Camlidl user's manual
Version 1.05
Xavier Leroy
INRIA Rocquencourt
1 Overview
Camlidl generates stub code for interfacing Caml with C
(as described in chapter ``Interfacing with C'' of the
Objective Caml
reference manual) from an IDL description of the C functions to be
made available in Caml.
Thus, Camlidl automates the most tedious task in interfacing C
libraries with Caml programs. It can also be used to interface Caml
programs with other languages, as long as those languages have a
well-defined C interface.
In addition, Camlidl provides basic support for COM interfaces and
components. It supports both using COM components (usually written in
C++ or C) from Caml programs, and packaging Caml objects as
COM components that can then be used from C++ or C.
1.1 What is IDL?
IDL stands for Interface Description Language. This is a generic term
for a family of small languages that have been developed to provide
type specifications for libraries written in C and C++. Those
languages resembles C declarations (as found in C header files), with
extra annotations to provide more precise types for the arguments and
results of the functions.
The particular IDL used by Camlidl is inspired by Microsoft's IDL,
which itself is an extension of the IDL found in DCE (The Open Group's
Distributed Common Environment). The initial motivation for those IDLs
was to automate the generation of stub code for remote procedure calls
and network objects, where the arguments to the function are marshaled
at the calling site, then sent across the network or through
interprocess communications to a server process, which unmarshals the
arguments, compute the function application, marshal the results,
sends them back to the calling site, where they are unmarshaled and
returned to the caller. IDLs were also found to be very useful for
inter-language communications, since the same type information that
guides the generation of marshaling stubs can be used to generate
stubs to convert between the data representations of several
languages.
1.2 What is COM?
COM is Microsoft's Common Object Model. It provides a set of
programming conventions as well as system support for packaging
C++ objects as executable components that can be used in other
programs, either by dynamic linking of the component inside the
program, or through interprocess or internetwork communications
between the program and a remote server. COM components implement one
or several interfaces, (similar to Caml object types or Java
interfaces) identified by unique 128-bit interface identifiers (IIDs).
COM specifies a standard protocol for reference counting of
components, and for asking a component which interfaces it
implements.
While the full range of COM services and third-party components is
available only on Microsoft's Windows operating systems, the basic COM
conventions can also be used on Unix and other operating systems
to exchange objects between Caml and C or C++. Of particular
interest is the encapsulation of Caml objects as COM components, which
can then be used inside larger C or C++ applications; those
applications do not need to know anything about Caml: they just call
the component methods as if they were C++ methods or C
functions, without knowing that they are actually implemented in Caml.
For more information about COM, see for instance Inside COM
by Dale Rogerson (Microsoft Press), or the
Microsoft developer Web site.
2 IDL syntax
This section describes the syntax of IDL files. IDL syntax is very
close to that of C declarations, with extra attributes between
brackets adding information to the C types. The following example
should give the flavor of the syntax:
int f([in,string] char * msg);
This reads: ``f is a function taking a character string as input and
returning an int''.
2.1 Lexical conventions
Blanks.
Space, newline, horizontal tabulation, carriage
return, line feed and form feed are considered as blanks. Blanks are
ignored, but they separate adjacent tokens.
Comments.
Both C-style comments /* ... */ and
Java-style comments // ... are supported. C-style comments are
introduced by /* and terminated by */. Java-style comments are
introduced by // and extend to the end of the line. Comments are
treated as blank characters. Comments do not occur inside string or
character literals. Nested C-style comments are not supported.
Identifiers.
Identifiers have the same syntax as in C.
ident |
::= |
(A ... Z | a ... z | _)
{A ... Z | a ... z | 0 ... 9 | _} |
Literals.
Integer literals, character literals and string
literals have the same syntax as in C.
integer |
::= |
[-] {0 ... 9}
[-] 0x {0 ... 9 | a ... f | A ... F}
[-] 0 {0 ... 7} |
character |
::= |
' (regular-char | escape-char) ' |
string |
::= |
" {regular-char | escape-char} " |
escape-char |
::= |
\ (b| n| r| t) |
|
| |
\ (0...7) [0...7] [0...7] |
UUID.
Unique identifiers are composed of 16 hexadecimal
digits, in groups of 8, 4, 4, 4 and 12, separated by dashes.
uuid |
::= |
hex8 - hex4 - hex4 - hex4 - hex4 hex4 hex4 |
hex |
::= |
0 ... 9 | a ... f | A ... F |
2.2 Limited expressions
Limited expressions are similar to C expressions, with the omission of
assignment operators (=, +=, etc), and the addition of the
unsigned (logical) right shift operator >>>. Operators have the same
precedences and associativities as in C. They are listed below in
decreasing priority order.
lexpr |
::= |
ident |
|
| |
integer |
|
| |
character |
|
| |
true |
|
| |
false |
|
| |
string |
|
| |
sizeof ( type-expr ) |
|
| |
( lexpr ) |
|
| |
lexpr (. | ->) ident |
|
| |
( type-expr ) lexpr |
|
| |
(&| *| !| ~| -| +) lexpr |
|
| |
lexpr (*| /| %) lexpr |
|
| |
lexpr (+| -) lexpr |
|
| |
lexpr (<<| >> |
|
| |
>>>) lexpr |
|
| |
lexpr (==| !=| >=| <=| >| <) |
|
| |
lexpr (&| ^| |) lexpr |
|
| |
lexpr (&&| ||) lexpr |
|
| |
lexpr ? lexpr : lexpr |
Constant limited expressions, written const-lexpr below, can only
reference identifiers that are bound by the IDL const declaration.
2.3 Attributes
attributes |
::= |
[ attribute { , attribute } ] |
attribute |
::= |
ident |
|
| |
ident ( [lexpr] { , [lexpr] } ) |
|
| |
ident ( uuid ) |
|
| |
attribute * |
|
| |
* attribute |
Attribute lists are written in brackets [...], and are always
optional. Each attribute is identified by a name, and may carry
optional arguments. Starred attributes apply to the element type of a
pointer or array type, rather than to the pointer or array type itself.
The following table summarizes the recognized attributes and their
arguments.
Attribute |
Context where it can appear |
abstract |
typedef |
bigarray |
array type |
blocking |
function declaration |
camlint |
int or long integer type |
compare(fun-name) |
typedef |
c2ml(fun-name) |
typedef |
errorcheck(fun-name) |
typedef |
errorcode |
typedef |
finalize(fun-name) |
typedef |
fortran |
array type with bigarray attribute |
hash(fun-name) |
typedef |
ignore |
any pointer type |
in |
function parameter |
int_default( camlint| nativeint| int32| int64) |
interface |
int32 |
int or long integer type |
int64 |
int or long integer type |
length_is(le1, le2,...) |
array type |
long_default( camlint| nativeint| int32| int64) |
interface |
managed |
array type with bigarray attribute |
ml2c(fun-name) |
typedef |
mlname( fun-or-field-name ) |
function declaration, struct field |
mltype(" caml-type-expr ") |
typedef |
nativeint |
int or long integer type |
null_terminated |
array of pointers |
object |
interface |
out |
function parameter |
pointer_default( ref| unique| ptr) |
interface |
propget |
function declaration |
propput |
function declaration |
propputref |
function declaration |
ptr |
any pointer type |
ref |
any pointer type |
set |
enum type |
size_is(le1, le2,...) |
array type |
string |
character array or pointer |
switch_is(le) |
union type or pointer to union |
switch_type(ty) |
union or pointer to union |
unique |
any pointer, array, or bigarray type |
uuid( uuid ) |
interface |
2.4 Types and declarators
The declaration of an identifier along with its type is as in C:
a type specification comes first, followed by the identifier possibly
decorated with * and [...] to denote pointers and array types.
For instance, int x declares an identifier x of type int,
while int (*x)[] declares an identifier x that is a pointer to an
array of integers.
type-spec |
::= |
[unsigned| signed]
(int| short| long| char| hyper| long long| __int64) |
|
| |
byte |
|
| |
float |
|
| |
double |
|
| |
boolean |
|
| |
void |
|
| |
ident |
|
| |
wchar_t |
|
| |
handle_t |
|
| |
struct ident |
|
| |
union ident |
|
| |
enum ident |
|
| |
struct-decl |
|
| |
union-decl |
|
| |
enum-decl |
declarator |
::= |
{*} direct-declarator |
direct-declarator |
::= |
ident |
|
| |
( declarator ) |
|
| |
direct-declarator [ [const-lexpr] ] |
2.5 Structures, unions and enumerations
struct-decl |
::= |
struct [ident] { {field-decl} } |
field-decl |
::= |
attributes type-spec declarator { , declarator } ; |
union-decl |
::= |
union [ident] { {union-case} } |
|
| |
union [ident] switch ( type-spec ident ) { {union-case} } |
union-case |
::= |
{case ident :}+ [field-decl] ;
default : [field-decl] ; |
enum-decl |
::= |
enum [ident] { enum-case {, enum-case} [,] } |
enum-case |
::= |
ident [= const-lexpr] |
IDL struct declarations are like those of C, with the addition of
optional attributes on each field. union declarations are also as
in C, except that each case of an union must be labeled by one or
several case ident :. The first form of union declaration
assumes that the discriminant of the union is provided separately
via a switch_is annotation on the union type, while the second form
encapsulates the discriminant along with the union itself
(like in Pascal's record case of construct).
2.6 Function declarations
function-decl |
::= |
attributes type-spec {*} ident ( params ) {quote(ident, string)} |
params |
::= |
e |
|
| |
void |
|
| |
param { , param } |
param |
::= |
attributes type-spec declarator |
Function declarations are like in ANSI C, with the addition of
attributes on each parameter and on the function itself.
Parameters must be named. The optional quote statements following the
declaration are user-provided calling sequences and deallocation
sequences that replaces the default sequences in the
camlidl-generated stub code for the function.
2.7 Constant definitions
constant-decl |
::= |
const attributes type-spec {*} ident = const-lexpr ; |
A constant declaration associates a name to a limited expression.
The limited expression can refer to constant names declared earlier,
but cannot refer to other kinds of identifiers. The optional
attributes influence the interpretation of the type specification,
e.g. const int x = 3 defines x with Caml type int, but
const [int64] long x = 5 defines x with Caml type int64.
2.8 IDL files
file |
::= |
{decl} |
decl |
::= |
function-decl ; |
|
| |
constant-decl ; |
|
| |
struct-decl ; |
|
| |
union-decl ; |
|
| |
enum-decl ; |
|
| |
typedef attributes type-spec declarator { , declarator } ; |
|
| |
attributes interface ident [ : ident ] { {decl} } |
|
| |
struct ident ; |
|
| |
union ident ; |
|
| |
union switch ( type-spec ident ) ; |
|
| |
attributes interface ident ; |
|
| |
import string ; |
|
| |
quote ( [ident ,] string ) |
|
| |
cpp_quote ( string ) |
An IDL file is a sequence of IDL declarations. Declarations include
function declarations, constant declarations, type declarations
(structs, unions, enums, as well as a C-style typedef declaration to
name a type expression), and interfaces.
An interface declaration gives a name and attributes to a collection
of declarations. For interfaces with the object attribute,
an optional super-interface can be provided, as in
interface intf : super-intf. The name of the interface can be
used as a type name in the remainder of the file.
Forward declarations of structs, unions and interfaces are supported
in the usual C manner, by just giving the name of the struct, union or
interface, but not its actual contents.
The import statement reads another IDL file and makes available its
type and constant declarations in the remainder of the file.
No code is generated for the functions and interfaces declared in the
imported file. The same file can be imported several times, but is
read in only the first time.
The quote ( ident , str ) diversion copies the string str
verbatim to one of the files generated by the camlidl compiler.
The ident determines the file where str is copied:
it can be ml for the Caml implementation file (.ml),
mli for the Caml interface file (.mli),
mlmli for both Caml files,
h for the C header file (.h),
and c for the C source file containing the generated stub code (.c
file). For backward compatibility, cpp_quote ( str ) is
recognized as synonymous for quote ( h , str ).
3 The Caml-IDL mapping
This section describes how IDL types, function declarations, and
interfaces are mapped to Caml types, functions and classes.
3.1 Base types
IDL type ty |
Caml type [[ty]] |
byte, short |
int |
int, long with [camlint] attribute |
int |
int, long with [nativeint] attribute |
nativeint |
int, long with [int32] attribute |
int32 |
int, long with [int64] attribute |
int64 |
hyper, long long, __int64 |
int64 |
char |
char |
float, double |
float |
boolean |
bool |
(For integer types, signed and unsigned variants of the same IDL
integer type translate to the same Caml type.)
Depending on the attributes, the int and long integer types are
converted to one of the Caml integer types int, nativeint,
int32, or int64. Values of Caml type int32 are exactly 32-bit wide
and values of type int64 are exactly 64-bit wide on all platforms.
Values of type nativeint have the natural word size of the platform,
and are large enough to accommodate any C int or long int without
loss of precision. Values of Caml type int have the natural word
size of the platform minus one bit of tag, hence the conversion from IDL
types int and long loses the most significant bit on 32-bit
platforms. On 64-bit platforms, the conversion from int is exact,
but the conversion from long loses the most significant bit.
If no explicit integer attribute is given for an int or long type,
the int_default or long_default attribute of the enclosing
interface, if any, determines the kind of the integer.
If no int_default or long_default attribute is in scope, the kind
camlint is assumed, which maps IDL int and long types to the
Caml int type.
3.2 Pointers
The mapping of IDL pointer types depends on their kinds. Writing
[[ty]] for the Caml type corresponding to the IDL type
ty, we have:
[ref] ty * Þ [[ty]]
[unique] ty * Þ [[ty]] option
[ptr] ty * Þ [[ty]] Com.opaque
In other terms, IDL pointers of kind ref are ignored during the mapping:
[ref] ty * is mapped to the same Caml type as ty.
A pointer p to a C value c = *p is translated to
the Caml value corresponding to c.
IDL pointers of kind unique are mapped to an option type. The
option value is None for a null pointer, and Some(v)
for a non-null pointer to a C value c that translates to the ML
value v.
IDL pointers of kind ptr are mapped to a Com.opaque type.
This is an abstract type that encapsulates the C pointer without
attempting to convert it to an ML data structure.
IDL pointers of kind ignore denote struct fields and function
parameters that need not be exposed in the Caml code. Those pointers
are simply set to null when converting from Caml to C, and ignored
when converting from C to Caml. They cannot occur elsewhere.
If no explicit pointer kind is given, the pointer_default attribute
of the enclosing interface, if any, determines the kind of the pointer.
If no pointer_default attribute is in scope, the kind unique is
assumed.
3.3 Arrays
IDL arrays of characters that carry the [string] attribute are mapped
to the Caml string type:
IDL type ty |
Caml type [[ty]] |
[string] char [] |
string |
[string] unsigned char [] |
string |
[string] signed char [] |
string |
[string] byte [] |
string |
Caml string values are translated to standard null-terminated C strings.
Be careful about embedded null characters in the Caml string, which
will be recognized as end of string by C functions.
IDL arrays carrying the [bigarray] attribute are translated to Caml
``big arrays'', as described in the next section.
All other IDL arrays are translated to ML arrays:
ty [] Þ [[ty]] array
For instance, double [] becomes float array.
Consequently, multi-dimensional arrays are translated to Caml arrays
of arrays. For instance, int [][] becomes int array array.
If the unique attribute is given, the IDL array is translated to an
ML option type:
[string,unique] char [] Þ string option
[unique] ty [] Þ [[ty]] array option
As in the case of pointers of kind unique, the option value is
None for a null C pointer, and Some(v) for a non-null
C pointer to a C array that translates to the ML string or array v.
Conversion between a C array and an ML array proceed element by
element. For the conversion from C to ML, the number of elements of
the ML array is determined as follows (in the order presented):
-
By the length_is attribute, if present.
- By the size_is attribute, if present.
- By the bound written in the array type, if any.
- By searching the first null element of the C array, if the
null_terminated attribute is present.
For instance, C values of IDL type [length_is(n)] double[] are
mapped to Caml float array of n elements. C values of IDL type
double[10] are mapped to Caml float array of 10 elements.
The length_is and size_is attributes take as argument one or
several limited expressions. Each expression applies to one dimension
of the array. For instance, [size_is(*dimx, *dimy)] double d[][]
specifies a matrix of double whose first dimension has size
*dimx and the second has size *dimy.
3.4 Big arrays
IDL arrays of integers or floats that carry the [bigarray] attribute
are mapped to one of the Caml Bigarray types: Array1.t for
one-dimensional arrays, Array2.t for 2-dimensional arrays,
Array3.t for 3-dimensional arrays, and Genarray.t for arrays of 4
dimensions or more.
If the [fortran] attribute is given, the big array is accessed
from Caml using the Fortran conventions (array indices start at 1;
column-major memory layout). By default, the big array is accessed
from Caml using the C conventions (array indices start at 0; row-major
memory layout).
If the [managed] attribute is given on a big array type that is
result type or out parameter type of a function, Caml assumes that the
corresponding C array was allocated using malloc(), and is not
referenced anywhere else; then, the Caml garbage collector will free
the C array when the corresponding Caml big array becomes unreachable.
By default, Caml assumes that result or out C arrays are statically or
permanently allocated, and keeps a pointer to them during conversion
to Caml big arrays, and does not free them when the Caml bigarrays
become unreachable.
Finally, the [unique] attribute applies to bigarrays as to arrays,
that is, it maps a null C pointer to None, and a non-null C pointer
p to Some(v) where v is the ML bigarray
resulting from the translation of p.
3.5 Structs
IDL structs are mapped to Caml record types. The names and types of
the IDL struct fields determine the names and types of the Caml record
type:
struct s { ... ; tyi idi ; ... } becomes type s = { ... ; idi : [[tyi]] ; ... }
Example: struct s { int n; double d[4]; } becomes
type s = {n: int; d: float array}.
Exceptions to this rule are as follows:
-
Fields of the IDL struct that are pointers with the [ignore]
attribute do not appear in the Caml record type.
Example: struct s { double x,y; [ignore] void * data; }
becomes type struct_s = {x : float; y: float}.
Those ignored pointer fields are set to NULL when converting from a
Caml record to a C struct.
- Integer fields of the IDL struct that appear in a length_is,
size_is or switch_is attribute of another field also do not appear
in the Caml record type. (We call those fields dependent fields.)
Example: struct s { int idx; int len; [size_is(len)] double d[]; }
is translated to the Caml record type
type struct_s = {idx: int; d: float array}.
The value of len is recovered from the size of the Caml array d,
and thus doesn't need to be represented explicitly in the Caml record.
- If, after elimination of ignored pointer fields and dependent
fields as described above, the IDL struct has only one field
ty id, we avoid creating a one-field Caml record type
and translate the IDL struct type directly to the Caml type
[[ty]].
Example: struct s { int len; [size_is(len)] double d[]; }
is translated to the Caml type abbreviation type struct_s = double array.
- The names of labels in the Caml record type can be changed by
using the mlname attribute on struct field declarations. For instance,
struct s { int n; [mlname(p)] int q; }
becomes type s = { n : int; p : int }
- The Caml type system makes it difficult to use two record types
defined in the same module and having some label names in common.
Thus, if CamlIDL encounters two or more structs having
identically-named fields, it prefixes the Caml label names by the names
of the structs in order to distinguish them. For instance:
struct s1 { int x; int y; }
struct s2 { double x; double t; }
struct s3 { int z; }
becomes type s1 = { s1_x: int; s1_y: int }
and s2 = { s2_x: float; s2_t: float }
and s3 = { z: int }
The labels for s1 and s2 have been prefixed by s1_ and
s2_ respectively, to avoid ambiguity on the x label. However, the
label z for s3 is not prefixed, since it is not used elsewhere.
The prefix added in front of multiply-defined labels is taken from the
struct name, if any, and otherwise from the name of the nearest
enclosing struct, union or typedef. For instance:
typedef struct { int x; } t;
struct s4 { struct { int x; } z; };
becomes type t = { t_x: int }
and s4 = { z: struct_1 }
and struct_1 = { s4_x: int }
The ``minimal prefixing'' strategy described above is the default
behavior of camlidl. If the -prefix-all-labels option is given,
all record labels are prefixed, whether they occur several times or
not. If the -keep-labels option is given, no automatic prefixing
takes place; the naming of record labels is left entirely under the
user's control, via mlname annotations.
3.6 Unions
IDL discriminated unions are translated to Caml sum types. Each case
of the union corresponds to a constructor of the sum type. The
constructor is constant if the union case has no associated field,
otherwise has one argument corresponding to the union case field. If
the union has a default case, an extra constructor
Default_unionname is added to the Caml sum type, carrying an
int argument (the value of the discriminating field),
and possibly another argument corresponding to the default field.
Examples:
union u1 { case A: int x; case B: case C: double d; case D: ; }
becomes type u1 = A of int | B of float | C of float | D
union u2 { case A: int x; case B: double d; default: ; }
becomes type u2 = A of int | B of float | Default_u of int
union u3 { case A: int x; default: double d; }
becomes type u3 = A of int | Default_v of int * double
All IDL unions must be discriminated, either via the special syntax
union name switch(int discr)..., or via the
attribute switch_is(discr), where discr is a C l-value
built from other parameters of the current function, or other fields
of the current struct. Both the discriminant and the
case labels must be of an integer type. Unless a default case is
given, the value of the discriminant must be one of the cases of the
union.
IDL enums are translated to Caml enumerated types (sum types with only
constant constructors). The names of the constructors are determined
by the names of the enum labels. The values attached to the enum
labels are ignored.
Example:
enum e { A, B = 2, C = 4 } becomes type enum_e = A | B | C.
The set attribute can be applied to a named enum to denote a
bitfield obtained by logical ``or'' of zero, one or several labels of
the enum. The corresponding ML value is a list of zero, one or
several constructors of the Caml enumerated type. Consider for
instance:
enum e { A = 1, B = 2, C = 4 };
typedef [set] enum e eset;
The Caml type eset is equal to enum_e list.
The C integer 6 (= B | C) is translated to the ML list [B; C].
The ML list [A; C] is translated to the C integer A | C, that is 5.
3.8 Type definitions
An IDL typedef statement is normally translated
to a Caml type abbreviation. For instance,
typedef [string] char * str becomes type str = string.
If the abstract attribute is given, a Caml abstract type is
generated instead of a type abbreviation, thus hinding from Caml the
representation of the type in question. For instance,
typedef [abstract] void * handle becomes type handle.
In this case, the IDL type in the typedef is ignored.
If the mltype ( " caml-type-expr " ) attribute is given,
the Caml type is made equal to caml-type-expr. This is often used
in conjunction with the ml2c and c2ml attributes to implement
custom translation of data structures between C and ML. For instance,
typedef [mltype("int list")] struct mylist_struct * mylist
becomes type mylist = int list.
If the c2ml(funct-name) and ml2c(funct-name) attributes are
given, the user-provided C functions given as attributes will be
called to perform Caml to C and C to Caml conversions for values of
the typedef-ed type, instead of using the camlidl-generated
conversion functions. This allows user-controlled translation of data
structures. The prototypes of the conversion functions must be
value c2ml(ty * input);
void ml2c(value input, ty * output);
where ty is the name of the type defined by typedef. In other
terms, the c2ml function is passed a reference to a ty and
returns the corresponding Caml value, while the ml2c function is
passed a Caml value as first argument and stores the corresponding C
value in the ty reference passed as second argument.
If the finalize(final-fn) attribute is given in combination with the
abstract attribute, the function final-fn is called when
the Caml block representing a value of this typedef becomes
unreachable from Caml and is reclaimed by the Caml garbage collector.
Similarly, compare(compare-fn) and hash(hash-fn) attach a
comparison function and a hashing function (respectively) to Caml
values for this typedef. The comparison function is called when two
Caml values of this typedef are compared using the generic comparisons
compare, =, <, etc. The hashing function is called when
Hashtbl.hash is applied to a Caml value of this typedef.
The prototype of the finalization, comparison and hashing functions are:
value final-fn(ty * x);
int compare-fn(ty * x, ty * y);
long hash-fn(ty * x);
That is, their arguments are passed by reference. The comparison
function must return an integer that is negative, zero, or positive
depending on whether its first argument is smaller, equal or greater
than its second argument. The hashing function returns a suitable
hash value for its argument.
If the errorcheck(fn) attribute is provided for the typedef ty,
the error checking function fn is called each time a function result
of type ty is converted from C to Caml. The function can then check
the ty value for values indicating an error condition, and raise the
appropriate exception. If in addition the errorcode attribute is
provided, the conversion from C to Caml is suppressed: values of type
ty are only passed to fn for error checking, then discarded.
3.9 Functions
IDL function declarations are translated to Caml functions.
The parameters and results of the Caml function are determined from
those of the IDL function according to the following rules:
-
First, dependent parameters (parameters that are size_is,
length_is or switch_is of other parameters) as well as parameters
that are ignored pointers are removed.
- The remaining parameters are split into Caml function inputs and
Caml function outputs. Parameters with the [in] attribute are added
to the inputs of the function. Parameters with the [out]
attribute are added to the outputs of the function. Parameters with
the [in,out] attribute are added both to the inputs and to the
outputs of the function, unless they are of type string or big array,
in which case they are added to the inputs of the function only.
(The reason for this exception is that strings and big arrays are
shared between Caml and C, thus allowing true in,out behavior on the
Caml function parameter, while other data types are copied during
Caml/C conversion, thus turning a C in,out parameter into a Caml
copy in, copy out parameter, that is, one parameter and one result.)
- The return value of the IDL function is added to the outputs of
the Caml function (in first position), unless it is of type void or
of a type name that carries the errorcode attribute. In the latter
two cases, the return value of the IDL function is not transmitted to
Caml.
- The Caml function is then given type
in1 -> ... -> inp -> out1 * ... * outq
where in1 ... inp are the types of its inputs
and out1 ... outq are the types of its outputs.
If there are no inputs, a unit parameter is added.
If there are no outputs, a unit result is added.
Examples:
int f([in] double x, [in] double y) f : float -> float -> int
Two double input, one int output
void g([in] int x) g : int -> unit
One int input, no output
int h() h : unit -> int
No input, one int result
void i([in] int x, [out] double * y) i : int -> double
One int input, one double output (as an out
parameter)
int j([in] int x, [out] double * y) j : int -> int * double
One int input, one int output (in the result), one
double output (as an out parameter)
void k([in,out,ref] int * x) k : int -> int
The in,out parameter is both one int input and one
int output.
HRESULT l([in] int x, [out] int * res1, [out] int * res2)
l : int -> int * int
HRESULT is a predefined type with the errorcode
attribute, hence it is ignored. It remains one int input and
two int outputs (out parameters)
void m([in] int len, [in,size_is(len)] double d[])
m : float array -> int
len is a dependent parameter, hence is ignored. The
only input is the double array
void n([in] int inputlen, [out] int * outputlen,
[in,out,size_is(inputlen),length_is(*outputlen)] double d[])
n : float array -> float array
The two parameters inputlen and outputlen are
dependent, hence ignored. The double array is both an input
and an output.
void p([in] int dimx, [in] int dimy,
[in,out,bigarray,size_is(dimx,dimy)] double d[][])
p : (float, Bigarray.float64_elt, Bigarray.c_layout) Bigarray.Array2.t -> unit
The two parameters dimx and dimy are dependent
(determined from the dimensions of the big array argument),
hence ignored. The two-dimensional array d, although marked [in,out],
is a big array, hence passed as an input that will be modified in
place by the C function p. The Caml function has no outputs.
Error checking:
For every output that is of a named type with the errorcheck(fn)
attribute, the error checking function fn is called after the C
function returns. That function is assumed to raise a Caml exception
if it finds an output denoting an error.
Custom calling and deallocation sequences:
The IDL declaration for a function can optionally specify a custom
calling sequence and/or a custom deallocation sequence, via quote
clauses following the function declaration:
function-decl |
::= |
attributes type-spec {*} ident ( params )
{ quote( ident , string ) } |
The general shape of a camlidl-generated stub function is as
follows:
value caml_wrapper(value camlparam1, ..., value camlparamK)
/* Convert the function parameters from Caml to C */
param1 = ...;
...
paramN = ...;
/* Call the C function 'ident' */
_res = ident(param1, ..., paramN);
/* Convert the function result and out parameters to Caml values */
camlres = ...;
/* Return result to Caml */
return camlres;
A quote(call, string ) clause causes the C statements in
string to be inserted in the generated stub code
instead of the default calling sequence _res = ident(param1, ..., paramN).
Thus, the statements in string find the converted parameters in
local variables that have the same names as the parameters in the IDL
declaration, and should leave the result of the function, if any, in
the local variable named _res.
A quote(dealloc, string ) clause causes the C statements in
string to be inserted in the generated stub code just before the
stub function returns, hence after the conversion of the C function
results to Caml values. Again, the statements in string have access
to the function result in the local variable named _res, and to out
parameters in local variables having the same names as the
parameters. Since the function results and out parameters have
already been converted to Caml values, the code in string can safely
deallocate the data structures they point to.
Custom calling sequences are typically used to rearrange or combine
function parameters, and to perform extra error checks on the
arguments and results. For instance, the Unix write system call can
be specified in IDL as follows:
int write([in] int fd,
[in,string,length_is(len)] char * data,
[in] int len,
[in] int ofs,
[in] int towrite)
quote(call,
" /* Validate the arguments */
if (ofs < 0 || ofs + towrite >= len) failwith(\"write\");
/* Perform the write */
_res = write(fd, data + ofs, towrite);
/* Validate the result */
if (_res == -1) failwith(\"write\"); ");
Custom deallocation sequences are useful to free data structures
dynamically allocated and returned by the C function. For instance,
a C function f that returns a malloc-ed string can be specified in
IDL as follows:
[string] char * f([in] int x)
quote(dealloc, "free(_res); ");
If the string is returned as an out parameter instead, we would write:
void f ([in] int x, [out, string*] char ** str)
quote(dealloc, "free(*str); ");
Blocking functions:
A function can be given the attribute blocking to indicate that it
may block on an input/output operation. The generated code will then
allow other Caml threads to execute concurrently with the operation.
3.10 Interfaces
IDL interfaces that do not have the object attribute are
essentially ignored. That is, the declarations contained in the
interface are processed as if they occurred at the top-level of the
IDL file. The pointer_default, int_default and
long_default attributes to the interface can be
used to specify the default pointer kind and integer mappings
for the declarations contained in the interface. Other attributes, as
well as the name of the super-interface if any, are ignored.
IDL interfaces having the object attribute specify COM-style object
interfaces. The function declarations contained in the interface
specify the methods of the COM interface. Other kinds of declarations
(type declarations, import statements, etc) are treated as if they
occurred at the top-level of the IDL file. An optional
super-interface can be given, in which case the COM interface
implements the methods of the super-interface in addition to those
specified in the IDL interface.
Example:
[object, uuid(...)] interface IA { typedef int t; int f(int x); }
[object] interface IB : IA { import "foo.idl"; void g([string] char * s); }
This defines a type t and imports the file foo.idl as usual. In
addition, two interfaces are declared: IA, containing one
method f from int to int, and IB, containing
two methods, f from int to int and g from string to unit.
The definition of an object interface i generates the following
Caml definitions:
-
An abstract type i identifying the interface.
COM interfaces of type i are represented in Caml
with type i Com.interface.
- If a super-interface s is given, a conversion function
s_of_i of type
i Com.interface -> s Com.interface.
- If the uuid(iid) attribute is given, a value
iid_i of type i Com.iid holding the given interface
identifier.
- A Caml class i_class, with the same methods as the COM
interface.
- A function use_i of type i Com.interface -> i_class, to transform a COM object into a Caml object. This
allows the methods of the COM object to be invoked from Caml.
- A function make_i of type #i_class -> i
Com.interface, to transform a Caml object into a COM object with
interface i. This allows the methods of the Caml object to be
invoked from any COM client.
Example: in the IA and IB example above, the following Caml
definitions are generated for IA:
type iA
val iid_iA : iA Com.iid
class iA_class : iA Com.interface -> object method f : int -> int end
val use_iA : iA Com.interface -> iA_class
val make_iA : #iA_class -> iA Com.interface
For IB, we get:
type iB
val iA_of_iB : iB Com.interface -> iA Com.interface
class iB_class :
iB Com.interface -> object inherit iA_class method g : string -> unit end
val use_iB : iB Com.interface -> iB_class
val make_iB : #iB_class -> iB Com.interface
Error handling in interfaces:
Conventionally, methods of
COM interfaces always return a result of type HRESULT that says
whether the method succeeded or failed, and in the latter case returns
an error code to its caller.
When calling an interface method from Caml, if the method returns an
HRESULT denoting failure, the exception Com.Error is raised with a
message describing the error. Successful HRESULT return values are
ignored. To make them available to Caml, camlidl defines the types
HRESULT_bool and HRESULT_int. If those types are used as return
types instead of HRESULT, failure results are mapped to
Com.Error exceptions as before, but successful results are mapped to
the Caml types bool and int respectively. (For HRESULT_bool,
the S_OK result is mapped to true and other successful results are
mapped to false. For HRESULT_int, the low 16 bits of the result
code are returned as a Caml int.)
When calling a Caml method from a COM client, any exception that
escapes the Caml method is mapped back to a failure HRESULT. A
textual description of the uncaught exception is saved using
SetLastError, and can be consulted by the COM client using
GetLastError (this is the standard convention for passing extended
error information in COM).
If the IDL return type of the method is not one of the HRESULT
types, any exception escaping the Caml method aborts the whole program
after printing a description of the exception. Hence, programmers of
Caml components should either use HRESULT as result type, or make
very sure that all exceptions are properly caught by the method.
4 Using camlidl
4.1 Overview
The camlidl stub generator is invoked as follows:
camlidl options file1.idl file2.idl ...
For each file f.idl given on the command line, camlidl
generates the following files:
-
A Caml interface file f.mli that defines the Caml view
of the IDL file. It contains Caml definitions for the types declared
in the IDL file, as well as declarations for the functions and the
interfaces.
- A Caml implementation file f.ml that implements the
f.mli file.
- A C source file f_stubs.c that contains the stub functions for
converting between C and Caml data representations.
- If the -header option is given, a C header file f.h
containing C declarations for the types declared in the IDL file.
The generated .ml and .c files must be compiled and linked with
the remainder of the Caml program.
4.2 Options
The following command-line options are recognized by camlidl.
- -cpp
-
Pre-process the source IDL files with the C preprocessor. This option
is set by default.
- -D symbol=value
-
Define a preprocessor symbol. The option -Dsymbol=value
is passed to the C preprocessor. The value can be omitted,
as in -D symbol, and defaults to 1.
- -header
-
Generate a C header file f.h containing C declarations for the
types and functions declared in the IDL file f.c.
- -I dir
-
Add the directory dir to the list of directories searched for
.idl files, as given on the command line or recursively loaded
by import statements.
- -keep-labels
-
Keep the Caml names of record labels as specified in the IDL file.
Do not prefix them with the name of the enclosing struct, even if they
appear in several struct definitions.
- -nocpp
-
Suppresses the pre-processing of source IDL files.
- -no-include
-
By default, camlidl emits a #include "f.h" statement in
the file f.c containing the generated C code.
The f.h header file being included is
either the one generated by camlidl -header, or generated by another
tool (such as Microsoft's midl compiler) from the IDL file, or
hand-written. The f.h file is assumed to provide all C type
declarations needed for compiling the stub code.
The -no-include option suppresses the automatic inclusion of the
f.h file. The IDL file should then include the right header
files and provide the right type declarations via quote statements.
- -prefix-all-labels
-
Prefix all Caml names of record labels with the name of the enclosing
struct. The default is to prefix only those labels that could cause
ambiguity because they appear in several struct definitions.
- -prepro preprocessing-command
-
Set the command that is executed to pre-process the source IDL files.
The default is the C preprocessor.
4.3 The camlidldll script
Under Windows, a bash script called camlidldll is provided to
automate the construction of a DLL containing a COM component written
in Caml.
The script camlidldll accepts essentially the same command-line
arguments and options as the ocamlc compiler. (It also accepts
.tlb type library files on the command-line; see
section 6.3, ``Dispatch interfaces'', for more
information on type libraries.)
It produces a DLL file that encapsulates the Caml and C object files
given on the command line.
Use regsvr32 /s file.dll to record the components in the
system registry once it is compiled to a DLL.
5 Module Com: run-time library for COM components
type 'a interface
The type of COM components implementing interface 'a
type 'a iid
The type of the interface identifier for interface 'a
type clsid
The type of component identifiers
type 'a opaque
The type representing opaque pointers to values of type 'a
.
Opaque pointers are pointers with attribute ptr
in IDL files.
exception Error of int * string * string
Exception raised to report Com errors.
The arguments are Error(errcode, who, what)
.
errcode
is the Com error code (HRESULT
code)
with the high bit clear.
who
identifies the function or method that raised the exception.
what
is a message explaining the cause of the error.
val initialize : unit -> unit
Initialize the COM library. Must be called once before
using any function in this module. Com.initialize
can be called several times, provided that Com.uninitialize
is called an equal number of times before the program exits.
val uninitialize : unit -> unit
Terminate the COM library.
val query_interface : 'a interface -> 'b iid -> 'b interface
Com.query_interface comp iid
asks the component comp
whether it supports the interface identified by iid
.
If yes, it returns the corresponding interface of the component.
If not, it raises Com.Error
.
type iUnknown
The type of the interface IUnknown
, from which all other
interfaces derive.
type iDispatch
The type of the interface IDispatch
, from which all
dispatch interfaces derive.
val iUnknown_of : 'a interface -> iUnknown interface
Return the IUnknown
interface of the given component.
This operation never fails, since all components support
the IUnknown
interface.
val combine : 'a interface -> 'b interface -> 'a interface
Combine the interfaces of two components.
Com.combine c1 c2
returns a component that supports the
union of the interfaces supported by c1
and c2
.
When queried for an interface, the resulting component
delegates its implementation to c1
if c1
implements that
interface, and otherwise delegates its implementation to c2
.
val clsid : string -> clsid
Parse the string representation of a component identifier
(hex8-hex4-hex4-hex4-hex12
, where hexN
represents N
hexadecimal digits).
val create_instance : clsid -> 'a iid -> 'a interface
Com.create_instance clsid iid
creates an instance of
the component identified by clsid
, and returns its iid
interface. The implementation of the component is searched
in the registry; if the component is implemented in a DLL,
the DLL is loaded in memory if necessary; if the component
is implemented in a separate server process, the server is
started if necessary. Raise Com.Error
if the component
clsid
cannot be found, or if it does not support interface
iid
.
type 'a component_factory =
{ create : unit -> 'a interface;
clsid : clsid;
friendly_name : string;
ver_ind_prog_id : string;
prog_id : string }
Informations required for registering a Caml implementation
of a component.
create
is a function that returns a fresh instance
of the component.
clsid
is the component identifier.
friendly_name
is a short description of the component
(for information only).
ver_ind_prog_id
and prog_id
are symbolic names for the
component. By convention, prog_id
is ver_ind_prog_id
plus
a version number at the end, i.e. ver_ind_prog_id
is
"MyCamlComponent"
while prog_id
is "MyCamlComponent.3"
.
val register_factory : 'a component_factory -> unit
Register a Caml implementation of a component.
Com.register_factory f
stores the component factory f
in the registry. Other programs can then create instances
of the component by calling CreateInstance
from C and C++
or Com.create_instance
from Caml.
type hRESULT_int = int
type hRESULT_bool = bool
type bSTR = string
The Caml types corresponding to the IDL types HRESULT_int
,
HRESULT_bool
and BSTR
, respectively.
6 Hints on writing IDL files
6.1 Writing an IDL file for a C library
When writing an IDL file for a C library that doesn't have an IDL interface
already, the include files for that library are a good starting point:
just copy the relevant type and functin declarations to the IDL file,
then annotate them with IDL attributes to describe more precisely
their actual behavior. The documentation of the library must be read
carefully to determine the mode of function parameters (in, out,
inout), the actual sizes of arrays, etc.
The type definitions in the IDL file need not correspond exactly with
those in the include files. Often, a cleaner Caml interface can be
obtained by omitting irrelevant struct fields, or changing their types.
For instance, the Unix library functions for reading library entries
may use the following structure:
struct dirent {
long int d_ino;
__off_t d_off;
unsigned short int d_reclen;
unsigned char d_type;
char d_name[256];
};
Of those fields, only d_name and d_ino are of interest to the
user; the other fields are internal information for the library
functions, are not specified in the POSIX specs, and therefore must
not be used. Thus, in the IDL file, you should declare:
struct dirent {
long int d_ino;
char d_name[256];
};
Thus, the Caml code will have
type struct_dirent = {d_ino: int; d_name: string}
as desired. However, the generated stub code, being
compiled against the ``true'' definition of struct dirent, will find
those two fields at the correct offsets in the actual struct.
Special attention must be paid to integer fields or variables.
By default, integer IDL types are mapped to the Caml type int,
which is convenient to use in Caml code, but loses one bit
when converting from a C long integer, and may lose one bit (on
32-bit platforms) when converting from a C int integer. When the
range of values represented by the C integer is small enough, this
loss is acceptable. Otherwise, you should use the attributes
nativeint, int32 or int64 so that integer IDL types are mapped
to one of the Caml boxed integer types. (We recommend that you use
int32 or int64 for integers that are specified as being exactly 32
bit wide or 64 bit wide, and nativeint for unspecified int or
long integers.)
Yet another possibility is to declare certain integer fields or variables
as double in the IDL file, so that they are represented by float
in Caml, and all 32 bits of the integer are preserved in Caml. For
instance, the Unix function to get the current type is declared as
time_t time(time_t * t);
where time_t is usually defined as long. We can nonetheless
pretend (in the IDL file) that time returns a double:
double time() quote(" _res = time(NULL); ");
This way, time will have the Caml type unit -> float.
Again, the stub code ``knows'' that time actually returns an integer,
and therefore will insert the right integer-float coercions.
6.2 Sharing IDL files between MIDL and CamlIDL
The Microsoft software development kit provides a number of IDL files
describing various libraries and components. In its current state,
camlidl cannot exploit those files directly: they use many
(often poorly documented) Microsoft IDL features that are not
implemented yet in camlidl; symmetrically, camlidl introduces
several new annotations that are not recognized by Microsoft's midl
compiler. So, significant editing work on the IDL files is required.
The C preprocessor can be used to alleviate the camlidl-midl
incompatibilities: camlidl defines the preprocessor symbol CAMLIDL
when preprocessing its input files, while midl does not. Hence,
one can bracket incompatible definitions in
#ifdef CAMLIDL ... #else ... #endif. Along these lines, a C
preprocessor header file, camlidlcompat.h, is provided: it uses
#define to remove camlidl-specific attributes when compiling with
midl, and to remove midl-specific attributes when compiling with
camlidl. Thus, an IDL file compatible with both midl and
camlidl would look like this:
#include <camlidlcompat.h>
#ifndef CAMLIDL
import "unknwn.idl"; // imports specific to MIDL
import "oaidl.idl";
#endif
import "mymodule.idl"; // imports common to MIDL and CamlIDL
typedef [abstract,marshal_as(int)] void * ptr;
...
#ifndef CAMLIDL
[...] library MyTypeLib {
importlib("stdole32.tlb");
[...] coclass MyComponent { [default] interface IX; }
}
#endif
Notice that since camlidl doesn't handle type libraries, the type
library part of an midl file must be enclosed in #ifndef CAMLIDL.
6.3 Dispatch interfaces and type libraries
A dispatch interface, in COM lingo, is an interface that supports
dynamic, interpreted dispatch of method interfaces. This form of
interpreted dispatch is used by Visual Basic and other scripting
languages to perform calls to methods of COM components.
CamlIDL provides minimal support for dispatch interfaces. To equip a
Caml component with a dispatch interface (thus making it callable from
Visual Basic), you need to do the following:
-
Use IDispatch instead of IUnknown as the super-interface of
the component's interfaces.
- Write a type library for your component and compile it using
midl. A type library is a run-time representation of the interfaces
supported by an object. The midl compiler can generate a type
library from the IDL description of the component, enriched with some
special-purpose declarations (the library and coclass
statements). Refer to the documentation of midl for more
information.
- Pass the type library files (.tlb files) generated by midl
as extra arguments to camlidldll when generating the DLL for your
Caml component.
7 Release notes
Here are some caveats and open issues that apply to the current
release.
Deallocation of function results and out parameters:
If a C function dynamically allocates some of its outputs (either
returned or stored in out parameters), its IDL declaration must
contain a quote(dealloc, string ) clause to properly free the
space occupied by those outputs after they have been converted to
Caml. Otherwise, memory leaks will occur. (The only exception is
results and output parameters of type [bigarray,managed] ty[],
where the Caml garbage collector takes care of deallocation.)
This does not conform to the MIDL and COM specifications, which say
that space for out data structures must be allocated
with CoTaskMemAlloc by the callee, and automatically freed
using CoTaskMemFree by the generated stub code. (The specs don't
say what happens with the return value of the function.)
However, there are many functions in Win32 (not to mention the
Unix world) that do not follow this convention, and
return data structures (e.g. strings) that are statically
allocated, or require special deallocation functions. Hence,
camlidl leaves deallocation of outputs entirely under user control.
Allocation and deallocation of in,out parameters:
For in,out parameters, the MIDL/COM rules are that the caller (the
stub code) should allocate the inputs, the callee should free them
and allocate again its outputs, and the caller should free the outputs.
As explained above, camlidl-generated stubs don't automatically free
the outputs. Worse, the inputs passed to the functions are allocated
partially on the stack and partially in the heap
(using CoTaskMemAlloc), so the callee may perform an incorrect
free on a stack-allocated argument. The best thing to do is avoid
in,out parameters entirely, and split them into one in and one
out parameter.
Reference-counting of COM interfaces:
Caml finalized objects are used to call Release automatically on COM
interfaces that become unreachable. The reference counting of
interfaces passed as in and out parameters is correctly
implemented. However, in,out parameters that are interfaces are not
correctly handled. Again, avoid in,out parameters.
COM support:
The support for COM is currently quite small. COM components
registered in the system registry can be imported via
Com.create_instance. Components written in Caml can be exported as
DLLs, but not yet as standalone servers. Preliminary support for
dispatch interfaces is available, however many of the data types used
in the Automation framework are not supported yet (e.g. SAFEARRAY).
This document was translated from LATEX by
HEVEA.