root/trunk/docsrc/htomodule.dd

Ddoc

$(D_S Converting C $(TT .h) Files to D Modules,

    While D cannot directly compile C source code, it can easily
    interface to C code, be linked with C object files, and call
    C functions in DLLs.
    The interface to C code is normally found in C $(TT .h) files.
    So, the trick to connecting with C code is in converting C
    $(TT .h) files to D modules.
    This turns out to be difficult to do mechanically since
    inevitably some human judgement must be applied.
    This is a guide to doing such conversions.

<h4>Preprocessor</h4>

    $(TT .h) files can sometimes be a bewildering morass of layers of
    macros, $(TT #include) files, $(TT #ifdef)'s, etc. D doesn't
    include a text preprocessor like the C preprocessor,
    so the first step is to remove the need for
    it by taking the preprocessed output. For DMC (the Digital
    Mars C/C++ compiler), the command:

$(CONSOLE
<a href="http://www.digitalmars.com/ctg/sc.html">dmc</a> <a href="http://www.digitalmars.com/ctg/sc.html#dashc">-c</a> program.h <a href="http://www.digitalmars.com/ctg/sc.html#dashe">-e</a> <a href="http://www.digitalmars.com/ctg/sc.html#dashl">-l</a>
)

    will create a file $(TT program.lst) which is the source file after
    all text preprocessing.
    <p>

    Remove all the $(TT #if), $(TT #ifdef), $(TT #include),
    etc. statements.

<h4>Linkage</h4>

    Generally, surround the entire module with:

---------------------------
extern (C)
{
     /* ...file contents... */
}
---------------------------

    to give it C linkage.

<h4>Types</h4>

    A little global search and replace will take care of renaming
    the C types to D types. The following table shows a typical mapping
    for 32 bit C code:
    <p>

    $(TABLE1
    <caption>Mapping C type to D type</caption>
    <tr>
    <th>C type
    <th>D type
    <tr>
    <td>long double
    <td>real
    <tr>
    <td>unsigned long long
    <td>ulong
    <tr>
    <td>long long
    <td>long
    <tr>
    <td>unsigned long
    <td>uint
    <tr>
    <td>long
    <td>int
    <tr>
    <td>unsigned
    <td>uint
    <tr>
    <td>unsigned short
    <td>ushort
    <tr>
    <td>signed char
    <td>byte
    <tr>
    <td>unsigned char
    <td>ubyte
    <tr>
    <td>wchar_t
    <td>wchar or dchar
    <tr>
    <td>bool
    <td>bool, byte, int
    <tr>
    <td>size_t
    <td>size_t
    <tr>
    <td>ptrdiff_t
    <td>ptrdiff_t
    )

<h4>NULL</h4>

    $(TT NULL) and $(TT ((void*)0)) should be replaced
    with $(TT null).

<h4>Numeric Literals</h4>

    Any $(SINGLEQUOTE L) or $(SINGLEQUOTE l) numeric literal suffixes should be removed,
    as a C $(TT long) is (usually) the same size as a D $(TT int).
    Similarly, $(SINGLEQUOTE LL) suffixes should be replaced with a
    single $(SINGLEQUOTE L).
    Any $(SINGLEQUOTE u) suffix will work the same in D.

<h4>String Literals</h4>

    In most cases, any $(SINGLEQUOTE L) prefix to a string can just be dropped,
    as D will implicitly convert strings to wide characters if
    necessary. However, one can also replace:

$(CCODE
L"string"
)

    with:

---------------------------
"string"w   // for 16 bit wide characters
"string"d   // for 32 bit wide characters
---------------------------

<h4>Macros</h4>

    Lists of macros like:

$(CCODE
#define FOO 1
#define BAR 2
#define ABC 3
#define DEF 40
)

    can be replaced with:

---------------------------
enum
{   FOO = 1,
    BAR = 2,
    ABC = 3,
    DEF = 40
}
---------------------------

    or with:

---------------------------
const int FOO = 1;
const int BAR = 2;
const int ABC = 3;
const int DEF = 40;
---------------------------

    Function style macros, such as:

$(CCODE
#define MAX(a,b) ((a) < (b) ? (b) : (a))
)

    can be replaced with functions:

---------------------------
int MAX(int a, int b) { return (a < b) ? b : a; }
---------------------------

    <!-- Thanks to Jarrett Billingsley for the following tip -->

    The functions, however, won't work if they appear inside static
    initializers that must be evaluated at compile time rather than
    runtime. To do it at compile time, a template can be used:

$(CCODE
#define GT_DEPTH_SHIFT  (0)
#define GT_SIZE_SHIFT   (8)
#define GT_SCHEME_SHIFT (24)
#define GT_DEPTH_MASK   (0xffU << GT_DEPTH_SHIFT)
#define GT_TEXT         ((0x01) << GT_SCHEME_SHIFT)

/* Macro that constructs a graphtype */
#define GT_CONSTRUCT(depth,scheme,size) \
    ((depth) | (scheme) | ((size) << GT_SIZE_SHIFT))

/* Common graphtypes */
#define GT_TEXT16  GT_CONSTRUCT(4, GT_TEXT, 16)
)

    The corresponding D version would be:

---------------------------
const uint GT_DEPTH_SHIFT  = 0;
const uint GT_SIZE_SHIFT   = 8;
const uint GT_SCHEME_SHIFT = 24;
const uint GT_DEPTH_MASK   = 0xffU << GT_DEPTH_SHIFT;
const uint GT_TEXT         = 0x01 << GT_SCHEME_SHIFT;

// Template that constructs a graphtype
template GT_CONSTRUCT(uint depth, uint scheme, uint size)
{
 // notice the name of the const is the same as that of the template
 const uint GT_CONSTRUCT = (depth | scheme | (size << GT_SIZE_SHIFT));
}

// Common graphtypes
const uint GT_TEXT16 = GT_CONSTRUCT!(4, GT_TEXT, 16);
---------------------------


<h4>Declaration Lists</h4>

    D doesn't allow declaration lists to change the type.
    Hence:

$(CCODE
int *p, q, t[3], *s;
)

    should be written as:

---------------------------
int* p, s;
int q;
int[3] t;
---------------------------

<h4>Void Parameter Lists</h4>

    Functions that take no parameters:

$(CCODE
int foo(void);
)

    are in D:

---------------------------
int foo();
---------------------------

$(V1

<h4>Const Type Modifiers</h4>

    D has $(TT const) as a storage class, not a type modifier. Hence, just
    drop any $(TT const) used as a type modifier:

$(CCODE
void foo(const int *p, char *const q);
)

    becomes:

---------------------------
void foo(int* p, char* q);
---------------------------
)
<h4>Extern Global C Variables</h4>

    Whenever a global variable is declared in D, it is also defined.
    But if it's also defined by the C object file being linked in,
    there will be a multiple definition error. To fix this problem,
    use the extern storage class.
    For example, given a C header file named
    $(TT foo.h):

$(CCODE
struct Foo { };
struct Foo bar;
)

    It can be replaced with the D modules, $(TT foo.d):

---------------------------
struct Foo { }
extern (C)
{
    extern Foo bar;
}
---------------------------


<h4>Typedef</h4>

    $(TT alias) is the D equivalent to the C $(TT typedef):

$(CCODE
typedef int foo;
)

    becomes:

---------------------------
alias int foo;
---------------------------

<h4>Structs</h4>

    Replace declarations like:

$(CCODE
typedef struct Foo
{   int a;
    int b;
} Foo, *pFoo, *lpFoo;
)

    with:

---------------------------
struct Foo
{   int a;
    int b;
}
alias Foo* pFoo, lpFoo;
---------------------------

<h4>Struct Member Alignment</h4>

    A good D implementation by default will align struct members the
    same way as the C compiler it was designed to work with. But
    if the $(TT .h) file has some $(TT #pragma)'s to control alignment, they
    can be duplicated with the D $(TT align) attribute:

$(CCODE
#pragma pack(1)
struct Foo
{
    int a;
    int b;
};
#pragma pack()
)

    becomes:

---------------------------
struct Foo
{
  align (1):
    int a;
    int b;
}
---------------------------

<h4>Nested Structs</h4>

$(CCODE
struct Foo
{
    int a;
    struct Bar
    {
    int c;
    } bar;
};

struct Abc
{
    int a;
    struct
    {
    int c;
    } bar;
};
)

    becomes:

---------------------------
struct Foo
{
    int a;
    struct Bar
    {
    int c;
    }
    Bar bar;
}

struct Abc
{
    int a;
    struct
    {
    int c;
    }
}
---------------------------

<h4>$(TT __cdecl), $(TT __pascal), $(TT __stdcall)</h4>

$(CCODE
int __cdecl x;
int __cdecl foo(int a);
int __pascal bar(int b);
int __stdcall abc(int c);
)

    becomes:

---------------------------
extern (C) int x;
extern (C) int foo(int a);
extern (Pascal) int bar(int b);
extern (Windows) int abc(int c);
---------------------------

<h4>$(TT __declspec(dllimport))</h4>

$(CCODE
__declspec(dllimport) int __stdcall foo(int a);
)

    becomes:

---------------------------
export extern (Windows) int foo(int a);
---------------------------

<h4>$(TT __fastcall)</h4>

    Unfortunately, D doesn't support the $(TT __fastcall) convention.
    Therefore, a shim will be needed, either written in C:

$(CCODE
int __fastcall foo(int a);

int myfoo(int a)
{
    return foo(int a);
}
)

    and compiled with a C compiler that supports $(TT __fastcall) and
    linked in, or compile the above, disassemble it with
    <a href="http://www.digitalmars.com/ctg/obj2asm.html">obj2asm</a>
    and insert it in a D $(TT myfoo) shim with
    <a href="iasm.html">inline assembler</a>.

)

Macros:
    TITLE=Converting C .h Files to D Modules
    WIKI=HToModule