Using GDC

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Simple Compilation

Creating an executable is quite easy.

gdc main.d -o main

This will attempt to compile and link the file 'main.d' and place the output into the file 'main'. If you do not use the -o switch, then your executable will be called 'a.out'.

On a typical Unix system, you can execute the resulting program with "./main" or "./a.out". On Windows, you can run the program with "main" or "a.out".(?)

To help make a transition from DMD to GDC easier, there is a script called 'gdmd' which maps DMD's command line options to GDC. To see the available options for gdmd, type 'gdmd' or 'gdmd -help' on the command line.

Command line switches

Many of the options in GCC may also be applicable to GDC, such as optimization flags, -O1, -O2, -Os, -O3, or flags such as -c, which compiles a file, but does not link it, and will send the object file to "main.o", if you file is main.d

Compiler Options

Switch Description
-debuglib=<lib> Link against a debug <lib> instead of Phobos.
-defaultlib=<lib> Link against <lib> instead of Phobos.
-fdeps Print information about module dependencies.
-fdeps=<file> Write module dependencies to <file>.
-fdoc Generate Ddoc documentation.
-fdoc-dir=<dir> Write Ddoc documentation files to <dir>.
-fdoc-file=<file> Write Ddoc documentation to <file>.
-fdoc-inc=<file> Include a Ddoc macro <file>.
-fintfc Generate D interface files,
-fintfc-dir=<dir> Write D interface files to directory <dir>.
-fintfc-file=<file> Write D interface file to <file>.
-fmake-deps Print information about module Makefile dependencies.
-fmake-deps=<file> Write Makefile dependency output to <file>.
-fmake-mdeps Like -fmake-deps but ignore system modules.
-fmake-mdeps=<file> Like -fmake-deps=<file> but ignore system modules.
-fonly=<file> Process all modules specified on the command line, but only generate code for the module <file>.
-fXf=<file> Write JSON documenation to <file>.
-imultilib <dir> Set <dir> to be the multilib include subdirectory.
-iprefix <path> Specify <path> as a prefix for next two options.
-isysroot <dir> Set <dir> to be the system root directory.
-isystem <dir> Add <dir> to the start of the system include path.
-I <dir> Add <dir> to the list of the module import paths.
-J <dir> Add <dir> to the list of string import paths.
-nophoboslib Do not link the standard D library in the compilation. The D standard library, Phobos, and the D runtime are compiled into a single library, libgphobos2. Therefore, this option prevents linking both Phobos and the D runtime.
-nostdinc Do not search standard system include directories.
-nostdlib Do not link the standard gcc libraries in the compilation.

Language Options

Most of these have both positive and negative forms; the negative form of -ffoo is -fno-foo. This page lists only one of these two forms, whichever one is not the default.

Switch Description
-fno-assert Don't generate runtime code for the assert keyword.
-fno-bounds-check Don't generate runtime code for checking array bounds before indexing.
-fno-builtin Don't recognize built-in functions.
-fno-debug Don't compile debug code.
-fdebug=<level> Compile in debug code less than or equal to that in <level>.
-fdebug=<ident> Compile in debug code identified by <ident>.
-fd-verbose Print information about D language processing to stdout.
-fd-vtls Print information about all variables going into thread local storage to stdout.
-femit-templates Generate code for all template instantiations, not just used instantiations.
-fno-in Don't compile in contracts.
-fno-invariants Don't compile invariant contracts.
-fno-emit-moduleinfo Don't generate any ModuleInfo.
-fno-out Don't compile out contracts.
-fproperty Enforce @property syntax of D code.
-frelease Compile release version. Equivalent to -fno-invariants -fno-in -fno-out -fno-assert -fno-bounds-check.
-funittest Compile unittest code.
-fversion=<level> Compile in version code greater than or equal to that in <level>.
-fversion=<ident> Compile in version code identified by <ident>.
-Wall Enable most warning messages.
-Werror Error out the compiler on warnings.
-Wdeprecated Enable warning of deprecated language features.
-Wunknown-pragmas Enable warning of unsupported pragmas.


Extended Assembler

GDC implements a GCC extension that allows inline assembler with D expression operands. It is available on nearly all targets, not just i386. The syntax differs from the C language extension in the following ways:

  • Statements start with 'asm { ...', just like the regular DMD inline assembler.
  • It is not necesary to put parentheses around operands.
  • Instruction templates can be compile-time string constants, not just string literals. If the template is not a string literal, use parenthesis to indicate that it is not an opcode.

Unlike i386 inline assembler statements, extended assembler statements do not prevent a function from being inlined.

See the GCC manual for more information about this extension.


uint invert(uint v)
    uint result;
       asm{ "notl %[iov]" : [iov] "=r" result : "0" v; }
    else version(PPC)
       asm{ "nor %[oresult],%[iv],%[iv]" : [oresult] "=r" result : [iv] "r" v; }
    return result;


GDC supports a small subset of the GCC attributes. The syntax differs from the C language __attribute__ extension in the following ways:

  • All attributes are recognised only through the 'gcc.attribute' module.
  • The attribute, and all its arguments are comma-dellimied CTFE strings packed in a tuple.
  • Nesting (brackets) for attribute arguments are optional.

Attribute Description
forceinline* Inlines the function even if no optimization level is specified.
flatten* Inlines every call inside this function, if possible.
noinline* Prevents the function from being considered for inlining.
target* Specify that the function is to be compiled with different target options than specified on the command line.
noclone* See GCC documentation.
section* Place symbol in specific section. See GCC documentation.
weak* Mark symbol as weak. See GCC documentation.
alias* Mark symbol as an alias (on object file level). See GCC documentation.
architecture specific attributes All target specific attributes are available. See GCC documentation.

* Being backend attributes, you can't enforce that these attributes actually take effect in user code (no static asserts!) - but you have some guarantee in that the backend will complain if it can't apply the attribute


import gcc.attribute;

@attribute("noinline") void foobar() { }

@attribute("target", ("sse3")) void sse3_func() { }

//Can be overwritten in other source files
@attribute("weak") extern(C) void c_func() {};
@attribute("alias", "c_func") void aliased_func();

//Place into "test" section
@attribute("section", "test") int value;

Known Issues

See bugzilla to see bugs that have been reported to GDC.

More bugs may be found here.

Some more known issues, taken from here:

  • See DStress for known failing cases. (Again, may be irrelevant)
  • Debugging information may have a few problems. For D symbol name demangling you need at least gdb 7.2.
  • Some targets do not support once-only linking. A workaround is to manually control template emission. See the -femit-templates option below. For Darwin, Apple's GCC 3.x compiler supports one-only linking, but GDC does not build with those sources. There are no problems with the stock GCC 4.x on Darwin.
  • Complex floating point operations may not work the same as DMD.
  • Some math functions behave differently due to different implementations of the extended floating-point type.
  • Volatile statements may not always do the right thing.
  • Because of a problem on AIX, the linker will pull in more modules than needed.
  • Some C libraries (Cygwin, MinGW, AIX) don't handle floating-point formatting and parsing in a standard way.

See also