Modules
-------
The binary encoding of modules is organized into *sections*.
Most sections correspond to one component of a :ref:`module <syntax-module>` record,
except that :ref:`function definitions <syntax-func>` are split into two sections, separating their type declarations in the :ref:`function section <binary-funcsec>` from their bodies in the :ref:`code section <binary-codesec>`.
.. note::
This separation enables *parallel* and *streaming* compilation of the functions in a module.
.. index:: index, type index, function index, table index, memory index, global index, element index, data index, local index, label index
pair: binary format; type index
pair: binary format; function index
pair: binary format; table index
pair: binary format; memory index
pair: binary format; global index
pair: binary format; element index
pair: binary format; data index
pair: binary format; local index
pair: binary format; label index
Indices
~~~~~~~
All :ref:`indices <syntax-index>` are encoded with their respective value.
.. math::
\begin{array}{llclll}
\production{type index} & \Btypeidx &::=& x{:}\Bu32 &\Rightarrow& x \\
\production{function index} & \Bfuncidx &::=& x{:}\Bu32 &\Rightarrow& x \\
\production{table index} & \Btableidx &::=& x{:}\Bu32 &\Rightarrow& x \\
\production{memory index} & \Bmemidx &::=& x{:}\Bu32 &\Rightarrow& x \\
\production{global index} & \Bglobalidx &::=& x{:}\Bu32 &\Rightarrow& x \\
\production{element index} & \Belemidx &::=& x{:}\Bu32 &\Rightarrow& x \\
\production{data index} & \Bdataidx &::=& x{:}\Bu32 &\Rightarrow& x \\
\production{local index} & \Blocalidx &::=& x{:}\Bu32 &\Rightarrow& x \\
\production{label index} & \Blabelidx &::=& l{:}\Bu32 &\Rightarrow& l \\
\end{array}
.. index:: ! section
pair: binary format; section
Sections
~~~~~~~~
Each section consists of
* a one-byte section *id*,
* the |U32| *size* of the contents, in bytes,
* the actual *contents*, whose structure is dependent on the section id.
Every section is optional; an omitted section is equivalent to the section being present with empty contents.
The following parameterized grammar rule defines the generic structure of a section with id :math:`N` and contents described by the grammar :math:`\B{B}`.
.. math::
\begin{array}{llclll@{\qquad}l}
\production{section} & \Bsection_N(\B{B}) &::=&
N{:}\Bbyte~~\X{size}{:}\Bu32~~\X{cont}{:}\B{B}
&\Rightarrow& \X{cont} & (\iff \X{size} = ||\B{B}||) \\ &&|&
\epsilon &\Rightarrow& \epsilon
\end{array}
For most sections, the contents :math:`\B{B}` encodes a :ref:`vector <binary-vec>`.
In these cases, the empty result :math:`\epsilon` is interpreted as the empty vector.
.. note::
Other than for unknown :ref:`custom sections <binary-customsec>`,
the :math:`\X{size}` is not required for decoding, but can be used to skip sections when navigating through a binary.
The module is malformed if the size does not match the length of the binary contents :math:`\B{B}`.
The following section ids are used:
== ===============================================
Id Section
== ===============================================
0 :ref:`custom section <binary-customsec>`
1 :ref:`type section <binary-typesec>`
2 :ref:`import section <binary-importsec>`
3 :ref:`function section <binary-funcsec>`
4 :ref:`table section <binary-tablesec>`
5 :ref:`memory section <binary-memsec>`
6 :ref:`global section <binary-globalsec>`
7 :ref:`export section <binary-exportsec>`
8 :ref:`start section <binary-startsec>`
9 :ref:`element section <binary-elemsec>`
10 :ref:`code section <binary-codesec>`
11 :ref:`data section <binary-datasec>`
12 :ref:`data count section <binary-datacountsec>`
== ===============================================
.. note::
Section ids do not always correspond to the :ref:`order of sections <binary-module>` in the encoding of a module.
.. index:: ! custom section
pair: binary format; custom section
single: section; custom
Custom Section
~~~~~~~~~~~~~~
*Custom sections* have the id 0.
They are intended to be used for debugging information or third-party extensions, and are ignored by the WebAssembly semantics.
Their contents consist of a :ref:`name <syntax-name>` further identifying the custom section, followed by an uninterpreted sequence of bytes for custom use.
.. math::
\begin{array}{llclll}
\production{custom section} & \Bcustomsec &::=&
\Bsection_0(\Bcustom) \\
\production{custom data} & \Bcustom &::=&
\Bname~~\Bbyte^\ast \\
\end{array}
.. note::
If an implementation interprets the data of a custom section, then errors in that data, or the placement of the section, must not invalidate the module.
.. index:: ! type section, type definition
pair: binary format; type section
pair: section; type
Type Section
~~~~~~~~~~~~
The *type section* has the id 1.
It decodes into a vector of :ref:`function types <syntax-functype>` that represent the |MTYPES| component of a :ref:`module <syntax-module>`.
.. math::
\begin{array}{llclll}
\production{type section} & \Btypesec &::=&
\X{ft}^\ast{:\,}\Bsection_1(\Bvec(\Bfunctype)) &\Rightarrow& \X{ft}^\ast \\
\end{array}
.. index:: ! import section, import, name, function type, table type, memory type, global type
pair: binary format; import
pair: section; import
Import Section
~~~~~~~~~~~~~~
The *import section* has the id 2.
It decodes into a vector of :ref:`imports <syntax-import>` that represent the |MIMPORTS| component of a :ref:`module <syntax-module>`.
.. math::
\begin{array}{llclll}
\production{import section} & \Bimportsec &::=&
\X{im}^\ast{:}\Bsection_2(\Bvec(\Bimport)) &\Rightarrow& \X{im}^\ast \\
\production{import} & \Bimport &::=&
\X{mod}{:}\Bname~~\X{nm}{:}\Bname~~d{:}\Bimportdesc
&\Rightarrow& \{ \IMODULE~\X{mod}, \INAME~\X{nm}, \IDESC~d \} \\
\production{import description} & \Bimportdesc &::=&
\hex{00}~~x{:}\Btypeidx &\Rightarrow& \IDFUNC~x \\ &&|&
\hex{01}~~\X{tt}{:}\Btabletype &\Rightarrow& \IDTABLE~\X{tt} \\ &&|&
\hex{02}~~\X{mt}{:}\Bmemtype &\Rightarrow& \IDMEM~\X{mt} \\ &&|&
\hex{03}~~\X{gt}{:}\Bglobaltype &\Rightarrow& \IDGLOBAL~\X{gt} \\
\end{array}
.. index:: ! function section, function, type index, function type
pair: binary format; function
pair: section; function
Function Section
~~~~~~~~~~~~~~~~
The *function section* has the id 3.
It decodes into a vector of :ref:`type indices <syntax-typeidx>` that represent the |FTYPE| fields of the :ref:`functions <syntax-func>` in the |MFUNCS| component of a :ref:`module <syntax-module>`.
The |FLOCALS| and |FBODY| fields of the respective functions are encoded separately in the :ref:`code section <binary-codesec>`.
.. math::
\begin{array}{llclll}
\production{function section} & \Bfuncsec &::=&
x^\ast{:}\Bsection_3(\Bvec(\Btypeidx)) &\Rightarrow& x^\ast \\
\end{array}
.. index:: ! table section, table, table type
pair: binary format; table
pair: section; table
Table Section
~~~~~~~~~~~~~
The *table section* has the id 4.
It decodes into a vector of :ref:`tables <syntax-table>` that represent the |MTABLES| component of a :ref:`module <syntax-module>`.
.. math::
\begin{array}{llclll}
\production{table section} & \Btablesec &::=&
\X{tab}^\ast{:}\Bsection_4(\Bvec(\Btable)) &\Rightarrow& \X{tab}^\ast \\
\production{table} & \Btable &::=&
\X{tt}{:}\Btabletype &\Rightarrow& \{ \TTYPE~\X{tt} \} \\
\end{array}
.. index:: ! memory section, memory, memory type
pair: binary format; memory
pair: section; memory
Memory Section
~~~~~~~~~~~~~~
The *memory section* has the id 5.
It decodes into a vector of :ref:`memories <syntax-mem>` that represent the |MMEMS| component of a :ref:`module <syntax-module>`.
.. math::
\begin{array}{llclll}
\production{memory section} & \Bmemsec &::=&
\X{mem}^\ast{:}\Bsection_5(\Bvec(\Bmem)) &\Rightarrow& \X{mem}^\ast \\
\production{memory} & \Bmem &::=&
\X{mt}{:}\Bmemtype &\Rightarrow& \{ \MTYPE~\X{mt} \} \\
\end{array}
.. index:: ! global section, global, global type, expression
pair: binary format; global
pair: section; global
Global Section
~~~~~~~~~~~~~~
The *global section* has the id 6.
It decodes into a vector of :ref:`globals <syntax-global>` that represent the |MGLOBALS| component of a :ref:`module <syntax-module>`.
.. math::
\begin{array}{llclll}
\production{global section} & \Bglobalsec &::=&
\X{glob}^\ast{:}\Bsection_6(\Bvec(\Bglobal)) &\Rightarrow& \X{glob}^\ast \\
\production{global} & \Bglobal &::=&
\X{gt}{:}\Bglobaltype~~e{:}\Bexpr
&\Rightarrow& \{ \GTYPE~\X{gt}, \GINIT~e \} \\
\end{array}
.. index:: ! export section, export, name, index, function index, table index, memory index, global index
pair: binary format; export
pair: section; export
Export Section
~~~~~~~~~~~~~~
The *export section* has the id 7.
It decodes into a vector of :ref:`exports <syntax-export>` that represent the |MEXPORTS| component of a :ref:`module <syntax-module>`.
.. math::
\begin{array}{llclll}
\production{export section} & \Bexportsec &::=&
\X{ex}^\ast{:}\Bsection_7(\Bvec(\Bexport)) &\Rightarrow& \X{ex}^\ast \\
\production{export} & \Bexport &::=&
\X{nm}{:}\Bname~~d{:}\Bexportdesc
&\Rightarrow& \{ \ENAME~\X{nm}, \EDESC~d \} \\
\production{export description} & \Bexportdesc &::=&
\hex{00}~~x{:}\Bfuncidx &\Rightarrow& \EDFUNC~x \\ &&|&
\hex{01}~~x{:}\Btableidx &\Rightarrow& \EDTABLE~x \\ &&|&
\hex{02}~~x{:}\Bmemidx &\Rightarrow& \EDMEM~x \\ &&|&
\hex{03}~~x{:}\Bglobalidx &\Rightarrow& \EDGLOBAL~x \\
\end{array}
.. index:: ! start section, start function, function index
pair: binary format; start function
single: section; start
single: start function; section
Start Section
~~~~~~~~~~~~~
The *start section* has the id 8.
It decodes into an optional :ref:`start function <syntax-start>` that represents the |MSTART| component of a :ref:`module <syntax-module>`.
.. math::
\begin{array}{llclll}
\production{start section} & \Bstartsec &::=&
\X{st}^?{:}\Bsection_8(\Bstart) &\Rightarrow& \X{st}^? \\
\production{start function} & \Bstart &::=&
x{:}\Bfuncidx &\Rightarrow& \{ \SFUNC~x \} \\
\end{array}
.. index:: ! element section, element, table index, expression, function index
pair: binary format; element
pair: section; element
single: table; element
single: element; segment
Element Section
~~~~~~~~~~~~~~~
The *element section* has the id 9.
It decodes into a vector of :ref:`element segments <syntax-elem>` that represent the |MELEMS| component of a :ref:`module <syntax-module>`.
.. math::
\begin{array}{llclll}
\production{element section} & \Belemsec &::=&
\X{seg}^\ast{:}\Bsection_9(\Bvec(\Belem)) &\Rightarrow& \X{seg}^\ast \\
\production{element segment} & \Belem &::=&
0{:}\Bu32~~e{:}\Bexpr~~y^\ast{:}\Bvec(\Bfuncidx)
&\Rightarrow& \\&&&\quad
\{ \ETYPE~\FUNCREF, \EINIT~((\REFFUNC~y)~\END)^\ast, \EMODE~\EACTIVE~\{ \ETABLE~0, \EOFFSET~e \} \} \\ &&|&
1{:}\Bu32~~\X{et}:\Belemkind~~y^\ast{:}\Bvec(\Bfuncidx)
&\Rightarrow& \\&&&\quad
\{ \ETYPE~\X{et}, \EINIT~((\REFFUNC~y)~\END)^\ast, \EMODE~\EPASSIVE \} \\ &&|&
2{:}\Bu32~~x{:}\Btableidx~~e{:}\Bexpr~~\X{et}:\Belemkind~~y^\ast{:}\Bvec(\Bfuncidx)
&\Rightarrow& \\&&&\quad
\{ \ETYPE~\X{et}, \EINIT~((\REFFUNC~y)~\END)^\ast, \EMODE~\EACTIVE~\{ \ETABLE~x, \EOFFSET~e \} \} \\ &&|&
3{:}\Bu32~~\X{et}:\Belemkind~~y^\ast{:}\Bvec(\Bfuncidx)
&\Rightarrow& \\&&&\quad
\{ \ETYPE~\X{et}, \EINIT~((\REFFUNC~y)~\END)^\ast, \EMODE~\EDECLARATIVE \} \\ &&|&
4{:}\Bu32~~e{:}\Bexpr~~\X{el}^\ast{:}\Bvec(\Bexpr)
&\Rightarrow& \\&&&\quad
\{ \ETYPE~\FUNCREF, \EINIT~\X{el}^\ast, \EMODE~\EACTIVE~\{ \ETABLE~0, \EOFFSET~e \} \} \\ &&|&
5{:}\Bu32~~\X{et}:\Breftype~~\X{el}^\ast{:}\Bvec(\Bexpr)
&\Rightarrow& \\&&&\quad
\{ \ETYPE~et, \EINIT~\X{el}^\ast, \EMODE~\EPASSIVE \} \\ &&|&
6{:}\Bu32~~x{:}\Btableidx~~e{:}\Bexpr~~\X{et}:\Breftype~~\X{el}^\ast{:}\Bvec(\Bexpr)
&\Rightarrow& \\&&&\quad
\{ \ETYPE~et, \EINIT~\X{el}^\ast, \EMODE~\EACTIVE~\{ \ETABLE~x, \EOFFSET~e \} \} \\ &&|&
7{:}\Bu32~~\X{et}:\Breftype~~\X{el}^\ast{:}\Bvec(\Bexpr)
&\Rightarrow& \\&&&\quad
\{ \ETYPE~et, \EINIT~\X{el}^\ast, \EMODE~\EDECLARATIVE \} \\
\production{element kind} & \Belemkind &::=&
\hex{00} &\Rightarrow& \FUNCREF \\
\end{array}
.. note::
The initial integer can be interpreted as a bitfield.
Bit 0 distinguishes a passive or declarative segment from an active segment,
bit 1 indicates the presence of an explicit table index for an active segment and otherwise distinguishes passive from declarative segments,
bit 2 indicates the use of element type and element :ref:`expressions <binary-expr>` instead of element kind and element indices.
Additional element kinds may be added in future versions of WebAssembly.
.. index:: ! code section, function, local, type index, function type
pair: binary format; function
pair: binary format; local
pair: section; code
Code Section
~~~~~~~~~~~~
The *code section* has the id 10.
It decodes into a vector of *code* entries that are pairs of :ref:`value type <syntax-valtype>` vectors and :ref:`expressions <syntax-expr>`.
They represent the |FLOCALS| and |FBODY| field of the :ref:`functions <syntax-func>` in the |MFUNCS| component of a :ref:`module <syntax-module>`.
The |FTYPE| fields of the respective functions are encoded separately in the :ref:`function section <binary-funcsec>`.
The encoding of each code entry consists of
* the |U32| *size* of the function code in bytes,
* the actual *function code*, which in turn consists of
* the declaration of *locals*,
* the function *body* as an :ref:`expression <binary-expr>`.
Local declarations are compressed into a vector whose entries consist of
* a |U32| *count*,
* a :ref:`value type <binary-valtype>`,
denoting *count* locals of the same value type.
.. math::
\begin{array}{llclll@{\qquad}l}
\production{code section} & \Bcodesec &::=&
\X{code}^\ast{:}\Bsection_{10}(\Bvec(\Bcode))
&\Rightarrow& \X{code}^\ast \\
\production{code} & \Bcode &::=&
\X{size}{:}\Bu32~~\X{code}{:}\Bfunc
&\Rightarrow& \X{code} & (\iff \X{size} = ||\Bfunc||) \\
\production{function} & \Bfunc &::=&
(t^\ast)^\ast{:}\Bvec(\Blocals)~~e{:}\Bexpr
&\Rightarrow& \concat((t^\ast)^\ast), e
& (\iff |\concat((t^\ast)^\ast)| < 2^{32}) \\
\production{locals} & \Blocals &::=&
n{:}\Bu32~~t{:}\Bvaltype &\Rightarrow& t^n \\
\end{array}
Here, :math:`\X{code}` ranges over pairs :math:`(\valtype^\ast, \expr)`.
The meta function :math:`\concat((t^\ast)^\ast)` concatenates all sequences :math:`t_i^\ast` in :math:`(t^\ast)^\ast`.
Any code for which the length of the resulting sequence is out of bounds of the maximum size of a :ref:`vector <syntax-vec>` is malformed.
.. note::
Like with :ref:`sections <binary-section>`, the code :math:`\X{size}` is not needed for decoding, but can be used to skip functions when navigating through a binary.
The module is malformed if a size does not match the length of the respective function code.
.. index:: ! data section, data, memory, memory index, expression, byte
pair: binary format; data
pair: section; data
single: memory; data
single: data; segment
Data Section
~~~~~~~~~~~~
The *data section* has the id 11.
It decodes into a vector of :ref:`data segments <syntax-data>` that represent the |MDATAS| component of a :ref:`module <syntax-module>`.
.. math::
\begin{array}{llclll}
\production{data section} & \Bdatasec &::=&
\X{seg}^\ast{:}\Bsection_{11}(\Bvec(\Bdata)) &\Rightarrow& \X{seg}^\ast \\
\production{data segment} & \Bdata &::=&
0{:}\Bu32~~e{:}\Bexpr~~b^\ast{:}\Bvec(\Bbyte)
&\Rightarrow& \{ \DINIT~b^\ast, \DMODE~\DACTIVE~\{ \DMEM~0, \DOFFSET~e \} \} \\ &&|&
1{:}\Bu32~~b^\ast{:}\Bvec(\Bbyte)
&\Rightarrow& \{ \DINIT~b^\ast, \DMODE~\DPASSIVE \} \\ &&|&
2{:}\Bu32~~x{:}\Bmemidx~~e{:}\Bexpr~~b^\ast{:}\Bvec(\Bbyte)
&\Rightarrow& \{ \DINIT~b^\ast, \DMODE~\DACTIVE~\{ \DMEM~x, \DOFFSET~e \} \} \\
\end{array}
.. note::
The initial integer can be interpreted as a bitfield.
Bit 0 indicates a passive segment,
bit 1 indicates the presence of an explicit memory index for an active segment.
In the current version of WebAssembly, at most one memory may be defined or
imported in a single module, so all valid :ref:`active <syntax-data>` data
segments have a |DMEM| value of :math:`0`.
.. index:: ! data count section, data count, data segment
pair: binary format; data count
pair: section; data count
Data Count Section
~~~~~~~~~~~~~~~~~~
The *data count section* has the id 12.
It decodes into an optional :ref:`u32 <syntax-uint>` that represents the number of :ref:`data segments <syntax-data>` in the :ref:`data section <binary-datasec>`. If this count does not match the length of the data segment vector, the module is malformed.
.. math::
\begin{array}{llclll}
\production{data count section} & \Bdatacountsec &::=&
\X{n}^?{:}\Bsection_{12}(\Bu32) &\Rightarrow& \X{n}^? \\
\end{array}
.. note::
The data count section is used to simplify single-pass validation. Since the
data section occurs after the code section, the :math:`\MEMORYINIT` and
:math:`\DATADROP` instructions would not be able to check whether the data
segment index is valid until the data section is read. The data count section
occurs before the code section, so a single-pass validator can use this count
instead of deferring validation.
.. index:: module, section, type definition, function type, function, table, memory, global, element, data, start function, import, export, context, version
pair: binary format; module
Modules
~~~~~~~
The encoding of a :ref:`module <syntax-module>` starts with a preamble containing a 4-byte magic number (the string :math:`\text{\backslash0asm}`) and a version field.
The current version of the WebAssembly binary format is 1.
The preamble is followed by a sequence of :ref:`sections <binary-section>`.
:ref:`Custom sections <binary-customsec>` may be inserted at any place in this sequence,
while other sections must occur at most once and in the prescribed order.
All sections can be empty.
The lengths of vectors produced by the (possibly empty) :ref:`function <binary-funcsec>` and :ref:`code <binary-codesec>` section must match up.
Similarly, the optional data count must match the length of the :ref:`data segment <binary-datasec>` vector.
Furthermore, it must be present if any :ref:`data index <syntax-dataidx>` occurs in the code section.
.. math::
\begin{array}{llcllll}
\production{magic} & \Bmagic &::=&
\hex{00}~\hex{61}~\hex{73}~\hex{6D} \\
\production{version} & \Bversion &::=&
\hex{01}~\hex{00}~\hex{00}~\hex{00} \\
\production{module} & \Bmodule &::=&
\Bmagic \\ &&&
\Bversion \\ &&&
\Bcustomsec^\ast \\ &&&
\functype^\ast{:\,}\Btypesec \\ &&&
\Bcustomsec^\ast \\ &&&
\import^\ast{:\,}\Bimportsec \\ &&&
\Bcustomsec^\ast \\ &&&
\typeidx^n{:\,}\Bfuncsec \\ &&&
\Bcustomsec^\ast \\ &&&
\table^\ast{:\,}\Btablesec \\ &&&
\Bcustomsec^\ast \\ &&&
\mem^\ast{:\,}\Bmemsec \\ &&&
\Bcustomsec^\ast \\ &&&
\global^\ast{:\,}\Bglobalsec \\ &&&
\Bcustomsec^\ast \\ &&&
\export^\ast{:\,}\Bexportsec \\ &&&
\Bcustomsec^\ast \\ &&&
\start^?{:\,}\Bstartsec \\ &&&
\Bcustomsec^\ast \\ &&&
\elem^\ast{:\,}\Belemsec \\ &&&
\Bcustomsec^\ast \\ &&&
m^?{:\,}\Bdatacountsec \\ &&&
\Bcustomsec^\ast \\ &&&
\X{code}^n{:\,}\Bcodesec \\ &&&
\Bcustomsec^\ast \\ &&&
\data^m{:\,}\Bdatasec \\ &&&
\Bcustomsec^\ast
\quad\Rightarrow\quad \{~
\begin{array}[t]{@{}l@{}}
\MTYPES~\functype^\ast, \\
\MFUNCS~\func^n, \\
\MTABLES~\table^\ast, \\
\MMEMS~\mem^\ast, \\
\MGLOBALS~\global^\ast, \\
\MELEMS~\elem^\ast, \\
\MDATAS~\data^m, \\
\MSTART~\start^?, \\
\MIMPORTS~\import^\ast, \\
\MEXPORTS~\export^\ast ~\} \\
\end{array} \\ &&&
(\iff m^? \neq \epsilon \vee \freedataidx(\X{code}^n) = \emptyset) \\
\end{array}
where for each :math:`t_i^\ast, e_i` in :math:`\X{code}^n`,
.. math::
\func^n[i] = \{ \FTYPE~\typeidx^n[i], \FLOCALS~t_i^\ast, \FBODY~e_i \} \\
.. note::
The version of the WebAssembly binary format may increase in the future
if backward-incompatible changes have to be made to the format.
However, such changes are expected to occur very infrequently, if ever.
The binary format is intended to be extensible,
such that future features can be added without incrementing its version.