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d++ - #include C and C++ headers in D files

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Goal

To directly #include C and C++ headers in D files and have the same semantics and ease-of-use as if the file had been #included from C or C++ themselves. Warts and all, meaning that C enum declarations will pollute the global namespace, just as it does "back home".

This work was supported by Symmetry Investments.

Example

// c.h
#ifndef C_H
#define C_H

#define FOO_ID(x) (x*3)

int twice(int i);

#endif

// c.c
int twice(int i) { return i * 2; }

// foo.dpp
#include "c.h"
void main() {
    import std.stdio;
    writeln(twice(FOO_ID(5)));  // yes, it's using a C macro here!
}

At the shell:

$ gcc -c c.c
$ d++ foo.dpp c.o
$ ./foo
$ 30

Open on run.dlang.io

C++ support

C++ support is currently limited. Including any header from the C++ standard library is unlikely to work. Simpler headers might, the probability rising with how similar the C++ dialect used is to C. Despite that, dpp currently does try to translate classes, templates and operator overloading. It's unlikely to work on production headers without judicious use of the --ignore-cursor and --ignore-namespace command-line options. When using these, the user can then define their own versions of problematic declarations such as std::vector.

Limitations

Success stories

Known project headers whose translations produce D code that compiles:

Compilation however doesn't guarantee they work as expected and YMMV. Please consult the examples.

Command-line arguments

It is likely that the header or headers need -I flags to indicate paths to be searched, both by this executable and by libclang itself. The --include-path option can be used for that, once for each such path.

Use -h or --help to learn more.

Details

d++ is an executable that wraps a D compiler such as dmd (the default) so that D files with #include directives can be compiled.

It takes a .dpp file and outputs a valid D file that can be compiled. The original can't since D has no preprocessor, so the .dpp file is "quasi-D", or "D with #include directives". The only supported C preprocessor directive is #include.

The input .dpp file may also use C preprocessor macros defined in the file(s) it #includes, just as a C/C++ program would (see the example above). It may not, however, define macros of its own.

d++ goes through the input file line-by-line, and upon encountering an #include directive, parses the file to be included with libclang, loops over the definitions of data structures and functions therein and expands in-place the relevant D translations. e.g. if a header contains:

uint16_t foo(uint32_t a);

The output file will contain:

ushort foo(uint a);

d++ will also enclose each one of these original #include directives with either extern(C) {} or extern(C++) {} depending on the header file name and/or command-line options.

As part of expanding the #include, and as well as translating declarations, d++ will also insert text to define macros originally defined in the #included translation unit so that these macros can be used by the D program. The reason for this is that nearly every non-trivial C API requires the preprocessor to use properly. It is possible to mimic this usage in D with enums and CTFE, but the result is not guaranteed to be the same. The only way to use a C or C++ API as it was intended is by leveraging the preprocessor.

Trivial literal macros however(e.g. #define THE_ANSWER 42) are translated as D enums.

As a final pass before writing the output D file, d++ will run the C preprocessor (currently the cpp binary installed on the system) on the intermediary result of expanding all the #include directives so that any used macros are expanded, and the result is a D file that can be compiled.

In this fashion a user can write code that's not-quite-D-but-nearly that can "natively" call into a C/C++ API by #includeing the appropriate header(s).

Translation notes

enum

For convenience, this declaration:

enum Enum { foo, bar, baz }

Will generate this translation:

enum Enum { foo, bar, baz }
enum foo = Enum.foo;
enum bar = Enum.bar;
enum baz = Enum.baz;

This is to mimic C semantics with regards to the global namespace whilst also allowing one to, say, reflect on the enum type.

Renaming enums

There is the ability to rename C enums. With the following C definition:

enum FancyWidget { Widget_foo,  Widget_bar }

Then adding this to your .dpp file after the #include directive:

mixin dpp.EnumD!("Widget",      // the name of the new D enum
                 FancyWidget,   // the name of the original C enum
                 "Widget_");    // the prefix to cut out

will yield this translation:

enum Widget { foo, bar }

Names of structs, enums and unions

C has a different namespace for the aforementioned user-defined types. As such, this is legal C:

struct foo { int i; };
extern int foo;

The D translations just use the short name for these aggregates, and if there is a name collision with a variable or function, the latter two get renamed and have a pragma(mangle) added to avoid linker failures:

struct foo { int i; }
pragma(mangle, "foo") extern export __gshared int foo_;

Functions or variables with a name that is a D keyword

Similary to name collisions with aggregates, they get an underscore appended and a pragma(mangle) added so they link:

void debug(const char* msg);

Becomes:

pragma(mangle, "debug")
void debug_(const(char)*);

Build Instructions

dub install dpp

After the instructions for your OS (see below), you can use this commands to run dpp:

dub run dpp -- yoursourcefilenamehere.dpp

Note: for a reproducible and cross-platform build environment, you can run setup-cpp with --llvm=11.0.0. This will set up LLVM 11.0.0 and the proper environment variables.

Windows

Install LLVM into C:\Program Files\LLVM\, making sure to tick the "Add LLVM to the system PATH for all users" option.

If libclang.lib was not found, put the lib folder of the llvm directory on the PATH.

Linux

If libclang is not installed, install libclang-10-dev with apt: sudo apt-get install -y -qq libclang-10-dev

If libclang.so was not found, link it using the following command (adjust the installation path and the llvm version):

sudo ln -s path_to_llvm/lib/libclang-12.so.1 /lib/x86_64-linux-gnu/libclang.so

MacOS

If using an external LLVM installation, add these to your ~/.bash_profile

LLVM_PATH="/usr/local/opt/llvm/" # or any other path
LLVM_VERSION="11.0.0"
export PATH="$LLVM_PATH:$PATH"
export SDKROOT=$(xcrun --sdk macosx --show-sdk-path)
export LD_LIBRARY_PATH="$LLVM_PATH/lib/:$LD_LIBRARY_PATH"
export DYLD_LIBRARY_PATH="$LLVM_PATH/lib/:$DYLD_LIBRARY_PATH"
export CPATH="$LLVM_PATH/lib/clang/$LLVM_VERSION/include/"
export LDFLAGS="-L$LLVM_PATH/lib"
export CPPFLAGS="-I$LLVM_PATH/include"
export CC="$LLVM_PATH/bin/clang"
export CXX="$LLVM_PATH/bin/clang++"

(adjust the clang version and the external llvm installation path.)

Then run source ~/.bash_profile

If libclang.dylib was not found, link it using the following command (adjust the installation path):

ln -s path_to_llvm/lib/libclang.dylib /usr/local/opt/llvm/lib/libclang.dylib