D Programming Language 2.0

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Last update Wed Jun 9 12:07:03 2010

# Interfacing to C

D is designed to fit comfortably with a C compiler for the target system. D makes up for not having its own VM by relying on the target environment's C runtime library. It would be senseless to attempt to port to D or write D wrappers for the vast array of C APIs available. How much easier it is to just call them directly.

This is done by matching the C compiler's data types, layouts, and function call/return sequences.

## Calling C Functions

C functions can be called directly from D. There is no need for wrapper functions, argument swizzling, and the C functions do not need to be put into a separate DLL.

The C function must be declared and given a calling convention, most likely the "C" calling convention, for example:

    extern (C) int strcmp(char* string1, char* string2);

and then it can be called within D code in the obvious way:

    import std.string;
    int myDfunction(char[] s)
    {
        return strcmp(std.string.toStringz(s), "foo");
    }

There are several things going on here:

- D understands how C function names are "mangled" and the correct C function call/return sequence.
- C functions cannot be overloaded with another C function with the same name.
- There are no __cdecl, __far, __stdcall, __declspec, or other such C type modifiers in D. These are handled by attributes, such as `extern (C)`.
- There are no const or volatile type modifiers in D. To declare a C function that uses those type modifiers, just drop those keywords from the declaration.
- Strings are not 0 terminated in D. See "Data Type Compatibility" for more information about this. However, string literals in D are 0 terminated.

C code can correspondingly call D functions, if the D functions use an attribute that is compatible with the C compiler, most likely the extern (C):

    // myfunc() can be called from any C function
    extern (C)
    {
        void myfunc(int a, int b)
        {
    	...
        }
    }

## Storage Allocation

C code explicitly manages memory with calls to malloc() and free(). D allocates memory using the D garbage collector, so no explicit free's are necessary.

D can still explicitly allocate memory using std.c.stdlib.malloc() and std.c.stdlib.free(), these are useful for connecting to C functions that expect malloc'd buffers, etc.

If pointers to D garbage collector allocated memory are passed to C functions, it's critical to ensure that that memory will not be collected by the garbage collector before the C function is done with it. This is accomplished by:

- Making a copy of the data using std.c.stdlib.malloc() and passing the copy instead.
- Leaving a pointer to it on the stack (as a parameter or automatic variable), as the garbage collector will scan the stack.
- Leaving a pointer to it in the static data segment, as the garbage collector will scan the static data segment.
- Registering the pointer with the garbage collector with the std.gc.addRoot() or std.gc.addRange() calls.

An interior pointer to the allocated memory block is sufficient to let the GC know the object is in use; i.e. it is not necessary to maintain a pointer to the beginning of the allocated memory.

The garbage collector does not scan the stacks of threads not created by the D Thread interface. Nor does it scan the data segments of other DLL's, etc.

## Data Type Compatibility

D And C Type Equivalence
D type | C type
--- | ---
void | void
byte | signed char
ubyte | unsigned char
char | char (chars are unsigned in D)
wchar | wchar_t (when sizeof(wchar_t) is 2)
dchar | wchar_t (when sizeof(wchar_t) is 4)
short | short
ushort | unsigned short
int | int
uint | unsigned
long | long long
ulong | unsigned long long
float | float
double | double
real | long double
ifloat | float _Imaginary
idouble | double _Imaginary
ireal | long double _Imaginary
cfloat | float _Complex
cdouble | double _Complex
creal | long double _Complex
struct | struct
union | union
enum | enum
class | no equivalent
type* | type *
type[dim] | type[dim]
type[dim]* | type(*)[dim]
type[] | no equivalent
type[type] | no equivalent
type function(parameters) | type(*)(parameters)
type delegate(parameters) | no equivalent

These equivalents hold for most 32 bit C compilers. The C standard does not pin down the sizes of the types, so some care is needed.

## Passing D Array Arguments to C Functions

In C, arrays are passed to functions as pointers even if the function prototype says its an array. In D, static arrays are passed by value, not by reference. Thus, the function prototype must be adjusted to match what C expects.

D And C Function Prototype Equivalence
D type | C type
--- | ---
T* | T[]
ref T[dim] | T[dim]

For example:

    void foo(int a[3]) { ... } // C code

    extern (C)
    {
      void foo(ref int[3] a); // D prototype
    }

## Calling printf()

This mostly means checking that the printf format specifier matches the corresponding D data type. Although printf is designed to handle 0 terminated strings, not D dynamic arrays of chars, it turns out that since D dynamic arrays are a length followed by a pointer to the data, the `%.*s` format works perfectly:

    void foo(char[] string)
    {
        printf("my string is: %.*s\n", string);
    }

The `printf` format string literal in the example doesn't end with `'\0'`. This is because string literals, when they are not part of an initializer to a larger data structure, have a `'\0'` character helpfully stored after the end of them.

An improved D function for formatted output is `std.stdio.writef()`.

## Structs and Unions

D structs and unions are analogous to C's.

C code often adjusts the alignment and packing of struct members with a command line switch or with various implementation specific #pragma's. D supports explicit alignment attributes that correspond to the C compiler's rules. Check what alignment the C code is using, and explicitly set it for the D struct declaration.

D does not support bit fields. If needed, they can be emulated with shift and mask operations. htod will convert bit fields to inline functions that do the right shift and masks.

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