Converting C .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 .h files. So, the trick to connecting with C code is in converting C .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.Preprocessor
.h files can sometimes be a bewildering morass of layers of macros, #include files, #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:dmc -c program.h -e -lwill create a file program.lst which is the source file after all text preprocessing.
Remove all the #if, #ifdef, #include, etc. statements.
Linkage
Generally, surround the entire module with:extern (C) { /* ...file contents... */ }to give it C linkage.
Types
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:
C type | D type |
---|---|
long double | real |
unsigned long long | ulong |
long long | long |
unsigned long | uint |
long | int |
unsigned | uint |
unsigned short | ushort |
signed char | byte |
unsigned char | ubyte |
wchar_t | wchar or dchar |
bool | bool, byte, int |
size_t | size_t |
ptrdiff_t | ptrdiff_t |
NULL
NULL and ((void*)0) should be replaced with null.Numeric Literals
Any 'L' or 'l' numeric literal suffixes should be removed, as a C long is (usually) the same size as a D int. Similarly, 'LL' suffixes should be replaced with a single 'L'. Any 'u' suffix will work the same in D.String Literals
In most cases, any '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:L"string"with:
"string"w // for 16 bit wide characters "string"d // for 32 bit wide characters
Macros
Lists of macros like:#define FOO 1 #define BAR 2 #define ABC 3 #define DEF 40can 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:
#define MAX(a,b) ((a) < (b) ? (b) : (a))can be replaced with functions:
int MAX(int a, int b) { return (a < b) ? b : a; }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:
#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);
Declaration Lists
D doesn't allow declaration lists to change the type. Hence:int *p, q, t[3], *s;should be written as:
int* p, s; int q; int[3] t;
Void Parameter Lists
Functions that take no parameters:int foo(void);are in D:
int foo();
Const Type Modifiers
D has const as a storage class, not a type modifier. Hence, just drop any const used as a type modifier:void foo(const int *p, char *const q);becomes:
void foo(int* p, char* q);
Extern Global C Variables
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 foo.h:struct Foo { }; struct Foo bar;It can be replaced with the D modules, foo.d:
struct Foo { } extern (C) { extern Foo bar; }
Typedef
alias is the D equivalent to the C typedef:typedef int foo;becomes:
alias int foo;
Structs
Replace declarations like:typedef struct Foo { int a; int b; } Foo, *pFoo, *lpFoo;with:
struct Foo { int a; int b; } alias Foo* pFoo, lpFoo;
Struct Member Alignment
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 .h file has some #pragma's to control alignment, they can be duplicated with the D align attribute:#pragma pack(1) struct Foo { int a; int b; }; #pragma pack()becomes:
struct Foo { align (1): int a; int b; }
Nested Structs
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; } }
__cdecl, __pascal, __stdcall
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);
__declspec(dllimport)
__declspec(dllimport) int __stdcall foo(int a);becomes:
export extern (Windows) int foo(int a);
__fastcall
Unfortunately, D doesn't support the __fastcall convention. Therefore, a shim will be needed, either written in C:int __fastcall foo(int a); int myfoo(int a) { return foo(int a); }and compiled with a C compiler that supports __fastcall and linked in, or compile the above, disassemble it with obj2asm and insert it in a D myfoo shim with inline assembler.