GDC(1)

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💖 GDC(1) · GNU Manual 💖 (part 1 of 2)

✨ NAME ✨

gdc is a GCC-based compiler for the D language, and it's giving main character energy 💅

✨ SYNOPSIS ✨

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

okay so this is just the cheat sheet 📝 Only the most useful options are listed here, so scroll down for the rest 👀

✨ DESCRIPTION ✨

The gdc command is the GNU compiler for the D language, and it slays because it supports a lot of the same options as gcc 🌸 This manual only covers the options that are specific to gdc 🫶

✨ OPTIONS ✨

🦋 Input and Output files

For any input file, the file name suffix decides what kind of compilation happens 🔥 These are the kinds of input file names that are supported:

  • file.d — D source files 💜
  • file.dd — Ddoc source files 🎀
  • file.di — D interface files 🧚

You can pass more than one input file on the gdc command line, and each one gets compiled separately 🍓 If you use -o file, then all the input files get compiled together and produce one single output file called file ✨ That works even with -S or -c, so no stress 💖

A D interface file only has what an import of the module actually needs, not the whole implementation of that module 🧁 gdc can make these from a D source file if you use the -H option 🌈 When the compiler resolves an import declaration, it searches for matching .di files first, and then for .d files 👀

A Ddoc source file has code in the D macro processor language 🎀 It's mainly for making user documentation out of embedded comments, with a slight HTML-generation crush 💕 If a .d source file starts with the string "Ddoc", then it's treated as general purpose documentation instead of D source code 😭

🌈 Runtime Options

These options change how programs compiled with gdc behave at runtime ⚡

  • -fall-instantiations — generates code for all template instantiations 🧁 By default, the compiler skips code for declarations that were instantiated speculatively, like from __traits(compiles, ...), or that come from an imported module that isn't being compiled 🙈
  • -fno-assert — turns off code generation for "assert" contracts 🚫
  • -fno-bounds-check — turns off array bounds checking for all functions 🏎️ This can boost performance for code that uses arrays a lot 💨
⚠️ Heads up: if the code actually violates array bounds, this can lead to unpredictable behavior. It's only safe to use if you're sure your code never throws a RangeError 🫣
  • -fbounds-check=value — an alternative to -fbounds-check that gives you more control over where bounds checking is turned on or off 🎛️ These 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 recognizes when a function in the core.stdc package is a built-in function ✨
  • -fdebug, -fdebug=value — turns on compiling conditional debug code into the program 🐛 -fdebug by itself sets the debug level to 1, while -fdebug= enables debug code identified by any of these values:
    • level — sets the debug level to level, so any debug code <= level gets compiled in.
    • ident — turns on compilation of any debug code identified by ident.
  • -fno-druntime — implements <https://dlang.org/spec/betterc.html> 📜 It assumes the compilation targets an environment without a D runtime library 🌵 This is equivalent to compiling with these options:
gdc -nophoboslib -fno-exceptions -fno-moduleinfo -fno-rtti
  • -fno-invariants — turns off code generation for class invariant contracts 🧊
  • -fno-moduleinfo — turns off generation of ModuleInfo and the related functions that would become unreferenced without it 🧹 This may allow linking to programs that aren't written in D 🔗 The functions that are not 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 🎯
  • -fno-postconditions — turns off code generation for postcondition out contracts 🚫
  • -fno-preconditions — turns off code generation for precondition in contracts 🚫
  • -frelease — turns on release mode 🚀 which means no emitted runtime checks for contracts and asserts. Array bounds checking isn't done for @system and @trusted functions, and assertion failures are undefined behavior 💀
💀 Undefined behavior alert: assertion failures in release mode are undefined behavior, so be extra careful, bestie.

This is equivalent to compiling with these options:

gdc -fno-assert -fbounds-check=safe -fno-invariants \
    -fno-postconditions -fno-preconditions -fno-switch-errors
  • -fno-rtti — turns off generation of run-time type information for all user defined types 🧬 Any code that uses language features requiring that info will result in an error 🚫
  • -fno-switch-errors — controls what code gets 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 the execution of the program is immediately halted instead 🛑
  • -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 these values:
    • level — sets the version level to level, so any version code >= level gets compiled in.
    • ident — turns on compilation of version code identified by ident.
  • -fno-weak — turns off emission of instantiated declarations that can be defined in multiple objects as weak or one-only symbols 🕊️ The default is to emit all public symbols as weak, unless the target lacks support for weak symbols 💪 Disabling this option means common symbols are instead put in COMDAT or become private 🔒

These options tell the compiler which directories to search for files, libraries, and other parts of the compiler itself 🗺️

  • -Idir — a directory to use when searching for imported modules at compile time 📂 Multiple -I options can be used, and the paths get searched in the same order 🔁
  • -Jdir — a 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 get searched in the same order 🔁
  • -Ldir — when linking, a library search directory, same as with gcc 🔗
  • -Bdir — where to find the compiler's own executables, libraries, source files, and data files, same as with gcc 🏠
  • -fmodule-file=module=spec — manipulates file paths of imported modules 🛤️ If an imported module matches all or the leftmost part of module, then the file path in spec is used as the location to search for D sources 🔎 This is handy when the source file paths and names don't match the package and module hierarchy 🧩 Check out these examples:
gdc test.d -fmodule-file=A.B=foo.d -fmodule-file=C=bar

This tells the compiler to search in all import paths for the source file foo.d when importing A.B, and to search the directory bar/ when importing C, as annotated in this 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 — uses dir as a subdirectory of the gcc directory containing target-specific D sources and interfaces 🎯
  • -iprefix prefix — specifies 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 '/' 📁
  • -nostdinc — don't search the standard system directories for D source and interface files 🙅 Only the directories specified with -I options (and the directory of the current file, if appropriate) get searched 🔍

(continued in part 2 👀)

  • -MD — okay so this is basically -M -MF file 💖 The driver figures out the file name by stripping any directory components and suffix off your input file, then tacking on a .deps suffix 🧚
  • -MMD — same energy as -MD, but it doesn't mention imported modules that come from the D standard library package directories 🙈
  • -X — dumps info about the contents of every source file being compiled, in JSON format, into a file 📦 The driver picks the name by removing directory components and suffix from the input file, then adding a .json suffix 🌸
  • -Xf file — same as -X, but all the JSON goes into the exact file you name 🎀
  • -fdoc — generates "Ddoc" documentation and writes it to a file ✨ The compiler names the file by stripping directory components and suffix from the input file, then adding a .html suffix 💅
  • -fdoc-dir=dir — same as -fdoc, but the docs go into the directory dir 🗂️ You can pair it with -fdoc-file=file to set the output file and directory path separately 🦋
  • -fdoc-file=file — same as -fdoc, but the docs go into file 🌈 It works with -fdoc-dir=dir so you can set the file and directory independently 💜
  • -fdoc-inc=file — reads file as a Ddoc macro file 📖 You can use several -fdoc-inc options, and the files get read and processed in the same order you gave them ⭐

🚨 Warnings 🚨

Warnings are diagnostic messages about constructions that aren't inherently erroneous but are risky, or that suggest there's probably a bug in the program. Unless -Werror is specified, they don't stop the program from compiling 💀

  • -Wall — turns on all warning messages 🔥 Warnings aren't a defined part of the D language, and every construct that might trigger a warning is still valid code 🫶
  • -Walloca — warns on all uses of alloca in the source 😭
  • -Walloca-larger-than=n — warns on unbounded uses of alloca, and on bounded uses whose bound can be larger than n bytes 📏 -Wno-alloca-larger-than turns off -Walloca-larger-than and is equivalent to -Walloca-larger-than=SIZE_MAX or larger 🙅
  • -Wcast-result — warns about casts that will produce a null or zero result 🫠 Right now it's only checked for casting between an imaginary and non-imaginary data type, or between a D and C++ class 👑
  • -Wno-deprecated — no warnings about usage of deprecated features and symbols with "deprecated" attributes 🙃
  • -Werror — turns all warnings into errors 💥
  • -Wspeculative — lists all error messages from speculative compiles, like __traits(compiles, ...) 🧐 It doesn't report them as warnings, so they never become errors even when -Werror is also used 🛡️
  • -Wtemplates — warns whenever a template instantiation is encountered 🚫 Some coding rules disallow templates, so this can help enforce that rule 📋
  • -Wunknown-pragmas — warns when a pragma() is encountered that gdc doesn't understand 🤷 This is different from -fignore-unknown-pragmas, where a pragma that's part of the D language but not implemented by the compiler won't get reported 📝
  • -fignore-unknown-pragmas — turns off errors for unsupported pragmas 😌
  • -fmax-errors=n — caps the number of error messages at n, at which point gdc bails out instead of trying to keep going 🛑 If n is 0 (the default), there's no limit on how many errors get produced ♾️
  • -fsyntax-only — checks the code for syntax errors but doesn't actually compile it 🔍 You can use it with -fdoc or -H to generate files for each module on the command line, but no other output file 📄
  • -ftransition=id — reports extra info about D language changes identified by id 🦄 These values are supported:
    • all — lists info on all language changes
    • checkimports — gives deprecation messages about -ftransition=import anomalies
    • complex — lists all usages of complex or imaginary types
    • dip1000 — implements http://wiki.dlang.org/DIP1000 (experimental) 🧪
    • dip25 — implements http://wiki.dlang.org/DIP25 (experimental) 🧪
    • field — lists all non-mutable fields which occupy an object instance
    • import — tells the compiler to go back to the old lookup behavior for resolving unqualified symbol names, which happened in a single pass and ignored any protection attributes 🕰️ The default lookup strategy uses two passes: the first ignores imported declarations, and the second only looks at imports 🔁 Protection ("private", "package", "protected") of symbols is also enforced to resolve conflicts between private and public symbols 🔐
    • nogc — lists all hidden GC allocations 🗑️
    • tls — lists 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're meaningless if the compiler isn't doing a link step, so don't bother with them otherwise 🙈

  • -defaultlib libname — uses libname instead of libphobos when linking 📚 Options that set the linkage of libphobos, like -static-libphobos or -shared-libphobos, get ignored 🙃
  • -debuglib — specifies the debug library to use instead of libphobos when linking 🐛 This does nothing unless the -g option was also given on the command line 💡 Linkage options like -static-libphobos or -shared-libphobos are ignored here too 🙅
  • -nophoboslib — doesn't use the Phobos or D runtime library when linking 🚫 Linkage options like -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 both a shared and a static library, this forces the shared version 🤝 If no shared version was built when the compiler was configured, it does nothing 🤷
  • -static-libphobos — on systems that provide libgphobos and libgdruntime as both a shared and a static library, this forces the static version 🧊 If no static version was built when the compiler was configured, it does nothing 🤷

🧪 Developer Options 🧪

This section is all about command-line options that are mainly for developers or language tooling 👩‍💻

  • -fdump-d-original — outputs the internal front-end AST after the "semantic3" stage 🌳 This is only useful for debugging the GNU D compiler itself 🛠️
  • -v — dumps info about the compiler's language processing stages while your source program is being compiled 📣 That includes listing all modules processed through the "parse", "semantic", "semantic2", and "semantic3" stages 📋; all "import" modules and their file paths 🗺️; and all "function" bodies being compiled 🧑‍🍳

💌 SEE ALSO 💌

gpl(7), gfdl(7), fsf-funding(7), gcc(1), and the Info entries for gdc and gcc 📚✨

Copyright (c) 2006-2019 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) 📜

Okay bestie, that's the whole GDC(1) page! 🌈 This one comes from the gcc-9 set and was last touched on 2022-05-27, so you're officially up to speed on your D compiler flags 💅🦋 bye bestie 💖

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