Module Ctypes

The core ctypes module.

The main points of interest are the set of functions for describing C types (see types) and the set of functions for accessing C values (see Values representing C values). The Foreign.foreign function uses C type descriptions to bind external C values.

Pointer types
type ('a, 'b) pointer = ('a, 'b) Ctypes_static.pointer

The type of pointer values. A value of type ('a, [`C]) pointer contains a C-compatible pointer, and a value of type ('a, [`OCaml]) pointer contains a pointer to a value that can be moved by OCaml runtime.

C-compatible pointers
type 'a ptr = ('a, [ `C ]) pointer

The type of C-compatible pointer values. A value of type t ptr can be used to read and write values of type t at particular addresses.

type 'a ocaml = 'a Ctypes_static.ocaml

The type of pointer values pointing directly into OCaml values. Pointers of this type should never be captured by external code. In particular, functions accepting 'a ocaml pointers must not invoke any OCaml code.

C array types
type 'a carray = 'a Ctypes_static.carray

The type of C array values. A value of type t carray can be used to read and write array objects in C-managed storage.

Bigarray types
type 'a bigarray_class = 'a Ctypes_static.bigarray_class

The type of Bigarray classes. There are four instances, one for each of the Bigarray submodules.

val genarray : < element : 'a; layout : 'l; ba_repr : 'b; bigarray : ('a, 'b, 'l) Bigarray.Genarray.t; carray : 'a carray; dims : int array; > bigarray_class

The class of Bigarray.Genarray.t values

val array1 : < element : 'a; layout : 'l; ba_repr : 'b; bigarray : ('a, 'b, 'l) Bigarray.Array1.t; carray : 'a carray; dims : int; > bigarray_class

The class of Bigarray.Array1.t values

val array2 : < element : 'a; layout : 'l; ba_repr : 'b; bigarray : ('a, 'b, 'l) Bigarray.Array2.t; carray : 'a carray carray; dims : int * int; > bigarray_class

The class of Bigarray.Array2.t values

val array3 : < element : 'a; layout : 'l; ba_repr : 'b; bigarray : ('a, 'b, 'l) Bigarray.Array3.t; carray : 'a carray carray carray; dims : int * int * int; > bigarray_class

The class of Bigarray.Array3.t values

Struct and union types

type ('a, 'kind) structured = ('a, 'kind) Ctypes_static.structured

The base type of values representing C struct and union types. The 'kind parameter is a polymorphic variant type indicating whether the type represents a struct (`Struct) or a union (`Union).

type 'a structure = ('a, [ `Struct ]) structured

The type of values representing C struct types.

type 'a union = ('a, [ `Union ]) structured

The type of values representing C union types.

type ('a, 't) field = ('a, 't) Ctypes_static.field

The type of values representing C struct or union members (called "fields" here). A value of type (a, s) field represents a field of type a in a struct or union of type s.

type 'a abstract = 'a Ctypes_static.abstract

The type of abstract values. The purpose of the abstract type is to represent values whose type varies from platform to platform.

For example, the type pthread_t is a pointer on some platforms, an integer on other platforms, and a struct on a third set of platforms. One way to deal with this kind of situation is to have possibly-platform-specific code which interrogates the C type in some way to help determine an appropriate representation. Another way is to use abstract, leaving the representation opaque.

(Note, however, that although pthread_t is a convenient example, since the type used to implement it varies significantly across platforms, it's not actually a good match for abstract, since values of type pthread_t are passed and returned by value.)

include Ctypes_types.TYPE with type 'a typ = 'a Ctypes_static.typ and type ('a, 's) field := ('a, 's) field
type 'a typ = 'a Ctypes_static.typ

The type of values representing C types. There are two types associated with each typ value: the C type used to store and pass values, and the corresponding OCaml type. The type parameter indicates the OCaml type, so a value of type t typ is used to read and write OCaml values of type t. There are various uses of typ values, including

  • constructing function types for binding native functions using Foreign.foreign
  • constructing pointers for reading and writing locations in C-managed storage using ptr
  • describing the fields of structured types built with structure and union.

The void type

val void : unit typ

Value representing the C void type. Void values appear in OCaml as the unit type, so using void in an argument or result type specification produces a function which accepts or returns unit.

Dereferencing a pointer to void is an error, as in C, and will raise IncompleteType.

Scalar types

The scalar types consist of the Arithmetic types and the Pointer types.

Arithmetic types

The arithmetic types consist of the signed and unsigned integer types (including character types) and the floating types. There are values representing both exact-width integer types (of 8, 16, 32 and 64 bits) and types whose size depend on the platform (signed and unsigned short, int, long, long long).

val char : char typ

Value representing the C type char.

Signed integer types
val schar : int typ

Value representing the C type signed char.

val short : int typ

Value representing the C type (signed) short.

val int : int typ

Value representing the C type (signed) int.

val long : Signed.long typ

Value representing the C type (signed) long.

val llong : Signed.llong typ

Value representing the C type (signed) long long.

val nativeint : nativeint typ

Value representing the C type (signed) int.

val int8_t : int typ

Value representing an 8-bit signed integer C type.

val int16_t : int typ

Value representing a 16-bit signed integer C type.

val int32_t : int32 typ

Value representing a 32-bit signed integer C type.

val int64_t : int64 typ

Value representing a 64-bit signed integer C type.

module Intptr : Signed.S
val intptr_t : Intptr.t typ

Value representing the C type intptr_t.

module Ptrdiff : Signed.S
val ptrdiff_t : Ptrdiff.t typ

Value representing the C type ptrdiff_t.

val camlint : int typ

Value representing an integer type with the same storage requirements as an OCaml int.

Unsigned integer types
val uchar : Unsigned.uchar typ

Value representing the C type unsigned char.

val bool : bool typ

Value representing the C type bool.

val uint8_t : Unsigned.uint8 typ

Value representing an 8-bit unsigned integer C type.

val uint16_t : Unsigned.uint16 typ

Value representing a 16-bit unsigned integer C type.

val uint32_t : Unsigned.uint32 typ

Value representing a 32-bit unsigned integer C type.

val uint64_t : Unsigned.uint64 typ

Value representing a 64-bit unsigned integer C type.

val size_t : Unsigned.size_t typ

Value representing the C type size_t, an alias for one of the unsigned integer types. The actual size and alignment requirements for size_t vary between platforms.

val ushort : Unsigned.ushort typ

Value representing the C type unsigned short.

val sint : Signed.sint typ

Value representing the C type int.

val uint : Unsigned.uint typ

Value representing the C type unsigned int.

val ulong : Unsigned.ulong typ

Value representing the C type unsigned long.

val ullong : Unsigned.ullong typ

Value representing the C type unsigned long long.

val uintptr_t : Uintptr.t typ

Value representing the C type uintptr_t.

Floating types
val float : float typ

Value representing the C single-precision float type.

val double : float typ

Value representing the C type double.

val ldouble : LDouble.t typ

Value representing the C type long double.

Complex types
val complex32 : Stdlib.Complex.t typ

Value representing the C99 single-precision float complex type.

val complex64 : Stdlib.Complex.t typ

Value representing the C99 double-precision double complex type.

val complexld : ComplexL.t typ

Value representing the C99 long-double-precision long double complex type.

Pointer types
C-compatible pointers
val ptr : 'a typ -> 'a Ctypes_static.ptr typ

Construct a pointer type from an existing type (called the reference type).

val ptr_opt : 'a typ -> 'a Ctypes_static.ptr option typ

Construct a pointer type from an existing type (called the reference type). This behaves like ptr, except that null pointers appear in OCaml as None.

val string : string typ

A high-level representation of the string type.

On the C side this behaves like char *; on the OCaml side values read and written using string are simply native OCaml strings.

To avoid problems with the garbage collector, values passed using string are copied into immovable C-managed storage before being passed to C.

When the memory is not owned by the C code, -- i.e. when creating or initializing a struct in OCaml before passing it to C -- then the string view isn't a good choice, because there's no way to manage the lifetime of the C copy of the generated OCaml string.

val string_opt : string option typ

A high-level representation of the string type. This behaves like string, except that null pointers appear in OCaml as None.

OCaml pointers
val ocaml_string : string Ctypes_static.ocaml typ

Value representing the directly mapped storage of an OCaml string.

val ocaml_bytes : Stdlib.Bytes.t Ctypes_static.ocaml typ

Value representing the directly mapped storage of an OCaml byte array.

Array types

C array types
val array : int -> 'a typ -> 'a Ctypes_static.carray typ

Construct a sized array type from a length and an existing type (called the element type).

Bigarray types
val bigarray : < element : 'a; layout : Bigarray.c_layout; ba_repr : 'b; dims : 'dims; bigarray : 'bigarray; carray : _; > Ctypes_static.bigarray_class -> 'dims -> ('a, 'b) Bigarray.kind -> 'bigarray typ

Construct a sized C-layout bigarray type representation from a bigarray class, the dimensions, and the Bigarray.kind.

val fortran_bigarray : < element : 'a; layout : Bigarray.fortran_layout; ba_repr : 'b; dims : 'dims; bigarray : 'bigarray; carray : _; > Ctypes_static.bigarray_class -> 'dims -> ('a, 'b) Bigarray.kind -> 'bigarray typ

Construct a sized Fortran-layout bigarray type representation from a bigarray class, the dimensions, and the Bigarray.kind.

val typ_of_bigarray_kind : ('a, 'b) Bigarray.kind -> 'a typ

typ_of_bigarray_kind k is the type corresponding to the Bigarray kind k.

Struct and union types

type ('a, 't) field
val structure : string -> 's Ctypes_static.structure typ

Construct a new structure type. The type value returned is incomplete and can be updated using field until it is passed to seal, at which point the set of fields is fixed.

The type ('_s structure typ) of the expression returned by the call structure tag includes a weak type variable, which can be explicitly instantiated to ensure that the OCaml values representing different C structure types have incompatible types. Typical usage is as follows:

type tagname

let tagname : tagname structure typ = structure "tagname"

val union : string -> 's Ctypes_static.union typ

Construct a new union type. This behaves analogously to structure; fields are added with field.

val field : 't typ -> string -> 'a typ -> ('a, ('s, [< `Struct | `Union ]) Ctypes_static.structured as t) field

field ty label ty' adds a field of type ty' with label label to the structure or union type ty and returns a field value that can be used to read and write the field in structure or union instances (e.g. using getf and setf).

Attempting to add a field to a union type that has been sealed with seal is an error, and will raise ModifyingSealedType.

val seal : (_, [< `Struct | `Union ]) Ctypes_static.structured typ -> unit

seal t completes the struct or union type t so that no further fields can be added. Struct and union types must be sealed before they can be used in a way that involves their size or alignment; see the documentation for IncompleteType for further details.

View types

val view : ?⁠format_typ:((Stdlib.Format.formatter -> unit) -> Stdlib.Format.formatter -> unit) -> ?⁠format:(Stdlib.Format.formatter -> 'b -> unit) -> read:('a -> 'b) -> write:('b -> 'a) -> 'a typ -> 'b typ

view ~read:r ~write:w t creates a C type representation t' which behaves like t except that values read using t' are subsequently transformed using the function r and values written using t' are first transformed using the function w.

For example, given suitable definitions of string_of_char_ptr and char_ptr_of_string, the type representation

view ~read:string_of_char_ptr ~write:char_ptr_of_string (ptr char)

can be used to pass OCaml strings directly to and from bound C functions, or to read and write string members in structs and arrays. (In fact, the string type representation is defined in exactly this way.)

The optional argument format_typ is used by the Ctypes.format_typ and string_of_typ functions to print the type at the top level and elsewhere. If format_typ is not supplied the printer for t is used instead.

The optional argument format is used by the Ctypes.format and string_of functions to print the values. If format_val is not supplied the printer for t is used instead.

val typedef : 'a typ -> string -> 'a typ

typedef t name creates a C type representation t' which is equivalent to t except its name is printed as name.

This is useful when generating C stubs involving "anonymous" types, for example: typedef struct { int f } typedef_name;

Abstract types

val abstract : name:string -> size:int -> alignment:int -> 'a Ctypes_static.abstract typ

Create an abstract type specification from the size and alignment requirements for the type.

Injection of concrete types

val lift_typ : 'a Ctypes_static.typ -> 'a typ

lift_typ t turns a concrete type representation into an abstract type representation.

For example, retrieving struct layout from C involves working with an abstract representation of types which do not support operations such as sizeof. The lift_typ function makes it possible to use concrete type representations wherever such abstract type representations are needed.

Function types

type 'a fn = 'a Ctypes_static.fn

The type of values representing C function types. A value of type t fn can be used to bind to C functions and to describe type of OCaml functions passed to C.

val (@->) : 'a typ -> 'b fn -> ('a -> 'b) fn

Construct a function type from a type and an existing function type. This corresponds to prepending a parameter to a C function parameter list. For example,

int @-> ptr void @-> returning float

describes a function type that accepts two arguments -- an integer and a pointer to void -- and returns a float.

val returning : 'a typ -> 'a fn

Give the return type of a C function. Note that returning is intended to be used together with (@->); see the documentation for (@->) for an example.

type 'a static_funptr = 'a Ctypes_static.static_funptr

Function pointer types

The type of values representing C function pointer types.

val static_funptr : 'a fn -> 'a Ctypes_static.static_funptr typ

Construct a function pointer type from an existing function type (called the reference type).

Operations on types

val sizeof : 'a typ -> int

sizeof t computes the size in bytes of the type t. The exception IncompleteType is raised if t is incomplete.

val alignment : 'a typ -> int

alignment t computes the alignment requirements of the type t. The exception IncompleteType is raised if t is incomplete.

val format_typ : ?⁠name:string -> Stdlib.Format.formatter -> 'a typ -> unit

Pretty-print a C representation of the type to the specified formatter.

val format_fn : ?⁠name:string -> Stdlib.Format.formatter -> 'a fn -> unit

Pretty-print a C representation of the function type to the specified formatter.

val string_of_typ : ?⁠name:string -> 'a typ -> string

Return a C representation of the type.

val string_of_fn : ?⁠name:string -> 'a fn -> string

Return a C representation of the function type.

Values representing C values

val format : 'a typ -> Stdlib.Format.formatter -> 'a -> unit

Pretty-print a representation of the C value to the specified formatter.

val string_of : 'a typ -> 'a -> string

Return a string representation of the C value.

Pointer values

val null : unit ptr

A null pointer.

val (!@) : 'a ptr -> 'a

!@ p dereferences the pointer p. If the reference type is a scalar type then dereferencing constructs a new value. If the reference type is an aggregate type then dereferencing returns a value that references the memory pointed to by p.

val (<-@) : 'a ptr -> 'a -> unit

p <-@ v writes the value v to the address p.

val (+@) : ('a, 'b) pointer -> int -> ('a, 'b) pointer

If p is a pointer to an array element then p +@ n computes the address of the nth next element.

val (-@) : ('a, 'b) pointer -> int -> ('a, 'b) pointer

If p is a pointer to an array element then p -@ n computes the address of the nth previous element.

val ptr_diff : ('a, 'b) pointer -> ('a, 'b) pointer -> int

ptr_diff p q computes q - p. As in C, both p and q must point into the same array, and the result value is the difference of the subscripts of the two array elements.

val from_voidp : 'a typ -> unit ptr -> 'a ptr

Conversion from void *.

val to_voidp : _ ptr -> unit ptr

Conversion to void *.

val allocate : ?⁠finalise:('a ptr -> unit) -> 'a typ -> 'a -> 'a ptr

allocate t v allocates a fresh value of type t, initialises it with v and returns its address. The argument ?finalise, if present, will be called just before the memory is freed. The value will be automatically freed after no references to the pointer remain within the calling OCaml program.

val allocate_n : ?⁠finalise:('a ptr -> unit) -> 'a typ -> count:int -> 'a ptr

allocate_n t ~count:n allocates a fresh array with element type t and length n, and returns its address. The argument ?finalise, if present, will be called just before the memory is freed. The array will be automatically freed after no references to the pointer remain within the calling OCaml program. The memory is allocated with libc's calloc and is guaranteed to be zero-filled.

val ptr_compare : 'a ptr -> 'a ptr -> int

If p and q are pointers to elements i and j of the same array then ptr_compare p q compares the indexes of the elements. The result is negative if i is less than j, positive if i is greater than j, and zero if i and j are equal.

val is_null : 'a ptr -> bool

is_null p is true when p is a null pointer.

val reference_type : 'a ptr -> 'a typ

Retrieve the reference type of a pointer.

val ptr_of_raw_address : nativeint -> unit ptr

Convert the numeric representation of an address to a pointer

val funptr_of_raw_address : nativeint -> (unit -> unit) Ctypes_static.static_funptr

Convert the numeric representation of an address to a function pointer

val raw_address_of_ptr : unit ptr -> nativeint

raw_address_of_ptr p returns the numeric representation of p.

Note that the return value remains valid only as long as the pointed-to object is alive. If p is a managed object (e.g. a value returned by make) then unless the caller retains a reference to p, the object may be collected, invalidating the returned address.

val string_from_ptr : char ptr -> length:int -> string

string_from_ptr p ~length creates a string initialized with the length characters at address p.

Raise Invalid_argument "Ctypes.string_from_ptr" if length is negative.

val ocaml_string_start : string -> string ocaml

ocaml_string_start s allows to pass a pointer to the contents of an OCaml string directly to a C function.

val ocaml_bytes_start : Stdlib.Bytes.t -> Stdlib.Bytes.t ocaml

ocaml_bytes_start s allows to pass a pointer to the contents of an OCaml byte array directly to a C function.

Array values

C array values
module CArray : sig ... end

Operations on C arrays.

Bigarray values
val bigarray_start : < element : 'a; layout : 'l; ba_repr : _; bigarray : 'b; carray : _; dims : _; > bigarray_class -> 'b -> 'a ptr

Return the address of the first element of the given Bigarray value.

val bigarray_of_ptr : < element : 'a; layout : Bigarray.c_layout; ba_repr : 'f; bigarray : 'b; carray : _; dims : 'i; > bigarray_class -> 'i -> ('a, 'f) Bigarray.kind -> 'a ptr -> 'b

bigarray_of_ptr c dims k p converts the C pointer p to a C-layout bigarray value. No copy is made; the bigarray references the memory pointed to by p.

val fortran_bigarray_of_ptr : < element : 'a; layout : Bigarray.fortran_layout; ba_repr : 'f; bigarray : 'b; carray : _; dims : 'i; > bigarray_class -> 'i -> ('a, 'f) Bigarray.kind -> 'a ptr -> 'b

fortran_bigarray_of_ptr c dims k p converts the C pointer p to a Fortran-layout bigarray value. No copy is made; the bigarray references the memory pointed to by p.

val array_of_bigarray : < element : _; layout : Bigarray.c_layout; ba_repr : _; bigarray : 'b; carray : 'c; dims : _; > bigarray_class -> 'b -> 'c

array_of_bigarray c b converts the bigarray value b to a value of type CArray.t. No copy is made; the result occupies the same memory as b.

val bigarray_of_array : < element : 'a; layout : Bigarray.c_layout; ba_repr : 'f; bigarray : 'b; carray : 'c carray; dims : 'i; > bigarray_class -> ('a, 'f) Bigarray.kind -> 'c carray -> 'b

bigarray_of_array c k a converts the CArray.t value a to a C-layout bigarray value. No copy is made; the result occupies the same memory as a.

Struct and union values

val make : ?⁠finalise:('s -> unit) -> (_, _) structured as s typ -> 's

Allocate a fresh, uninitialised structure or union value. The argument ?finalise, if present, will be called just before the underlying memory is freed.

val setf : (_, _) structured as s -> ('a, 's) field -> 'a -> unit

setf s f v overwrites the value of the field f in the structure or union s with v.

val getf : (_, _) structured as s -> ('a, 's) field -> 'a

getf s f retrieves the value of the field f in the structure or union s. The semantics for non-scalar types are non-copying, as for (!@).

val (@.) : (_, _) structured as s -> ('a, 's) field -> 'a ptr

s @. f computes the address of the field f in the structure or union value s.

val (|->) : (_, _) structured as s ptr -> ('a, 's) field -> 'a ptr

p |-> f computes the address of the field f in the structure or union value pointed to by p.

val offsetof : (_, _ structure) field -> int

offsetof f returns the offset, in bytes, of the field f from the beginning of the associated struct type.

val field_type : ('a, _) field -> 'a typ

field_type f returns the type of the field f.

val field_name : (_, _) field -> string

field_name f returns the name of the field f.

val addr : (_, _) structured as s -> 's ptr

addr s returns the address of the structure or union s.

Coercions

val coerce : 'a typ -> 'b typ -> 'a -> 'b

coerce t1 t2 returns a coercion function between the types represented by t1 and t2. If t1 cannot be coerced to t2, coerce raises Uncoercible.

The following coercions are currently supported:

  • All function and object pointer types are intercoercible.
  • Any type may be coerced to void
  • There is a coercion between a view and another type t (in either direction) if there is a coercion between the representation type underlying the view and t.
  • Coercion is transitive: if t1 is coercible to t2 and t2 is coercible to t3, then t1 is directly coercible to t3.

The set of supported coercions is subject to change. Future versions of ctypes may both add new types of coercion and restrict the existing coercions.

val coerce_fn : 'a fn -> 'b fn -> 'a -> 'b

coerce_fn f1 f2 returns a coercion function between the function types represented by f1 and f2. If f1 cannot be coerced to f2, coerce_fn raises Uncoercible.

A function type f1 may be coerced to another function type f2 if all of the following hold:

  • the C types described by f1 and f2 have the same arity
  • each argument of f2 may be coerced to the corresponding argument of f1
  • the return type of f1 may be coerced to the return type of f2

The set of supported coercions is subject to change. Future versions of ctypes may both add new types of coercion and restrict the existing coercions.

binding interfaces

.

module type FOREIGN = sig ... end

Foreign function binding interface.

module type TYPE = sig ... end

Foreign types binding interface.

module Root : sig ... end

Exceptions

exception Unsupported of string

An attempt was made to use a feature not currently supported by ctypes. In practice this refers to attempts to use an union, array or abstract type as an argument or return type of a function.

exception ModifyingSealedType of string

An attempt was made to modify a sealed struct or union type description.

exception IncompleteType

An attempt was made to compute the size or alignment of an incomplete type.

The incomplete types are struct and union types that have not been sealed, and the void type.

It is not permitted to compute the size or alignment requirements of an incomplete type, to use it as a struct or union member, to read or write a value of the type through a pointer or to use it as the referenced type in pointer arithmetic. Additionally, incomplete struct and union types cannot be used as argument or return types.

type uncoercible_info
exception Uncoercible of uncoercible_info

An attempt was made to coerce between uncoercible types.