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https://github.com/PMunch/futhark

Automatic wrapping of C headers in Nim
https://github.com/PMunch/futhark

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Automatic wrapping of C headers in Nim

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README 

        
![Futhark logo](https://github.com/PMunch/futhark/blob/master/futhark.png)



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 a beta state, but it

already wraps most header files without any rewrites or pre-processing and has

successfully been used to wrap complex projects.



```nim

import futhark, strutils



# Remove the `stbi_` prefix since Nim doesn't struggle as much with collisions as C

proc renameCb(n, k: string, p = ""): string = n.replace "stbi_", ""



# 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 # This define is required by the STB library

  renameCallback renameCb

  rename FILE, CFile # Rename `FILE` that STB uses to `CFile` which is the Nim equivalent

  "stb_image.h"



# Tell Nim how to compile against the library. If you have a dynamic library

# this would simply be a `--passL:"-l`

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 = load("futhark.png", width.addr, height.addr, channels.addr, STBI_default.cint)

if image.isNil:

  echo "Error in loading the image"

  quit 1



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

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

A lot of people coming from other wrapping tools start out a bit confused about

Futhark. With other tools it is common to wrap things into a Nim module first,

then import that module into your program. This is not the way Futhark is

supposed to be used. With Futhark you handle your C imports in the `importc`

block and just import the module in which you have that block. This is to ensure

that all defines, configurations, and platform specific things will match up

between your code and the C code. It is however possible to use Futhark to

generate a wrapper, this is detailed in the "Shipping wrappers" section of this

README.



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::` if you want to automatically generate these. By

  default Futhark tries to find the `sysPath` automatically and you don't need

  to specify this yourself.

- `path` denotes library paths, these will also be passed to Øpir, but anything

  found in these files which is used by anything in the explicitly imported

  files will be wrapped by Futhark as well.

- `compilerArgs` 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 definitions found in these paths. `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 or ending with `_`, 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.



| Name issue       | Nim rename                                          |

| ---------------- | --------------------------------------------------- |

| struct type      | `struct_` prefix                                    |

| union type       | `union_` prefix                                     |

| enum type        | `enum_` prefix                                      |

| `_` prefix       | `internal_` prefix                                  |

| `__` prefix      | `compiler_` prefix                                  |

| `_` postfix      | `_private` postfix                                  |

| `__` in name     | All consecutive underscores collapsed into one      |

| Reserved keyword | Append kind to name, `proc`, `const`, `struct` etc. |



Apart from this the name is kept as it appears in the C sources, but note that

because of Nims style insensitivity you can still call `some_proc` as `someProc`

without any issues. This renaming scheme, along with Nims style-insensitivity,

does however mean that some identifiers might collide. In this case 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 , ` along with your other options. `` can be

either just an object name (before any other renaming) as a string or ident, or

a field in the format `.` both the original C names either as

two identifiers, or the whole thing as a single string. `` 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 ., ` 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:



| Define        | Effect                                                      |

| ------------- | ----------------------------------------------------------- |

| nodeclguards  | Disables the object rename/override functionality           |

| noopaquetypes | Disables opaque types used for unknown objects              |

| exportall     | Exports all fields (including renamed ones)                 |



### 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 = 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

```c

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](https://jameshfisher.com/2016/11/27/c-k-and-r/),

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

### Rename callback

If you want to rename identifiers, for example removing common prefixes,

postfixes, or other similar things you can use a rename callback. This can be

defined by adding `renameCallback ` to your `importc` block. The

signature of the callback is:

```

proc(name: string, kind: string, partof = ""): string

```

where `name` is the original name as present in the C code, `kind` is the kind

of the identifier such as "enum", "proc", "arg", "field", etc. The argument

`partof` is only used for field names in a structure.



Please note that in order to avoid name collisions the normal anti-collision

transformations are done after the rename callback.



### Øpir callbacks

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 `addOpirCallback ` 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:

```nim

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:



```nim

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:



```nim

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

You've built wrappers with Futhark, expanded them with a nice Nim interface and

it's time to share them with the world! 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 an `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`.



A complete sample would look a bit something like this:



```nim

when defined(useFuthark) or defined(useFutharkForExample):

  import futhark, os



  importc:

    outputPath currentSourcePath.parentDir / "generated.nim"

    path ""

    "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_.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).



## Project mode

Futhark is best used when it is allowed to see the C files while compiling. This

allows it to make sure that all platform specific stuff matches up perfectly.

However this is not always possible, or for whatever other reason wanted. One

scenario where this is the case is for embedded projects. The Futhark macro

doesn't work when targeting platforms without proper OS support, so using it the

recommended way is simply not possible.



Because of this it is also possible to use Futhark in "project mode". In project

mode you simply don't specify any files to import. Instead you only specify

which `path`'s you want to search. Futhark will then recreate the original

folder structure of the header files it finds and generate Nim files in their

place. These try to properly import and export each other so the behaviour is as

similar to C and if Futhark finds `.c` files next to the `.h` files of the same

name it also adds `{.passL:"-I".}` and `{.compile: "".}` pragmas to

automatically import and compile the required C files. This means that a

separate dummy wrapper project can be set up which just generates this folder

structure, and the main project can simply import files from this folder as if

they where normal modules.



While this is quite a nice feature it isn't as bullet-proof as the traditional

way to use Futhark. It might require some pre-processing of the C project, and

it often means more work with getting everything compiled properly. But when

normal Futhark usage isn't possible or desired this is a decent option.



To minimize the pre-processing burden this mode also adds a new option `ignore`

to the `importc` block. This is simply a file or folder to ignore while

traversing the given paths. Futhark will not try to generate any input for this

file/folder, but it will look at the headers inside to resolve other parts of

the project. This is mainly useful if you have folders with examples or other

code you're not actually supposed to import.



# 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. 

(Debian-based distributions: `sudo apt install clang libclang-dev`,

Void Linux: `sudo xbps-install clang clang17-devel libclang`). 

To install clang on Windows you need to install

[LLVM](https://github.com/llvm/llvm-project/releases/tag/llvmorg-15.0.7) (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](src/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" futhark

#e.g.: nimble install --passL:"-L/usr/lib/llvm-6.0/lib" futhark

```



# Known wrappers made with Futhark

By the nature of Futhark wrappers can be made on the fly for a specific project.

I've wrapped many large C projects with Futhark, but just for use in specific,

and sometimes proprietary, codebases. That being said this list contains

wrappers that are made with Futhark which have been published in some shape or

form. These can very from just a simple file with some Futhark instructions, to

full projects that add nice-to-have features on top of an existing C library. If

you have a project of your own you want to show off, please make a PR!



- [MAPM](https://github.com/PMunch/mapm-nim): Fully wrapped with destructors and a nice Nim interface

- [libfuse](https://github.com/PMunch/libfuse-nim/blob/master/libfuse.nim): Quick and dirty wrapper for libfuse to play around with filesystems

- [VapourSynth](https://github.com/mantielero/vs.nim): Uses Futhark to create the initial wrapper, then builds a nice Nim library on top

- [python_nim_libmem](https://github.com/Hypnootika/python_nim_libmem): Futhark and helper scripts to generate Nim and Python bindings for "Libmem"

- [box2d.nim](https://github.com/jon-edward/box2d.nim): Nim bindings for Erin Catto's Box2D physics engine. 



## TODO

- Proper handling of C macros (inherently hard because C macros are typeless)

- Find way to not require C compiler include paths