10.3. Writing Haskell interfaces to C code: hsc2hs
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## 10.3. Writing Haskell interfaces to C code: hsc2hs

The hsc2hs command can be used to automate some parts of the process of writing Haskell bindings to C code. It reads an almost-Haskell source with embedded special constructs, and outputs a real Haskell file with these constructs processed, based on information taken from some C headers. The extra constructs deal with accessing C data from Haskell.

It may also output a C file which contains additional C functions to be linked into the program, together with a C header that gets included into the C code to which the Haskell module will be compiled (when compiled via C) and into the C file. These two files are created when the `#def` construct is used (see below).

Actually hsc2hs does not output the Haskell file directly. It creates a C program that includes the headers, gets automatically compiled and run. That program outputs the Haskell code.

In the following, “Haskell file” is the main output (usually a `.hs` file), “compiled Haskell file” is the Haskell file after ghc has compiled it to C (i.e. a `.hc` file), “C program” is the program that outputs the Haskell file, “C file” is the optionally generated C file, and “C header” is its header file.

### 10.3.1. command line syntax

hsc2hs takes input files as arguments, and flags that modify its behavior:

`-o FILE` or `––output=FILE`

  Name of the Haskell file.

`-t FILE` or `––template=FILE`

  The template file (see below).

`-c PROG` or `––cc=PROG`

  The C compiler to use (default: ghc)

`-l PROG` or `––ld=PROG`

  The linker to use (default: gcc).

`-C FLAG` or `––cflag=FLAG`

  An extra flag to pass to the C compiler.

`-I DIR`

  Passed to the C compiler.

`-L FLAG` or `––lflag=FLAG`

  An extra flag to pass to the linker.

`-i FILE` or `––include=FILE`

  As if the appropriate `#include` directive was placed in the source.

`-D NAME[=VALUE]` or `––define=NAME[=VALUE]`

  As if the appropriate `#define` directive was placed in the source.

`––no-compile`

  Stop after writing out the intermediate C program to disk. The file name for the intermediate C program is the input file name with `.hsc` replaced with `_hsc_make.c`.

`-?` or `––help`

  Display a summary of the available flags and exit successfully.

`-V` or `––version`

  Output version information and exit successfully.

The input file should end with .hsc (it should be plain Haskell source only; literate Haskell is not supported at the moment). Output files by default get names with the `.hsc` suffix replaced:

`.hs` | Haskell file
`_hsc.h` | C header
`_hsc.c` | C file

The C program is compiled using the Haskell compiler. This provides the include path to `HsFFI.h` which is automatically included into the C program.

### 10.3.2. Input syntax

All special processing is triggered by the `#` operator. To output a literal `#`, write it twice: `##`. Inside string literals and comments `#` characters are not processed.

A `#` is followed by optional spaces and tabs, an alphanumeric keyword that describes the kind of processing, and its arguments. Arguments look like C expressions separated by commas (they are not written inside parens). They extend up to the nearest unmatched `)`, `]` or `}`, or to the end of line if it occurs outside any `() [] {} '' "" /**/` and is not preceded by a backslash. Backslash-newline pairs are stripped.

In addition `#{stuff}` is equivalent to `#stuff` except that it's self-delimited and thus needs not to be placed at the end of line or in some brackets.

Meanings of specific keywords:

`#include <file.h>`, `#include "file.h"`,

  The specified file gets included into the C program, the compiled Haskell file, and the C header. `<HsFFI.h>` is included automatically.

`#define name`, `#define name value`, `#undef name`,

  Similar to `#include`. Note that `#includes` and `#defines` may be put in the same file twice so they should not assume otherwise.

`#let name parameters = "definition"`

  Defines a macro to be applied to the Haskell source. Parameter names are comma-separated, not inside parens. Such macro is invoked as other `#`-constructs, starting with `#name`. The definition will be put in the C program inside parens as arguments of `printf`. To refer to a parameter, close the quote, put a parameter name and open the quote again, to let C string literals concatenate. Or use `printf`'s format directives. Values of arguments must be given as strings, unless the macro stringifies them itself using the C preprocessor's `#parameter` syntax.

`#def C_definition`

  The definition (of a function, variable, struct or typedef) is written to the C file, and its prototype or extern declaration to the C header. Inline functions are handled correctly. struct definitions and typedefs are written to the C program too. The `inline`, `struct` or `typedef` keyword must come just after `def`.

  ### Note

  A `foreign import` of a C function may be inlined across a module boundary, in which case you must arrange for the importing module to `#include` the C header file generated by hsc2hs (see Section 8.2.2, “Using function headers”). For this reason we avoid using `#def` in the libraries.

`#if condition`, `#ifdef name`, `#ifndef name`, `#elif condition`, `#else`, `#endif`, `#error message`, `#warning message`,

  Conditional compilation directives are passed unmodified to the C program, C file, and C header. Putting them in the C program means that appropriate parts of the Haskell file will be skipped.

`#const C_expression`

  The expression must be convertible to `long` or `unsigned long`. Its value (literal or negated literal) will be output.

`#const_str C_expression`

  The expression must be convertible to const char pointer. Its value (string literal) will be output.

`#type C_type`

  A Haskell equivalent of the C numeric type will be output. It will be one of `{Int,Word}{8,16,32,64}`, `Float`, `Double`, `LDouble`.

`#peek struct_type, field`

  A function that peeks a field of a C struct will be output. It will have the type `Storable b => Ptr a -> IO b`. The intention is that `#peek` and `#poke` can be used for implementing the operations of class `Storable` for a given C struct (see the `Foreign.Storable` module in the library documentation).

`#poke struct_type, field`

  Similarly for poke. It will have the type `Storable b => Ptr a -> b -> IO ()`.

`#ptr struct_type, field`

  Makes a pointer to a field struct. It will have the type `Ptr a -> Ptr b`.

`#offset struct_type, field`

  Computes the offset, in bytes, of `field` in `struct_type`. It will have type `Int`.

`#size struct_type`

  Computes the size, in bytes, of `struct_type`. It will have type `Int`.

`#enum type, constructor, value, value, ...`

  A shortcut for multiple definitions which use `#const`. Each `value` is a name of a C integer constant, e.g. enumeration value. The name will be translated to Haskell by making each letter following an underscore uppercase, making all the rest lowercase, and removing underscores. You can supply a different translation by writing `hs_name = c_value` instead of a `value`, in which case `c_value` may be an arbitrary expression. The `hs_name` will be defined as having the specified `type`. Its definition is the specified `constructor` (which in fact may be an expression or be empty) applied to the appropriate integer value. You can have multiple `#enum` definitions with the same `type`; this construct does not emit the type definition itself.

### 10.3.3. Custom constructs

`#const`, `#type`, `#peek`, `#poke` and `#ptr` are not hardwired into the hsc2hs, but are defined in a C template that is included in the C program: `template-hsc.h`. Custom constructs and templates can be used too. Any `#`-construct with unknown key is expected to be handled by a C template.

A C template should define a macro or function with name prefixed by `hsc_` that handles the construct by emitting the expansion to stdout. See `template-hsc.h` for examples.

Such macros can also be defined directly in the source. They are useful for making a `#let`-like macro whose expansion uses other `#let` macros. Plain `#let` prepends `hsc_` to the macro name and wraps the definition in a `printf` call.

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