Otter

OS	arch	Build Status
Ubuntu 20.04	x86_64
macOS 11 Big Sur	x86_64

Dependencies

• pkg-config (for finding libffi)
• libffi-dev

For macOS, libffi can be installed by HomeBrew

brew install libffi

For debian/ubuntu, libffi can be installed using the following command

sudo apt update
sudo apt install libffi-dev

Type Correspondences

Generally, we can extern a function using the following syntax

extern func_name(:return_type)
extern func_name(:return_type, arg_type, ...)
extern func_name(:return_type, arg_name :: arg_type, ...)

Note that arg_name is optional, which means rule 2 and 3 can be rewritten as

extern func_name(:return_type, [arg_name :: ]arg_type, ...)

and if we want to further simplify (or complify?) it, we have

extern func_name(:return_type[, [arg_name :: ]arg_type, ...])

Function return type

Basic types

defmodule Foo do
  import Otter, except: [{:&, 1}]

  # see their implementations in test/test.cpp
  extern pass_through_u8(:u8, val :: u8)
  extern pass_through_u16(:u16, val :: u16)
  extern pass_through_u32(:u32, val :: u32)
  extern pass_through_u64(:u64, val :: u64)
  extern pass_through_s8(:s8, val :: s8)
  extern pass_through_s16(:s16, val :: s16)
  extern pass_through_s32(:s32, val :: s32)
  extern pass_through_s64(:s64, val :: s64)
  extern pass_through_f32(:f32, val :: f32)
  extern pass_through_f64(:f64, val :: f64)
  extern pass_through_c_ptr(:u64, ptr :: c_ptr)
end

We'll use {T} to indicate any basic types from now on. For example, {T}-size(42) could be u32-size(42).

ND-array types

ND-array is not supported as a function argument yet. It can be only used in structs (see below) for now.

We'll use {NDA} to indicate any ND-array types from now on.

Struct types

To declare C structs, we'll have to use the cstruct macro.

We'll use {FT} to indicate the type of a field in the struct.

{FT} = {T}
       | {NDA}

Say you have a struct named name,

Todo

• [ ] Create struct instances using c_struct. Maybe merge code in c_struct to here?

Demo

defmodule Ctypes do
  import Otter, except: [{:&, 1}]

  # module level default shared library name/path
  @default_from (case :os.type() do
                   {:unix, :darwin} -> "libSystem.B.dylib"
                   {:unix, _} -> "libc.so"
                   {:win32, _} -> raise "Windows is not supported yet"
                 end)
  # module level default dlopen mode
  @default_mode :RTLD_NOW

  # specify shared library name and/or load mode for a single function
  @load_from (case :os.type() do
                {:unix, :darwin} -> "libSystem.B.dylib"
                {:unix, _} -> "libc.so"
                {:win32, _} -> raise "Windows is not supported yet"
              end)
  @load_mode :RTLD_NOW
  extern sin(:f64, f64)

  # or using module level default shared library name and load mode
  extern puts(:s32, c_ptr)
  extern dlopen(:c_ptr, c_ptr, s32)
  extern dlsym(:c_ptr, c_ptr, c_ptr)

  # explict mark argument name and type
  extern cos(:f64, theta :: f64)
  
  # also support functions with variadic arguments
  extern printf(:u64, fmt :: c_ptr, args :: va_args)
end

# one extern will define two function, 
# - one returns ok-error tuple, 
# - the other is the bang version, which returns unwrapped value on :ok, and raise RuntimeError on :error  

iex> CtypesDemo.puts("hello \r")
hello
{:ok, 10}
iex> CtypesDemo.puts!("hello \r")
hello
10
iex> CtypesDemo.printf!("world!\r\n")
world!
9
iex> CtypesDemo.sin(3.1415926535)
{:ok, 8.979318433952318e-11}
iex> CtypesDemo.sin!(3.1415926535)
8.979318433952318e-11
iex> CtypesDemo.cos!(0)
1.0
iex> CtypesDemo.cos!(0.0)
1.0
iex> CtypesDemo.printf!("%s-%.5lf-0x%08x-%c\r\n\0", [
...>   as_type!("hello world!\0", :c_ptr),
...>   as_type!(123.456789, :f64),
...>   as_type!(0xdeadbeef, :u32),
...>   as_type!(65, :u8)
...> ])
hello world!-123.45679-0xdeadbeef-A
37
iex> handle = CtypesDemo.dlopen!("/usr/lib/libSystem.B.dylib", 2) # or "libc.so" for Linux
20152781936
iex> dlsym_addr = CtypesDemo.dlsym!(handle, "dlsym")
7023526352

Note that we have CtypesDemo.dlopen and CtypesDemo.dlsym in the demo code above. They are declared in the examples/ctypes_demo.ex file.

  extern dlopen(:c_ptr, c_ptr, s32)
  extern dlsym(:c_ptr, c_ptr, c_ptr)

And they are different from the ones in module Otter, namely, Otter.dlopen and Otter.dlsym. CtypesDemo.dl* are obtained by Otter.dlopen and Otter.dlsym.

Just like the sin and cos functions in CtypesDemo, dlopen and dlsym are also C functions that can be dlsym'ed.

Otter.dl* calls go to NIFs otter_dl* functions while CtypesDemo.dl* calls going to Otter.invoke which redirects to the otter_invoke NIF.

Support for C struct

basic example

defmodule Foo do
  import Otter
  
  @default_from Path.join([__DIR__, "test.so"])
  @default_mode :RTLD_NOW
  
  # #pragma pack(push, 4)
  # struct alignas(4) s_u8_u16 {
  #     uint8_t u8;
  #     uint16_t u16;
  # };
  # #pragma pack(pop)
  cstruct(s_u8_u16(u8 :: u8, u16 :: u16))

  # please see test/test.cpp for these extern functions
  extern create_s_u8_u16(s_u8_u16())
  extern receive_s_u8_u16(:u32, t :: s_u8_u16())
end

nd-array in C struct

defmodule Foo do
  import Otter

  @default_from Path.join([__DIR__, "test.so"])
  @default_mode :RTLD_NOW
  
  # struct matrix16x16 {
  #     uint32_t m[16][16];
  # };
  cstruct(matrix16x16(m :: u32-size(16, 16)))

  # please see test/test.cpp for these extern functions
  extern create_matrix16x16(matrix16x16())
  extern receive_matrix16x16(:u32, t :: matrix16x16())
end

Installation

If available in Hex, the package can be installed by adding otter to your list of dependencies in mix.exs:

def deps do
  [
    {:otter, "~> 0.1.0"}
  ]
end

Documentation can be generated with ExDoc and published on HexDocs. Once published, the docs can be found at https://hexdocs.pm/otter.