interfaceToC.dd

Ddoc

$(SPEC_S Interfacing to C,

	$(P 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.
	)

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

<h2>Calling C Functions</h2>

	$(P 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.
	)

	$(P 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);
------

	$(P 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");
}
------

	$(P There are several things going on here:)

	$(UL 
	$(LI D understands how C function names are "mangled" and the
	correct C function call/return sequence.)

	$(LI C functions cannot be overloaded with another C function
	with the same name.)

	$(LI There are no __cdecl, __far, __stdcall, __declspec, or other
	such C type modifiers in D. These are handled by attributes, such
	as $(TT extern (C)).)

	$(LI 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.)

	$(LI Strings are not 0 terminated in D. See "Data Type Compatibility"
	for more information about this. However, string literals in D are
	0 terminated.)

	)

	$(P 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)
    {
	...
    }
}
------

<h2>Storage Allocation</h2>

	$(P 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.
	)

	$(P 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.
	)

	$(P 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:
	)

	$(UL 

	$(LI Making a copy of the data using std.c.stdlib.malloc() and passing
	the copy instead.)

	$(LI Leaving a pointer to it on the stack (as a parameter or
	automatic variable), as the garbage collector will scan the stack.)

	$(LI Leaving a pointer to it in the static data segment, as the
	garbage collector will scan the static data segment.)

	$(LI Registering the pointer with the garbage collector with the
	std.gc.addRoot() or std.gc.addRange() calls.)

	)

	$(P 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.
	)

	$(P 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.
	)

<h2>Data Type Compatibility</h2>

	$(TABLE1
	<caption>D And C Type Equivalence</caption>

	$(TR
	$(TH D type)
	$(TH C type)
	)

	$(TR
	$(TD $(B void))
	$(TD $(B void))
	)

	$(TR
	$(TD $(B byte))
	$(TD $(B signed char))
	)

	$(TR
	$(TD $(B ubyte))
	$(TD $(B unsigned char))
	)

	$(TR
	$(TD $(B char))
	$(TD $(B char) (chars are unsigned in D))
	)

	$(TR
	$(TD $(B wchar))
	$(TD $(B wchar_t) (when sizeof(wchar_t) is 2))
	)

	$(TR
	$(TD $(B dchar))
	$(TD $(B wchar_t) (when sizeof(wchar_t) is 4))
	)

	$(TR
	$(TD $(B short))
	$(TD $(B short))
	)

	$(TR
	$(TD $(B ushort))
	$(TD $(B unsigned short))
	)

	$(TR
	$(TD $(B int))
	$(TD $(B int))
	)

	$(TR
	$(TD $(B uint))
	$(TD $(B unsigned))
	)

	$(TR
	$(TD $(B long))
	$(TD $(B long long))
	)

	$(TR
	$(TD $(B ulong))
	$(TD $(B unsigned long long))
	)

	$(TR
	$(TD $(B float))
	$(TD $(B float))
	)

	$(TR
	$(TD $(B double))
	$(TD $(B double))
	)

	$(TR
	$(TD $(B real))
	$(TD $(B long double))
	)

	$(TR
	$(TD $(B ifloat))
	$(TD $(B float _Imaginary))
	)

	$(TR
	$(TD $(B idouble))
	$(TD $(B double _Imaginary))
	)

	$(TR
	$(TD $(B ireal))
	$(TD $(B long double _Imaginary))
	)

	$(TR
	$(TD $(B cfloat))
	$(TD $(B float _Complex))
	)

	$(TR
	$(TD $(B cdouble))
	$(TD $(B double _Complex))
	)

	$(TR
	$(TD $(B creal))
	$(TD $(B long double _Complex))
	)

	$(TR
	$(TD $(B struct))
	$(TD $(B struct))
	)

	$(TR
	$(TD $(B union))
	$(TD $(B union))
	)

	$(TR
	$(TD $(B enum))
	$(TD $(B enum))
	)

	$(TR
	$(TD $(B class))
	$(TD no equivalent)
	)

	$(TR
	$(TD $(I type)$(B *))
	$(TD $(I type) $(B *))
	)

	$(TR
	$(TD $(I type)$(B [)$(I dim)$(B ]))
	$(TD $(I type)$(B [)$(I dim)$(B ]))
	)

	$(TR
	$(TD $(I type)$(B [)$(I dim)$(B ]*))
	$(TD $(I type)$(B (*)[)$(I dim)$(B ]))
	)

	$(TR
	$(TD $(I type)$(B []))
	$(TD no equivalent)
	)

	$(TR
	$(TD $(I type)$(B [)$(I type)$(B ]))
	$(TD no equivalent)
	)

	$(TR
	$(TD $(I type) $(B function)$(B $(LPAREN))$(I parameters)$(B $(RPAREN)))
	$(TD $(I type)$(B (*))$(B $(LPAREN))$(I parameters)$(B $(RPAREN)))
	)

	$(TR
	$(TD $(I type) $(B delegate)$(B $(LPAREN))$(I parameters)$(B $(RPAREN)))
	$(TD no equivalent)
	)

	)

	$(P 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.
	)

$(V2
<h2>Passing D Array Arguments to C Functions</h2>

	$(P 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.)

	$(TABLE1
	<caption>D And C Function Prototype Equivalence</caption>

	$(TR
	$(TH D type)
	$(TH C type)
	)

	$(TR
	$(TD $(I T)$(B *))
	$(TD $(I T)$(B []))
	)

	$(TR
	$(TD $(B ref) $(I T)$(B [)$(I dim)$(B ]))
	$(TD $(I T)$(B [)$(I dim)$(B ]))
	)

	)

	$(P For example:)

$(CCODE
void foo(int a[3]) { ... } // C code
)
---
extern (C)
{
  void foo(ref int[3] a); // D prototype
}
---
)

<h2>Calling printf()</h2>

	$(P 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 $(TT %.*s) format works perfectly:
	)

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

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

	$(P An improved D function for formatted output is
	$(CODE std.stdio.writef()).
	)

<h2>Structs and Unions</h2>

	$(P D structs and unions are analogous to C's.
	)

	$(P 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.
	)

	$(P D does not support bit fields. If needed, they can be emulated
	with shift and mask operations.
	$(LINK2 htod.html, htod) will convert bit fields to inline functions that
	do the right shift and masks.
	)

$(V1
<hr>
<h1>Interfacing to C++</h1>

	$(P D does not provide an interface to C++, other than
	through $(LINK2 ../COM.html, COM programming). Since D, however,
	interfaces directly to C, it can interface directly to
	C++ code if it is declared as having C linkage.
	)

	$(P D class objects are incompatible with C++ class objects.
	)
)

)

Macros:
	TITLE=Interfacing to C
	WIKI=InterfaceToC
	CATEGORY_SPEC=$0