's gonna be a whole thing. 😭
  • -fno-switch-errors — This option controls what code is generated when no case is matched in a final switch statement. The default run time behavior is to throw a SwitchError. Turning off -fswitch-errors means that instead the execution of the program is immediately halted.
  • -funittest — Turns on compilation of unittest code, and turns on the version(unittest) identifier. This implies -fassert.
  • -fversion=value — Turns on compilation of conditional version code into the program identified by any of the following values:
    • ident — Turns on compilation of version code identified by ident.
  • -fno-weak-templates — Turns off emission of declarations that can be defined in multiple objects as weak symbols. The default is to emit all public symbols as weak, unless the target lacks support for weak symbols. Disabling this option means that common symbols are instead put in COMDAT or become private.

These options specify directories to search for files, libraries, and other parts of the compiler:

okay bestie, we’re still deep in option land — here’s the next batch, and yes every path matters 💅

  • -Idir — tells gdc which directory to use when searching for imported modules at compile time. You can stack multiple -I options, and the paths are searched in the same order you give them. path order matters, bestie ✨
  • -Jdir — tells gdc which directory to use when searching for files in string imports at compile time. This switch is required in order to use import(file) expressions. Multiple -J options can be used, and the paths are searched in the same order. 🎀
  • -Ldir — when linking, specify a library search directory, as with gcc. 🔗
  • -Bdir — this option specifies where to find the executables, libraries, source files, and data files of the compiler itself, as with gcc. 🛠️
  • -fmodule-file=module=spec — this option manipulates file paths of imported modules, such that if an imported module matches all or the leftmost part of module, the file path in spec is used as the location to search for D sources. This is used when the source file path and names are not the same as the package and module hierarchy. Consider the following examples:
gdc test.d -fmodule-file=A.B=foo.d -fmodule-file=C=bar

This will tell the compiler to search in all import paths for the source file foo.d when importing A.B, and the directory bar/ when importing C, as annotated in the following D code:

module test;
import A.B;     // Matches A.B, searches for foo.d
import C.D.E;   // Matches C, searches for bar/D/E.d
import A.B.C;   // No match, searches for A/B/C.d
  • -imultilib dir — use dir as a subdirectory of the gcc directory containing target-specific D sources and interfaces. 🗂️
  • -iprefix prefix — specify prefix as the prefix for the gcc directory containing target-specific D sources and interfaces. If the prefix represents a directory, you should include the final '/'. don’t forget the slash, queen 💛
  • -nostdinc — do not search the standard system directories for D source and interface files. Only the directories that have been specified with -I options (and the directory of the current file, if appropriate) are searched. 🚫📁

🛠️ Code Generation 🛠️

On top of the many gcc options controlling code generation, gdc has several options specific to itself. main character compiler energy 👑

  • -H — generates D interface files for all modules being compiled. The compiler determines the output file based on the name of the input file, removes any directory components and suffix, and applies the .di suffix. ✨
  • -Hd dir — same as -H, but writes interface files to directory dir. This option can be used with -Hf file to independently set the output file and directory path. 🎀
  • -Hf file — same as -H but writes interface files to file. This option can be used with -Hd dir to independently set the output file and directory path. 🎀
  • -M — output the module dependencies of all source files being compiled in a format suitable for make. The compiler outputs one make rule containing the object file name for that source file, a colon, and the names of all imported files. 📦
  • -MM — like -M but does not mention imported modules from the D standard library package directories. Same vibe, less noise. 🌸
  • -MF file — when used with -M or -MM, specifies a file to write the dependencies to. When used with the driver options -MD or -MMD, -MF overrides the default dependency output file. 📝
  • -MG — this option is for compatibility with gcc, and is ignored by the compiler. it’s literally just vibing 💙
  • -MP — outputs a phony target for each dependency other than the modules being compiled, causing each to depend on nothing. 👻
  • -MT target — change the target of the rule emitted by dependency generation to be exactly the string you specify. If you want multiple targets, you can specify them as a single argument to -MT, or use multiple -MT options. 🎯
  • -MQ target — same as -MT, but it quotes any characters which are special to make. 🧚
  • -MD — this option is equivalent to -M -MF file. The driver determines file by removing any directory components and suffix from the input file, and then adding a .deps suffix. 🗃️
  • -MMD — like -MD but does not mention imported modules from the D standard library package directories. 🍓
  • -X — output information describing the contents of all source files being compiled in JSON format to a file. The driver determines file by removing any directory components and suffix from the input file, and then adding a .json suffix. 🧾
  • -Xf file — same as -X, but writes all JSON contents to the specified file. ✨
  • -fdoc — generates "Ddoc" documentation and writes it to a file. The compiler determines file by removing any directory components and suffix from the input file, and then adding a .html suffix. 📚
  • -fdoc-dir=dir — same as -fdoc, but writes documentation to directory dir. This option can be used with -fdoc-file=file to independently set the output file and directory path. 🎀
  • -fdoc-file=file — same as -fdoc, but writes documentation to file. This option can be used with -fdoc-dir=dir to independently set the output file and directory path. 🎀
  • -fdoc-inc=file — specify file as a Ddoc macro file to be read. Multiple -fdoc-inc options can be used, and files are read and processed in the same order. 📖
  • -fdump-c++-spec=file — for D source files, generate corresponding C++ declarations in file. 💅
  • -fdump-c++-spec-verbose — in conjunction with -fdump-c++-spec= above, add comments for ignored declarations in the generated C++ header. 🗒️
  • -fsave-mixins=file — generates code expanded from D "mixin" statements and writes the processed sources to file. This is useful to debug errors in compilation and provides source for debuggers to show when requested. 🐛✨

⚠️ Warnings ⚠️

Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there is likely to be a bug in the program. Unless -Werror is specified, they do not prevent compilation of the program. they’re like little red flags, not full stops 🚩

  • -Wall — turns on all warnings messages. Warnings are not a defined part of the D language, and all constructs for which this may generate a warning message are valid code. 📣
  • -Walloca — this option warns on all uses of alloca in the source. 😬
  • -Walloca-larger-than=n — warn on unbounded uses of alloca, and on bounded uses of alloca whose bound can be larger than n bytes. -Wno-alloca-larger-than disables -Walloca-larger-than warning and is equivalent to -Walloca-larger-than=SIZE_MAX or larger. big number energy 🔥
  • -Wno-builtin-declaration-mismatch — warn if a built-in function is declared with an incompatible signature. 🧩
  • -Wcast-result — warn about casts that will produce a null or zero result. Currently this is only done for casting between an imaginary and non-imaginary data type, or casting between a D and C++ class. 🎭
  • -Wno-deprecated — do not warn about usage of deprecated features and symbols with "deprecated" attributes. 🙈
  • -Werror — turns all warnings into errors. no more warnings, bestie — now they’re crimes 💀
  • -Wextra — this enables some extra warning flags that are not enabled by -Wall: -Waddress, -Wcast-result, -Wmismatched-special-enum, -Wunknown-pragmas. ✨
  • -Wmismatched-special-enum — warn when an enum the compiler recognizes as special is declared with a different size to the built-in type it is representing. 🧠
  • -Wspeculative — list all error messages from speculative compiles, such as __traits(compiles, ...). This option does not report messages as warnings, and these messages therefore never become errors when the -Werror option is also used. 🔍
  • -Wunknown-pragmas — warn when a pragma() is encountered that is not understood by gdc. This differs from -fignore-unknown-pragmas where a pragma that is part of the D language, but not implemented by the compiler, won't get reported. 🫶
  • -Wno-varargs — do not warn upon questionable usage of the macros used to handle variable arguments like va_start. 🌀
  • -fno-ignore-unknown-pragmas — do not recognize unsupported pragmas. Any pragma() encountered that is not part of the D language will result in an error. > 🚨 heads up: this option is now deprecated and will be removed in a future release. don’t get attached, bestie 💔
  • -fmax-errors=n — limits the maximum number of error messages to n, at which point gdc bails out rather than attempting to continue processing the source code. If n is 0 (the default), there is no limit on the number of error messages produced. 🛑
  • -fsyntax-only — check the code for syntax errors, but do not actually compile it. This can be used in conjunction with -fdoc or -H to generate files for each module present on the command-line, but no other output file. 👀
  • -ftransition=id — report additional information about D language changes identified by id. The following values are supported:
    • all — list information on all D language transitions. 🌈
    • complex — list all usages of complex or imaginary types. 🔮
    • field — list all non-mutable fields which occupy an object instance. 🧱
    • in — list all usages of in on parameter. 📥
    • nogc — list all hidden GC allocations. 🗑️
    • templates — list statistics on template instantiations. 🧬
    • tls — list all variables going into thread local storage. 🧵

🔗 Options for Linking 🔗

These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler is not doing a link step. link o’clock, bestie 🕒

  • -defaultlib=libname — specify the library to use instead of libphobos when linking. Options specifying the linkage of libphobos, such as -static-libphobos or -shared-libphobos, are ignored. 📚
  • -debuglib=libname — specify the debug library to use instead of libphobos when linking. This option has no effect unless the -g option was also given on the command line. Options specifying the linkage of libphobos, such as -static-libphobos or -shared-libphobos, are ignored. 🐞
  • -nophoboslib — do not use the Phobos or D runtime library when linking. Options specifying the linkage of libphobos, such as -static-libphobos or -shared-libphobos, are ignored. The standard system libraries are used normally, unless -nostdlib or -nodefaultlibs is used. 🚫
  • -shared-libphobos — on systems that provide libgphobos and libgdruntime as a shared and a static library, this option forces the use of the shared version. If no shared version was built when the compiler was configured, this option has no effect. 🦋
  • -static-libphobos — okay so on systems where libgphobos and libgdruntime come in both a shared and a static flavor, this flag makes the compiler pick the static one, periodt. But heads up: if no static version was built when the compiler was configured, this option literally does nothing. No effect, no drama, no notes. 💅

🛠️ Developer Options 🛠️

This bit is all about the command-line options that are mostly for developers or for the language-tooling girlies — the people poking at the compiler's insides. If that's not you, you can absolutely skip, but stay for the vibes anyway. 🫶

  • -fdump-d-original — dumps out the internal front-end AST after the semantic3 stage. Not gonna lie, this one is only useful for debugging the GNU D compiler itself. It's giving "compiler surgeon only" energy, iykyk. 🩺✨
  • -v — this is the gossip flag, fr fr. It dumps information about the compiler's language processing stages while your source program is being compiled. What's in the dump? So much:
    • a listing of all modules processed through the parse, semantic, semantic2, and semantic3 stages
    • all import modules, plus their file paths
    • all function bodies that are being compiled

Basically the compiler narrating its own main character journey. Obsessed. 👑

🔗 SEE ALSO 🔗

Go check out gpl(7), gfdl(7), fsf-funding(7), gcc(1), and the Info entries for gdc and gcc. Bestie reading list, slay. 📚💖

Copyright (c) 2006-2026 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the man page gfdl(7).

Fr fr, that's the actual license text — keep it exactly as-is if you're passing this doc around. Respect the paperwork, it ate. 🎀

and that's a wrap on the whole page, bestie!! 💖✨ This little masterpiece is from gcc-16, dated 2026-08-07, and it's living its best life as GDC(1). Go compile something iconic. 🦋🌸💅

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