D programming Language

Calling C Functions

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 string;

int myDfunction(char[] s)

{

return strcmp(string.toCharz(s), "foo\0");

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.
  

// 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 c.stdlib.malloc() and 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 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 gc.addRoot() or 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 type	C type
void	void
bit	no equivalent
byte	signed char
ubyte	unsigned char
char	char (chars are unsigned in D)
wchar	wchar_t
short	short
ushort	unsigned short
int	int
uint	unsigned
long	long long
ulong	unsigned long long
float	float
double	double
extended	long double
imaginary	long double _Imaginary
complex	long double _Complex
type*	type *
type[dim]	type[dim]
type[]	no equivalent
type[type]	no equivalent
"string\0"	"string" or L"string"
class	no equivalent
type(*)(parameters)	type(*)(parameters)

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.

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