• Introduction
• Lexical
• Interpolation Expression Sequence
• Grammar
• Modules
• Declarations
• Types
• Properties
• Attributes
• Pragmas
• Expressions
• Statements
• Arrays
• Associative Arrays
• Structs and Unions
• Classes
• Interfaces
• Enums
• Type Qualifiers
• Functions
• Operator Overloading
• Templates
• Template Mixins
• Contract Programming
• Conditional Compilation
• Traits
• Error Handling
• Unit Tests
• Garbage Collection
• Floating Point
• D x86 Inline Assembler
• Embedded Documentation
• Interfacing to C
• Interfacing to C++
• Interfacing to Objective-C
• Portability Guide
• Named Character Entities
• Memory Safety
• Application Binary Interface
• Vector Extensions
• Better C
• ImportC
• Live Functions
• Windows Programming
• Glossary
• Legacy Code

Interfacing to C

D is designed to fit comfortably with a C compiler for the target system. It is able to call C functions directly without requiring wrapper functions.

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

TheImportC compiler extension allows importing or compiling.c files directly.

Bindings for popular C libraries can be found in thestandard library andpackage repository.

The rest of this page describes the manual, low-level side of interfacing with C.

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(constchar* string1,constchar* string2);

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 Cextended type modifiers in D. These are handled bylinkage attributes, such asextern (C).
• There is no volatile type modifier in D. To declare a C function that uses volatile, just drop the keyword 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 functionextern (C){void myfunc(int a,int b)    {        ...    }}

Storage Allocation

C code explicitly manages memory with calls tomalloc() andfree(). D allocates memory using the D garbage collector, so no explicit frees are necessary.

D can still explicitly allocate memory using core.stdc.stdlib.malloc() and core.stdc.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 the 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 core.stdc.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 DLLs, etc.

Data Type Compatibility

These equivalents hold for most 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.

For example:

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

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

Callingprintf()

printf can be directly called from D code:

import core.stdc.stdio;int main(){    printf("hello world\n");return 0;}

Printing values works as it does in C:

int apples;printf("there are %d apples\n", apples);

Correctly matching the format specifier to the D type is necessary. The D compiler recognizes the printf formats and diagnoses mismatches with the supplied arguments. The specification for the formats used by D is the C99 specification 7.19.6.1.

A generous interpretation of what is a match between the argument and format specifier is taken, for example, an unsigned type can be printed with a signed format specifier. Diagnosed incompatibilites are:

• incompatible sizes which may cause argument misalignment
• dereferencing arguments that are not pointers
• insufficient number of arguments
• struct, array and slice arguments are not allowed
• non-pointer arguments tos specifier
• non-Standard formats
• undefined behavior per C99

Strings

A string cannot be printed directly. But%.*s can be used:

string s ="betty";printf("hello %.*s\n",cast(int) s.length, s.ptr);

The cast toint is required.

size_t andptrdiff_t

These use thezu andtd format specifiers respectively:

import core.stdc.stdio : printf;int* p =newint, q =newint;printf("size of an int is %zu, pointer difference is %td\n",int.sizeof, p - q);

Non-Standard Format Specifiers

Non-Standard format specifiers will be rejected by the compiler. Since the checking is only done for formats as string literals, non-Standard ones can be used:

constchar* format ="value: %K\n";printf(format, value);

Modern Formatted Writing

An improved D function for formatted output isstd.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 #pragmas. 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, or use thestd.bitmanip.bitfields library type.htod will convert bit fields to inline functions that do the right shift and masks.

D does not support declaring variables of anonymous struct types. In such a case, define a named struct in D and make it private:

union Info  // C code{    struct    {        char *name;    } file;};

union Info// D code{privatestruct File    {char* name;    }    File file;}

Callbacks

D can easily call C callbacks (function pointers), and C can call callbacks provided by D code if the callback is anextern(C) function, or some other linkage that both sides have agreed to (e.g.extern(Windows)).

Here's an example of C code providing a callback to D code:

void someFunc(void *arg) { printf("Called someFunc!\n"); }  // C codetypedef void (*Callback)(void *);extern "C" Callback getCallback(void){    return someFunc;}

extern(C)alias Callback =intfunction(int,int);// D codeextern(C) Callback getCallback();void main(){    Callback cb = getCallback();    cb();// invokes the callback}

And an example of D code providing a callback to C code:

extern "C" void printer(int (*callback)(int, int))  // C code{    printf("calling callback with 2 and 4 returns: %d\n", callback(2, 4));}

extern(C)alias Callback =intfunction(int,int);// D codeextern(C)void printer(Callback callback);extern(C)int sum(int x,int y) {return x + y; }void main(){    printer(&sum);}

For more info about callbacks read theclosures section.

Using Existing C Libraries

Since D can call C code directly, it can also call any C library functions, giving D access to the smorgasbord of existing C libraries. To do so, however, one needs to write a D interface (.di) file, which is a translation of the C .h header file for the C library into D.

For popular C libraries, the first place to look for the corresponding D interface file is theDeimos Project. If it isn't there already, please write and contribute one to the Deimos Project.

Accessing C Globals

C globals can be accessed directly from D. C globals have the C naming convention, and so must be in anextern (C) block. Use theextern storage class to indicate that the global is allocated in the C code, not the D code. C globals default to being in global, not thread local, storage. To reference global storage from D, use the__gshared storage class.

extern (C)extern__gsharedint x;