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<h1>Module <a href="type_Marshal.html">Marshal</a></h1>
<pre><span id="MODULEMarshal"><span class="keyword">module</span> Marshal</span>: <code class="code"><span class="keyword">sig</span></code> <a href="Marshal.html">..</a> <code class="code"><span class="keyword">end</span></code></pre><div class="info module top">
<div class="info-desc">
<p>Marshaling of data structures.</p>
<p>This module provides functions to encode arbitrary data structures
as sequences of bytes, which can then be written on a file or
sent over a pipe or network connection. The bytes can then
be read back later, possibly in another process, and decoded back
into a data structure. The format for the byte sequences
is compatible across all machines for a given version of OCaml.</p>
<p>Warning: marshaling is currently not type-safe. The type
of marshaled data is not transmitted along the value of the data,
making it impossible to check that the data read back possesses the
type expected by the context. In particular, the result type of
the <code class="code"><span class="constructor">Marshal</span>.from_*</code> functions is given as <code class="code"><span class="keywordsign">'</span>a</code>, but this is
misleading: the returned OCaml value does not possess type <code class="code"><span class="keywordsign">'</span>a</code>
for all <code class="code"><span class="keywordsign">'</span>a</code>; it has one, unique type which cannot be determined
at compile-time. The programmer should explicitly give the expected
type of the returned value, using the following syntax:</p>
<ul>
<li><code class="code">(<span class="constructor">Marshal</span>.from_channel chan : <span class="keyword">type</span>)</code>.
Anything can happen at run-time if the object in the file does not
belong to the given type.</li>
</ul>
<p>Values of extensible variant types, for example exceptions (of
extensible type <code class="code">exn</code>), returned by the unmarshaller should not be
pattern-matched over through <code class="code"><span class="keyword">match</span> ... <span class="keyword">with</span></code> or <code class="code"><span class="keyword">try</span> ... <span class="keyword">with</span></code>,
because unmarshalling does not preserve the information required for
matching their constructors. Structural equalities with other
extensible variant values does not work either. Most other uses such
as Printexc.to_string, will still work as expected.</p>
<p>The representation of marshaled values is not human-readable,
and uses bytes that are not printable characters. Therefore,
input and output channels used in conjunction with <code class="code"><span class="constructor">Marshal</span>.to_channel</code>
and <code class="code"><span class="constructor">Marshal</span>.from_channel</code> must be opened in binary mode, using e.g.
<code class="code">open_out_bin</code> or <code class="code">open_in_bin</code>; channels opened in text mode will
cause unmarshaling errors on platforms where text channels behave
differently than binary channels, e.g. Windows.</p>
</div>
</div>
<hr width="100%">
<pre><code><span id="TYPEextern_flags"><span class="keyword">type</span> <code class="type"></code>extern_flags</span> = </code></pre><table class="typetable">
<tr>
<td align="left" valign="top" >
<code><span class="keyword">|</span></code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTextern_flags.No_sharing"><span class="constructor">No_sharing</span></span></code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>Don't preserve sharing</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code><span class="keyword">|</span></code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTextern_flags.Closures"><span class="constructor">Closures</span></span></code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>Send function closures</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code><span class="keyword">|</span></code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTextern_flags.Compat_32"><span class="constructor">Compat_32</span></span></code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>Ensure 32-bit compatibility</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr></table>
<div class="info ">
<div class="info-desc">
<p>The flags to the <code class="code"><span class="constructor">Marshal</span>.to_*</code> functions below.</p>
</div>
</div>
<pre><span id="VALto_channel"><span class="keyword">val</span> to_channel</span> : <code class="type"><a href="Stdlib.html#TYPEout_channel">out_channel</a> -> 'a -> <a href="Marshal.html#TYPEextern_flags">extern_flags</a> list -> unit</code></pre><div class="info ">
<div class="info-desc">
<p><code class="code"><span class="constructor">Marshal</span>.to_channel chan v flags</code> writes the representation
of <code class="code">v</code> on channel <code class="code">chan</code>. The <code class="code">flags</code> argument is a
possibly empty list of flags that governs the marshaling
behavior with respect to sharing, functional values, and compatibility
between 32- and 64-bit platforms.</p>
<p>If <code class="code">flags</code> does not contain <code class="code"><span class="constructor">Marshal</span>.<span class="constructor">No_sharing</span></code>, circularities
and sharing inside the value <code class="code">v</code> are detected and preserved
in the sequence of bytes produced. In particular, this
guarantees that marshaling always terminates. Sharing
between values marshaled by successive calls to
<code class="code"><span class="constructor">Marshal</span>.to_channel</code> is neither detected nor preserved, though.
If <code class="code">flags</code> contains <code class="code"><span class="constructor">Marshal</span>.<span class="constructor">No_sharing</span></code>, sharing is ignored.
This results in faster marshaling if <code class="code">v</code> contains no shared
substructures, but may cause slower marshaling and larger
byte representations if <code class="code">v</code> actually contains sharing,
or even non-termination if <code class="code">v</code> contains cycles.</p>
<p>If <code class="code">flags</code> does not contain <code class="code"><span class="constructor">Marshal</span>.<span class="constructor">Closures</span></code>, marshaling fails
when it encounters a functional value inside <code class="code">v</code>: only 'pure' data
structures, containing neither functions nor objects, can safely be
transmitted between different programs. If <code class="code">flags</code> contains
<code class="code"><span class="constructor">Marshal</span>.<span class="constructor">Closures</span></code>, functional values will be marshaled as a the
position in the code of the program together with the values
corresponding to the free variables captured in the closure. In
this case, the output of marshaling can only be read back in
processes that run exactly the same program, with exactly the same
compiled code. (This is checked at un-marshaling time, using an MD5
digest of the code transmitted along with the code position.)</p>
<p>The exact definition of which free variables are captured in a
closure is not specified and can vary between bytecode and native
code (and according to optimization flags). In particular, a
function value accessing a global reference may or may not include
the reference in its closure. If it does, unmarshaling the
corresponding closure will create a new reference, different from
the global one.</p>
<p>If <code class="code">flags</code> contains <code class="code"><span class="constructor">Marshal</span>.<span class="constructor">Compat_32</span></code>, marshaling fails when
it encounters an integer value outside the range <code class="code">[-2{^30}, 2{^30}-1]</code>
of integers that are representable on a 32-bit platform. This
ensures that marshaled data generated on a 64-bit platform can be
safely read back on a 32-bit platform. If <code class="code">flags</code> does not
contain <code class="code"><span class="constructor">Marshal</span>.<span class="constructor">Compat_32</span></code>, integer values outside the
range <code class="code">[-2{^30}, 2{^30}-1]</code> are marshaled, and can be read back on
a 64-bit platform, but will cause an error at un-marshaling time
when read back on a 32-bit platform. The <code class="code"><span class="constructor">Mashal</span>.<span class="constructor">Compat_32</span></code> flag
only matters when marshaling is performed on a 64-bit platform;
it has no effect if marshaling is performed on a 32-bit platform.</p>
</div>
</div>
<pre><span id="VALto_bytes"><span class="keyword">val</span> to_bytes</span> : <code class="type">'a -> <a href="Marshal.html#TYPEextern_flags">extern_flags</a> list -> bytes</code></pre><div class="info ">
<div class="info-desc">
<p><code class="code"><span class="constructor">Marshal</span>.to_bytes v flags</code> returns a byte sequence containing
the representation of <code class="code">v</code>.
The <code class="code">flags</code> argument has the same meaning as for
<a href="Marshal.html#VALto_channel"><code class="code"><span class="constructor">Marshal</span>.to_channel</code></a>.</p>
</div>
<ul class="info-attributes">
<li><b>Since</b> 4.02.0</li>
</ul>
</div>
<pre><span id="VALto_string"><span class="keyword">val</span> to_string</span> : <code class="type">'a -> <a href="Marshal.html#TYPEextern_flags">extern_flags</a> list -> string</code></pre><div class="info ">
<div class="info-desc">
<p>Same as <code class="code">to_bytes</code> but return the result as a string instead of
a byte sequence.</p>
</div>
</div>
<pre><span id="VALto_buffer"><span class="keyword">val</span> to_buffer</span> : <code class="type">bytes -> int -> int -> 'a -> <a href="Marshal.html#TYPEextern_flags">extern_flags</a> list -> int</code></pre><div class="info ">
<div class="info-desc">
<p><code class="code"><span class="constructor">Marshal</span>.to_buffer buff ofs len v flags</code> marshals the value <code class="code">v</code>,
storing its byte representation in the sequence <code class="code">buff</code>,
starting at index <code class="code">ofs</code>, and writing at most
<code class="code">len</code> bytes. It returns the number of bytes
actually written to the sequence. If the byte representation
of <code class="code">v</code> does not fit in <code class="code">len</code> characters, the exception <code class="code"><span class="constructor">Failure</span></code>
is raised.</p>
</div>
</div>
<pre><span id="VALfrom_channel"><span class="keyword">val</span> from_channel</span> : <code class="type"><a href="Stdlib.html#TYPEin_channel">in_channel</a> -> 'a</code></pre><div class="info ">
<div class="info-desc">
<p><code class="code"><span class="constructor">Marshal</span>.from_channel chan</code> reads from channel <code class="code">chan</code> the
byte representation of a structured value, as produced by
one of the <code class="code"><span class="constructor">Marshal</span>.to_*</code> functions, and reconstructs and
returns the corresponding value.</p>
<p>It raises <code class="code"><span class="constructor">End_of_file</span></code> if the function has already reached the
end of file when starting to read from the channel, and raises
<code class="code"><span class="constructor">Failure</span> <span class="string">"input_value: truncated object"</span></code> if it reaches the end
of file later during the unmarshalling.</p>
</div>
</div>
<pre><span id="VALfrom_bytes"><span class="keyword">val</span> from_bytes</span> : <code class="type">bytes -> int -> 'a</code></pre><div class="info ">
<div class="info-desc">
<p><code class="code"><span class="constructor">Marshal</span>.from_bytes buff ofs</code> unmarshals a structured value
like <a href="Marshal.html#VALfrom_channel"><code class="code"><span class="constructor">Marshal</span>.from_channel</code></a> does, except that the byte
representation is not read from a channel, but taken from
the byte sequence <code class="code">buff</code>, starting at position <code class="code">ofs</code>.
The byte sequence is not mutated.</p>
</div>
<ul class="info-attributes">
<li><b>Since</b> 4.02.0</li>
</ul>
</div>
<pre><span id="VALfrom_string"><span class="keyword">val</span> from_string</span> : <code class="type">string -> int -> 'a</code></pre><div class="info ">
<div class="info-desc">
<p>Same as <code class="code">from_bytes</code> but take a string as argument instead of a
byte sequence.</p>
</div>
</div>
<pre><span id="VALheader_size"><span class="keyword">val</span> header_size</span> : <code class="type">int</code></pre><div class="info ">
<div class="info-desc">
<p>The bytes representing a marshaled value are composed of
a fixed-size header and a variable-sized data part,
whose size can be determined from the header.
<a href="Marshal.html#VALheader_size"><code class="code"><span class="constructor">Marshal</span>.header_size</code></a> is the size, in bytes, of the header.
<a href="Marshal.html#VALdata_size"><code class="code"><span class="constructor">Marshal</span>.data_size</code></a><code class="code"> buff ofs</code> is the size, in bytes,
of the data part, assuming a valid header is stored in
<code class="code">buff</code> starting at position <code class="code">ofs</code>.
Finally, <a href="Marshal.html#VALtotal_size"><code class="code"><span class="constructor">Marshal</span>.total_size</code></a> <code class="code">buff ofs</code> is the total size,
in bytes, of the marshaled value.
Both <a href="Marshal.html#VALdata_size"><code class="code"><span class="constructor">Marshal</span>.data_size</code></a> and <a href="Marshal.html#VALtotal_size"><code class="code"><span class="constructor">Marshal</span>.total_size</code></a> raise <code class="code"><span class="constructor">Failure</span></code>
if <code class="code">buff</code>, <code class="code">ofs</code> does not contain a valid header.</p>
<p>To read the byte representation of a marshaled value into
a byte sequence, the program needs to read first
<a href="Marshal.html#VALheader_size"><code class="code"><span class="constructor">Marshal</span>.header_size</code></a> bytes into the sequence,
then determine the length of the remainder of the
representation using <a href="Marshal.html#VALdata_size"><code class="code"><span class="constructor">Marshal</span>.data_size</code></a>,
make sure the sequence is large enough to hold the remaining
data, then read it, and finally call <a href="Marshal.html#VALfrom_bytes"><code class="code"><span class="constructor">Marshal</span>.from_bytes</code></a>
to unmarshal the value.</p>
</div>
</div>
<pre><span id="VALdata_size"><span class="keyword">val</span> data_size</span> : <code class="type">bytes -> int -> int</code></pre><div class="info ">
<div class="info-desc">
<p>See <a href="Marshal.html#VALheader_size"><code class="code"><span class="constructor">Marshal</span>.header_size</code></a>.</p>
</div>
</div>
<pre><span id="VALtotal_size"><span class="keyword">val</span> total_size</span> : <code class="type">bytes -> int -> int</code></pre><div class="info ">
<div class="info-desc">
<p>See <a href="Marshal.html#VALheader_size"><code class="code"><span class="constructor">Marshal</span>.header_size</code></a>.</p>
</div>
</div>
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