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 the standalone program 'gdmd', distributed with GDC releases, 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

External links