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Application Binary Interface

A D implementation that conforms to the D ABI (Application Binary Interface) will be able to generate libraries, DLLs, etc., that can interoperate with D binaries built by other implementations.


The C ABI referred to in this specification means the C Application Binary Interface of the target system. C and D code should be freely linkable together, in particular, D code shall have access to the entire C ABI runtime library.


The endianness (byte order) of the layout of the data will conform to the endianness of the target machine. The Intel x86 CPUs are little endian meaning that the value 0x0A0B0C0D is stored in memory as: 0D 0C 0B 0A.

Basic Types

bool8 bit byte with the values 0 for false and 1 for true
byte8 bit signed value
ubyte8 bit unsigned value
short16 bit signed value
ushort16 bit unsigned value
int32 bit signed value
uint32 bit unsigned value
long64 bit signed value
ulong64 bit unsigned value
cent128 bit signed value
ucent128 bit unsigned value
float32 bit IEEE 754 floating point value
double64 bit IEEE 754 floating point value
realimplementation defined floating point value, for x86 it is 80 bit IEEE 754 extended real
char8 bit unsigned value
wchar16 bit unsigned value
dchar32 bit unsigned value


Delegates are fat pointers with two parts:

Delegate Layout
0.ptrcontext pointer
ptrsize.funcptrpointer to function

The context pointer can be a class this reference, a struct this pointer, a pointer to a closure (nested functions) or a pointer to an enclosing function's stack frame (nested functions).


Conforms to the target's C ABI struct layout.


An object consists of:

Class Object Layout
ptrsize.__vptrpointer to vtable
ptrsize... vptrs for any interfaces implemented by this class in left to right, most to least derived, order
......super's non-static fields and super's interface vptrs, from least to most derived
...named fieldsnon-static fields

The vtable consists of:

Virtual Function Pointer Table Layout
ptrsizepointer to instance of TypeInfo
ptrsize...pointers to virtual member functions

Casting a class object to an interface consists of adding the offset of the interface's corresponding vptr to the address of the base of the object. Casting an interface ptr back to the class type it came from involves getting the correct offset to subtract from it from the object.Interface entry at vtbl[0]. Adjustor thunks are created and pointers to them stored in the method entries in the vtbl[] in order to set the this pointer to the start of the object instance corresponding to the implementing method.

An adjustor thunk looks like:

  ADD EAX,offset
  JMP method

The leftmost side of the inheritance graph of the interfaces all share their vptrs, this is the single inheritance model. Every time the inheritance graph forks (for multiple inheritance) a new vptr is created and stored in the class' instance. Every time a virtual method is overridden, a new vtbl[] must be created with the updated method pointers in it.

The class definition:

class XXXX

Generates the following:


An interface is a pointer to a pointer to a vtbl[]. The vtbl[0] entry is a pointer to the corresponding instance of the object.Interface class. The rest of the vtbl[1..$] entries are pointers to the virtual functions implemented by that interface, in the order that they were declared.

A COM interface differs from a regular interface in that there is no object.Interface entry in vtbl[0]; the entries vtbl[0..$] are all the virtual function pointers, in the order that they were declared. This matches the COM object layout used by Windows.

A C++ interface differs from a regular interface in that it matches the layout of a C++ class using single inheritance on the target machine.


A dynamic array consists of:

Dynamic Array Layout
0.lengtharray dimension
size_t.ptrpointer to array data

A dynamic array is declared as:

type[] array;
whereas a static array is declared as:
type[dimension] array;

Thus, a static array always has the dimension statically available as part of the type, and so it is implemented like in C. Static arrays and Dynamic arrays can be easily converted back and forth to each other.

Associative Arrays

Associative arrays consist of a pointer to an opaque, implementation defined type.

The current implementation is contained in and defined by rt/aaA.d.

Reference Types

D has reference types, but they are implicit. For example, classes are always referred to by reference; this means that class instances can never reside on the stack or be passed as function parameters.

Name Mangling

D accomplishes typesafe linking by mangling a D identifier to include scope and type information.

    _D QualifiedName Type
    _D QualifiedName Z        // Internal

The Type above is the type of a variable or the return type of a function. This is never a TypeFunction, as the latter can only be bound to a value via a pointer to a function or a delegate.

    SymbolFunctionName QualifiedName
SymbolFunctionName: SymbolName SymbolName TypeFunctionNoReturn SymbolName M TypeModifiersopt TypeFunctionNoReturn

The M means that the symbol is a function that requires a this pointer. Class or struct fields are mangled without M. To disambiguate M from being a Parameter with modifier scope, the following type needs to be checked for being a TypeFunction.

    0                         // anonymous symbols

Template Instance Names have the types and values of its parameters encoded into it:

    TemplateID LName TemplateArgs Z
TemplateID: __T __U // for symbols declared inside template constraint
TemplateArgs: TemplateArg TemplateArg TemplateArgs
TemplateArg: TemplateArgX H TemplateArgX

If a template argument matches a specialized template parameter, the argument is mangled with prefix H.

    T Type
    V Type Value
    S QualifiedName
    X Number ExternallyMangledName

ExternallyMangledName can be any series of characters allowed on the current platform, e.g. generated by functions with C++ linkage or annotated with pragma(mangle,...).

    Value Values
Value: n i Number N Number e HexFloat c HexFloat c HexFloat CharWidth Number _ HexDigits A Number Values S Number Values f MangledName
HexFloat: NAN INF NINF N HexDigits P Exponent HexDigits P Exponent
Exponent: N Number Number
HexDigits: HexDigit HexDigit HexDigits
HexDigit: Digit A B C D E F
CharWidth: a w d
is for null arguments.
i Number
is for positive numeric literals (including character literals).
N Number
is for negative numeric literals.
e HexFloat
is for real and imaginary floating point literals.
c HexFloat c HexFloat
is for complex floating point literals.
CharWidth Number _ HexDigits
CharWidth is whether the characters are 1 byte (a), 2 bytes (w) or 4 bytes (d) in size. Number is the number of characters in the string. The HexDigits are the hex data for the string.
A Number Values
An array or asssociative array literal. Number is the length of the array. Value is repeated Number times for a normal array, and 2 * Number times for an associative array.
S Number Values
A struct literal. Value is repeated Number times.
    Namestart Namechars
Namestart: _ Alpha
Namechar: Namestart Digit
Namechars: Namechar Namechar Namechars

A Name is a standard D identifier.

    Number Name
    Number __S Number    // function-local parent symbols
Number: Digit Digit Number
Digit: 0 1 2 3 4 5 6 7 8 9

An LName is a name preceded by a Number giving the number of characters in the Name.

Back references

Any LName or non-basic Type (i.e. any type that does not encode as a fixed one or two character sequence) that has been emitted to the mangled symbol before will not be emitted again, but is referenced by a special sequence encoding the relative position of the original occurrence in the mangled symbol name.

Numbers in back references are encoded with base 26 by upper case letters A - Z for higher digits but lower case letters a - z for the last digit.

    Q NumberBackRef
IdentifierBackRef: Q NumberBackRef
NumberBackRef: lower-case-letter upper-case-letter NumberBackRef

To distinguish between the type of the back reference a look-up of the back referenced character is necessary: An identifier back reference always points to a digit 0 to 9, while a type back reference always points to a letter.

Type Mangling

Types are mangled using a simple linear scheme:

    TypeModifiersopt TypeX
TypeX: TypeArray TypeStaticArray TypeAssocArray TypePointer TypeFunction TypeIdent TypeClass TypeStruct TypeEnum TypeTypedef TypeDelegate TypeVoid TypeByte TypeUbyte TypeShort TypeUshort TypeInt TypeUint TypeLong TypeUlong TypeCent TypeUcent TypeFloat TypeDouble TypeReal TypeIfloat TypeIdouble TypeIreal TypeCfloat TypeCdouble TypeCreal TypeBool TypeChar TypeWchar TypeDchar TypeNoreturn TypeNull TypeTuple TypeVector
TypeModifiers: Const Wild Wild Const Shared Shared Const Shared Wild Shared Wild Const Immutable
Shared: O
Const: x
Immutable: y
Wild: Ng
TypeArray: A Type
TypeStaticArray: G Number Type
TypeAssocArray: H Type Type
TypePointer: P Type
TypeVector: Nh Type
TypeFunction: TypeFunctionNoReturn Type
TypeFunctionNoReturn: CallConvention FuncAttrsopt Parametersopt ParamClose
CallConvention: F // D U // C W // Windows R // C++ Y // Objective-C
FuncAttrs: FuncAttr FuncAttr FuncAttrs
FuncAttr: FuncAttrPure FuncAttrNothrow FuncAttrRef FuncAttrProperty FuncAttrNogc FuncAttrReturn FuncAttrScope FuncAttrTrusted FuncAttrSafe FuncAttrLive

Function attributes are emitted in the order as listed above, with the exception of return and scope. return comes before scope when this is a return scope parameter, and after scope when this is a scope and return ref parameter.

FuncAttrNogc: Ni
FuncAttrNothrow: Nb
FuncAttrProperty: Nd
FuncAttrRef: Nc
FuncAttrReturn: Nj
FuncAttrScope: Nl
FuncAttrTrusted: Ne
FuncAttrSafe: Nf
FuncAttrLive: Nm
Parameters: Parameter Parameter Parameters
Parameter: Parameter2 M Parameter2 // scope Nk Parameter2 // return
Parameter2: Type I Type // in J Type // out K Type // ref L Type // lazy
ParamClose: X // variadic T t...) style Y // variadic T t,...) style Z // not variadic
TypeIdent: I QualifiedName
TypeClass: C QualifiedName
TypeStruct: S QualifiedName
TypeEnum: E QualifiedName
TypeTypedef: T QualifiedName
TypeDelegate: D TypeModifiersopt TypeFunction
TypeVoid: v
TypeByte: g
TypeUbyte: h
TypeShort: s
TypeUshort: t
TypeInt: i
TypeUint: k
TypeLong: l
TypeUlong: m
TypeCent: zi
TypeUcent: zk
TypeFloat: f
TypeDouble: d
TypeReal: e
TypeIfloat: o
TypeIdouble: p
TypeIreal: j
TypeCfloat: q
TypeCdouble: r
TypeCreal: c
TypeBool: b
TypeChar: a
TypeWchar: u
TypeDchar: w
TypeNoreturn: Nn
TypeNull: n
TypeTuple: B Parameters Z

Function Calling Conventions

The extern (C) and extern (D) calling convention matches the C calling convention used by the supported C compiler on the host system. Except that the extern (D) calling convention for Windows x86 is described here.

Register Conventions

Return Value


The parameters to the non-variadic function:

foo(a1, a2, ..., an);
are passed as follows:

where hidden is present if needed to return a struct value, and this is present if needed as the this pointer for a member function or the context pointer for a nested function.

The last parameter is passed in EAX rather than being pushed on the stack if the following conditions are met:

Parameters are always pushed as multiples of 4 bytes, rounding upwards, so the stack is always aligned on 4 byte boundaries. They are pushed most significant first. out and ref are passed as pointers. Static arrays are passed as pointers to their first element. On Windows, a real is pushed as a 10 byte quantity, a creal is pushed as a 20 byte quantity. On Linux, a real is pushed as a 12 byte quantity, a creal is pushed as two 12 byte quantities. The extra two bytes of pad occupy the ‘most significant’ position.

The callee cleans the stack.

The parameters to the variadic function:

void foo(int p1, int p2, int[] p3...)
foo(a1, a2, ..., an);
are passed as follows:

The variadic part is converted to a dynamic array and the rest is the same as for non-variadic functions.

The parameters to the variadic function:

void foo(int p1, int p2, ...)
foo(a1, a2, a3, ..., an);
are passed as follows:

The caller is expected to clean the stack. _argptr is not passed, it is computed by the callee.

Exception Handling

Windows 32 bit

Conforms to the Microsoft Windows Structured Exception Handling conventions.

Linux, FreeBSD and OS X

Conforms to the DWARF (debugging with attributed record formats) Exception Handling conventions.

Windows 64 bit

Uses static address range/handler tables. It is not compatible with the MSVC x64 exception handling tables. The stack is walked assuming it uses the EBP/RBP stack frame convention. The EBP/RBP convention must be used for every function that has an associated EH (Exception Handler) table.

For each function that has exception handlers, an EH table entry is generated.

EH Table Entry
void*pointer to start of function
DHandlerTable*pointer to corresponding EH data
uintsize in bytes of the function

The EH table entries are placed into the following special segments, which are concatenated by the linker.

EH Table Segment
Operating SystemSegment Name
OS X__deh_eh, __DATA

The rest of the EH data can be placed anywhere, it is immutable.

void*pointer to start of function
uintoffset of ESP/RSP from EBP/RBP
uintoffset from start of function to return code
uintnumber of entries in DHandlerInfo[]
DHandlerInfo[]array of handler information

uintoffset from function address to start of guarded section
uintoffset of end of guarded section
intprevious table index
uintif != 0 offset to DCatchInfo data from start of table
void*if not null, pointer to finally code to execute

uintnumber of entries in DCatchBlock[]
DCatchBlock[]array of catch information

void*, catch handler code
ClassInfocatch type
uintoffset from EBP/RBP to catch variable

Garbage Collection

The interface to this is found in Druntime's core/gc/gcinterface.d.

ModuleInfo Instance

An instance of ModuleInfo is generated by the compiler and inserted into the object file for every module. ModuleInfo contains information about the module that is useful to the D runtime library:

ModuleInfo is defined in Druntime's object.d, which must match the compiler's output in both the values of flags and layout of fields.

Modules compiled with -betterC do not have a ModuleInfo instance generated, because such modules must work without the D runtime library. Similarly, ImportC modules do not generate a ModuleInfo.

Module Initialization and Termination

All the static constructors for a module are aggregated into a single function, and a pointer to that function is inserted into the ctor member of the ModuleInfo instance for that module.

All the static destructors for a module are aggregated into a single function, and a pointer to that function is inserted into the dtor member of the ModuleInfo instance for that module.

Unit Testing

All the unit tests for a module are aggregated into a single function, and a pointer to that function is inserted into the unitTest member of the ModuleInfo instance for that module.

Runtime Helper Functions

These are found in Druntime's rt/.

Symbolic Debugging

D has types that are not represented in existing C or C++ debuggers. These are dynamic arrays, associative arrays, and delegates. Representing these types as structs causes problems because function calling conventions for structs are often different than that for these types, which causes C/C++ debuggers to misrepresent things. For these debuggers, they are represented as a C type which does match the calling conventions for the type.

Types for C Debuggers
D typeC representation
dynamic arrayunsigned long long
associative arrayvoid*
delegatelong long
dcharunsigned long

For debuggers that can be modified to accept new types, the following extensions help them fully support the types.

Codeview Debugger Extensions

The D dchar type is represented by the special primitive type 0x78.

D makes use of the Codeview OEM generic type record indicated by LF_OEM (0x0015). The format is:

Codeview OEM Extensions for D
field size222222
D TypeLeaf IndexOEM IdentifierrecOEMnum indices type indextype index
dynamic arrayLF_OEMOEM12@index@element
associative arrayLF_OEMOEM22@key@element
indextype index of array index
keytype index of key
elementtype index of array element
thistype index of context pointer
functiontype index of function

These extensions can be pretty-printed by obj2asm. The Ddbg debugger supports them.

Memory Safety
Vector Extensions