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<h2 id="sec18" class="section">3 The Caml-IDL mapping</h2>
<p>This section describes how IDL types, function declarations, and
interfaces are mapped to Caml types, functions and classes.</p>
<h3 id="sec19" class="subsection">3.1 Base types</h3>
<div class="tableau">
<div class="center"><table border=1 style="border-spacing:0;width:80%" class="cellpadding1"><tr><td style="text-align:center;border:solid 1px;white-space:nowrap" ><span class=font-bold>IDL type </span><span class=font-bold><span class=font-it>ty</span></span></td><td style="text-align:center;border:solid 1px;white-space:nowrap" ><span class=font-bold>Caml type </span>[[<span class=font-it>ty</span>]] </td></tr>
<tr><td style="text-align:left;border:solid 1px;white-space:nowrap" >
<span class=machine><span class=font-tt>byte</span></span>, <span class=machine><span class=font-tt>short</span></span></td><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>int</span></span> </td></tr>
<tr><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>int</span></span>, <span class=machine><span class=font-tt>long</span></span> with <span class=machine><span class=font-tt>[camlint]</span></span> attribute</td><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>int</span></span> </td></tr>
<tr><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>int</span></span>, <span class=machine><span class=font-tt>long</span></span> with <span class=machine><span class=font-tt>[nativeint]</span></span> attribute</td><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>nativeint</span></span> </td></tr>
<tr><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>int</span></span>, <span class=machine><span class=font-tt>long</span></span> with <span class=machine><span class=font-tt>[int32]</span></span> attribute</td><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>int32</span></span> </td></tr>
<tr><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>int</span></span>, <span class=machine><span class=font-tt>long</span></span> with <span class=machine><span class=font-tt>[int64]</span></span> attribute</td><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>int64</span></span> </td></tr>
<tr><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>hyper</span></span>, <span class=machine><span class=font-tt>long long</span></span>, <span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>int64</span></span></td><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>int64</span></span> </td></tr>
<tr><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>char</span></span></td><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>char</span></span> </td></tr>
<tr><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>float</span></span>, <span class=machine><span class=font-tt>double</span></span></td><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>float</span></span> </td></tr>
<tr><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>boolean</span></span></td><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>bool</span></span> </td></tr>
</table></div></div><p>
(For integer types, <span class=machine><span class=font-tt>signed</span></span> and <span class=machine><span class=font-tt>unsigned</span></span> variants of the same IDL
integer type translate to the same Caml type.)</p><p>Depending on the attributes, the <span class=machine><span class=font-tt>int</span></span> and <span class=machine><span class=font-tt>long</span></span> integer types are
converted to one of the Caml integer types <span class=machine><span class=font-tt>int</span></span>, <span class=machine><span class=font-tt>nativeint</span></span>,
<span class=machine><span class=font-tt>int32</span></span>, or <span class=machine><span class=font-tt>int64</span></span>. Values of Caml type <span class=machine><span class=font-tt>int32</span></span> are exactly 32-bit wide
and values of type <span class=machine><span class=font-tt>int64</span></span> are exactly 64-bit wide on all platforms.
Values of type <span class=machine><span class=font-tt>nativeint</span></span> have the natural word size of the platform,
and are large enough to accommodate any C <span class=machine><span class=font-tt>int</span></span> or <span class=machine><span class=font-tt>long int</span></span> without
loss of precision. Values of Caml type <span class=machine><span class=font-tt>int</span></span> have the natural word
size of the platform minus one bit of tag, hence the conversion from IDL
types <span class=machine><span class=font-tt>int</span></span> and <span class=machine><span class=font-tt>long</span></span> loses the most significant bit on 32-bit
platforms. On 64-bit platforms, the conversion from <span class=machine><span class=font-tt>int</span></span> is exact,
but the conversion from <span class=machine><span class=font-tt>long</span></span> loses the most significant bit.</p><p>If no explicit integer attribute is given for an <span class=machine><span class=font-tt>int</span></span> or <span class=machine><span class=font-tt>long</span></span> type,
the <span class=machine><span class=font-tt>int</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>default</span></span> or <span class=machine><span class=font-tt>long</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>default</span></span> attribute of the enclosing
interface, if any, determines the kind of the integer.
If no <span class=machine><span class=font-tt>int</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>default</span></span> or <span class=machine><span class=font-tt>long</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>default</span></span> attribute is in scope, the kind
<span class=machine><span class=font-tt>camlint</span></span> is assumed, which maps IDL <span class=machine><span class=font-tt>int</span></span> and <span class=machine><span class=font-tt>long</span></span> types to the
Caml <span class=machine><span class=font-tt>int</span></span> type.</p>
<h3 id="sec20" class="subsection">3.2 Pointers</h3>
<p>The mapping of IDL pointer types depends on their kinds. Writing
[[<span class=font-it>ty</span>]] for the Caml type corresponding to the IDL type
<span class=font-it>ty</span>, we have:
</p><pre>
[ref] <span class=font-it>ty</span> * ⇒ [[<span class=font-it>ty</span>]]
[unique] <span class=font-it>ty</span> * ⇒ [[<span class=font-it>ty</span>]] option
[ptr] <span class=font-it>ty</span> * ⇒ [[<span class=font-it>ty</span>]] Com.opaque
</pre><p>
In other terms, IDL pointers of kind <span class=machine><span class=font-tt>ref</span></span> are ignored during the mapping:
<span class=machine><span class=font-tt>[ref] </span></span><span class=font-it>ty</span><span class=machine><span class=font-tt> *</span></span> is mapped to the same Caml type as <span class=font-it>ty</span>.
A pointer <span class=font-it>p</span> to a C value <span class=font-it>c</span><span class=machine><span class=font-tt> = *</span></span><span class=font-it>p</span> is translated to
the Caml value corresponding to <span class=font-it>c</span>.</p><p>IDL pointers of kind <span class=machine><span class=font-tt>unique</span></span> are mapped to an <span class=machine><span class=font-tt>option</span></span> type. The
option value is <span class=machine><span class=font-tt>None</span></span> for a null pointer, and <span class=machine><span class=font-tt>Some(</span></span><span class=font-it>v</span><span class=machine><span class=font-tt>)</span></span>
for a non-null pointer to a C value <span class=font-it>c</span> that translates to the ML
value <span class=font-it>v</span>.</p><p>IDL pointers of kind <span class=machine><span class=font-tt>ptr</span></span> are mapped to a <span class=machine><span class=font-tt>Com.opaque</span></span> type.
This is an abstract type that encapsulates the C pointer without
attempting to convert it to an ML data structure.</p><p>IDL pointers of kind <span class=machine><span class=font-tt>ignore</span></span> 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.</p><p>If no explicit pointer kind is given, the <span class=machine><span class=font-tt>pointer</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>default</span></span> attribute
of the enclosing interface, if any, determines the kind of the pointer.
If no <span class=machine><span class=font-tt>pointer</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>default</span></span> attribute is in scope, the kind <span class=machine><span class=font-tt>unique</span></span> is
assumed.</p>
<h3 id="sec21" class="subsection">3.3 Arrays</h3>
<p>IDL arrays of characters that carry the <span class=machine><span class=font-tt>[string]</span></span> attribute are mapped
to the Caml <span class=machine><span class=font-tt>string</span></span> type:
</p><div class="tableau">
<div class="center"><table border=1 style="border-spacing:0;width:80%" class="cellpadding1"><tr><td style="text-align:center;border:solid 1px;white-space:nowrap" ><span class=font-bold>IDL type </span><span class=font-bold><span class=font-it>ty</span></span></td><td style="text-align:center;border:solid 1px;white-space:nowrap" ><span class=font-bold>Caml type </span>[[<span class=font-it>ty</span>]] </td></tr>
<tr><td style="text-align:left;border:solid 1px;white-space:nowrap" >
<span class=machine><span class=font-tt>[string] char []</span></span></td><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>string</span></span> </td></tr>
<tr><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>[string] unsigned char []</span></span></td><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>string</span></span> </td></tr>
<tr><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>[string] signed char []</span></span></td><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>string</span></span> </td></tr>
<tr><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>[string] byte []</span></span></td><td style="text-align:left;border:solid 1px;white-space:nowrap" ><span class=machine><span class=font-tt>string</span></span> </td></tr>
</table></div></div><p>
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.</p><p>IDL arrays of characters that carry the <span class=machine><span class=font-tt>[byte]</span></span> attribute are mapped
to the Caml <span class=machine><span class=font-tt>bytes</span></span> type of byte arrays.</p><p>IDL arrays carrying the <span class=machine><span class=font-tt>[bigarray]</span></span> attribute are translated to Caml
“big arrays”, as described in the next section.</p><p>All other IDL arrays are translated to ML arrays:
</p><pre>
<span class=font-it>ty</span> [] ⇒ [[<span class=font-it>ty</span>]] array
</pre><p>
For instance, <span class=machine><span class=font-tt>double []</span></span> becomes <span class=machine><span class=font-tt>float array</span></span>.
Consequently, multi-dimensional arrays are translated to Caml arrays
of arrays. For instance, <span class=machine><span class=font-tt>int [][]</span></span> becomes <span class=machine><span class=font-tt>int array array</span></span>.</p><p>If the <span class=machine><span class=font-tt>unique</span></span> attribute is given, the IDL array is translated to an
ML option type:
</p><pre>
[string,unique] char [] ⇒ string option
[unique] <span class=font-it>ty</span> [] ⇒ [[<span class=font-it>ty</span>]] array option
</pre><p>
As in the case of pointers of kind <span class=machine><span class=font-tt>unique</span></span>, the option value is
<span class=machine><span class=font-tt>None</span></span> for a null C pointer, and <span class=machine><span class=font-tt>Some(</span></span><span class=font-it>v</span><span class=machine><span class=font-tt>)</span></span> for a non-null
C pointer to a C array that translates to the ML string or array <span class=font-it>v</span>.</p><p>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):
</p><ul class="itemize"><li class="li-itemize">
By the <span class=machine><span class=font-tt>length</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>is</span></span> attribute, if present.
</li><li class="li-itemize">By the <span class=machine><span class=font-tt>size</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>is</span></span> attribute, if present.
</li><li class="li-itemize">By the bound written in the array type, if any.
</li><li class="li-itemize">By searching the first null element of the C array, if the
<span class=machine><span class=font-tt>null</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>terminated</span></span> attribute is present.
</li></ul><p>
For instance, C values of IDL type <span class=machine><span class=font-tt>[length</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>is(n)] double[]</span></span> are
mapped to Caml <span class=machine><span class=font-tt>float array</span></span> of <span class=machine><span class=font-tt>n</span></span> elements. C values of IDL type
<span class=machine><span class=font-tt>double[10]</span></span> are mapped to Caml <span class=machine><span class=font-tt>float array</span></span> of 10 elements.</p><p>The <span class=machine><span class=font-tt>length</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>is</span></span> and <span class=machine><span class=font-tt>size</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>is</span></span> attributes take as argument one or
several limited expressions. Each expression applies to one dimension
of the array. For instance, <span class=machine><span class=font-tt>[size</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>is(*dimx, *dimy)] double d[][]</span></span>
specifies a matrix of <span class=machine><span class=font-tt>double</span></span> whose first dimension has size
<span class=machine><span class=font-tt>*dimx</span></span> and the second has size <span class=machine><span class=font-tt>*dimy</span></span>.</p>
<h3 id="sec22" class="subsection">3.4 Big arrays</h3>
<p>IDL arrays of integers or floats that carry the <span class=machine><span class=font-tt>[bigarray]</span></span> attribute
are mapped to one of the Caml <span class=machine><span class=font-tt>Bigarray</span></span> types: <span class=machine><span class=font-tt>Array1.t</span></span> for
one-dimensional arrays, <span class=machine><span class=font-tt>Array2.t</span></span> for 2-dimensional arrays,
<span class=machine><span class=font-tt>Array3.t</span></span> for 3-dimensional arrays, and <span class=machine><span class=font-tt>Genarray.t</span></span> for arrays of 4
dimensions or more.</p><p>If the <span class=machine><span class=font-tt>[fortran]</span></span> 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).</p><p>If the <span class=machine><span class=font-tt>[managed]</span></span> 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 <span class=machine><span class=font-tt>malloc()</span></span>, 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.</p><p>Finally, the <span class=machine><span class=font-tt>[unique]</span></span> attribute applies to bigarrays as to arrays,
that is, it maps a null C pointer to <span class=machine><span class=font-tt>None</span></span>, and a non-null C pointer
<span class=font-it>p</span> to <span class=machine><span class=font-tt>Some(</span></span><span class=font-it>v</span><span class=machine><span class=font-tt>)</span></span> where <span class=font-it>v</span> is the ML bigarray
resulting from the translation of <span class=font-it>p</span>.</p>
<h3 id="sec23" class="subsection">3.5 Structs</h3>
<p>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:
</p><pre>
struct <span class=font-it>s</span> { ... ; <span class=font-it>ty</span><sub><span class=font-it>i</span></sub> <span class=font-it>id</span><sub><span class=font-it>i</span></sub> ; ... } becomes type <span class=font-it>s</span> = { ... ; <span class=font-it>id</span><sub><span class=font-it>i</span></sub> : [[<span class=font-it>ty</span><sub><span class=font-it>i</span></sub>]] ; ... }
</pre><p>
Example: <span class=machine><span class=font-tt>struct s </span></span><span class=machine><span class=font-tt>{</span></span><span class=machine><span class=font-tt> int n; double d[4]; </span></span><span class=machine><span class=font-tt>}</span></span> becomes
<span class=machine><span class=font-tt>type s = </span></span><span class=machine><span class=font-tt>{</span></span><span class=machine><span class=font-tt>n: int; d: float array</span></span><span class=machine><span class=font-tt>}</span></span>.</p><p>Exceptions to this rule are as follows:
</p><ul class="itemize"><li class="li-itemize">
Fields of the IDL struct that are pointers with the <span class=machine><span class=font-tt>[ignore]</span></span>
attribute do not appear in the Caml record type.
Example: <span class=machine><span class=font-tt>struct s </span></span><span class=machine><span class=font-tt>{</span></span><span class=machine><span class=font-tt> double x,y; [ignore] void * data; </span></span><span class=machine><span class=font-tt>}</span></span>
becomes <span class=machine><span class=font-tt>type struct</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>s = </span></span><span class=machine><span class=font-tt>{</span></span><span class=machine><span class=font-tt>x : float; y: float</span></span><span class=machine><span class=font-tt>}</span></span>.
Those ignored pointer fields are set to <span class=machine><span class=font-tt>NULL</span></span> when converting from a
Caml record to a C struct.</li><li class="li-itemize">Integer fields of the IDL struct that appear in a <span class=machine><span class=font-tt>length</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>is</span></span>,
<span class=machine><span class=font-tt>size</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>is</span></span> or <span class=machine><span class=font-tt>switch</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>is</span></span> attribute of another field also do not appear
in the Caml record type. (We call those fields <em>dependent</em> fields.)
Example: <span class=machine><span class=font-tt>struct s </span></span><span class=machine><span class=font-tt>{</span></span><span class=machine><span class=font-tt> int idx; int len; [size</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>is(len)] double d[]; </span></span><span class=machine><span class=font-tt>}</span></span>
is translated to the Caml record type
<span class=machine><span class=font-tt>type struct</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>s = </span></span><span class=machine><span class=font-tt>{</span></span><span class=machine><span class=font-tt>idx: int; d: float array</span></span><span class=machine><span class=font-tt>}</span></span>.
The value of <span class=machine><span class=font-tt>len</span></span> is recovered from the size of the Caml array <span class=machine><span class=font-tt>d</span></span>,
and thus doesn’t need to be represented explicitly in the Caml record.</li><li class="li-itemize">If, after elimination of ignored pointer fields and dependent
fields as described above, the IDL struct has only one field
<span class=font-it>ty</span> <span class=font-it>id</span>, we avoid creating a one-field Caml record type
and translate the IDL struct type directly to the Caml type
[[<span class=font-it>ty</span>]].
Example: <span class=machine><span class=font-tt>struct s </span></span><span class=machine><span class=font-tt>{</span></span><span class=machine><span class=font-tt> int len; [size</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>is(len)] double d[]; </span></span><span class=machine><span class=font-tt>}</span></span>
is translated to the Caml type abbreviation <span class=machine><span class=font-tt>type struct</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>s = double array</span></span>.</li><li class="li-itemize">The names of labels in the Caml record type can be changed by
using the <span class=syntax-token>mlname</span> attribute on struct field declarations. For instance,
<pre>
struct s { int n; [mlname(p)] int q; }
becomes type s = { n : int; p : int }
</pre></li><li class="li-itemize">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:
<pre>
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 }
</pre>
The labels for <span class=machine><span class=font-tt>s1</span></span> and <span class=machine><span class=font-tt>s2</span></span> have been prefixed by <span class=machine><span class=font-tt>s1</span></span><span class=machine><span class=font-tt>_</span></span> and
<span class=machine><span class=font-tt>s2</span></span><span class=machine><span class=font-tt>_</span></span> respectively, to avoid ambiguity on the <span class=machine><span class=font-tt>x</span></span> label. However, the
label <span class=machine><span class=font-tt>z</span></span> for <span class=machine><span class=font-tt>s3</span></span> is not prefixed, since it is not used elsewhere.<p>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:
</p><pre>
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 }
</pre><p>The “minimal prefixing” strategy described above is the default
behavior of <span class=machine><span class=font-tt>camlidl</span></span>. If the <span class=machine><span class=font-tt>-prefix-all-labels</span></span> option is given,
all record labels are prefixed, whether they occur several times or
not. If the <span class=machine><span class=font-tt>-keep-labels</span></span> option is given, no automatic prefixing
takes place; the naming of record labels is left entirely under the
user’s control, via <span class=syntax-token>mlname</span> annotations.</p></li></ul>
<h3 id="sec24" class="subsection">3.6 Unions</h3>
<p>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 <span class=machine><span class=font-tt>default</span></span> case, an extra constructor
<span class=machine><span class=font-tt>Default</span></span><span class=machine><span class=font-tt>_</span></span><span class=font-it>unionname</span> is added to the Caml sum type, carrying an
<span class=machine><span class=font-tt>int</span></span> argument (the value of the discriminating field),
and possibly another argument corresponding to the default field.
Examples:
</p><pre>
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
</pre><p>
All IDL unions must be discriminated, either via the special syntax
<span class=machine><span class=font-tt>union </span></span><span class=font-it>name</span><span class=machine><span class=font-tt> switch(int </span></span><span class=font-it>discr</span><span class=machine><span class=font-tt>)</span></span>…, or via the
attribute <span class=machine><span class=font-tt>switch</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>is(</span></span><span class=font-it>discr</span><span class=machine><span class=font-tt>)</span></span>, where <span class=font-it>discr</span> is a C l-value
built from other parameters of the current function, or other fields
of the current <span class=machine><span class=font-tt>struct</span></span>. Both the discriminant and the
case labels must be of an integer type. Unless a <span class=machine><span class=font-tt>default</span></span> case is
given, the value of the discriminant must be one of the cases of the
union.</p>
<h3 id="sec25" class="subsection">3.7 Enums</h3>
<p>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:
<span class=machine><span class=font-tt>enum e </span></span><span class=machine><span class=font-tt>{</span></span><span class=machine><span class=font-tt> A, B = 2, C = 4 </span></span><span class=machine><span class=font-tt>}</span></span> becomes <span class=machine><span class=font-tt>type enum</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>e = A | B | C</span></span>.</p><p>The <span class=syntax-token>set</span> 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:
</p><pre>enum e { A = 1, B = 2, C = 4 };
typedef [set] enum e eset;
</pre><p>The Caml type <span class=machine><span class=font-tt>eset</span></span> is equal to <span class=machine><span class=font-tt>enum</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>e list</span></span>.
The C integer 6 (= <span class=machine><span class=font-tt>B | C</span></span>) is translated to the ML list <span class=machine><span class=font-tt>[B; C]</span></span>.
The ML list <span class=machine><span class=font-tt>[A; C]</span></span> is translated to the C integer <span class=machine><span class=font-tt>A | C</span></span>, that is <span class=machine><span class=font-tt>5</span></span>.</p>
<h3 id="sec26" class="subsection">3.8 Type definitions</h3>
<p>An IDL <span class=machine><span class=font-tt>typedef</span></span> statement is normally translated
to a Caml type abbreviation. For instance,
<span class=machine><span class=font-tt>typedef [string] char * str</span></span> becomes <span class=machine><span class=font-tt>type str = string</span></span>.</p><p>If the <span class=syntax-token>abstract</span> 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,
<span class=machine><span class=font-tt>typedef [abstract] void * handle</span></span> becomes <span class=machine><span class=font-tt>type handle</span></span>.
In this case, the IDL type in the <span class=machine><span class=font-tt>typedef</span></span> is ignored.</p><p>If the <span class=syntax-token>mltype</span> <span class=syntax-token>(</span> <span class=syntax-token>"</span> <span class=nonterminal>caml-type-expr</span> <span class=syntax-token>"</span> <span class=syntax-token>)</span> attribute is given,
the Caml type is made equal to <span class=nonterminal>caml-type-expr</span>. This is often used
in conjunction with the <span class=syntax-token>ml2c</span> and <span class=syntax-token>c2ml</span> attributes to implement
custom translation of data structures between C and ML. For instance,
<span class=machine><span class=font-tt>typedef [mltype("int list")] struct mylist</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>struct * mylist</span></span>
becomes <span class=machine><span class=font-tt>type mylist = int list</span></span>.</p><p>If the <span class=syntax-token>c2ml(</span><span class=nonterminal>funct-name</span><span class=syntax-token>)</span> <span class=nonterminal>and</span> <span class=machine><span class=font-tt>ml2c(</span></span>funct-name<span class=machine><span class=font-tt>)</span></span> 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 <span class=machine><span class=font-tt>camlidl</span></span>-generated
conversion functions. This allows user-controlled translation of data
structures. The prototypes of the conversion functions must be
</p><pre>
value c2ml(<span class=font-it>ty</span> * input);
void ml2c(value input, <span class=font-it>ty</span> * output);
</pre><p>
where <span class=font-it>ty</span> is the name of the type defined by <span class=machine><span class=font-tt>typedef</span></span>. In other
terms, the <span class=machine><span class=font-tt>c2ml</span></span> function is passed a reference to a <span class=font-it>ty</span> and
returns the corresponding Caml value, while the <span class=machine><span class=font-tt>ml2c</span></span> function is
passed a Caml value as first argument and stores the corresponding C
value in the <span class=font-it>ty</span> reference passed as second argument.</p><p>If the <span class=syntax-token>finalize(</span><span class=nonterminal>final-fn</span><span class=syntax-token>)</span> attribute is given in combination with the
<span class=syntax-token>abstract</span> attribute, the function <span class=nonterminal>final-fn</span> 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, <span class=syntax-token>compare(</span><span class=nonterminal>compare-fn</span><span class=syntax-token>)</span> and <span class=syntax-token>hash(</span><span class=nonterminal>hash-fn</span><span class=syntax-token>)</span> 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
<span class=machine><span class=font-tt>compare</span></span>, <span class=machine><span class=font-tt>=</span></span>, <span class=machine><span class=font-tt><</span></span>, etc. The hashing function is called when
<span class=machine><span class=font-tt>Hashtbl.hash</span></span> is applied to a Caml value of this typedef.
The prototype of the finalization, comparison and hashing functions are:
</p><pre>
value <span class=font-it>final-fn</span>(<span class=font-it>ty</span> * x);
int <span class=font-it>compare-fn</span>(<span class=font-it>ty</span> * x, <span class=font-it>ty</span> * y);
long <span class=font-it>hash-fn</span>(<span class=font-it>ty</span> * x);
</pre><p>
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.</p><p>If the <span class=syntax-token>errorcheck(</span><span class=nonterminal>fn</span><span class=syntax-token>)</span> attribute is provided for the <span class=machine><span class=font-tt>typedef</span></span> <span class=font-it>ty</span>,
the error checking function <span class=nonterminal>fn</span> is called each time a function result
of type <span class=font-it>ty</span> is converted from C to Caml. The function can then check
the <span class=font-it>ty</span> value for values indicating an error condition, and raise the
appropriate exception. If in addition the <span class=syntax-token>errorcode</span> attribute is
provided, the conversion from C to Caml is suppressed: values of type
<span class=font-it>ty</span> are only passed to <span class=nonterminal>fn</span> for error checking, then discarded.</p>
<h3 id="sec27" class="subsection">3.9 Functions</h3>
<p>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:
</p><ul class="itemize"><li class="li-itemize">
First, dependent parameters (parameters that are <span class=machine><span class=font-tt>size</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>is</span></span>,
<span class=machine><span class=font-tt>length</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>is</span></span> or <span class=machine><span class=font-tt>switch</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>is</span></span> of other parameters) as well as parameters
that are ignored pointers are removed.</li><li class="li-itemize">The remaining parameters are split into Caml function inputs and
Caml function outputs. Parameters with the <span class=machine><span class=font-tt>[in]</span></span> attribute are added
to the inputs of the function. Parameters with the <span class=machine><span class=font-tt>[out]</span></span>
attribute are added to the outputs of the function. Parameters with
the <span class=machine><span class=font-tt>[in,out]</span></span> attribute are added both to the inputs and to the
outputs of the function, unless they are of type <span class=machine><span class=font-tt>bytes</span></span> or big array,
in which case they are added to the inputs of the function only.
(The reason for this exception is that byte arrays and big arrays are
shared between Caml and C, thus allowing true <span class=machine><span class=font-tt>in,out</span></span> behavior on the
Caml function parameter, while other data types are copied during
Caml/C conversion, thus turning a C <span class=machine><span class=font-tt>in,out</span></span> parameter into a Caml
<span class=machine><span class=font-tt>copy in, copy out</span></span> parameter, that is, one parameter and one result.)</li><li class="li-itemize">The return value of the IDL function is added to the outputs of
the Caml function (in first position), unless it is of type <span class=machine><span class=font-tt>void</span></span> or
of a type name that carries the <span class=machine><span class=font-tt>errorcode</span></span> attribute. In the latter
two cases, the return value of the IDL function is not transmitted to
Caml.
</li><li class="li-itemize">The Caml function is then given type
<span class=nonterminal>in</span><sub>1</sub> <span class=syntax-token>-></span> … <span class=syntax-token>-></span> <span class=nonterminal>in</span><sub><span class=font-it>p</span></sub> <span class=syntax-token>-></span> <span class=nonterminal>out</span><sub>1</sub> <span class=syntax-token>*</span> … <span class=syntax-token>*</span> <span class=nonterminal>out</span><sub><span class=font-it>q</span></sub>
where <span class=nonterminal>in</span><sub>1</sub> … <span class=nonterminal>in</span><sub><span class=font-it>p</span></sub> are the types of its inputs
and <span class=nonterminal>out</span><sub>1</sub> … <span class=nonterminal>out</span><sub><span class=font-it>q</span></sub> are the types of its outputs.
If there are no inputs, a <span class=machine><span class=font-tt>unit</span></span> parameter is added.
If there are no outputs, a <span class=machine><span class=font-tt>unit</span></span> result is added.
</li></ul><p>
Examples:
</p><pre>
int f([in] double x, [in] double y) f : float -> float -> int
</pre><blockquote class="quote"> Two <span class=machine><span class=font-tt>double</span></span> input, one <span class=machine><span class=font-tt>int</span></span> output </blockquote><pre>
void g([in] int x) g : int -> unit
</pre><blockquote class="quote"> One <span class=machine><span class=font-tt>int</span></span> input, no output </blockquote><pre>
int h() h : unit -> int
</pre><blockquote class="quote"> No input, one <span class=machine><span class=font-tt>int</span></span> result </blockquote><pre>
void i([in] int x, [out] double * y) i : int -> double
</pre><blockquote class="quote"> One <span class=machine><span class=font-tt>int</span></span> input, one <span class=machine><span class=font-tt>double</span></span> output (as an <span class=machine><span class=font-tt>out</span></span>
parameter) </blockquote><pre>
int j([in] int x, [out] double * y) j : int -> int * double
</pre><blockquote class="quote"> One <span class=machine><span class=font-tt>int</span></span> input, one <span class=machine><span class=font-tt>int</span></span> output (in the result), one
<span class=machine><span class=font-tt>double</span></span> output (as an <span class=machine><span class=font-tt>out</span></span> parameter) </blockquote><pre>
void k([in,out,ref] int * x) k : int -> int
</pre><blockquote class="quote"> The <span class=machine><span class=font-tt>in,out</span></span> parameter is both one <span class=machine><span class=font-tt>int</span></span> input and one
<span class=machine><span class=font-tt>int</span></span> output. </blockquote><pre>
HRESULT l([in] int x, [out] int * res1, [out] int * res2)
l : int -> int * int
</pre><blockquote class="quote"> <span class=machine><span class=font-tt>HRESULT</span></span> is a predefined type with the <span class=machine><span class=font-tt>errorcode</span></span>
attribute, hence it is ignored. It remains one <span class=machine><span class=font-tt>int</span></span> input and
two <span class=machine><span class=font-tt>int</span></span> outputs (<span class=machine><span class=font-tt>out</span></span> parameters) </blockquote><pre>
void m([in] int len, [in,size_is(len)] double d[])
m : float array -> int
</pre><blockquote class="quote"> <span class=machine><span class=font-tt>len</span></span> is a dependent parameter, hence is ignored. The
only input is the <span class=machine><span class=font-tt>double</span></span> array </blockquote><pre>
void n([in] int inputlen, [out] int * outputlen,
[in,out,size_is(inputlen),length_is(*outputlen)] double d[])
n : float array -> float array
</pre><blockquote class="quote"> The two parameters <span class=machine><span class=font-tt>inputlen</span></span> and <span class=machine><span class=font-tt>outputlen</span></span> are
dependent, hence ignored. The <span class=machine><span class=font-tt>double</span></span> array is both an input
and an output. </blockquote><pre>
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
</pre><blockquote class="quote"> The two parameters <span class=machine><span class=font-tt>dimx</span></span> and <span class=machine><span class=font-tt>dimy</span></span> are dependent
(determined from the dimensions of the big array argument),
hence ignored. The two-dimensional array <span class=machine><span class=font-tt>d</span></span>, although marked <span class=machine><span class=font-tt>[in,out]</span></span>,
is a big array, hence passed as an input that will be modified in
place by the C function <span class=machine><span class=font-tt>p</span></span>. The Caml function has no outputs.
</blockquote>
<h4 id="sec28" class="paragraph">Error checking:</h4>
<p>
For every output that is of a named type with the <span class=syntax-token>errorcheck(</span><span class=nonterminal>fn</span><span class=syntax-token>)</span>
attribute, the error checking function <span class=nonterminal>fn</span> is called after the C
function returns. That function is assumed to raise a Caml exception
if it finds an output denoting an error.</p>
<h4 id="sec29" class="paragraph">Custom calling and deallocation sequences:</h4>
<p>
The IDL declaration for a function can optionally specify a custom
calling sequence and/or a custom deallocation sequence, via <span class=nonterminal>quote</span>
clauses following the function declaration:
</p><div class="syntax"><table class="display dcenter"><tr style="vertical-align:middle"><td class="dcell"><table style="border-spacing:6px;border-collapse:separate;" class="cellpading0"><tr><td style="text-align:right;white-space:nowrap" ><span class=nonterminal>function-decl</span></td><td style="text-align:center;white-space:nowrap" >::=</td><td style="text-align:left;white-space:nowrap" >
<a href="main002.html#attributes" class="syntax"><span class=nonterminal>attributes</span></a> <a href="main002.html#type-spec" class="syntax"><span class=nonterminal>type-spec</span></a> {<span class=syntax-token>*</span>} <a href="main002.html#ident" class="syntax"><span class=nonterminal>ident</span></a> <span class=syntax-token>(</span> <a href="main002.html#params" class="syntax"><span class=nonterminal>params</span></a> <span class=syntax-token>)</span>
{ <span class=syntax-token>quote</span><span class=syntax-token>(</span> <a href="main002.html#ident" class="syntax"><span class=nonterminal>ident</span></a> <span class=syntax-token>,</span> <a href="main002.html#string" class="syntax"><span class=nonterminal>string</span></a> <span class=syntax-token>)</span> } </td></tr>
<tr><td style="text-align:right;white-space:nowrap" > </td></tr>
</table></td></tr>
</table></div><p>
The general shape of a <span class=machine><span class=font-tt>camlidl</span></span>-generated stub function is as
follows:
</p><pre>
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;
</pre><p>
A <span class=syntax-token>quote(call,</span> <a href="main002.html#string" class="syntax"><span class=nonterminal>string</span></a> <span class=syntax-token>)</span> clause causes the C statements in
<a href="main002.html#string" class="syntax"><span class=nonterminal>string</span></a> to be inserted in the generated stub code
instead of the default calling sequence <span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>res = ident(param1, ..., paramN)</span></span>.
Thus, the statements in <a href="main002.html#string" class="syntax"><span class=nonterminal>string</span></a> 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 <span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>res</span></span>.</p><p>A <span class=syntax-token>quote(dealloc,</span> <a href="main002.html#string" class="syntax"><span class=nonterminal>string</span></a> <span class=syntax-token>)</span> clause causes the C statements in
<a href="main002.html#string" class="syntax"><span class=nonterminal>string</span></a> 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 <a href="main002.html#string" class="syntax"><span class=nonterminal>string</span></a> have access
to the function result in the local variable named <span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>res</span></span>, 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 <a href="main002.html#string" class="syntax"><span class=nonterminal>string</span></a> can safely
deallocate the data structures they point to.</p><p>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 <span class=machine><span class=font-tt>write</span></span> system call can
be specified in IDL as follows:
</p><pre> 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\"); ");
</pre><p>Custom deallocation sequences are useful to free data structures
dynamically allocated and returned by the C function. For instance,
a C function <span class=machine><span class=font-tt>f</span></span> that returns a <span class=machine><span class=font-tt>malloc</span></span>-ed string can be specified in
IDL as follows:
</p><pre> [string] char * f([in] int x)
quote(dealloc, "free(_res); ");
</pre><p>If the string is returned as an <span class=machine><span class=font-tt>out</span></span> parameter instead, we would write:
</p><pre> void f ([in] int x, [out, string*] char ** str)
quote(dealloc, "free(*str); ");
</pre>
<h4 id="sec30" class="paragraph">Blocking functions:</h4>
<p>
A function can be given the attribute <span class=machine><span class=font-tt>blocking</span></span> 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.</p>
<h3 id="sec31" class="subsection">3.10 Interfaces</h3>
<p>IDL interfaces that do not have the <span class=syntax-token>object</span> 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 <span class=syntax-token>pointer_default</span>, <span class=syntax-token>int_default</span> and
<span class=syntax-token>long_default</span> 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.</p><p>IDL interfaces having the <span class=syntax-token>object</span> 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, <span class=syntax-token>import</span> 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:
</p><pre>[object, uuid(...)] interface IA { typedef int t; int f(int x); }
[object] interface IB : IA { import "foo.idl"; void g([string] char * s); }
</pre><p>This defines a type <span class=machine><span class=font-tt>t</span></span> and imports the file <span class=machine><span class=font-tt>foo.idl</span></span> as usual. In
addition, two interfaces are declared: <span class=machine><span class=font-tt>IA</span></span>, containing one
method <span class=machine><span class=font-tt>f</span></span> from <span class=machine><span class=font-tt>int</span></span> to <span class=machine><span class=font-tt>int</span></span>, and <span class=machine><span class=font-tt>IB</span></span>, containing
two methods, <span class=machine><span class=font-tt>f</span></span> from <span class=machine><span class=font-tt>int</span></span> to <span class=machine><span class=font-tt>int</span></span> and <span class=machine><span class=font-tt>g</span></span> from <span class=machine><span class=font-tt>string</span></span> to <span class=machine><span class=font-tt>unit</span></span>.</p><p>The definition of an object interface <span class=font-it>i</span> generates the following
Caml definitions:
</p><ul class="itemize"><li class="li-itemize">
An abstract type <span class=font-it>i</span> identifying the interface.
COM interfaces of type <span class=font-it>i</span> are represented in Caml
with type <span class=font-it>i</span> <span class=machine><span class=font-tt> Com.interface</span></span>.
</li><li class="li-itemize">If a super-interface <span class=font-it>s</span> is given, a conversion function
<span class=font-it>s</span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>of</span></span><span class=machine><span class=font-tt>_</span></span><span class=font-it>i</span> of type
<span class=font-it>i</span> <span class=machine><span class=font-tt> Com.interface -> </span></span> <span class=font-it>s</span> <span class=machine><span class=font-tt> Com.interface</span></span>.
</li><li class="li-itemize">If the <span class=machine><span class=font-tt>uuid(</span></span><span class=font-it>iid</span><span class=machine><span class=font-tt>)</span></span> attribute is given, a value
<span class=machine><span class=font-tt>iid</span></span><span class=machine><span class=font-tt>_</span></span><span class=font-it>i</span> of type <span class=font-it>i</span><span class=machine><span class=font-tt> Com.iid</span></span> holding the given interface
identifier.
</li><li class="li-itemize">A Caml class <span class=font-it>i</span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>class</span></span>, with the same methods as the COM
interface.
</li><li class="li-itemize">A function <span class=machine><span class=font-tt>use</span></span><span class=machine><span class=font-tt>_</span></span><span class=font-it>i</span> of type <span class=font-it>i</span> <span class=machine><span class=font-tt> Com.interface -> </span></span><span class=font-it>i</span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>class</span></span>, to transform a COM object into a Caml object. This
allows the methods of the COM object to be invoked from Caml.
</li><li class="li-itemize">A function <span class=machine><span class=font-tt>make</span></span><span class=machine><span class=font-tt>_</span></span><span class=font-it>i</span> of type <span class=machine><span class=font-tt>#</span></span><span class=font-it>i</span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>class -> </span></span><span class=font-it>i</span>
<span class=machine><span class=font-tt> Com.interface</span></span>, to transform a Caml object into a COM object with
interface <span class=font-it>i</span>. This allows the methods of the Caml object to be
invoked from any COM client.
</li></ul><p>Example: in the <span class=machine><span class=font-tt>IA</span></span> and <span class=machine><span class=font-tt>IB</span></span> example above, the following Caml
definitions are generated for <span class=machine><span class=font-tt>IA</span></span>:
</p><pre>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
</pre><p>For <span class=machine><span class=font-tt>IB</span></span>, we get:
</p><pre>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
</pre>
<h4 id="sec32" class="paragraph">Error handling in interfaces:</h4>
<p> Conventionally, methods of
COM interfaces always return a result of type <span class=machine><span class=font-tt>HRESULT</span></span> that says
whether the method succeeded or failed, and in the latter case returns
an error code to its caller.</p><p>When calling an interface method from Caml, if the method returns an
<span class=machine><span class=font-tt>HRESULT</span></span> denoting failure, the exception <span class=machine><span class=font-tt>Com.Error</span></span> is raised with a
message describing the error. Successful <span class=machine><span class=font-tt>HRESULT</span></span> return values are
ignored. To make them available to Caml, <span class=machine><span class=font-tt>camlidl</span></span> defines the types
<span class=machine><span class=font-tt>HRESULT</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>bool</span></span> and <span class=machine><span class=font-tt>HRESULT</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>int</span></span>. If those types are used as return
types instead of <span class=machine><span class=font-tt>HRESULT</span></span>, failure results are mapped to
<span class=machine><span class=font-tt>Com.Error</span></span> exceptions as before, but successful results are mapped to
the Caml types <span class=machine><span class=font-tt>bool</span></span> and <span class=machine><span class=font-tt>int</span></span> respectively. (For <span class=machine><span class=font-tt>HRESULT</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>bool</span></span>,
the <span class=machine><span class=font-tt>S</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>OK</span></span> result is mapped to <span class=machine><span class=font-tt>true</span></span> and other successful results are
mapped to <span class=machine><span class=font-tt>false</span></span>. For <span class=machine><span class=font-tt>HRESULT</span></span><span class=machine><span class=font-tt>_</span></span><span class=machine><span class=font-tt>int</span></span>, the low 16 bits of the result
code are returned as a Caml <span class=machine><span class=font-tt>int</span></span>.)</p><p>When calling a Caml method from a COM client, any exception that
escapes the Caml method is mapped back to a failure <span class=machine><span class=font-tt>HRESULT</span></span>. A
textual description of the uncaught exception is saved using
<span class=machine><span class=font-tt>SetLastError</span></span>, and can be consulted by the COM client using
<span class=machine><span class=font-tt>GetLastError</span></span> (this is the standard convention for passing extended
error information in COM).</p><p>If the IDL return type of the method is not one of the <span class=machine><span class=font-tt>HRESULT</span></span>
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 <span class=machine><span class=font-tt>HRESULT</span></span> as result type, or make
very sure that all exceptions are properly caught by the method.</p>
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