Calling C Functions
Starting from the 1.3 beta version, Luna comes equipped with the ability to call foreign C functions from object files. This document describes how to use this system to create C based libraries.
Object files contain compiled code that can be dynamically loaded and used by other programs. The most common types are
.so files on Linux,
.dylib files on macOS and
.dll on Windows.
Luna is able to work with object files coming from two sources:
- System-wide available dynamic libraries. If a library is installed on your machine, Luna will be able to use it. Unfortunately, this requires all the users of your library to have the dynamic library installed in their systems. For most libraries this is not a good solution, it will, however, work fine for bindings to the most popular libraries and for quick prototyping.
- Object files inside the project tree. You can drop any shared object file into the
native_libs/PLATFORMdirectory inside your project (
windows) and use it from Luna. This is a great solution for making sure users are able to use your library without any external dependencies. It's also the only way to include your own C extension or wrapper.
Using a shared library
[info] Changes ahead!
This describes the bare-bones way of interacting with foreign objects. There are some syntactic changes coming soon to cut down on boilerplate. The basic concepts, however, will remain unchanged.
OK, so you have your shared library in place, how to actually use it from Luna? For this example, we'll assume that we're working with the following directory structure:
MyProject/ MyProject.lunaproject src/ Main.luna native_libs/ macos/ libawesome.dylib linux/ libawesome.so windows/ libawesome.dll
The object files in this example contain just a single function, with the following C signature:
How do we use it?
Resolving the symbol
The workhorse of our FFI (Foreign Function Interface) is the
lookupSymbol function from
Std.Foreign module. It takes a shared object file name and a name of the symbol to lookup, and returns a
FunPtr object. The library resolution takes into account the most common naming conventions, so that it is possible to use the same call with a file named
awesome.dylib as well as
So, in our example, you can just write:
def awesomeFunction i: funPtr = lookupSymbol "awesome" "awesomeFunction"
Calling the FunPtr
Now that we've obtained the function pointer, it's time to call the function.
This is what the
call method of
FunPtr does. It takes two arguments – a representation for the output type and a list of objects of type
CArg, representing the function arguments. Note that not every type can be represented as a C argument. The types that can (numeric types, pointers and a few more) are converted using their
So, back to our example. The
awesomeFunction expects and
int and returns an
int. Note that Luna's
Ints are different from C's, so the latter are represented by the class
CInt. This is what our function call looks like:
def awesomeFunction i: funPtr = lookupSymbol "awesome" "awesomeFunction" result = funPtr . call CInt [i.toCArg]
Translating between C and Luna data types
We have successfully called a C function. It is not very usable for most Luna code though – it requires us to use the C types throughout the program. We'll fix that with a few conversion methods. First of all, the function argument needs to be a
CInt. Most Luna programs, however, use standard
Ints and these cannot easily be represented in C. This can be fixed with the help of
CInt.fromInt function, which converts an ordinary
Int object into a
It also returns a
CInt, which again is not very handy. This can be fixed with the help of
toInt method of
CInt. The final version of our wrapper, which operates on
Ints and from the outside is nearly indistinguishable from pure Luna code looks like this:
def awesomeFunction i: iArg = CInt.fromInt i funPtr = lookupSymbol "awesome" "awesomeFunction" result = funPtr . call CInt [iArg.toCArg] result.toInt
Basic C Types
Luna defines counterparts of the standard C types in the
Std.Foreign.C.Value module. This section is a short overview of all of them.
There is a counterpart for integral types commonly used throughout C codebases. All these wrappers have a common API – they define the
toInt methods, support comparison and basic arithmetic operators. The following table shows the C types and their Luna counterparts.
|C Type||Luna Class|
Floating point numbers
Luna defines the
CFloat classes as counterparts of C's
float, respectively. Both classes define
toReal methods for conversion between them and Luna's
Real. They also support basic arithmetic and comparison operators.
The basic pointer type is just
Pointer. It takes a single argument denoting the type of its content. So, for example,
Pointer CInt corresponds to
int* in C, while
Pointer (Pointer None) is
Creating and freeing pointers
To create a pointer able to hold a single value of type
X use the
malloc method on pointer class:
ptr = Pointer X . malloc
For this to work, the
X object must define a
byteSize method, returning the size in bytes of the structure.
You can also use
mallocElems to create arrays like so:
arr = Pointer CInt . mallocElems 40
Any pointer can be freed using its
free method – if you're done with your
ptr just call
Reading and writing
Reading a pointer can be accomplished by its
read method. It works on a
Ptr X and returns a value of type
X. Keep in mind that in order for this to work, the
X type must define a
readPtr method that takes a bare pointer and plucks the fields one by one.
Similarly, to write a value to
ptr of type
Pointer X, call
ptr.write x. Again, this requires the type
X to define
For the basic C types, the required methods are already defined in the standard library.
Any pointer can be moved by a specified number of bytes using the
ptr.moveBytes i method – it returns a new pointer, resulting from adding
i bytes to
There is also a
moveElems method, that will move the pointer by the specified number of elements (i.e. by
number of elements * element.byteSize bytes).
With standard pointers we need to think about freeing unused memory, or it will clutter up our RAM. We can fix that issue with managed pointers – pointers that can be automatically garbage collected when no longer needed. Since managed pointers are available the regular pointers should not be used any more.
To create managed pointer for single value of
X type call, like for pointer,
malloc method just on the managed pointer class:
ptr = ManagedPointer X . malloc
Allocating multiple elements with
mallocElems works just like for regular poinetrs. To create array with managed pointer use:
arr = ManagedPointer CInt . mallocElems 40
It is also possible to create managed pointer from existing pointer
ptr. For this finalizer function
fin is required. Finalizer will be run when the pointer will be garbage collected:
ptr = ManagedPointer X . fromPtr fin ptr
moveElems works the same way for managed pointers like for regular pointers.
Real life example
Now that we've covered all the basics, let's dive into a more involved example – using the
SHA1 function from
openssl. It takes an input buffer of type
unsigned char*, a
size_t denoting the length of input and an output buffer of type
unsigned char* and length 20.
Suppose you have a list of Luna
Ints and want to compute the SHA1 digest of this list, as another list of
Ints. This is how this can be done with Luna's FFI:
import Std.Foreign import Std.Foreign.C.Value def sha1Digest inputList: # Inputs preparation inputLength = inputList . length inBuf = ManagedPointer CUChar . mallocElems inputLength # Allocate the input buffer. inSize = CSize.fromInt inputLength # Convert the length to a CSize. outBuf = ManagedPointer CUChar . mallocElems 20 # Allocate the output buffer. indexed = 0 . upto inputLength . zip inputList indexed . each (ix, elem): inBuf . moveElems ix . write (CUChar.fromInt elem) # Write each element to the buffer at correct position. # Calling the foreign function sha1FunPtr = lookupSymbol "openssl" "SHA1" # Get the function from dynamic library. sha1FunPtr . call None [inBuf.toCArg, inSize.toCArg, outBuf.toCArg] # Call the function passing all the arguments # and specifying the return type as None # Getting the final results result = 0 . upto 19 . each i: outBuf . moveElems i . read . toInt # Read from the output buffer at consecutive positions # and convert the values back to Ints. result