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Note: This document is under construction. Please excuse the dust and noise.

ImportC is a C compiler embedded into the D implementation. Its purpose is to enable direct importation of C files, without needing to manually prepare a D file corresponding to the declarations in the C file. It enables directly compiling C files into modules, and linking them in with D code to form an executable. It can be used as a C compiler to compile and link 100% C programs.

This is not a C reference manual nor programming tutorial. It describes the specifics of the dialect of C that ImportC is, and how to use it effectively.

ImportC and BetterC are very different. ImportC is an actual C compiler. BetterC is a subset of D that relies only on the existence of the C Standard library.

ImportC Dialect

There are many versions of C. ImportC is designed to be an implementation of ISO/IEC 9899:2011, which will hereafter be referred to as C11. References to the C11 Standard will be C11 followed by the paragraph number. Prior versions, such as C99, C89, and K+R C, are not supported.

Adjustment to the ImportC dialect is made to match the behavior of the C compiler that the D compiler is matched to i.e. the associated C compiler. For example, on Win32 D is matched to the Digital Mars C compiler, and can be matched to the Visual C compiler using the -m32mscoff switch. Win64 D is matched to the Visual C compiler. On Posix targets, the matching C compiler is Gnu C or Clang C.

Further adjustment is made to take advantage of some of the D implementation's capabilities.

This is all covered in the rest of this document.

Invoking ImportC

The ImportC compiler can be invoked in two ways:

ImportC Files on the Command Line

Importing C Files


ImportC does not have a preprocessor. It is designed to compile C files after they have been first run through the C preprocessor.

Digital Mars C Preprocessor sppn.exe

sppn.exe runs on Win32 and is invoked as:

    sppn file.c

and the preprocessed output is written to file.i.

Gnu C Preprocessor

The Gnu C Preprocessor can be invoked as:

    gcc -E file.c > file.i

Clang C Preprocessor

The Clang Preprocessor can be invoked as:

    clang -E file.c > file.i

dmpp C Preprocessor

The dmpp C Preprocessor can be invoked as:

    dmpp file.c

and the preprocessed output is written to file.i.

Predefined Macros

ImportC does not predefine any macros.

Best Practices: To distinguish an ImportC compile vs some other C compiler, define __IMPORTC__ to be 1 when the preprocessor is run.

Preprocessor Directives

Nevertheless, ImportC supports these preprocessor directives:

Line control

C11 6.10.4


linemarker directives are normally embedded in the output of C preprocessors.


The implementation defined characteristics of ImportC are:


enumeration-constants are always typed as int.

The expression that defines the value of an enumeration-constant must be an integral type and evaluate to an integer value that fits in an int.

enum E { -10, 0x81231234 }; // ok
enum F {  0x812312345678 }; // error, doesn't fit in int
enum G { 1.0 };             // error, not integral type

The enumerated type is int.

Bit Fields

There are many implementation defined aspects of C11 bit fields. ImportC's behavior adjusts to match the behavior of the associated C compiler on the target platform.

Implicit Function Declarations

Implicit function declarations:

    int main()
        func();  // implicit declaration of func()

were allowed in K+R C and C89, but were invalidated in C99 and C11. Although many C compilers still support them, ImportC does not.

Rationale: Implicit function declarations are very error-prone and cause hard to find bugs.


Exception Handling

ImportC is assumed to never throw exceptions. setjmp and longjmp are not supported.


C11 specifies that const only applies locally. const in ImportC applies transitively, meaning that although

int *const p;
means in C11 that p is a const pointer to int, in ImportC it means p is a const pointer to a const int.


The volatile type-qualifier (C11 6.7.3) is ignored. Use of volatile to implement shared memory access is unlikely to work anyway, _Atomic is for that. To use volatile as a device register, call a function to do it that is compiled separately, or use inline assembler.


The restrict type-qualifier (C11 6.7.3) is ignored.


The _Atomic type-qualifier (C11 6.7.3) is ignored. To do atomic operations, use an externally compiled function for that, or the inline assembler.

Compatible Types

Compatible Types (C11 6.7.2) are identical types in ImportC.

Same only Different Types

On some platforms, C long and unsigned long are the same size as int and unsigned int, respectively. On other platforms, C long and unsigned long are the same size as long long and unsigned long long. long double and long double _Complex can be same size as double and double _Complex. In ImportC, these types that are the same size and signed-ness are treated as the same types.


Generic selection expressions (C11 differ from ImportC. The types in Same only Different Types are indistinguishable in the type-name parts of generic-association. Instead of giving an error for duplicate types per C11, ImportC will select the first compatible type-name in the generic-assoc-list.


Forward References

Any declarations in scope can be accessed, not just declarations that lexically precede a reference.

Compile Time Function Execution

Evaluating constant expressions includes executing functions in the same manner as D's CTFE can.

Function Inlining

Functions for which the function body is present can be inlined by ImportC as well as by the D code that calls them.

Register Storage Class

Objects with register storage class are treated as auto declarations.

Objects with register storage class may have their address taken. C11

Arrays can have register storage class, and may be enregistered by the compiler. C11

Gnu and Clang Extensions

gcc and clang are presumed to have the same behavior w.r.t. extensions, so gcc as used here refers to both.


__attribute__((noreturn)) marks a function as never returning. gcc set this as an attribute of the function, it is not part of the function's type. In D, a function that never returns has the return type noreturn. The difference can be seen with the code:

    attribute((noreturn)) int foo();
    size_t x = sizeof(foo());

This code is accepted by gcc, but makes no sense for D. Hence, although it works in ImportC, it is not representable as D code, meaning one must use judgement in creating a .di file to interface with C noreturn functions.

Furthermore, the D compiler takes advantage of noreturn functions by issuing compile time errors for unreachable code. Such unreachable code, however, is valid C11, and the ImportC compiler will accept it.

Best Practices: C code that uses the noreturn attribute should at the very least set the return type to void.

Visual C Extensions

Digital Mars C Extensions

ImportC from D's Point of View

There is no one-to-one mapping of C constructs to D constructs, although it is very close. What follows is a description of how the D side views the C declarations that are imported.

Module Name

The module name assigned to the ImportC file is the filename stripped of its path and extension. This is just like the default module name assigned to a D module that does not have a module declaration.

extern (C)

All C symbols are extern (C).


The C enum:

enum E { A, B = 2 };

appears to D code as:

enum E : int { A, B = 2 }
alias A = E.A;
alias B = E.B;

The .min and .max properties are available:

static assert(E.min == 0 && E.max == 2);

Wrapping C Code

Many difficulties with adapting C code to ImportC can be done without editing the C code itself. Wrap the C code in another C file and then

it. Consider the following problematic C file file.c:

    void func(int *__restrict p);
    int S;
    struct S { int a, b; };

The problems are that

is not a type qualifier recognized by ImportC (or C11), and the struct S is hidden from D by the declaration
int S;
. To wrap file.c with a fix, create the file file_ic.c with the contents:

    #define __restrict restrict
    #include "file.c"
    typedef struct S S_t;

Then, import file_ic; instead of import file;, and use S_t when

struct S
is desired.


Many suspicious C constructs normally cause warnings to be emitted by default by typical compilers, such as:

int *p = 3; // Warning: integer implicitly converted to pointer

ImportC does not emit warnings. The presumption is the user will be importing existing C code developed using another C compiler, and it is written as intended. If C11 says it is legal, ImportC accepts it.


ImportC will not compile C++ code. For that, use dpp.

Other Solutions

dpp by Atila Neves

dpp code

dpp Article

From the Article:

dpp is a compiler wrapper that will parse a D source file with the .dpp extension and expand in place any #include directives it encounters, translating all of the C or C++ symbols to D, and then pass the result to a D compiler (DMD by default).

Like DStep, dpp relies on libclang.

DStep by Jacob Carlborg

DStep code

DStep Article

From the Article:

DStep is a tool for automatically generating D bindings for C and Objective-C libraries. This is implemented by processing C or Objective-C header files and outputting D modules. DStep uses the Clang compiler as a library (libclang) to process the header files.

htod by Walter Bright

htod converts a C .h file to a D source file, suitable for importing into D code. htod is built from the front end of the Digital Mars C and C++ compiler. It works just like a C or C++ compiler except that its output is source code for a D module rather than object code.

How ImportC Works

This is a description of how ImportC is implemented, intended to remove the mystery of various design choices.

Better C
Live Functions