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C.scala: A Shallow DSL for C

This project defines a Scala library for the core C language and some useful C libraries. It also provides implementations of Scala collections such as HashMap or List in the C DSL.

Installation

The shallow C definitions are accompanied by implementations which use the JNI. sbt compile will take care of generating and compiling the required C code, provided that:

  • GLib is installed on your system. You can check this by making sure that pkg-config --list-all | grep glib prints something.

  • clang is installed on your system (which clang should not return an error).

  • The JNI headers for your system are as specified in project/Build.scala.

Please note that this has been tested only on Mac OS 10.9+.

The core C language

The functionality of the C language itself is provided in the cscala.CLang package. This includes functions such as sizeof, address-taking and dereferencing.

This functionality is contained in the cscala.CLang package. The following import is also needed in order to use it:

import cscala.CLangTypes._

To use the JNI implementations, the calling code needs to load the library generated during compilation:

System.loadLibrary("shallow")

Pointers

C.scala deals with pointers by providing the Scala type Pointer[T]. Pointers to values can be obtained using the & method and those pointers can be dereferenced using the * method:

val x: Int = 5
val px: Pointer[Int] = &(x)
println(*(x))
// => 5

Note: currently, calling & on the same value twice will return pointers to different copies of the value.

It is possible to perform pointer arithmetic using the + and - methods. Assignment is possible using pointer_assign or update. For example, using malloc from the cscala.StdLib package it is possible to write array-like operations:

val array: Pointer[Int] = malloc[Int](5) // Allocates space for 5 integers
for (i <- 0 until 5) {
  array(i) = i
}

println(*(array + 3))
// => 3

Structs

It is possible to declare and use structs with C.scala, but declaration is currently cumbersome. A struct is declared by declaring both a subclass of CStruct and an instance of CStructInfo[T] for your new struct T. For example:

class CTimeVal(val bytes: Array[Byte]) extends CStruct {
  def this(sec: Long, usec: Long) =
    this(ByteBuffer.allocate(16).order(ByteOrder.LITTLE_ENDIAN).putLong(sec).putLong(usec).array)
}
implicit val CTimeValInfo: CStructInfo[CTimeVal] = new CStructInfo[CTimeVal] {
  val sizes = List(
    'tv_sec -> sizeof[Long],
    'tv_usec -> sizeof[Long])
  def create(bytes: Array[Byte]) = new CTimeVal(bytes)
}

Structs can be created using malloc, pointer_assign and +, or by simply creating a new instance of the CStruct. Member selection in pointers to structs can be done using the -> method:

val tv = new CTimeVal(12 ,45)
val p = &(tv)
println(->[CTimeVal, Long](p, 'tv_sec))
// => 12

Casts

C programmers expect to be able to cast between types without runtime errors. Since Scala casts will fail at runtime when attempting, for example, a cast from a Pointer[Int] to a Int, C.scala provides an alternative way to cast via the as method. as[Int](p: Pointer[Int]) will cast the pointer to an int (the underlying address pointed to) without failing at runtime.

C libraries

A handful of useful C libraries are provided by C.scala. Currently functions in the following libraries are provided:

  • stdlib.h via the cscala.CStdLib package (only free and malloc).
  • stdio.h via the cscala.CStdIO package.
  • sys/time.h via the cscala.CSysTime package (only gettimeofday, timeval_subtract and the timeval struct
  • string.h via the CString pacakge.

Some collections from GLib are also provided:

To use the GLib libraries, the following import is needed:

import cscala.GLibTypes._

Scala collections

The libraries presented above can be used to prototype C implementations of Scala collections. This has already been done for a few collections:

  • HashMaps in cscala.collections.CHashMap.
  • ArrayBuffers in cscala.collections.CArrayBuffer.
  • Lists in cscala.collections.CList.
  • Sets in cscala.collections.CSet (partially).
  • TreeSets in cscala.collections.CTreeSet (partially).

To use these collections, the following import is needed:

import cscala.collections.CCollectionsTypes._

Issues

  • Struct declaration and selection should be cleaned up so that -> no longer needs type parameters all the time, and declaration can be done more easily (e.g. just a case class).

  • The -> method should be on the PointerOps implicit class.

  • Casting is only partly implemented.

  • Varargs functions such as printf aren't implemented.