Have your eyes set on the perfect C library for your project? Can't find a wrapper for it in Nim? Look no further! Futhark aims to allow you to simply import C header files directly into Nim, and allow you to use them like you would from C without any manual intervention. It's still in an alpha state, but it can already wrap many complex header files without any rewrites or pre-processing.

import futhark

# Tell futhark where to find the C libraries you will compile with, and what
# header files you wish to import.
importc:
  path "../stb"
  define STB_IMAGE_IMPLEMENTATION
  "stb_image.h"

# Tell Nim how to compile against the library. If you have a dynamic library
# this would simply be a `--passL:"-l<library name>`
static:
  writeFile("test.c", """
  #define STB_IMAGE_IMPLEMENTATION
  #include "../stb/stb_image.h"
  """)
{.compile: "test.c".}

# Use the library just like you would in C!
var width, height, channels: cint

var image = stbi_load("futhark.png", width.addr, height.addr, channels.addr, STBI_default.cint)
if image == nil:
  echo "Error in loading the image"
  quit 1

echo "Loaded image with a width of ", width, ", a height of ", height, " and ", channels, " channels"
stbi_image_free(image)

So are all C wrappers now obsolete?

Not quite. Futhark only tells you what the C headers define and allows you to use them. This means that the interface is still very C-like. A lot of great Nim wrappers will take a C library and wrap it into something that is a little more simple to use from Nim land. But Futhark can definitely be used to help with wrapping C libraries. Since it reads the C files directly you are guaranteed that all the types match up with their C counterparts, no matter what platform you're on, or what defines you want to pass. This is a huge benefit over hand-wrapped code. Futhark and Øpir will also cache their results, so after the initial compilation it's just as fast to use as it simply grabs the pre-generated Nim file from the cache. Both files could of course also be edited or included as-is in a project if you want users to not have to run Øpir or Futhark themselves.

How does it work?

Basically Futhark comprises of two parts, a helper program called Øpir (or opir just to ensure that it works everywhere) and a module called futhark that exposes a importc macro. Øpir is compiled with libclang and uses Clang to parse and understand the C files, it then creates a big JSON output of everything that is defined in the headers with Nim friendly types. The macro then reads this file and applies any overrides to types and names before it generates all the Nim definitions.

Basic usage

The four main things you need to know to use Futhark is sysPath, path, compilerArgs, and normal imports (the "stb_image.h" part in the above example).

• sysPath denotes system paths, these will be passed to Øpir to make sure Clang knows where to find all the definitions. This can also be passed with -d:sysPath:<path 1>:<path 2> if you want to automatically generate these.
• path denotes library paths, these will also be passed to Øpir, but anything found in these paths which is used by the files you have in your project will be wrapped by Futhark as well.
• comppilerArgs specifies additional flags that should be passed to Clang when parsing the C headers.
• Files listed in quotes in the importc are equivalent to #include "file.h" in C. Futhark will generate all definitions in these files, and if file.h imports more files found in any of the paths passed in by path these files will also be imported.

Note: The difference between sysPath and path is simply about how Futhark handles the files. sysPath are paths which are fed to Øpir and Clang in order to make Clang able to read all the types. path are the paths Futhark takes into account when generating definitions. This difference exists to make sure Futhark doesn't import all kinds of low-level system stuff which is already available in Nim. A subpath of sysPath can be passed in with path without trouble. So sysPath "/usr/include" followed by path "/usr/include/X11" is fine and Futhark will only generate code for the explicitly mentioned files, and any files it requires from /usr/include/X11.

Hard names and overrides

Nim, unlike C, is case and underscore insensitive and doesn't allow you to have identifiers starting with _ or __, or identifiers that have more than one consecutive _ in them. Nim also has a set of reserved keywords like proc, addr, and type which would be inconvenient to have as names. Because of this Futhark will rename these according to some fairly simple rules.

Since this, along with Nims style-insensitivity, means that some identifiers might still collide, the name will further have the kind appended, and if it still collides it will append the hash of the original identifier. This shouldn't happen often in real projects and exists mostly to create a foolproof renaming scheme. Note that struct and union types also get a prefix, this is normally resolved automatically by C typedef-ing the struct struct_name to struct_name_t, but in case you need to use a struct struct_name type just keep in mind that in Nim it will be struct_struct_name.

If you want to rename an object or a field you can use the rename directive. Simply put rename <from>, <to> along with your other options. <from> can be either just an object name (before any other renaming) as a string or ident, or a field in the format <object>.<field> both the original C names either as two identifiers, or the whole thing as a single string. <to> is always a single identifier and is the new name.

If you want to implement more complex renaming you can use the renameCallback directive and pass in a callback function that takes the original name, a string denoting what kind of identifier this is, and an optional string denoting which object or procedure this identifier occurs in, and which returns a new string. This callback will be inserted into the renaming logic and will be called on the original C identifier before all the other rules are applied.

Redefining types

C tends to use a lot of void pointers, pointers to characters, and pointers to a single element which is supposed to be a collection of said element. In Nim we like to be a bit more strict about our types. For this you can use the retype directive. It takes the form retype <object>.<field>, <Nim type> so for example to retype the C array type defined as some_element* some_field to an indexable type in Nim you can use retype some_object.some_field, ptr UncheckedArray[some_element]. The names for the object and field are both their renamed Nim identifiers.

If you need to redefine an entire object, instead of just specific fields Futhark by default also guards every type and procedure definiton in simple when declared(SomeType) statements so that if you want to override a definition you can simply define your type before the importc macro invocation and Futhark won't override your definition. It is up to you however to ensure that this type actually matches in size and layout with the original C type.

Compatibility features and readability

Futhark by default tries to ensure the highest amount of compatibility with pre-wrapped libraries (e.g. the posix standard library module) and other user code. Because of this the output which Futhark generates isn't very pretty, being littered with when defined statements and weird numbered identifiers for renaming things. These features are intended to make Futhark easier to use in a mostly automatic fashion, but you might not need them. If you want to read the generated output, build documentation of a Futhark generated module, or possibly get an improved editor experience you might want to disable some of these features for a prettier, more readable output.

There are basically three things you can control with define switches:

Object rename/override functionality

This declares types in such a way that earlier declarations by the same name will not be overriden or collide. With this feature you can declare an object, function, enum, etc. before the call to Futhark and these declarations will be used instead of the auto-generated ones. This is also what enables the feature at the end of the "Redifining types" section. Disabling this feature will remove all of the when declared but you might run into Futhark trying to redeclare existing things, including built in types and names.

Opaque type functionality

If a type is not fully declared in your C headers but is still required for your project Futhark will generate a type SomeType = distinct object dummy type for it. Since most C libraries will pass things by pointer this makes sure that a ptr SomeType can exist and be passed around without having to know anything about SomeType. Disabling this feature will remove these definitions but might mean some procedure definitions now have invalid parameters or return types. This is mostly useful in conjunction with nodeclguards and manually declaring these types.

Hiding symbols functionality

To avoid editors showing the renamed identifiers used by the object rename/override functionality they are hidden by default. If however you want to generate documentation for a Futhark generated module these fields won't be visible and the documentation mostly useless. With exportall these symbols will be exported as well and documentation will be readable. This is mostly useful if you want to export documentation but can't use nodeclguards (which makes even more readable documentation).

Inline functions

When using Futhark with dynamic libraries it doesn't make sense to wrap inline functions. However if you are compiling your code directly against some C code these might be useful to you. In this case you can pass -d:generateInline to generate function definitions for inline functions.

Pre-ANSI C function declarations

Also known as K&R style functions. By definition C code like

int* myfunc();

is a pre-ansi C function declaration that says myfunc returns a pointer to an integer and takes any number of arguments. This last part is a historic thing you can read more about here, but suffice to say this is misused in quite a lot of C libraries to mean that the function takes no arguments. Since this is fairly obscure, and Nim will create bad C code if the varargs pragma is attached to a function without arguments this is ignored by default. However if you for some reason require this you might add -d:preAnsiFuncDecl while compiling.

Deeper control

In case you face issues that aren't easily solveable there is one last option for making modifications, and this is Øpir hooks. Since Øpir converts your C imports to a JSON format you're able to register hooks that will be run before Futhark consumes this JSON. These are simple procedures which takes a JsonNode and returns a JsonNode. With this you're able to change every aspect of the JSON, and even add or remove definitions. The callbacks are a list, so modules to perform certain commonly done transformations (e.g. combine similarly named constants into an enum) could be added to the list of callbacks easily. To add these simply add in outputPath <procedure name> to your importc block.

Destructors

If you are using a C library you will probably want to wrap destructor calls. Futhark makes all C objects {.pure, inheritable.} which means you can quite easily use somewhat idiomatic Nim to achieve destructors.

An example usecase from MiniAudio bindings is as follows:

type TAudioEngine = object of maEngine # Creates a new object in this scope
                                       # maEngine is a type wrapped by Futhark
                                       # TAudioEngine can now have a destructor
                                       # attached to it

proc `=destroy`(engine: var TAudioEngine) = # Define a destructor as normal
  maEngineUninit(engine.addr)

Dynamic libraries and implementing headers

If you are making a dynamic or static library to be loaded or linked with another program you are often given a header file to implement. This file typically includes the types and functions you are able to call from the main program, along with the procedures that your application has to implement in order to be loaded and run correctly. Futhark normally imports all headers on the assumption that things will be made available from C while compiling, ie. it adds the importc pragma to them. But in order to support this scenario you can also get it to create forward declarations with the exportc pragma. This allows Nim to know that there has to exist an implementation for a given procedure in your application, and as such will fail to compile if you're missing an implementation. It will also make sure that your signature is exported correctly and matches the intended C header. To do this simply define the procedures to be forward declared like this along with your other options:

importc:
  forward "proc_to_forward"

Futark will automatically add the dynlib pragma to this declaration if you're buildin with --app:lib, but if you need to add more pragmas you can list them after the name like so:

macro customPragma(msg: static[string], body: untyped): untyped =
  echo "Saying: ", msg
  return body

importc:
  forward "proc_to_forward", customPragma("Hello world"), used

If you want to see what code Futhark generated for your forward declarations, and therefore the signature you need to match (even the argument names), you can pass -d:echoForwards and they will be written out in the terminal while compiling.

NOTE: Since the forward declaration has all the pragmas for passing it on as C compatible symbols you don't actually need to have these pragmas attached to your implementation which makes it a bit cleaner. And the procedure can of course be written in camelCase if you prefer, it will still match the forward declaration.

Shipping wrappers

If you've built wrappers with Futhark, and expanded them with a Nim interface and now it's time to share them. This section will give some best-practices on how to ship wrappers. Since the Øpir tool requires Clang to be installed it can be a bit tricky to get Futhark installed. In addition to this Futhark obviously requires access to the C header files, which might be installed in different places based on the system. Because of these things you might not want to have Futhark as a dependency for your bindings. To help with this Futhark has a outputPath argument which can be added to the importc block. This path is where the completed bindings will be stored, and also where Futhark will look for existing bindings to avoid rebuilding them. This means that with the outputPath set to a file you will need to use -d:futharkRebuild to update the file when you make changes in the importc block. If you set outputPath to a folder then futhark will store the files with the appended hash in this folder instead of in the nimcache folder and caching will work as usual. By using this feature you will be able to set a path local to your project and check the generated Futhark file into your version control system. But that is only half the job, because to be aware of the cache file Futhark still needs to be installed. The recommended way to get around this is to do a when defined(useFuthark) switch to check whether the user wants to use Futhark directly or to use the shipped wrapper. It is recommended to use the exact name useFuthark, this way the user can turn on Futhark for the entire project (in case you have imported another library which also uses Futhark). If you want to give the user the option to switch on Futhark for only your project it is recommended to use an additional switch with useFutharkFor<project name>.

A complete sample would look a bit something like this:

when defined(useFuthark) or defined(useFutharkForExample):
  import futhark, os

  importc:
    outputPath currentSourcePath.parentDir / "generated.nim"
    path "<path to library>"
    "libfile.h"
else:
  include "generated.nim"

Keep in mind that when your package is installed the generated Futhark output would be placed in the folder of your package using this code. If the / "generated.nim" part is left of then the file would be named futhark_<hash>.nim as described above, this means that your include could use the one specified in your package installation, while users doing useFuthark would generate one based on its hash (or reuse yours if the hash matches).

But why not use c2nim or nimterop?

Both c2nim and nimterop have failed me in the past when wrapping headers. This boils down to how they are built. c2nim tries to read and understand C files on its own, something which might appear simple, but C is notoriously hard to parse and c2nim fails on macros and other slightly complex things. nimterop uses treesitter and performs slightly better. It is theoretically able to parse all C syntax, but the C semantics is still up to nimterop to implement. Which means it can't do macros or things like IFDEF automatically.

Futhark on the other hand uses clang, which is very good at both understand C syntax, but also C semantics. This means that it resolves all macros and IFDEF statements, and just gives us the definitions for everything. This means much less work in actually trying to understand C, which means that all this work can be spent on quality Nim translation.

Sounds great, what's the catch?

Futhark is currently in an beta state, it works really well but you might run into occasional bugs or hickups. It also doesn't support C++ at the moment, and it doesn't understand things like function-style macros. It might also mess up on strange C things which haven't been encountered yet, although this is more and more rare as people use it. All of these shortcomings are things I hope to get fixed up over time.

Installation

To install Futhark you first need to have clang installed. Installing clang on Linux is as simple as just grabbing it from your package manager (e.g. sudo apt install clang libclang-dev). To install clang on Windows you need to install LLVM (you probably want to grab the LLVM-15.0.7-win64.exe version). To install clang on macOS, run xcode-select --install in the terminal. Opir should then detect it automatically. Have a look at opir.nims if you're curious how the Windows and macOS detection works.

If you have Clang installed in your system path you can now simply run:

nimble install futhark

Otherwise you need to tell Opir how to link with libclang. Do this by either copying the libclang.lib and libclang.dll into the Futhark project dir or use passL to pass the folder that libclang.lib (or libclang.so on Linux machines) lives in to the linker:

nimble install --passL:"-L<path to lib directory containing libclang.so file>" futhark
#e.g.: nimble install --passL:"-L/usr/lib/llvm-6.0/lib" futhark

TODO

• Proper handling of C macros (inherently hard because C macros are typeless)
• Find way to not require C compiler include paths