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<link title="Bitstring_persistent" rel="Chapter" href="Bitstring_persistent.html"><link title="Introduction" rel="Section" href="#2_Introduction">
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<h1>Module <a href="type_Bitstring_persistent.html">Bitstring_persistent</a></h1>
<pre><span id="MODULEBitstring_persistent"><span class="keyword">module</span> Bitstring_persistent</span>: <code class="code">sig</code> <a href="Bitstring_persistent.html">..</a> <code class="code">end</code></pre><div class="info module top">
<div class="info-desc">
<p>Bitstring persistent patterns.</p>
</div>
</div>
<hr width="100%">
<p><b>Warning:</b> This documentation is for ADVANCED USERS ONLY.
If you are not an advanced user, you are probably looking
for <a href="Bitstring.html">the Bitstring documentation</a>.</p>
<p><a href="#reference">Jump straight to the reference section for
documentation on types and functions</a>.</p>
<h3 id="2_Introduction">Introduction</h3>
<p>Bitstring allows you to name sets of fields and reuse them
elsewhere. For example if you frequently need to parse
Pascal-style strings in the form length byte + string, then you
could name the <code class="code">{ strlen : 8 : int; str : strlen*8 : string }</code>
pattern and reuse it everywhere by name.</p>
<p>These are called <b>persistent patterns</b>.</p>
<p>The basic usage is:</p>
<pre class="verbatim">(* Create a persistent pattern called 'pascal_string' which
* matches Pascal-style strings (length byte + string).
*)
let bitmatch pascal_string =
{ strlen : 8 : int;
str : strlen*8 : string }
let is_pascal_string bits =
bitmatch bits with
| { :pascal_string } ->
printf "matches a Pascal string %s, len %d bytes\n"
str strlen</pre>
<p>or:</p>
<pre class="verbatim">(* Load a persistent pattern from a file. *)
open bitmatch "pascal.bmpp"
let is_pascal_string bits =
bitmatch bits with
| { :pascal_string } ->
printf "matches a Pascal string %s, len %d bytes\n"
str strlen</pre>
<h4 id="3_Importantnotes">Important notes</h4>
<p>There are some important things you should know about
persistent patterns before you decide to use them:</p>
<p>'Persistent' refers to the fact that they can be saved into binary
files. However these binary files use OCaml <code class="code">Marshal</code> module and
depend (sometimes) on the version of OCaml used to generate them
and (sometimes) the version of bitstring used. So your build system
should rebuild these files from source when your code is rebuilt.</p>
<p>Persistent patterns are syntactic. They work in the same way
as cutting and pasting (or <code class="code">#include</code>-ing) code. For example
if a persistent pattern binds a field named <code class="code">len</code>, then any
uses of <code class="code">len</code> following in the surrounding pattern could
be affected.</p>
<p>Programs which generate and manipulate persistent patterns have to
link to camlp4. Since camlp4 in OCaml >= 3.10 is rather large, we
have placed this code into this separate submodule, so that
programs which just use bitstring don't need to pull in the whole of
camlp4. This restriction does not apply to code which only uses
persistent patterns but does not generate them. If the distinction
isn't clear, use <code class="code">ocamlobjinfo</code> to look at the dependencies of your
<code class="code">*.cmo</code> files.</p>
<p>Persistent patterns can be generated in several ways, but they
can only be <i>used</i> by the <code class="code">pa_bitstring</code> syntax extension.
This means they are purely compile-time constructs. You
cannot use them to make arbitrary patterns and run those
patterns (not unless your program runs <code class="code">ocamlc</code> to make a <code class="code">*.cmo</code>
file then dynamically links to the <code class="code">*.cmo</code> file).</p>
<h3 id="2_Namedpatterns">Named patterns</h3>
<p>A named pattern is a way to name a pattern and use it later
in the same source file. To name a pattern, use:</p>
<p><code class="code">let bitmatch name = { fields ... } ;;</code></p>
<p>and you can then use the name later on inside another pattern,
by prefixing the name with a colon.
For example:</p>
<p><code class="code">bitmatch bits with { :name } -> ...</code></p>
<p>You can nest named patterns within named patterns to any depth.</p>
<p>Currently the use of named patterns is somewhat limited.
The restrictions are:</p>
<p>Named patterns can only be used within the same source file, and
the names occupy a completely separate namespace from anything
else in the source file.</p>
<p>The <code class="code">let bitmatch</code> syntax only works at the top level. We may
add a <code class="code">let bitmatch ... in</code> for inner levels later.</p>
<p>Because you cannot rename the bound identifiers in named
patterns, you can effectively only use them once in a
pattern. For example, <code class="code">{ :name; :name }</code> is legal, but
any bindings in the first name would be overridden by
the second name.</p>
<p>There are no "named constructors" yet, but the machinery
is in place to do this, and we may add them later.</p>
<h3 id="2_Persistentpatternsinfiles">Persistent patterns in files</h3>
<p>More useful than just naming patterns, you can load
persistent patterns from external files. The patterns
in these external files can come from a variety of sources:
for example, in the <code class="code">cil-tools</code> subdirectory are some
<a href="http://cil.sf.net/">Cil-based</a> tools for importing C
structures from header files. You can also generate
your own files or write your own tools, as described below.</p>
<p>To use the persistent pattern(s) from a file do:</p>
<p><code class="code">open bitmatch "filename.bmpp" ;;</code></p>
<p>A list of zero or more <a href="Bitstring_persistent.html#TYPEnamed"><code class="code">Bitstring_persistent.named</code></a> patterns are read from the file
and each is bound to a name (as contained in the file),
and then the patterns can be used with the usual <code class="code">:name</code>
syntax described above.</p>
<h4 id="3_Extension">Extension</h4>
<p>The standard extension is <code class="code">.bmpp</code>. This is just a convention
and you can use any extension you want.</p>
<h4 id="3_Directorysearchorder">Directory search order</h4>
<p>If the filename is an absolute or explicit path, then we try to
load it from that path and stop if it fails. See the <code class="code">Filename</code>
module in the standard OCaml library for the definitions of
"absolute path" and "explicit path". Otherwise we use the
following directory search order:</p>
<ul>
<li>Relative to the current directory</li>
<li>Relative to the OCaml library directory</li>
</ul>
<h4 id="3_bitstringobjinfo">bitstring-objinfo</h4>
<p>The <code class="code">bitstring-objinfo</code> command can be run on a file in order
to print out the patterns in the file.</p>
<h4 id="3_Constructors">Constructors</h4>
<p>We haven't implemented persistent constructors yet, although
the machinery is in place to make this happen. Any constructors
found in the file are ignored.</p>
<h3 id="2_Creatingyourownpersistentpatterns">Creating your own persistent patterns</h3>
<p>If you want to write a tool to import bitstrings from an
exotic location or markup language, you will need
to use the functions found in the <a href="#reference">reference section</a>.</p>
<p>I will describe using an example here of how you would
programmatically create a persistent pattern which
matches Pascal-style "length byte + data" strings.
Firstly note that there are two fields, so our pattern
will be a list of length 2 and type <a href="Bitstring_persistent.html#TYPEpattern"><code class="code">Bitstring_persistent.pattern</code></a>.</p>
<p>You will need to create a camlp4 location object (<code class="code">Loc.t</code>)
describing the source file. This source file is used
to generate useful error messages for the user, so
you may want to set it to be the name and location in
the file that your tool reads for input. By convention,
locations are bound to name <code class="code">_loc</code>:</p>
<pre class="verbatim"> let _loc = Loc.move_line 42 (Loc.mk "input.xml")
</pre>
<p>Create a pattern field representing a length field which is 8 bits wide,
bound to the identifier <code class="code">len</code>:</p>
<pre class="verbatim"> let len_field = create_pattern_field _loc
let len_field = set_length_int len_field 8
let len_field = set_lident_patt len_field "len"
</pre>
<p>Create a pattern field representing a string of <code class="code">len*8</code> bits.
Note that the use of <code class="code"><:expr< >></code> quotation requires
you to preprocess your source with <code class="code">camlp4of</code>
(see <a href="http://brion.inria.fr/gallium/index.php/Reflective_OCaml">this
page on Reflective OCaml</a>).</p>
<pre class="verbatim"> let str_field = create_pattern_field _loc
let str_field = set_length str_field <:expr< len*8 >>
let str_field = set_lident_patt str_field "str"
let str_field = set_type_string str_field
</pre>
<p>Join the two fields together and name it:</p>
<pre class="verbatim"> let pattern = [len_field; str_field]
let named_pattern = "pascal_string", Pattern pattern
</pre>
<p>Save it to a file:</p>
<pre class="verbatim"> let chan = open_out "output.bmpp" in
named_to_channel chan named_pattern;
close_out chan
</pre>
<p>You can now use this pattern in another program like this:</p>
<pre class="verbatim"> open bitmatch "output.bmpp" ;;
let parse_pascal_string bits =
bitmatch bits with
| { :pascal_string } -> str, len
| { _ } -> invalid_arg "not a Pascal string"
</pre>
<p>You can write more than one named pattern to the output file, and
they will all be loaded at the same time by <code class="code">open bitmatch ".."</code>
(obviously you should give each pattern a different name). To do
this, just call <a href="Bitstring_persistent.html#VALnamed_to_channel"><code class="code">Bitstring_persistent.named_to_channel</code></a> as many times as needed.</p>
<h3 id="reference">Reference</h3>
<h4 id="3_Types">Types</h4>
<pre><span id="TYPEpatt"><span class="keyword">type</span> <code class="type"></code>patt</span> = <code class="type">Camlp4.PreCast.Syntax.Ast.patt</code> </pre>
<pre><span id="TYPEexpr"><span class="keyword">type</span> <code class="type"></code>expr</span> = <code class="type">Camlp4.PreCast.Syntax.Ast.expr</code> </pre>
<pre><span id="TYPEloc_t"><span class="keyword">type</span> <code class="type"></code>loc_t</span> = <code class="type">Camlp4.PreCast.Syntax.Ast.Loc.t</code> </pre>
<div class="info ">
<div class="info-desc">
<p>Just short names for the camlp4 types.</p>
</div>
</div>
<pre><span id="TYPEfield"><span class="keyword">type</span> <code class="type">'a</code> field</span> </pre>
<div class="info ">
<div class="info-desc">
<p>A field in a persistent pattern or persistent constructor.</p>
</div>
</div>
<pre><span id="TYPEpattern"><span class="keyword">type</span> <code class="type"></code>pattern</span> = <code class="type"><a href="Bitstring_persistent.html#TYPEpatt">patt</a> <a href="Bitstring_persistent.html#TYPEfield">field</a> list</code> </pre>
<div class="info ">
<div class="info-desc">
<p>A persistent pattern (used in <code class="code">bitmatch</code> operator), is just a
list of pattern fields.</p>
</div>
</div>
<pre><span id="TYPEconstructor"><span class="keyword">type</span> <code class="type"></code>constructor</span> = <code class="type"><a href="Bitstring_persistent.html#TYPEexpr">expr</a> <a href="Bitstring_persistent.html#TYPEfield">field</a> list</code> </pre>
<div class="info ">
<div class="info-desc">
<p>A persistent constructor (used in <code class="code">BITSTRING</code> operator), is just a
list of constructor fields.</p>
</div>
</div>
<pre><span id="TYPEnamed"><span class="keyword">type</span> <code class="type"></code>named</span> = <code class="type">string * <a href="Bitstring_persistent.html#TYPEalt">alt</a></code> </pre>
<pre><code><span id="TYPEalt"><span class="keyword">type</span> <code class="type"></code>alt</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="TYPEELTalt.Pattern"><span class="constructor">Pattern</span></span> <span class="keyword">of</span> <code class="type"><a href="Bitstring_persistent.html#TYPEpattern">pattern</a></code></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>Pattern</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="TYPEELTalt.Constructor"><span class="constructor">Constructor</span></span> <span class="keyword">of</span> <code class="type"><a href="Bitstring_persistent.html#TYPEconstructor">constructor</a></code></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>Constructor</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr></table>
<div class="info ">
<div class="info-desc">
<p>A named pattern or constructor.</p>
<p>The name is used when binding a pattern from a file, but
is otherwise ignored.</p>
</div>
</div>
<h4 id="3_Printers">Printers</h4>
<pre><span id="VALstring_of_pattern"><span class="keyword">val</span> string_of_pattern</span> : <code class="type"><a href="Bitstring_persistent.html#TYPEpattern">pattern</a> -> string</code></pre>
<pre><span id="VALstring_of_constructor"><span class="keyword">val</span> string_of_constructor</span> : <code class="type"><a href="Bitstring_persistent.html#TYPEconstructor">constructor</a> -> string</code></pre>
<pre><span id="VALstring_of_pattern_field"><span class="keyword">val</span> string_of_pattern_field</span> : <code class="type"><a href="Bitstring_persistent.html#TYPEpatt">patt</a> <a href="Bitstring_persistent.html#TYPEfield">field</a> -> string</code></pre>
<pre><span id="VALstring_of_constructor_field"><span class="keyword">val</span> string_of_constructor_field</span> : <code class="type"><a href="Bitstring_persistent.html#TYPEexpr">expr</a> <a href="Bitstring_persistent.html#TYPEfield">field</a> -> string</code></pre><div class="info ">
<div class="info-desc">
<p>Convert patterns, constructors or individual fields
into printable strings for debugging purposes.</p>
<p>The strings look similar to the syntax used by bitmatch, but
some things cannot be printed fully, eg. length expressions.</p>
</div>
</div>
<h4 id="3_Persistence">Persistence</h4>
<pre><span id="VALnamed_to_channel"><span class="keyword">val</span> named_to_channel</span> : <code class="type">Stdlib.out_channel -> <a href="Bitstring_persistent.html#TYPEnamed">named</a> -> unit</code></pre><div class="info ">
<div class="info-desc">
<p>Save a pattern/constructor to an output channel.</p>
</div>
</div>
<pre><span id="VALnamed_to_string"><span class="keyword">val</span> named_to_string</span> : <code class="type"><a href="Bitstring_persistent.html#TYPEnamed">named</a> -> string</code></pre><div class="info ">
<div class="info-desc">
<p>Serialize a pattern/constructor to a string.</p>
</div>
</div>
<pre><span id="VALnamed_to_buffer"><span class="keyword">val</span> named_to_buffer</span> : <code class="type">string -> int -> int -> <a href="Bitstring_persistent.html#TYPEnamed">named</a> -> int</code></pre><div class="info ">
<div class="info-desc">
<p>Serialize a pattern/constructor to part of a string, return the length.</p>
</div>
</div>
<pre><span id="VALnamed_from_channel"><span class="keyword">val</span> named_from_channel</span> : <code class="type">Stdlib.in_channel -> <a href="Bitstring_persistent.html#TYPEnamed">named</a></code></pre><div class="info ">
<div class="info-desc">
<p>Load a pattern/constructor from an output channel.</p>
<p>Note: This is not type safe. The pattern/constructor must
have been written out under the same version of OCaml and
the same version of bitstring.</p>
</div>
</div>
<pre><span id="VALnamed_from_string"><span class="keyword">val</span> named_from_string</span> : <code class="type">string -> int -> <a href="Bitstring_persistent.html#TYPEnamed">named</a></code></pre><div class="info ">
<div class="info-desc">
<p>Load a pattern/constructor from a string at offset within the string.</p>
<p>Note: This is not type safe. The pattern/constructor must
have been written out under the same version of OCaml and
the same version of bitstring.</p>
</div>
</div>
<h4 id="3_Createpatternfields">Create pattern fields</h4>
<p>These fields are used in pattern matches (<code class="code">bitmatch</code>).</p>
<pre><span id="VALcreate_pattern_field"><span class="keyword">val</span> create_pattern_field</span> : <code class="type"><a href="Bitstring_persistent.html#TYPEloc_t">loc_t</a> -><br> <a href="Bitstring_persistent.html#TYPEpatt">patt</a> <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Create a pattern field.</p>
<p>The pattern is unbound, the type is set to <code class="code">int</code>, bit length to <code class="code">32</code>,
endianness to <code class="code">BigEndian</code>, signedness to unsigned (<code class="code">false</code>),
source code location to the <code class="code">_loc</code> parameter, and no offset expression.</p>
<p>To create a complete field you need to call the <code class="code">set_*</code>
functions. For example, to create <code class="code">{ len : 8 : int }</code>
you would do:</p>
<pre class="verbatim"> let field = create_pattern_field _loc in
let field = set_lident_patt field "len" in
let field = set_length_int field 8 in</pre></div>
</div>
<pre><span id="VALset_lident_patt"><span class="keyword">val</span> set_lident_patt</span> : <code class="type"><a href="Bitstring_persistent.html#TYPEpatt">patt</a> <a href="Bitstring_persistent.html#TYPEfield">field</a> -><br> string -> <a href="Bitstring_persistent.html#TYPEpatt">patt</a> <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Sets the pattern to the pattern binding an identifier
given in the string.</p>
<p>The effect is that the field <code class="code">{ len : 8 : int }</code> could
be created by calling <code class="code">set_lident_patt field "len"</code>.</p>
</div>
</div>
<pre><span id="VALset_int_patt"><span class="keyword">val</span> set_int_patt</span> : <code class="type"><a href="Bitstring_persistent.html#TYPEpatt">patt</a> <a href="Bitstring_persistent.html#TYPEfield">field</a> -><br> int -> <a href="Bitstring_persistent.html#TYPEpatt">patt</a> <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Sets the pattern field to the pattern which matches an integer.</p>
<p>The effect is that the field <code class="code">{ 2 : 8 : int }</code> could
be created by calling <code class="code">set_int_patt field 2</code>.</p>
</div>
</div>
<pre><span id="VALset_string_patt"><span class="keyword">val</span> set_string_patt</span> : <code class="type"><a href="Bitstring_persistent.html#TYPEpatt">patt</a> <a href="Bitstring_persistent.html#TYPEfield">field</a> -><br> string -> <a href="Bitstring_persistent.html#TYPEpatt">patt</a> <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Sets the pattern field to the pattern which matches a string.</p>
<p>The effect is that the field <code class="code">{ "MAGIC" : 8*5 : string }</code> could
be created by calling <code class="code">set_int_patt field "MAGIC"</code>.</p>
</div>
</div>
<pre><span id="VALset_unbound_patt"><span class="keyword">val</span> set_unbound_patt</span> : <code class="type"><a href="Bitstring_persistent.html#TYPEpatt">patt</a> <a href="Bitstring_persistent.html#TYPEfield">field</a> -><br> <a href="Bitstring_persistent.html#TYPEpatt">patt</a> <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Sets the pattern field to the unbound pattern (usually written <code class="code">_</code>).</p>
<p>The effect is that the field <code class="code">{ _ : 8 : int }</code> could
be created by calling <code class="code">set_unbound_patt field</code>.</p>
</div>
</div>
<pre><span id="VALset_patt"><span class="keyword">val</span> set_patt</span> : <code class="type"><a href="Bitstring_persistent.html#TYPEpatt">patt</a> <a href="Bitstring_persistent.html#TYPEfield">field</a> -><br> <a href="Bitstring_persistent.html#TYPEpatt">patt</a> -><br> <a href="Bitstring_persistent.html#TYPEpatt">patt</a> <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Sets the pattern field to an arbitrary OCaml pattern match.</p>
</div>
</div>
<pre><span id="VALset_length_int"><span class="keyword">val</span> set_length_int</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -> int -> 'a <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Sets the length in bits of a field to a constant integer.</p>
<p>The effect is that the field <code class="code">{ len : 8 : string }</code> could
be created by calling <code class="code">set_length field 8</code>.</p>
</div>
</div>
<pre><span id="VALset_length"><span class="keyword">val</span> set_length</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -><br> <a href="Bitstring_persistent.html#TYPEexpr">expr</a> -> 'a <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Sets the length in bits of a field to an OCaml expression.</p>
<p>The effect is that the field <code class="code">{ len : 2*i : string }</code> could
be created by calling <code class="code">set_length field <:expr< 2*i >></code>.</p>
</div>
</div>
<pre><span id="VALset_endian"><span class="keyword">val</span> set_endian</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -><br> <a href="Bitstring.html#TYPEendian">Bitstring.endian</a> -> 'a <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Sets the endianness of a field to the constant endianness.</p>
<p>The effect is that the field <code class="code">{ _ : 16 : bigendian }</code> could
be created by calling <code class="code">set_endian field Bitstring.BigEndian</code>.</p>
</div>
</div>
<pre><span id="VALset_endian_expr"><span class="keyword">val</span> set_endian_expr</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -><br> <a href="Bitstring_persistent.html#TYPEexpr">expr</a> -> 'a <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Sets the endianness of a field to an endianness expression.</p>
<p>The effect is that the field <code class="code">{ _ : 16 : endian(e) }</code> could
be created by calling <code class="code">set_endian_expr field e</code>.</p>
</div>
</div>
<pre><span id="VALset_signed"><span class="keyword">val</span> set_signed</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -> bool -> 'a <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Sets the signedness of a field to a constant signedness.</p>
<p>The effect is that the field <code class="code">{ _ : 16 : signed }</code> could
be created by calling <code class="code">set_signed field true</code>.</p>
</div>
</div>
<pre><span id="VALset_type_int"><span class="keyword">val</span> set_type_int</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -> 'a <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Sets the type of a field to <code class="code">int</code>.</p>
<p>The effect is that the field <code class="code">{ _ : 16 : int }</code> could
be created by calling <code class="code">set_type_int field</code>.</p>
</div>
</div>
<pre><span id="VALset_type_string"><span class="keyword">val</span> set_type_string</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -> 'a <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Sets the type of a field to <code class="code">string</code>.</p>
<p>The effect is that the field <code class="code">{ str : 16 : string }</code> could
be created by calling <code class="code">set_type_string field</code>.</p>
</div>
</div>
<pre><span id="VALset_type_bitstring"><span class="keyword">val</span> set_type_bitstring</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -> 'a <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Sets the type of a field to <code class="code">bitstring</code>.</p>
<p>The effect is that the field <code class="code">{ _ : 768 : bitstring }</code> could
be created by calling <code class="code">set_type_bitstring field</code>.</p>
</div>
</div>
<pre><span id="VALset_location"><span class="keyword">val</span> set_location</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -><br> <a href="Bitstring_persistent.html#TYPEloc_t">loc_t</a> -> 'a <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Sets the source code location of a field. This is used when
pa_bitstring displays error messages.</p>
</div>
</div>
<pre><span id="VALset_offset_int"><span class="keyword">val</span> set_offset_int</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -> int -> 'a <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Set the offset expression for a field to the given number.</p>
<p>The effect is that the field <code class="code">{ _ : 8 : offset(160) }</code> could
be created by calling <code class="code">set_offset_int field 160</code>.</p>
</div>
</div>
<pre><span id="VALset_offset"><span class="keyword">val</span> set_offset</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -><br> <a href="Bitstring_persistent.html#TYPEexpr">expr</a> -> 'a <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Set the offset expression for a field to the given expression.</p>
<p>The effect is that the field <code class="code">{ _ : 8 : offset(160) }</code> could
be created by calling <code class="code">set_offset_int field <:expr< 160 >></code>.</p>
</div>
</div>
<pre><span id="VALset_no_offset"><span class="keyword">val</span> set_no_offset</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -> 'a <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Remove the offset expression from a field. The field will
follow the previous field, or if it is the first field will
be at offset zero.</p>
</div>
</div>
<pre><span id="VALset_check"><span class="keyword">val</span> set_check</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -><br> <a href="Bitstring_persistent.html#TYPEexpr">expr</a> -> 'a <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Set the check expression for a field to the given expression.</p>
</div>
</div>
<pre><span id="VALset_no_check"><span class="keyword">val</span> set_no_check</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -> 'a <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Remove the check expression from a field.</p>
</div>
</div>
<pre><span id="VALset_bind"><span class="keyword">val</span> set_bind</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -><br> <a href="Bitstring_persistent.html#TYPEexpr">expr</a> -> 'a <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Set the bind-expression for a field to the given expression.</p>
</div>
</div>
<pre><span id="VALset_no_bind"><span class="keyword">val</span> set_no_bind</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -> 'a <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Remove the bind-expression from a field.</p>
</div>
</div>
<pre><span id="VALset_save_offset_to"><span class="keyword">val</span> set_save_offset_to</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -><br> <a href="Bitstring_persistent.html#TYPEpatt">patt</a> -> 'a <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Set the save_offset_to pattern for a field to the given pattern.</p>
</div>
</div>
<pre><span id="VALset_save_offset_to_lident"><span class="keyword">val</span> set_save_offset_to_lident</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -> string -> 'a <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Set the save_offset_to pattern for a field to identifier.</p>
</div>
</div>
<pre><span id="VALset_no_save_offset_to"><span class="keyword">val</span> set_no_save_offset_to</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -> 'a <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Remove the save_offset_to from a field.</p>
</div>
</div>
<h4 id="3_Createconstructorfields">Create constructor fields</h4>
<p>These fields are used in constructors (<code class="code">BITSTRING</code>).</p>
<pre><span id="VALcreate_constructor_field"><span class="keyword">val</span> create_constructor_field</span> : <code class="type"><a href="Bitstring_persistent.html#TYPEloc_t">loc_t</a> -><br> <a href="Bitstring_persistent.html#TYPEexpr">expr</a> <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Create a constructor field.</p>
<p>The defaults are the same as for <a href="Bitstring_persistent.html#VALcreate_pattern_field"><code class="code">Bitstring_persistent.create_pattern_field</code></a>
except that the expression is initialized to <code class="code">0</code>.</p>
</div>
</div>
<pre><span id="VALset_lident_expr"><span class="keyword">val</span> set_lident_expr</span> : <code class="type"><a href="Bitstring_persistent.html#TYPEexpr">expr</a> <a href="Bitstring_persistent.html#TYPEfield">field</a> -><br> string -> <a href="Bitstring_persistent.html#TYPEexpr">expr</a> <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Sets the expression in a constructor field to an expression
which uses the identifier.</p>
<p>The effect is that the field <code class="code">{ len : 8 : int }</code> could
be created by calling <code class="code">set_lident_expr field "len"</code>.</p>
</div>
</div>
<pre><span id="VALset_int_expr"><span class="keyword">val</span> set_int_expr</span> : <code class="type"><a href="Bitstring_persistent.html#TYPEexpr">expr</a> <a href="Bitstring_persistent.html#TYPEfield">field</a> -><br> int -> <a href="Bitstring_persistent.html#TYPEexpr">expr</a> <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Sets the expression to the value of the integer.</p>
<p>The effect is that the field <code class="code">{ 2 : 8 : int }</code> could
be created by calling <code class="code">set_int_expr field 2</code>.</p>
</div>
</div>
<pre><span id="VALset_string_expr"><span class="keyword">val</span> set_string_expr</span> : <code class="type"><a href="Bitstring_persistent.html#TYPEexpr">expr</a> <a href="Bitstring_persistent.html#TYPEfield">field</a> -><br> string -> <a href="Bitstring_persistent.html#TYPEexpr">expr</a> <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Sets the expression to the value of the string.</p>
<p>The effect is that the field <code class="code">{ "MAGIC" : 8*5 : string }</code> could
be created by calling <code class="code">set_int_expr field "MAGIC"</code>.</p>
</div>
</div>
<pre><span id="VALset_expr"><span class="keyword">val</span> set_expr</span> : <code class="type"><a href="Bitstring_persistent.html#TYPEexpr">expr</a> <a href="Bitstring_persistent.html#TYPEfield">field</a> -><br> <a href="Bitstring_persistent.html#TYPEexpr">expr</a> -><br> <a href="Bitstring_persistent.html#TYPEexpr">expr</a> <a href="Bitstring_persistent.html#TYPEfield">field</a></code></pre><div class="info ">
<div class="info-desc">
<p>Sets the expression field to an arbitrary OCaml expression.</p>
</div>
</div>
<h4 id="3_Accessors">Accessors</h4>
<pre><span id="VALget_patt"><span class="keyword">val</span> get_patt</span> : <code class="type"><a href="Bitstring_persistent.html#TYPEpatt">patt</a> <a href="Bitstring_persistent.html#TYPEfield">field</a> -><br> <a href="Bitstring_persistent.html#TYPEpatt">patt</a></code></pre><div class="info ">
<div class="info-desc">
<p>Get the pattern from a pattern field.</p>
</div>
</div>
<pre><span id="VALget_expr"><span class="keyword">val</span> get_expr</span> : <code class="type"><a href="Bitstring_persistent.html#TYPEexpr">expr</a> <a href="Bitstring_persistent.html#TYPEfield">field</a> -><br> <a href="Bitstring_persistent.html#TYPEexpr">expr</a></code></pre><div class="info ">
<div class="info-desc">
<p>Get the expression from an expression field.</p>
</div>
</div>
<pre><span id="VALget_length"><span class="keyword">val</span> get_length</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -> <a href="Bitstring_persistent.html#TYPEexpr">expr</a></code></pre><div class="info ">
<div class="info-desc">
<p>Get the length in bits from a field. Note that what is returned
is an OCaml expression, since lengths can be non-constant.</p>
</div>
</div>
<pre><code><span id="TYPEendian_expr"><span class="keyword">type</span> <code class="type"></code>endian_expr</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="TYPEELTendian_expr.ConstantEndian"><span class="constructor">ConstantEndian</span></span> <span class="keyword">of</span> <code class="type"><a href="Bitstring.html#TYPEendian">Bitstring.endian</a></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="TYPEELTendian_expr.EndianExpr"><span class="constructor">EndianExpr</span></span> <span class="keyword">of</span> <code class="type"><a href="Bitstring_persistent.html#TYPEexpr">expr</a></code></code></td>
</tr></table>
<pre><span id="VALget_endian"><span class="keyword">val</span> get_endian</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -> <a href="Bitstring_persistent.html#TYPEendian_expr">endian_expr</a></code></pre><div class="info ">
<div class="info-desc">
<p>Get the endianness of a field. This is an <a href="Bitstring_persistent.html#TYPEendian_expr"><code class="code">Bitstring_persistent.endian_expr</code></a> which
could be a constant or an OCaml expression.</p>
</div>
</div>
<pre><span id="VALget_signed"><span class="keyword">val</span> get_signed</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -> bool</code></pre><div class="info ">
<div class="info-desc">
<p>Get the signedness of a field.</p>
</div>
</div>
<pre><code><span id="TYPEfield_type"><span class="keyword">type</span> <code class="type"></code>field_type</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="TYPEELTfield_type.Int"><span class="constructor">Int</span></span></code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code><span class="keyword">|</span></code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTfield_type.String"><span class="constructor">String</span></span></code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code><span class="keyword">|</span></code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTfield_type.Bitstring"><span class="constructor">Bitstring</span></span></code></td>
</tr></table>
<pre><span id="VALget_type"><span class="keyword">val</span> get_type</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -> <a href="Bitstring_persistent.html#TYPEfield_type">field_type</a></code></pre><div class="info ">
<div class="info-desc">
<p>Get the type of a field, <code class="code">Int</code>, <code class="code">String</code> or <code class="code">Bitstring</code>.</p>
</div>
</div>
<pre><span id="VALget_location"><span class="keyword">val</span> get_location</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -> <a href="Bitstring_persistent.html#TYPEloc_t">loc_t</a></code></pre><div class="info ">
<div class="info-desc">
<p>Get the source code location of a field.</p>
</div>
</div>
<pre><span id="VALget_offset"><span class="keyword">val</span> get_offset</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -> <a href="Bitstring_persistent.html#TYPEexpr">expr</a> option</code></pre><div class="info ">
<div class="info-desc">
<p>Get the offset expression of a field, or <code class="code">None</code> if there is none.</p>
</div>
</div>
<pre><span id="VALget_check"><span class="keyword">val</span> get_check</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -> <a href="Bitstring_persistent.html#TYPEexpr">expr</a> option</code></pre><div class="info ">
<div class="info-desc">
<p>Get the check expression of a field, or <code class="code">None</code> if there is none.</p>
</div>
</div>
<pre><span id="VALget_bind"><span class="keyword">val</span> get_bind</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -> <a href="Bitstring_persistent.html#TYPEexpr">expr</a> option</code></pre><div class="info ">
<div class="info-desc">
<p>Get the bind expression of a field, or <code class="code">None</code> if there is none.</p>
</div>
</div>
<pre><span id="VALget_save_offset_to"><span class="keyword">val</span> get_save_offset_to</span> : <code class="type">'a <a href="Bitstring_persistent.html#TYPEfield">field</a> -> <a href="Bitstring_persistent.html#TYPEpatt">patt</a> option</code></pre><div class="info ">
<div class="info-desc">
<p>Get the save_offset_to pattern of a field, or <code class="code">None</code> if there is none.</p>
</div>
</div>
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