ezyang’s blog

the arc of software bends towards understanding

First steps in c2hs

This is part four of a six part tutorial series on c2hs. Today we discuss the simple things in c2hs, namely the type, enum, pointer, import and context directives.

Prior art. All of the directives c2hs supports are tersely described in the “tutorial” page (which would perhaps be more accurately described as a “reference manual”, not tutorial.) There is also (paradoxically) a much more informal introduction for most of the directives in c2hs's research paper.

Type. C code will occasionally contain macro conditionals redefining a type depending on some build condition (the following is real code):

#if       defined(__ccdoc__)
typedef platform_dependent_type ABC_PTRUINT_T;
#elif     defined(LIN64)
typedef unsigned long ABC_PTRUINT_T;
#elif     defined(NT64)
typedef unsigned long long ABC_PTRUINT_T;
#elif     defined(NT) || defined(LIN) || defined(WIN32)
typedef unsigned int ABC_PTRUINT_T;
   #error unknown platform
#endif /* defined(PLATFORM) */

If you wanted to refer to write FFI code that referenced functions that used ABC_PTRUINT_T, you might have to have performed some guess on what the value truly is in Haskell or used the C preprocessor to reimplement the conditions. With c2hs you can retrieve the true value of a typedef with type:

type ABC_PTRUINT_T = {#type ABC_PTRUINT_T #}

Consider the case of a 64-bit Linux system (such that __ccdoc__ is undefined and LIN64 is defined), then the result is:

type ABC_PTRUINT_T = CLong

Enum. Enums show up frequently in well-written (i.e. eschewing magic numbers) C code:

enum Abc_VerbLevel
   ABC_PROMPT   = -2,
   ABC_ERROR    = -1,
   ABC_WARNING  =  0,
   ABC_VERBOSE  =  2

However, underneath the hood, these are really just ints, so Haskell code that wants to pass an enum value to a function has to:

  1. Create a new datatype to represent the enumeration, and
  2. Write a function that maps from that datatype to the C integer and back again for the Enum instance.

We can have c2hs do all the work for us:

{#enum Abc_VerbLevel {underscoreToCase} deriving (Show, Eq) #}

which becomes:

data Abc_VerbLevel = AbcPrompt | AbcError | AbcWarning | AbcStandard | AbcVerbose
  deriving (Show, Eq)
instance Enum Abc_VerbLevel
  fromEnum AbcPrompt = -2
  -- ...

Note that, as ABC_PROMPT is a very unsightly constructor in Haskell, we transform the names using the underscoreToCase algorithm as mentioned before. You can also explicitly list out the renamings:

{#enum Abc_VerbLevel {AbcPrompt, AbcError, AbcWarning, AbcStandard, AbcVerbose} #}

or change the name of the datatype:

{#enum Abc_VerbLevel as AbcVerbLevel {underscoreToCase} #}

There are two other transforms (which can combine with underscoreToCase: upcaseFirstLetter and downcaseFirstLetter, though I'm not sure when the latter would result in working Haskell code.

Pointer. Unlike C primitives, which are specified in Foreign.C.Types, Haskell needs to be told how to map pointer types (foo*) into Haskell types. Consider some struct:

struct foobar {
  int foo;
  int bar;

It is quite conceivable that there exists a data Foobar = Foobar Int Int in the Haskell codebase, in which case we would like Ptr Foobar to represent a struct foobar* in the original C code. c2hs has no way of deriving this information directly, so we give it this information:

{#pointer *foobar as FoobarPtr -> Foobar #}

This generates the code:

type FoobarPtr = Ptr Foobar

But more importantly, allows c2hs to place more specific types in the signatures it writes for FFI bindings (which we will see in the next post in the series.)

Some variations on the theme:

  • If you want to represent an opaque pointer whose contents will not be marshalled, you can either do empty data declarations:

    data Foobar
    {#pointer *foobar as FoobarPtr -> Foobar #}

    or you can have c2hs generate code using the newtype trick:

    {#pointer *foobar as FoobarPtr newtype #}

    I prefer empty data declarations, since there’s no need to wrap and unwrap a newtype in that case: the newtype will generate:

    newtype FoobarPtr = FoobarPtr (Ptr FoobarPtr)

    which, for any code expecting Ptr a, needs to be unwrapped.

  • If you do not care for the name FoobarPtr and would just like to explicitly say Ptr Foobar, you can tell c2hs not to emit the type definition with nocode:

    {#pointer *foobar -> Foobar nocode #}
  • If no Haskell name mapping is specified, it will simply use the C name:

    -- if it was struct Foobar...
    {#pointer *Foobar #}
  • If you would like to refer to a typedef in C which is already a pointer, just omit the asterisk:

    typedef struct Foobar*   FoobarPtr
    {#pointer FoobarPtr #}
  • c2hs also has limited support for declaring pointers as foreign or stable, and generating code accordingly. I've not used this, except in one case where I found the generated bindings for the pointer were not flexible enough. Your mileage may vary.

Import. A C library that contains multiple header files will probably have some headers including others to get vital type definitions. If you organize your Haskell modules similarly, you need to mimic these includes: this can be done with import.

{#import Foobar.Internal.Common #}

In particular, this sets up the pointer mappings from the other module, as well as generating the usual import statement.

Context (optional). Context has two purported purposes. The first is to specify what library the FFI declarations in the file should be linked against; however, in Cabal, this doesn’t actually do anything—so you need to still add the library to Extra-libraries. The second is to save you keystrokes by adding an implicit prefix to every C identifier you reference, in the case that the original C code was namespaced gtk_ or similarly. I personally like not needing to have to qualify my imports to the lower level API and like the visual distinction of C prefixes, so I tend to omit this. Some directives let you change the prefix locally, in particular enum.

Next time. Marshalling with get and set.