duktape/website/guide/stacktypes.html

<h1 id="types">Stack types</h1>

<p>Duktape stack types are:</p>

<div class="table-wrap">
<table>
<tr class="header"><th>Type</th><th>Type constant</th><th>Type mask constant</th><th>Description</th></tr>
<tr><td>(none)</td><td>DUK_TYPE_NONE</td><td>DUK_TYPE_MASK_NONE</td><td>no type (missing value, invalid index, etc)</td></tr>
<tr><td>undefined</td><td>DUK_TYPE_UNDEFINED</td><td>DUK_TYPE_MASK_UNDEFINED</td><td><code>undefined</code></td></tr>
<tr><td>null</td><td>DUK_TYPE_NULL</td><td>DUK_TYPE_MASK_NULL</td><td><code>null</code></td></tr>
<tr><td>boolean</td><td>DUK_TYPE_BOOLEAN</td><td>DUK_TYPE_MASK_BOOLEAN</td><td><code>true</code> and <code>false</code></td></tr>
<tr><td>number</td><td>DUK_TYPE_NUMBER</td><td>DUK_TYPE_MASK_NUMBER</td><td>IEEE double</td></tr>
<tr><td>string</td><td>DUK_TYPE_STRING</td><td>DUK_TYPE_MASK_STRING</td><td>immutable string</td></tr>
<tr><td>object</td><td>DUK_TYPE_OBJECT</td><td>DUK_TYPE_MASK_OBJECT</td><td>object with properties</td></tr>
<tr><td>buffer</td><td>DUK_TYPE_BUFFER</td><td>DUK_TYPE_MASK_BUFFER</td><td>mutable byte buffer, fixed/dynamic</td></tr>
<tr><td>pointer</td><td>DUK_TYPE_POINTER</td><td>DUK_TYPE_MASK_POINTER</td><td>opaque pointer (void *)</td></tr>
</table>
</div>

<h2>Memory allocations</h2>

<p>The following stack types involve additional heap allocations:</p>

<ul>
<li>String: a single allocation contains a combined heap and string header,
    followed by the immutable string data.</li>
<li>Object: one allocation is used for a combined heap and object header,
    and another allocation is used for object properties.  The property
    allocation contains both array entries and normal properties, and if
    the object is large enough, a hash table to speed up lookups.</li>
<li>Buffer: for fixed buffers a single allocation contains a combined heap
    and buffer header, followed by the mutable fixed-size buffer.  For
    dynamic buffers the current buffer is allocated separately.</li>
</ul>
 
<p>Note that while strings are considered a primitive (pass-by-value)
type in Ecmascript, they are a heap allocated type from a memory allocation
viewpoint.</p>

<h2>Pointer stability</h2>

<p>Heap objects allocated by Duktape have stable pointers: the objects are
not relocated in memory while they are reachable from a garbage collection
point of view.  This is the case for the main heap object, but not
necessarily for any additional allocations related to the object, such as
dynamic property tables or dynamic buffer data area.  A heap object is
reachable e.g. when it resides on the value stack of a reachable thread or
is reachable through the global object.  Once a heap object becomes
unreachable any pointers held by user C code referring to the object are
unsafe and should no longer be dereferenced.</p>

<p>In practice the only heap allocated data directly referenced by user code
are strings, fixed buffers, and dynamic buffers.  The data area of strings
and fixed buffers is stable; it is safe to keep a C pointer referring to the
data even after a Duktape/C function returns as long the string or fixed
buffer remains reachable from a garbage collection point of view at all times.
Note that this is not usually <i>not</i> the case for Duktape/C value stack
arguments, for instance, unless specific arrangements are made.</p>

<p>The data area of a dynamic buffer does <b>not</b> have a stable pointer.
The buffer itself has a heap header with a stable address but the current
buffer is allocated separately and potentially relocated when the buffer
is resized.  It is thus unsafe to hold a pointer to a dynamic buffer's data
area across a buffer resize, and it's probably best not to hold a pointer
after a Duktape/C function returns (how would you reliably know when the
buffer is resized?).</p>

<h2>Type masks</h2>

<p>Type masks allows calling code to easily check whether a type belongs to
a certain type set.  For instance, to check that a certain stack value is
a number, string, or an object:</p>

<pre class="c-code">
if (duk_get_type_mask(ctx, -3) &amp; (DUK_TYPE_MASK_NUMBER |
                                  DUK_TYPE_MASK_STRING |
                                  DUK_TYPE_MASK_OBJECT)) {
    printf("type is number, string, or object\n");
}
</pre>

<p>There is a specific API call for matching a set of types even more
conveniently:</p>

<pre class="c-code">
if (duk_check_type_mask(ctx, -3, DUK_TYPE_MASK_NUMBER |
                                 DUK_TYPE_MASK_STRING |
                                 DUK_TYPE_MASK_OBJECT)) {
    printf("type is number, string, or object\n");
}
</pre>

<p>These are faster and more compact than the alternatives:</p>

<pre class="c-code">
// alt 1
if (duk_is_number(ctx, -3) || duk_is_string(ctx, -3) || duk_is_object(ctx, -3)) {
    printf("type is number, string, or object\n");
}

// alt 2
int t = duk_get_type(ctx, -3);
if (t == DUK_TYPE_NUMBER || t == DUK_TYPE_STRING || t == DUK_TYPE_OBJECT) {
    printf("type is number, string, or object\n");
}
</pre>

<h2>None</h2>

<p>The <b>none</b> type is not actually a type but is used in the API to
indicate that a value does not exist, a stack index is invalid, etc.</p>

<h2>Undefined</h2>

<p>The <b>undefined</b> type maps to Ecmascript <code>undefined</code>, which is
distinguished from a <code>null</code>.</p>

<p>Values read from outside the active value stack range read back as
<b>undefined</b>.</p>

<h2>Null</h2>

<p>The <b>null</b> type maps to Ecmascript <code>null</code>.</p>

<h2>Boolean</h2>

<p>The <b>boolean</b> type is represented in the C API as an integer: zero for false,
and non-zero for true.</p>

<p>Whenever giving boolean values as arguments in API calls, any non-zero value is
accepted as a "true" value.  Whenever API calls return boolean values, the value
<code>1</code> is always used for a "true" value.  This allows certain C idioms to be
used.  For instance, a bitmask can be built directly based on API call return values,
as follows:
</p>

<pre class="c-code">
// this works and generates nice code
int bitmask = (duk_get_boolean(ctx, -3) &lt;&lt; 2) |
              (duk_get_boolean(ctx, -2) &lt;&lt; 1) |
              duk_get_boolean(ctx, -1);

// more verbose variant not relying on "true" being represented by 1
int bitmask = ((duk_get_boolean(ctx, -3) ? 1 : 0) &lt;&lt; 2) |
              ((duk_get_boolean(ctx, -2) ? 1 : 0) &lt;&lt; 1) |
              (duk_get_boolean(ctx, -1) ? 1 : 0);

// another verbose variant
int bitmask = (duk_get_boolean(ctx, -3) ? (1 &lt;&lt; 2) : 0) |
              (duk_get_boolean(ctx, -2) ? (1 &lt;&lt; 1) : 0) |
              (duk_get_boolean(ctx, -1) ? 1 : 0);
</pre>

<h2>Number</h2>

<p>The <b>number</b> type is an IEEE double, including +/- Infinity and NaN values.
Zero sign is also preserved.  An IEEE double represents all integers up to 53 bits
accurately.</p>

<p>IEEE double allows NaN values to have additional signaling bits.  Because these
bits are used by Duktape internal tagged type representation (when using 8-byte
packed values), NaN values in the Duktape API are normalized.  Concretely, if you
push a certain NaN value to the value stack, another (normalized) NaN value may
come out.  Don't rely on NaNs preserving their exact form.</p>

<h2>String</h2>

<p>The <b>string</b> type is a raw byte sequence of a certain length which may
contain internal NUL (0x00) values.  Strings are always automatically NUL
terminated for C coding convenience.  The NUL terminator is not counted as part
of the string length.  For instance, the string <code>"foo"</code> has byte length 3
and is stored in memory as <code>{ 'f', 'o', 'o', '\0' }</code>.  Because of the
guaranteed NUL termination, strings can always be pointed to using a simple
<code>const char *</code> as long as internal NULs are not an issue; if they are,
the explicit byte length of the string can be queried with the API.  Calling code
can refer directly to the string data held by Duktape.  Such string data
pointers are valid (and stable) for as long as a string is reachable in the
Duktape heap.</p>

<p>Strings are <a href="http://en.wikipedia.org/wiki/String_interning">interned</a>
for efficiency: only a single copy of a certain string ever exists at a time.
Strings are immutable and must NEVER be changed by calling C code.  Doing so will
lead to very mysterious issues which are hard to diagnose.</p>

<p>Calling code most often deals with Ecmascript strings, which may contain
arbitrary 16-bit codepoints (the whole range 0x0000 to 0xFFFF) but cannot represent
non-<a href="http://en.wikipedia.org/wiki/Basic_Multilingual_Plane#Basic_Multilingual_Plane">BMP</a>
codepoints (this is how strings are defined in the Ecmascript standard).
In Duktape, Ecmascript strings are encoded with
<a href="http://en.wikipedia.org/wiki/CESU-8">CESU-8</a> encoding.  CESU-8
matches <a href="http://en.wikipedia.org/wiki/UTF-8">UTF-8</a> except that it
allows codepoints in the surrogate pair range (U+D800 to U+DFFF) to be encoded
directly; these are prohibited in UTF-8.  CESU-8, like UTF-8, encodes all 7-bit
ASCII characters as-is which is convenient for C code.  For example:</p>

<ul>
<li>U+0041 ("A") encodes to <code>41</code>.</li>
<li>U+1234 (ETHIOPIC SYLLABLE SEE) encodes to <code>e1 88 b4</code>.</li>
<li>U+D812 (high surrogate) encodes to <code>ed a0 92</code> (this would be
    <a href="http://en.wikipedia.org/wiki/UTF-8#Invalid_code_points">invalid UTF-8</a>).</li>
</ul>

<p>Duktape also allows extended strings internally.  Codepoints up to U+10FFFF
can be represented with UTF-8, and codepoints above that up to full 32 bits
can be represented with
<a href="http://en.wikipedia.org/wiki/UTF-8#Extending_from_31_bit_to_36_bit_range">"extended UTF-8"</a>.
Non-standard strings are used for storing internal object properties; using a
non-standard string ensures that such properties never conflict with properties
accessible using standard Ecmascript strings.  Non-standard strings can be given
to Ecmascript built-in functions, but since behavior may not be exactly
specified, results may vary.</p>

<p>Duktape uses internal object properties to record internal implementation
related fields in e.g. function objects.  For example, a finalizer reference is
stored in an internal finalizer property.  Such internal keys are kept separate
from valid Ecmascript keys by using a byte sequence which can never occur in
a valid CESU-8 string; consequently, standard Ecmascript code cannot accidentally
reference such fields.  Internal properties are never enumerable, and are not
returned by e.g. <code>Object.getOwnPropertyNames()</code>.  Currently, internal
property names begin with an <code>0xFF</code> byte followed by the property name.
For instance the finalizer property key consists of the byte <code>0xFF</code>
followed by the ASCII string <code>"finalizer"</code>.  In internal documentation
this property would usually be referred to as <code>_finalizer</code> for convenience.
You should never read or write internal properties directly.</p>

<p>The "extended UTF-8" encoding used by Duktape is described in the table below.
The leading byte is shown in binary (with "x" marking data bits) while
continuation bytes are marked with "C" (indicating the bit sequence 10xxxxxx):</p>

<table>
<thead>
<tr class="header"><th>Codepoint range</th><th>Bits</th><th>Byte sequence</th></tr>
</tr>
</thead>
<tbody>
<tr><td>U+0000 to U+007F</td><td>7</td><td>0xxxxxxx</td></tr>
<tr><td>U+0080 to U+07FF</td><td>11</td><td>110xxxxx C</td></tr>
<tr><td>U+0800 to U+FFFF</td><td>16</td><td>1110xxxx C C</td></tr>
<tr><td>U+1 0000 to U+1F FFFF</td><td>21</td><td>11110xxx C C C</td></tr>
<tr><td>U+20 0000 to U+3FF FFFF</td><td>26</td><td>111110xx C C C C</td></tr>
<tr><td>U+400 0000 to U+7FFF FFFF</td><td>31</td><td>1111110x C C C C C</td></tr>
<tr><td>U+8000 0000 to U+F FFFF FFFF</td><td>36 (32 used)</td><td>11111110 C C C C C C</td></tr>
</tbody>
</table>

<p>The downside of the encoding for codepoints above U+7FFFFFFF is that
the leading byte will be <code>0xFE</code> which conflicts with Unicode byte order
marker encoding.  This is not a practical concern in Duktape's internal use.</p>

<h2>Object</h2>

<p>The <b>object</b> type includes Ecmascript objects and arrays, functions, and
threads (coroutines).  In other words, anything with properties is an object.
Properties are key-value pairs with a string key and an arbitrary value
(including <b>undefined</b>).</p>

<p>Objects may participate in garbage collection finalization.</p>

<h2>Buffer</h2>

<p>The <b>buffer</b> type is a raw buffer for user data of either fixed or dynamic
size.  The size of a fixed buffer is given at its creation, and fixed buffers
have an unchanging (stable) data pointer.  Dynamic buffers may change during their
life time at the cost of having a (potentially) changing data pointer.  Dynamic
buffers also need two memory allocations internally, while fixed buffers only
need one.</p>

<p>Buffers are automatically garbage collected.  This also means that C code
must not hold onto a buffer data pointer unless the buffer is reachable to
Duktape, e.g. resides in an active value stack.</p>

<p>The buffer type is not standard Ecmascript.  There are a few
different Ecmascript typed array specifications, though, see e.g.
<a href="http://www.khronos.org/registry/typedarray/specs/latest/">Typed Array Specification</a>.
These will be implemented on top of raw arrays, most likely.
</p>

<h2>Pointer</h2>

<p>The <b>pointer</b> type is a raw, uninterpreted C pointer, essentially
a <code>void *</code>.  Pointers can be used to point to native objects (memory
allocations, handles, etc), but because Duktape doesn't know their use, they
are not automatically garbage collected.  You can, however, put one or more
pointers inside an object and use the object finalizer to free the
native resources related to the pointer(s).</p>