.. _settings-reference:

============================
Emscripten Compiler Settings
============================

The following is a complete list of settings that can be passed to emscripten
via ``-s`` on the command line.  For example ``-sASSERTIONS`` or
``-sASSERTIONS=0``.  For more details see the :ref:`emcc <emcc-s-option-value>`
documentation.

Unless otherwise noted these settings only apply when linking and have no effect
during compilation.

.. Auto-generated by update_settings_docs.py. **DO NOT EDIT**

.. _assertions:

ASSERTIONS
==========

Whether we should add runtime assertions. This affects both JS and how
system libraries are built.
ASSERTIONS == 2 gives even more runtime checks, that may be very slow. That
includes internal dlmalloc assertions, for example.
ASSERTIONS defaults to 0 in optimized builds (-O1 and above).

Default value: 1

.. _stack_overflow_check:

STACK_OVERFLOW_CHECK
====================

Chooses what kind of stack smash checks to emit to generated code:
Building with ASSERTIONS=1 causes STACK_OVERFLOW_CHECK default to 1.
Since ASSERTIONS=1 is the default at -O0, which itself is the default
optimization level this means that this setting also effectively
defaults to 1, absent any other settings:

- 0: Stack overflows are not checked.
- 1: Adds a security cookie at the top of the stack, which is checked at end
  of each tick and at exit (practically zero performance overhead)
- 2: Same as above, but also runs a binaryen pass which adds a check to all
  stack pointer assignments. Has a small performance cost.

Default value: 0

.. _check_null_writes:

CHECK_NULL_WRITES
=================

When :ref`STACK_OVERFLOW_CHECK` is enabled we also check writes to address
zero. This can help detect NULL pointer usage.  If you want to skip this
extra check (for example, if you want reads from the address zero to always
return zero) you can disable this here.  This setting has no effect when
:ref:`STACK_OVERFLOW_CHECK` is disabled.

Default value: true

.. _verbose:

VERBOSE
=======

When set to 1, will generate more verbose output during compilation.
[general]

Default value: false

.. _invoke_run:

INVOKE_RUN
==========

Whether we will run the main() function. Disable if you embed the generated
code in your own, and will call main() yourself at the right time (which you
can do with Module.callMain(), with an optional parameter of commandline args).

Default value: true

.. _exit_runtime:

EXIT_RUNTIME
============

If 0, support for shutting down the runtime is not emitted into the build.
This means that the program is not quit when main() completes, but execution
will yield to the event loop of the JS environment, allowing event handlers
to run afterwards. If 0, C++ global destructors will not be emitted into the
build either, to save on code size. Calling exit() will throw an unwinding
exception, but will not shut down the runtime.

Set this to 1 if you do want to retain the ability to shut down the program.
If 1, then completing main() will by default call exit(), unless a refcount
keeps the runtime alive. Call emscripten_exit_with_live_runtime() to finish
main() while keeping the runtime alive. Calling emscripten_force_exit() will
shut down the runtime, invoking atexit()s, and flushing stdio streams.
This setting is controlled automatically in :ref:`STANDALONE_WASM` mode:

- For a command (has a main function) this is always 1
- For a reactor (no main function) this is always 0

For more details, see documentation for emscripten_force_exit() and
emscripten_exit_with_live_runtime().

Default value: false

.. _stack_size:

STACK_SIZE
==========

The total stack size. There is no way to enlarge the stack, so this
value must be large enough for the program's requirements. If
assertions are on, we will assert on not exceeding this, otherwise,
it will fail silently.

Default value: 64*1024

.. _malloc:

MALLOC
======

What malloc()/free() to use, out of:

  - dlmalloc - a powerful general-purpose malloc
  - emmalloc - a simple and compact malloc designed for emscripten
  - emmalloc-debug - use emmalloc and add extra assertion checks
  - emmalloc-memvalidate - use emmalloc with assertions+heap consistency
    checking.
  - emmalloc-verbose - use emmalloc with assertions + verbose logging.
  - emmalloc-memvalidate-verbose - use emmalloc with assertions + heap
    consistency checking + verbose logging.
  - mimalloc - a powerful multithreaded allocator. This is recommended in
    large applications that have malloc() contention, but it is
    larger and uses more memory.
  - none - no malloc() implementation is provided, but you must implement
    malloc() and free() yourself.

dlmalloc is necessary for split memory and other special modes, and will be
used automatically in those cases.
In general, if you don't need one of those special modes, and if you don't
allocate very many small objects, you should use emmalloc since it's
smaller. Otherwise, if you do allocate many small objects, dlmalloc
is usually worth the extra size. dlmalloc is also a good choice if you want
the extra security checks it does (such as noticing metadata corruption in
its internal data structures, which emmalloc does not do).

Default value: "dlmalloc"

.. _aborting_malloc:

ABORTING_MALLOC
===============

If 1, then when malloc would fail we abort(). This is nonstandard behavior,
but makes sense for the web since we have a fixed amount of memory that
must all be allocated up front, and so (a) failing mallocs are much more
likely than on other platforms, and (b) people need a way to find out
how big that initial allocation (INITIAL_MEMORY) must be.
If you set this to 0, then you get the standard malloc behavior of
returning NULL (0) when it fails.

Setting ALLOW_MEMORY_GROWTH turns this off, as in that mode we default to
the behavior of trying to grow and returning 0 from malloc on failure, like
a standard system would. However, you can still set this flag to override
that.  This is a mostly-backwards-compatible change. Previously this option
was ignored when growth was on. The current behavior is that growth turns it
off by default, so for users that never specified the flag nothing changes.
But if you do specify it, it will have an effect now, which it did not
previously. If you don't want that, just stop passing it in at link time.

Note that this setting does not affect the behavior of operator new in C++.
This function will always abort on allocation failure if exceptions are
disabled.
If you want new to return 0 on failure, use it with std::nothrow.

Default value: true

.. _initial_heap:

INITIAL_HEAP
============

The initial amount of heap memory available to the program.  This is the
memory region available for dynamic allocations via `sbrk`, `malloc` and `new`.

Unlike INITIAL_MEMORY, this setting allows the static and dynamic regions of
your program's memory to independently grow. In most cases we recommend using
this setting rather than `INITIAL_MEMORY`. However, this setting does not work
for imported memories (e.g. when dynamic linking is used).

Default value: 16777216

.. _initial_memory:

INITIAL_MEMORY
==============

The initial amount of memory to use. Using more memory than this will
cause us to expand the heap, which can be costly with typed arrays:
we need to copy the old heap into a new one in that case.
If ALLOW_MEMORY_GROWTH is set, this initial amount of memory can increase
later; if not, then it is the final and total amount of memory.

By default, this value is calculated based on INITIAL_HEAP, STACK_SIZE,
as well the size of static data in input modules.

(This option was formerly called TOTAL_MEMORY.)

Default value: -1

.. _maximum_memory:

MAXIMUM_MEMORY
==============

Set the maximum size of memory in the wasm module (in bytes). This is only
relevant when ALLOW_MEMORY_GROWTH is set, as without growth, the size of
INITIAL_MEMORY is the final size of memory anyhow.

Note that the default value here is 2GB, which means that by default if you
enable memory growth then we can grow up to 2GB but no higher. 2GB is a
natural limit for several reasons:

  * If the maximum heap size is over 2GB, then pointers must be unsigned in
    JavaScript, which increases code size. We don't want memory growth builds
    to be larger unless someone explicitly opts in to >2GB+ heaps.
  * Historically no VM has supported more >2GB+, and only recently (Mar 2020)
    has support started to appear. As support is limited, it's safer for
    people to opt into >2GB+ heaps rather than get a build that may not
    work on all VMs.

To use more than 2GB, set this to something higher, like 4GB.

(This option was formerly called WASM_MEM_MAX and BINARYEN_MEM_MAX.)

Default value: 2147483648

.. _allow_memory_growth:

ALLOW_MEMORY_GROWTH
===================

If false, we abort with an error if we try to allocate more memory than
we can (INITIAL_MEMORY). If true, we will grow the memory arrays at
runtime, seamlessly and dynamically.
See https://code.google.com/p/v8/issues/detail?id=3907 regarding
memory growth performance in chrome.
Note that growing memory means we replace the JS typed array views, as
once created they cannot be resized. (In wasm we can grow the Memory, but
still need to create new views for JS.)
Setting this option on will disable ABORTING_MALLOC, in other words,
ALLOW_MEMORY_GROWTH enables fully standard behavior, of both malloc
returning 0 when it fails, and also of being able to allocate more
memory from the system as necessary.

Default value: false

.. _memory_growth_geometric_step:

MEMORY_GROWTH_GEOMETRIC_STEP
============================

If ALLOW_MEMORY_GROWTH is true, this variable specifies the geometric
overgrowth rate of the heap at resize. Specify MEMORY_GROWTH_GEOMETRIC_STEP=0
to disable overgrowing the heap at all, or e.g.
MEMORY_GROWTH_GEOMETRIC_STEP=1.0 to double the heap (+100%) at every grow step.
The larger this value is, the more memory the WebAssembly heap overreserves
to reduce performance hiccups coming from memory resize, and the smaller
this value is, the more memory is conserved, at the performance of more
stuttering when the heap grows. (profiled to be on the order of ~20 msecs)

Default value: 0.20

.. _memory_growth_geometric_cap:

MEMORY_GROWTH_GEOMETRIC_CAP
===========================

Specifies a cap for the maximum geometric overgrowth size, in bytes. Use
this value to constrain the geometric grow to not exceed a specific rate.
Pass MEMORY_GROWTH_GEOMETRIC_CAP=0 to disable the cap and allow unbounded
size increases.

Default value: 96*1024*1024

.. _memory_growth_linear_step:

MEMORY_GROWTH_LINEAR_STEP
=========================

If ALLOW_MEMORY_GROWTH is true and MEMORY_GROWTH_LINEAR_STEP == -1, then
geometric memory overgrowth is utilized (above variable). Set
MEMORY_GROWTH_LINEAR_STEP to a multiple of WASM page size (64KB), eg. 16MB to
replace geometric overgrowth rate with a constant growth step size. When
MEMORY_GROWTH_LINEAR_STEP is used, the variables MEMORY_GROWTH_GEOMETRIC_STEP
and MEMORY_GROWTH_GEOMETRIC_CAP are ignored.

Default value: -1

.. _memory64:

MEMORY64
========

The "architecture" to compile for. 0 means the default wasm32, 1 is
the full end-to-end wasm64 mode, and 2 is wasm64 for clang/lld but lowered to
wasm32 in Binaryen (such that it can run on wasm32 engines, while internally
using i64 pointers).
Nowadays we recommend using the more standard `-m64` or `--target=wasm64`
flags, which do the same thing.
Assumes WASM_BIGINT.

.. note:: Applicable during both linking and compilation

.. note:: This setting is deprecated

Default value: 0

.. _initial_table:

INITIAL_TABLE
=============

Sets the initial size of the table when MAIN_MODULE or SIDE_MODULE is used
(and not otherwise). Normally Emscripten can determine the size of the table
at link time, but in SPLIT_MODULE mode, wasm-split often needs to grow the
table, so the table size baked into the JS for the instrumented build will be
too small after the module is split. This is a hack to allow users to specify
a large enough table size that can be consistent across both builds. This
setting may be removed at any time and should not be used except in
conjunction with SPLIT_MODULE and dynamic linking.

Default value: -1

.. _allow_table_growth:

ALLOW_TABLE_GROWTH
==================

If true, allows more functions to be added to the table at runtime. This is
necessary for dynamic linking, and set automatically in that mode.

Default value: false

.. _global_base:

GLOBAL_BASE
===========

Where global data begins; the start of static memory.
A GLOBAL_BASE of 1024 or above is useful for optimizing load/store offsets,
as it enables the --low-memory-unused pass

Default value: 1024

.. _table_base:

TABLE_BASE
==========

Where table slots (function addresses) are allocated.
This must be at least 1 to reserve the zero slot for the null pointer.

Default value: 1

.. _use_closure_compiler:

USE_CLOSURE_COMPILER
====================

Whether closure compiling is being run on this output

Default value: false

.. _closure_warnings:

CLOSURE_WARNINGS
================

Deprecated: Use the standard warnings flags instead. e.g. ``-Wclosure``,
``-Wno-closure``, ``-Werror=closure``.
options: 'quiet', 'warn', 'error'. If set to 'warn', Closure warnings are
printed out to console. If set to 'error', Closure warnings are treated like
errors, similar to -Werror compiler flag.

.. note:: This setting is deprecated

Default value: 'quiet'

.. _ignore_closure_compiler_errors:

IGNORE_CLOSURE_COMPILER_ERRORS
==============================

Ignore closure warnings and errors (like on duplicate definitions)

Default value: false

.. _declare_asm_module_exports:

DECLARE_ASM_MODULE_EXPORTS
==========================

If set to 1, each wasm module export is individually declared with a
JavaScript "var" definition. This is the simple and recommended approach.
However, this does increase code size (especially if you have many such
exports), which can be avoided in an unsafe way by setting this to 0. In that
case, no "var" is created for each export, and instead a loop (of small
constant code size, no matter how many exports you have) writes all the
exports received into the global scope. Doing so is dangerous since such
modifications of the global scope can confuse external JS minifier tools, and
also things can break if the scope the code is in is not the global scope
(e.g. if you manually enclose them in a function scope).

Default value: true

.. _inlining_limit:

INLINING_LIMIT
==============

If set to 1, prevents inlining. If 0, we will inline normally in LLVM.
This does not affect the inlining policy in Binaryen.

.. note:: Only applicable during compilation

Default value: false

.. _support_big_endian:

SUPPORT_BIG_ENDIAN
==================

If set to 1, perform an acorn pass that converts each HEAP access into a
function call that uses DataView to enforce LE byte order for HEAP buffer;
This makes generated JavaScript run on BE as well as LE machines. (If 0, only
LE systems are supported). Does not affect generated wasm.

.. note:: This is an experimental setting

Default value: false

.. _safe_heap:

SAFE_HEAP
=========

Check each write to the heap, for example, this will give a clear
error on what would be segfaults in a native build (like dereferencing
0). See runtime_safe_heap.js for the actual checks performed.
Set to value 1 to test for safe behavior for both Wasm+Wasm2JS builds.
Set to value 2 to test for safe behavior for only Wasm builds. (notably,
Wasm-only builds allow unaligned memory accesses. Note, however, that
on some architectures unaligned accesses can be very slow, so it is still
a good idea to verify your code with the more strict mode 1)

Default value: 0

.. _safe_heap_log:

SAFE_HEAP_LOG
=============

Log out all SAFE_HEAP operations

Default value: false

.. _emulate_function_pointer_casts:

EMULATE_FUNCTION_POINTER_CASTS
==============================

Allows function pointers to be cast, wraps each call of an incorrect type
with a runtime correction.  This adds overhead and should not be used
normally.  Aside from making calls not fail, this tries to convert values as
best it can.  We use 64 bits (i64) to represent values, as if we wrote the
sent value to memory and loaded the received type from the same memory (using
truncs/extends/ reinterprets). This means that when types do not match the
emulated values may not match (this is true of native too, for that matter -
this is all undefined behavior). This approach appears good enough to
support Python (the original motivation for this feature) and Glib (the
continued motivation).

Default value: false

.. _exception_debug:

EXCEPTION_DEBUG
===============

Print out exceptions in emscriptened code.

Default value: false

.. _library_debug:

LIBRARY_DEBUG
=============

Print out when we enter a library call (library*.js). You can also unset
runtimeDebug at runtime for logging to cease, and can set it when you want
it back. A simple way to set it in C++ is::

  emscripten_run_script("runtimeDebug = ...;");

Default value: false

.. _syscall_debug:

SYSCALL_DEBUG
=============

Print out all musl syscalls, including translating their numeric index
to the string name, which can be convenient for debugging. (Other system
calls are not numbered and already have clear names; use :ref:`LIBRARY_DEBUG`
to get logging for all of them.)

Default value: false

.. _socket_debug:

SOCKET_DEBUG
============

Log out socket/network data transfer.

Default value: false

.. _dylink_debug:

DYLINK_DEBUG
============

Log dynamic linker information

Default value: 0

.. _fs_debug:

FS_DEBUG
========

Register file system callbacks using trackingDelegate in library_fs.js

Default value: false

.. _socket_webrtc:

SOCKET_WEBRTC
=============

As well as being configurable at compile time via the "-s" option the
WEBSOCKET_URL and WEBSOCKET_SUBPROTOCOL
settings may be configured at run time via the Module object e.g.
Module['websocket'] = {subprotocol: 'base64, binary, text'};
Module['websocket'] = {url: 'wss://', subprotocol: 'base64'};
You can set 'subprotocol' to null, if you don't want to specify it.
Run time configuration may be useful as it lets an application select
multiple different services.

.. note:: This setting is deprecated

Default value: false

.. _websocket_url:

WEBSOCKET_URL
=============

A string containing either a WebSocket URL prefix (ws:// or wss://) or a
complete RFC 6455 URL - "ws[s]:" "//" host [ ":" port ] path [ "?" query ].
In the (default) case of only a prefix being specified the URL will be
constructed from prefix + addr + ':' + port
where addr and port are derived from the socket connect/bind/accept calls.

Default value: 'ws://'

.. _proxy_posix_sockets:

PROXY_POSIX_SOCKETS
===================

If 1, the POSIX sockets API uses a native bridge process server to proxy
sockets calls from browser to native world.

Default value: false

.. _noderawsockets:

NODERAWSOCKETS
==============

If enabled, the POSIX sockets API is backed by Node.js's ``node:net``
module, giving real non-blocking outgoing TCP sockets with no WebSockets,
proxy process or pthreads. This is the sockets counterpart to
:ref:`NODERAWFS`: where :ref:`NODERAWFS` gives direct access to the host
filesystem, this gives direct access to host sockets. It only works under
node and is ignored elsewhere.

It supports full TCP (outgoing connect plus bind, listen and accept for
servers) and UDP. TCP clients use the public ``node:net`` API when possible,
falling back to the private ``tcp_wrap``/``udp_wrap`` handles on older
Node.js.

It is event-driven. Socket readiness comes through the same
``emscripten_set_socket_*_callback`` hooks the WebSocket backend uses, so it
works with existing readiness reactors. It cannot be combined with the
WebSocket emulation, :ref:`PROXY_POSIX_SOCKETS` or :ref:`SOCKET_WEBRTC`.

It works under -pthread with :ref:`PROXY_TO_PTHREAD`, where main() and every socket
syscall run on a single worker alongside the node handles and their event
loop. As with the WebSocket backend, sharing a socket across threads under a
plain -pthread build (without PROXY_TO_PTHREAD) is not supported.

Default value: false

.. _websocket_subprotocol:

WEBSOCKET_SUBPROTOCOL
=====================

A string containing a comma separated list of WebSocket subprotocols
as would be present in the Sec-WebSocket-Protocol header.
You can set 'null', if you don't want to specify it.

Default value: 'binary'

.. _openal_debug:

OPENAL_DEBUG
============

Print out debugging information from our OpenAL implementation.

Default value: false

.. _websocket_debug:

WEBSOCKET_DEBUG
===============

If 1, prints out debugging related to calls from ``emscripten_web_socket_*``
functions in ``emscripten/websocket.h``.
If 2, additionally traces bytes communicated via the sockets.

Default value: false

.. _gl_assertions:

GL_ASSERTIONS
=============

Adds extra checks for error situations in the GL library. Can impact
performance.

Default value: false

.. _trace_webgl_calls:

TRACE_WEBGL_CALLS
=================

If enabled, prints out all API calls to WebGL contexts. (*very* verbose)

Default value: false

.. _gl_debug:

GL_DEBUG
========

Enables more verbose debug printing of WebGL related operations. As with
LIBRARY_DEBUG, this is toggleable at runtime with option GL.debug.

Default value: false

.. _gl_testing:

GL_TESTING
==========

When enabled, sets preserveDrawingBuffer in the context, to allow tests to
work (but adds overhead)

Default value: false

.. _gl_max_temp_buffer_size:

GL_MAX_TEMP_BUFFER_SIZE
=======================

How large GL emulation temp buffers are

Default value: 2097152

.. _gl_unsafe_opts:

GL_UNSAFE_OPTS
==============

Enables some potentially-unsafe optimizations in GL emulation code

Default value: true

.. _full_es2:

FULL_ES2
========

Forces support for all GLES2 features, not just the WebGL-friendly subset.

Default value: false

.. _gl_emulate_gles_version_string_format:

GL_EMULATE_GLES_VERSION_STRING_FORMAT
=====================================

If true, glGetString() for GL_VERSION and GL_SHADING_LANGUAGE_VERSION will
return strings OpenGL ES format "Open GL ES ... (WebGL ...)" rather than the
WebGL format. If false, the direct WebGL format strings are returned. Set
this to true to make GL contexts appear like an OpenGL ES context in these
version strings (at the expense of a little bit of added code size), and to
false to make GL contexts appear like WebGL contexts and to save some bytes
from the output.

Default value: true

.. _gl_extensions_in_prefixed_format:

GL_EXTENSIONS_IN_PREFIXED_FORMAT
================================

If true, all GL extensions are advertised in unprefixed WebGL extension
format, but also in desktop/mobile GLES/GL extension format with ``GL_``
prefix.

Default value: true

.. _gl_support_automatic_enable_extensions:

GL_SUPPORT_AUTOMATIC_ENABLE_EXTENSIONS
======================================

If true, adds support for automatically enabling all GL extensions for
GLES/GL emulation purposes. This takes up code size. If you set this to 0,
you will need to manually enable the extensions you need.

Default value: true

.. _gl_support_simple_enable_extensions:

GL_SUPPORT_SIMPLE_ENABLE_EXTENSIONS
===================================

If true, the function ``emscripten_webgl_enable_extension()`` can be called
to enable any WebGL extension. If false, to save code size,
``emscripten_webgl_enable_extension()`` cannot be called to enable any of
extensions 'ANGLE_instanced_arrays', 'OES_vertex_array_object',
'WEBGL_draw_buffers', 'WEBGL_multi_draw',
'WEBGL_draw_instanced_base_vertex_base_instance', or
'WEBGL_multi_draw_instanced_base_vertex_base_instance',
but the dedicated functions ``emscripten_webgl_enable_*()``
found in html5.h are used to enable each of those extensions.
This way code size is increased only for the extensions that are actually used.
N.B. if setting this to 0, GL_SUPPORT_AUTOMATIC_ENABLE_EXTENSIONS must be set
to zero as well.

Default value: true

.. _gl_track_errors:

GL_TRACK_ERRORS
===============

If set to 0, Emscripten GLES2->WebGL translation layer does not track the
kind of GL errors that exist in GLES2 but do not exist in WebGL. Setting
this to 0 saves code size. (Good to keep at 1 for development)

Default value: true

.. _gl_support_explicit_swap_control:

GL_SUPPORT_EXPLICIT_SWAP_CONTROL
================================

If true, GL contexts support the explicitSwapControl context creation flag.
Set to 0 to save a little bit of space on projects that do not need it.

Default value: false

.. _gl_pool_temp_buffers:

GL_POOL_TEMP_BUFFERS
====================

If true, calls to glUniform*fv and glUniformMatrix*fv utilize a pool of
preallocated temporary buffers for common small sizes to avoid generating
temporary garbage for WebGL 1. Disable this to optimize generated size of the
GL library a little bit, at the expense of generating garbage in WebGL 1. If
you are only using WebGL 2 and do not support WebGL 1, this is not needed and
you can turn it off.

Default value: true

.. _gl_explicit_uniform_location:

GL_EXPLICIT_UNIFORM_LOCATION
============================

If true, enables support for the EMSCRIPTEN_explicit_uniform_location WebGL
extension. See docs/EMSCRIPTEN_explicit_uniform_location.txt

Default value: false

.. _gl_explicit_uniform_binding:

GL_EXPLICIT_UNIFORM_BINDING
===========================

If true, enables support for the EMSCRIPTEN_uniform_layout_binding WebGL
extension. See docs/EMSCRIPTEN_explicit_uniform_binding.txt

Default value: false

.. _use_webgl2:

USE_WEBGL2
==========

Deprecated. Pass -sMAX_WEBGL_VERSION=2 to target WebGL 2.0.

Default value: false

.. _min_webgl_version:

MIN_WEBGL_VERSION
=================

Specifies the lowest WebGL version to target. Pass -sMIN_WEBGL_VERSION=1
to enable targeting WebGL 1, and -sMIN_WEBGL_VERSION=2 to drop support
for WebGL 1.0

Default value: 1

.. _max_webgl_version:

MAX_WEBGL_VERSION
=================

Specifies the highest WebGL version to target. Pass -sMAX_WEBGL_VERSION=2
to enable targeting WebGL 2. If WebGL 2 is enabled, some APIs (EGL, GLUT, SDL)
will default to creating a WebGL 2 context if no version is specified.
Note that there is no automatic fallback to WebGL1 if WebGL2 is not supported
by the user's device, even if you build with both WebGL1 and WebGL2
support, as that may not always be what the application wants. If you want
such a fallback, you can try to create a context with WebGL2, and if that
fails try to create one with WebGL1.

Default value: 1

.. _webgl2_backwards_compatibility_emulation:

WEBGL2_BACKWARDS_COMPATIBILITY_EMULATION
========================================

If true, emulates some WebGL 1 features on WebGL 2 contexts, meaning that
applications that use WebGL 1/GLES 2 can initialize a WebGL 2/GLES3 context,
but still keep using WebGL1/GLES 2 functionality that no longer is supported
in WebGL2/GLES3. Currently this emulates GL_EXT_shader_texture_lod extension
in GLSL ES 1.00 shaders, support for unsized internal texture formats, and the
GL_HALF_FLOAT_OES != GL_HALF_FLOAT mixup.

Default value: false

.. _full_es3:

FULL_ES3
========

Forces support for all GLES3 features, not just the WebGL2-friendly subset.
This automatically turns on FULL_ES2 and WebGL2 support.

Default value: false

.. _legacy_gl_emulation:

LEGACY_GL_EMULATION
===================

Includes code to emulate various desktop GL features. Incomplete but useful
in some cases, see
http://kripken.github.io/emscripten-site/docs/porting/multimedia_and_graphics/OpenGL-support.html

Default value: false

.. _gl_ffp_only:

GL_FFP_ONLY
===========

If you specified LEGACY_GL_EMULATION = 1 and only use fixed function pipeline
in your code, you can also set this to 1 to signal the GL emulation layer
that it can perform extra optimizations by knowing that the user code does
not use shaders at all. If LEGACY_GL_EMULATION = 0, this setting has no
effect.

Default value: false

.. _gl_preinitialized_context:

GL_PREINITIALIZED_CONTEXT
=========================

If you want to create the WebGL context up front in JS code, set this to 1
and set Module['preinitializedWebGLContext'] to a precreated WebGL context.
WebGL initialization afterwards will use this GL context to render.

Default value: false

.. _stb_image:

STB_IMAGE
=========

Enables building of stb-image, a tiny public-domain library for decoding
images, allowing decoding of images without using the browser's built-in
decoders. The benefit is that this can be done synchronously, however, it
will not be as fast as the browser itself.  When enabled, stb-image will be
used automatically from IMG_Load and IMG_Load_RW. You can also call the
``stbi_*`` functions directly yourself.

Default value: false

.. _gl_disable_half_float_extension_if_broken:

GL_DISABLE_HALF_FLOAT_EXTENSION_IF_BROKEN
=========================================

From Safari 8 (where WebGL was introduced to Safari) onwards, OES_texture_half_float and OES_texture_half_float_linear extensions
are broken and do not function correctly, when used as source textures.
See https://webkit.org/b/183321, https://webkit.org/b/169999,
https://stackoverflow.com/questions/54248633/cannot-create-half-float-oes-texture-from-uint16array-on-ipad

Default value: false

.. _gl_workaround_safari_getcontext_bug:

GL_WORKAROUND_SAFARI_GETCONTEXT_BUG
===================================

Workaround Safari WebGL issue: After successfully acquiring WebGL context on a canvas,
calling .getContext() will always return that context independent of which 'webgl' or 'webgl2'
context version was passed. See https://webkit.org/b/222758 and
https://github.com/emscripten-core/emscripten/issues/13295.
Set this to 0 to force-disable the workaround if you know the issue will not affect you.

Default value: true

.. _gl_enable_get_proc_address:

GL_ENABLE_GET_PROC_ADDRESS
==========================

If 1, link with support to glGetProcAddress() functionality.
In WebGL, glGetProcAddress() causes a substantial code size and performance impact, since WebGL
does not natively provide such functionality, and it must be emulated. Using glGetProcAddress()
is not recommended. If you still need to use this, e.g. when porting an existing renderer,
you can link with -sGL_ENABLE_GET_PROC_ADDRESS to get support for this functionality.

Default value: true

.. _js_math:

JS_MATH
=======

Use JavaScript math functions like Math.tan. This saves code size as we can avoid shipping
compiled musl code. However, it can be significantly slower as it calls out to JS. It
also may give different results as JS math is specced somewhat differently than libc, and
can also vary between browsers.

Default value: false

.. _legacy_vm_support:

LEGACY_VM_SUPPORT
=================

Set this to enable compatibility emulations for old JavaScript engines. This gives you
the highest possible probability of the code working everywhere, even in rare old
browsers and shell environments. Specifically:

- Disable WebAssembly. (Must be paired with -sWASM=0)
- Adjusts MIN_X_VERSION settings to 0 to include support for all browser versions.
- Avoid TypedArray.fill, if necessary, in zeroMemory utility function.

You can also configure the above options individually.

Default value: false

.. _environment:

ENVIRONMENT
===========

Specify which runtime environments the JS output will be capable of running
in.  For maximum portability this can be configured to support all
environments or it can be limited to reduce overall code size.  The supported
environments are:

- 'web'     - the normal web environment.
- 'webview' - just like web, but in a webview like Cordova; considered to be
  same as "web" in almost every place
- 'worker'  - a web worker environment.
- 'worklet' - Audio Worklet environment.
- 'node'    - Node.js.
- 'shell'   - a JS shell like d8, js, or jsc.

This setting can be a comma-separated list of these environments, e.g.,
"web,worker". If this is the empty string, then all environments are
supported.

Certain settings will automatically add to this list.  For examble, building
with pthreads will automatically add `worker` and building with
``AUDIO_WORKLET`` will automatically add `worklet`.

Note that the set of environments recognized here is not identical to the
ones we identify at runtime using ``ENVIRONMENT_IS_*``. Specifically:

- We detect whether we are a pthread at runtime, but that's set for workers
  and not for the main file so it wouldn't make sense to specify here.
- The webview target is basically a subset of web. It must be specified
  alongside web (e.g. "web,webview") and we only use it for code generation
  at compile time, there is no runtime behavior change.

Note that by default we do not include the 'shell' environment since direct
usage of d8, spidermonkey and jsc is extremely rare.

Default value: ['web', 'webview', 'worker', 'node']

.. _lz4:

LZ4
===

Enable this to support lz4-compressed file packages. They are stored compressed in memory, and
decompressed on the fly, avoiding storing the entire decompressed data in memory at once.
If you run the file packager separately, you still need to build the main program with this flag,
and also pass --lz4 to the file packager.
(You can also manually compress one on the client, using LZ4.loadPackage(), but that is less
recommended.)
Limitations:

- LZ4-compressed files are only decompressed when needed, so they are not available
  for special preloading operations like pre-decoding of images using browser codecs,
  preloadPlugin stuff, etc.
- LZ4 files are read-only.

Default value: false

.. _disable_exception_catching:

DISABLE_EXCEPTION_CATCHING
==========================

Disables generating code to actually catch exceptions. This disabling is on
by default as the overhead of exceptions is quite high in size and speed
currently (in the future, wasm should improve that). When exceptions are
disabled, if an exception actually happens then it will not be caught
and the program will halt (so this will not introduce silent failures).

.. note::

  This removes *catching* of exceptions, which is the main
  issue for speed, but you should build source files with
  -fno-exceptions to really get rid of all exceptions code overhead,
  as it may contain thrown exceptions that are never caught (e.g.
  just using std::vector can have that). -fno-rtti may help as well.

This option is mutually exclusive with :ref:`EXCEPTION_CATCHING_ALLOWED`.

This option only applies to Emscripten (JavaScript-based) exception handling
and does not control the native Wasm exception handling.

.. note:: Applicable during both linking and compilation

Default value: 1

.. _exception_catching_allowed:

EXCEPTION_CATCHING_ALLOWED
==========================

Enables catching exception but only in the listed functions.  This
option acts like a more precise version of ``DISABLE_EXCEPTION_CATCHING=0``.

This option is mutually exclusive with :ref:`DISABLE_EXCEPTION_CATCHING`.

This option only applies to Emscripten (JavaScript-based) exception handling
and does not control the native Wasm exception handling.

.. note:: Applicable during both linking and compilation

Default value: []

.. _disable_exception_throwing:

DISABLE_EXCEPTION_THROWING
==========================

Internal: Tracks whether Emscripten should link in exception throwing (C++
'throw') support library. This does not need to be set directly, but pass
-fno-exceptions to the build disable exceptions support. (This is basically
-fno-exceptions, but checked at final link time instead of individual .cpp
file compile time) If the program *does* contain throwing code (some source
files were not compiled with ``-fno-exceptions``), and this flag is set at link
time, then you will get errors on undefined symbols, as the exception
throwing code is not linked in. If so you should either unset the option (if
you do want exceptions) or fix the compilation of the source files so that
indeed no exceptions are used).

This option only applies to Emscripten (JavaScript-based) exception handling
and does not control the native Wasm exception handling.

.. note:: Applicable during both linking and compilation

Default value: false

.. _export_exception_handling_helpers:

EXPORT_EXCEPTION_HANDLING_HELPERS
=================================

Make the exception message printing function, 'getExceptionMessage' available
in the JS library for use, by adding necessary symbols to
:ref:`EXPORTED_FUNCTIONS`.

This works with both Emscripten EH and Wasm EH. When you catch an exception
from JS, that gives you a user-thrown value in case of Emscripten EH, and a
WebAssembly.Exception object in case of Wasm EH. 'getExceptionMessage' takes
the user-thrown value in case of Emscripten EH and the WebAssembly.Exception
object in case of Wasm EH, meaning in both cases you can pass a caught
exception directly to the function.

When used with Wasm EH, this option additionally provides these functions in
the JS library:

- getCppExceptionTag: Returns the C++ tag
- getCppExceptionThrownObjectFromWebAssemblyException:
  Given an WebAssembly.Exception object, returns the actual user-thrown C++
  object address in Wasm memory.

Setting this option also adds refcount incrementing and decrementing
functions ('incrementExceptionRefcount' and 'decrementExceptionRefcount') in
the JS library because if you catch an exception from JS, you may need to
manipulate the refcount manually to avoid memory leaks.

See test_EXPORT_EXCEPTION_HANDLING_HELPERS in test/test_core.py for an
example usage.

.. note:: This setting is deprecated

Default value: false

.. _exception_stack_traces:

EXCEPTION_STACK_TRACES
======================

When this is enabled, exceptions will contain stack traces and uncaught
exceptions will display stack traces upon exiting. This defaults to true when
ASSERTIONS is enabled. This option is for users who want exceptions' stack
traces but do not want other overheads ASSERTIONS can incur.
This option implies :ref:`EXPORT_EXCEPTION_HANDLING_HELPERS`.

Default value: false

.. _wasm_legacy_exceptions:

WASM_LEGACY_EXCEPTIONS
======================

If true, emit instructions for the legacy Wasm exception handling proposal:
https://github.com/WebAssembly/exception-handling/blob/main/proposals/exception-handling/legacy/Exceptions.md
If false, emit instructions for the standardized exception handling proposal:
https://github.com/WebAssembly/exception-handling/blob/main/proposals/exception-handling/Exceptions.md

.. note:: Applicable during both linking and compilation

Default value: true

.. _asyncify:

ASYNCIFY
========

Whether to support async operations in the compiled code. This makes it
possible to call JS functions from synchronous-looking code in C/C++.

- 1 (default): Run binaryen's Asyncify pass to transform the code using
  asyncify. This emits a normal wasm file in the end, so it works everywhere,
  but it has a significant cost in terms of code size and speed.
  See https://emscripten.org/docs/porting/asyncify.html
- 2 (deprecated): Use ``-sJSPI`` instead.

Default value: 0

.. _asyncify_imports:

ASYNCIFY_IMPORTS
================

Imports which can do an async operation, in addition to the default ones that
emscripten defines like emscripten_sleep. If you add more you will need to
mention them to here, or else they will not work (in :ref:`ASSERTIONS` builds
an error will be shown).
Note that this list used to contain the default ones, which meant that you
had to list them when adding your own; the default ones are now added
automatically.

Default value: []

.. _asyncify_ignore_indirect:

ASYNCIFY_IGNORE_INDIRECT
========================

Whether indirect calls can be on the stack during an unwind/rewind.
If you know they cannot, then setting this can be extremely helpful, as
otherwise asyncify must assume an indirect call can reach almost everywhere.

Default value: false

.. _asyncify_stack_size:

ASYNCIFY_STACK_SIZE
===================

The size of the asyncify stack - the region used to store unwind/rewind
info. This must be large enough to store the call stack and locals. If it is
too small, you will see a wasm trap due to executing an "unreachable"
instruction. In that case, you should increase this size.

Default value: 4096

.. _asyncify_remove:

ASYNCIFY_REMOVE
===============

If the Asyncify remove-list is provided, then the functions in it will not
be instrumented even if it looks like they need to. This can be useful
if you know things the whole-program analysis doesn't, like if you
know certain indirect calls are safe and won't unwind. But if you
get the list wrong things will break (and in a production build user
input might reach code paths you missed during testing, so it's hard
to know you got this right), so this is not recommended unless you
really know what are doing, and need to optimize every bit of speed
and size.

The names in this list are names from the WebAssembly Names section. The
wasm backend will emit those names in *human-readable* form instead of
typical C++ mangling. For example, you should write ``Struct::func()``
instead of ``_ZN6Struct4FuncEv``. C is also different from C++, as C
names don't end with parameters; as a result foo(int) in C++ would appear
as just foo in C (C++ has parameters because it needs to differentiate
overloaded functions). You will see warnings in the console if a name in the
list is missing (these are not errors because inlining etc. may cause
changes which would mean a single list couldn't work for both -O0 and -O1
builds, etc.). You can inspect the wasm binary to look for the actual names,
either directly or using wasm-objdump or wasm-dis, etc.

Simple ``*`` wildcard matching is supported.

To avoid dealing with limitations in operating system shells or build system
escaping, the following substitutions can be made:

- ' ' -> ``.``,
- ``&`` -> ``#``,
- ``,`` -> ``?``.

That is, the function `"foo(char const*, int&)"` can be inputted as
`"foo(char.const*?.int#)"` on the command line instead.

Note: Whitespace is part of the function signature! I.e.
"foo(char const *, int &)" will not match "foo(char const*, int&)", and
neither would "foo(const char*, int &)".

Default value: []

.. _asyncify_add:

ASYNCIFY_ADD
============

Functions in the Asyncify add-list are added to the list of instrumented
functions, that is, they will be instrumented even if otherwise asyncify
thinks they don't need to be. As by default everything will be instrumented
in the safest way possible, this is only useful if you use IGNORE_INDIRECT
and use this list to fix up some indirect calls that *do* need to be
instrumented.

See :ref:`ASYNCIFY_REMOVE` about the names, including wildcard matching and
character substitutions.

Default value: []

.. _asyncify_propagate_add:

ASYNCIFY_PROPAGATE_ADD
======================

If enabled, instrumentation status will be propagated from the add-list, ie.
their callers, and their callers' callers, and so on. If disabled then all
callers must be manually added to the add-list (like the only-list).

Default value: true

.. _asyncify_only:

ASYNCIFY_ONLY
=============

If the Asyncify only-list is provided, then *only* the functions in the list
will be instrumented. Like the remove-list, getting this wrong will break
your application.

See :ref:`ASYNCIFY_REMOVE` about the names, including wildcard matching and
character substitutions.

Default value: []

.. _asyncify_advise:

ASYNCIFY_ADVISE
===============

If enabled will output which functions have been instrumented and why.

Default value: false

.. _asyncify_debug:

ASYNCIFY_DEBUG
==============

Runtime debug logging from asyncify internals.

- 1: Minimal logging.
- 2: Verbose logging.

Default value: 0

.. _asyncify_exports:

ASYNCIFY_EXPORTS
================

Deprecated, use JSPI_EXPORTS instead.

.. note:: This setting is deprecated

Default value: []

.. _jspi:

JSPI
====

Use VM support for the JavaScript Promise Integration proposal. This allows
async operations to happen without the overhead of modifying the wasm. This
is experimental at the moment while spec discussion is ongoing, see
https://github.com/WebAssembly/js-promise-integration/ TODO: document which
of the following flags are still relevant in this mode (e.g. IGNORE_INDIRECT
etc. are not needed)

Default value: 0

.. _jspi_exports:

JSPI_EXPORTS
============

A list of exported module functions that will be asynchronous. Each export
will return a ``Promise`` that will be resolved with the result. Any exports
that will call an asynchronous import (listed in ``JSPI_IMPORTS``) must be
included here.

By default this includes ``main``.

Default value: []

.. _jspi_imports:

JSPI_IMPORTS
============

A list of imported module functions that will potentially do asynchronous
work. The imported function should return a ``Promise`` when doing
asynchronous work.

Note when using JS library files, the function can be marked with
``<function_name>_async:: true`` in the library instead of this setting.

Default value: []

.. _exported_runtime_methods:

EXPORTED_RUNTIME_METHODS
========================

Runtime elements that are exported on Module by default. We used to export
quite a lot here, but have removed them all. You should use
EXPORTED_RUNTIME_METHODS for things you want to export from the runtime.
Note that the name may be slightly misleading, as this is for any JS library
element, and not just methods. For example, we can export the FS object by
having "FS" in this list.

Default value: []

.. _incoming_module_js_api:

INCOMING_MODULE_JS_API
======================

A list of incoming values on the Module object in JS that we care about. If
a value is not in this list, then we don't emit code to check if you provide
it on the Module object. For example, if
you have this::

  var Module = {
    print: (x) => console.log('print: ' + x),
    preRun: [() => console.log('pre run')]
  };

Then INCOMING_MODULE_JS_API must contain 'print' and 'preRun'; if it does not
then we may not emit code to read and use that value. In other words, this
option lets you set, statically at compile time, the list of which Module
JS values you will be providing at runtime, so the compiler can better
optimize.

Setting this list to [], or at least a short and concise set of names you
actually use, can be very useful for reducing code size. By default, the
list contains a set of commonly used symbols.

In addition to the default symbols, the following are also available:

- fetchSettings
- logReadFiles
- loadSplitModule
- onMalloc
- onRealloc
- onFree
- onSbrkGrow
- onCOSCacheHit
- onCOSCacheMiss
- onCOSStore
- GL_MAX_TEXTURE_IMAGE_UNITS
- SDL_canPlayWithWebAudio
- SDL_numSimultaneouslyQueuedBuffers
- freePreloadedMediaOnUse
- preinitializedWebGLContext
- keyboardListeningElement
- doNotCaptureKeyboard
- extraStackTrace
- preloadPlugins
- preMainLoop
- postMainLoop
- forcedAspectRatio
- mainScriptUrlOrBlob
- onFullScreen
- INITIAL_MEMORY
- wasmMemory
- wasmBinary

Default value: (multi-line value, see settings.js)

.. _case_insensitive_fs:

CASE_INSENSITIVE_FS
===================

If set to nonzero, the provided virtual filesystem is treated
case-insensitive, like Windows and macOS do. If set to 0, the VFS is
case-sensitive, like on Linux.

Default value: false

.. _filesystem:

FILESYSTEM
==========

If set to 0, does not build in any filesystem support. Useful if you are just
doing pure computation, but not reading files or using any streams (including
fprintf, and other stdio.h things) or anything related. The one exception is
there is partial support for printf, and puts, hackishly.  The compiler will
automatically set this if it detects that syscall usage (which is static)
does not require a full filesystem. If you still want filesystem support, use
FORCE_FILESYSTEM

Default value: true

.. _force_filesystem:

FORCE_FILESYSTEM
================

Makes full filesystem support be included, even if statically it looks like
it is not used. For example, if your C code uses no files, but you include
some JS that does, you might need this.

Default value: false

.. _noderawfs:

NODERAWFS
=========

Enables support for the ``NODERAWFS`` filesystem backend. This is a special
backend as it replaces all normal filesystem access with direct Node.js
operations, without the need to do ``FS.mount()``, and this backend only
works with Node.js. The initial working directory will be same as
process.cwd() instead of VFS root directory.  Because this mode directly uses
Node.js to access the real local filesystem on your OS, the code will not
necessarily be portable between OSes - it will be as portable as a Node.js
program would be, which means that differences in how the underlying OS
handles permissions and errors and so forth may be noticeable.

Enabling this setting will also enable :ref:`NODE_HOST_ENV` by default.

Default value: false

.. _node_host_env:

NODE_HOST_ENV
=============

When running under Node, expose the underlying OS environment variables.
This is similar to how ``NODERAWFS`` exposes the underlying FS.
This setting gets enabled by default when ``NODERAWFS`` is enabled, but can
also be controlled separately.

Default value: false

.. _node_code_caching:

NODE_CODE_CACHING
=================

This saves the compiled wasm module in a file with name
``$WASM_BINARY_NAME.$V8_VERSION.cached``
and loads it on subsequent runs. This caches the compiled wasm code from
v8 in node, which saves compiling on subsequent runs, making them start up
much faster.
The V8 version used in node is included in the cache name so that we don't
try to load cached code from another version, which fails silently (it seems
to load ok, but we do actually recompile).

- The only version known to work for sure is node 12.9.1, as this has
  regressed, see
  https://github.com/nodejs/node/issues/18265#issuecomment-622971547
- The default location of the .cached files is alongside the wasm binary,
  as mentioned earlier. If that is in a read-only directory, you may need
  to place them elsewhere. You can use the locateFile() hook to do so.

Default value: false

.. _exported_functions:

EXPORTED_FUNCTIONS
==================

Symbols that are explicitly exported. These symbols are kept alive through
LLVM dead code elimination, and also made accessible outside of the
generated code even after running closure compiler (on "Module").  Native
symbols listed here require an ``_`` prefix.

By default if this setting is not specified on the command line the
``_main`` function will be implicitly exported.  In :ref:`STANDALONE_WASM`
mode the default export is ``__start`` (or ``__initialize`` if ``--no-entry``
is specified).

JS Library symbols can also be added to this list (without the leading `$`).

Default value: []

.. _export_all:

EXPORT_ALL
==========

If true, we export all the symbols that are present in JS onto the Module
object. This does not affect which symbols will be present - it does not
prevent DCE or cause anything to be included in linking. It only does
``Module['X'] = X;``
for all X that end up in the JS file. This is useful to export the JS
library functions on Module, for things like dynamic linking.

Default value: false

.. _export_keepalive:

EXPORT_KEEPALIVE
================

If true, we export the symbols that are present in JS onto the Module
object.
It only does ``Module['X'] = X;``

Default value: true

.. _retain_compiler_settings:

RETAIN_COMPILER_SETTINGS
========================

Remembers the values of these settings, and makes them accessible
through getCompilerSetting and emscripten_get_compiler_setting.
To see what is retained, look for compilerSettings in the generated code.

Default value: false

.. _default_library_funcs_to_include:

DEFAULT_LIBRARY_FUNCS_TO_INCLUDE
================================

JS library elements (C functions implemented in JS) that we include by
default.  If you want to make sure something is included by the JS compiler,
add it here.  For example, if you do not use some ``emscripten_*`` C API call
from C, but you want to call it from JS, add it here.
Note that the name may be slightly misleading, as this is for any JS
library element, and not just functions. For example, you can include the
Browser object by adding "$Browser" to this list.

If you want to both include and export a JS library symbol, it is enough to
simply add it to EXPORTED_FUNCTIONS, without also adding it to
DEFAULT_LIBRARY_FUNCS_TO_INCLUDE.

Default value: []

.. _include_full_library:

INCLUDE_FULL_LIBRARY
====================

Include all JS library functions instead of the sum of
DEFAULT_LIBRARY_FUNCS_TO_INCLUDE + any functions used by the generated code.
This is needed when dynamically loading (i.e. dlopen) modules that make use
of runtime library functions that are not used in the main module.  Note that
this only applies to js libraries, *not* C. You will need the main file to
include all needed C libraries.  For example, if a module uses malloc or new,
you will need to use those in the main file too to pull in malloc for use by
the module.

Default value: false

.. _main_module:

MAIN_MODULE
===========

A main module is a file compiled in a way that allows us to link it to
a side module at runtime.

- 1: Normal main module.
- 2: DCE'd main module. We eliminate dead code normally. If a side
  module needs something from main, it is up to you to make sure
  it is kept alive.

.. note:: Applicable during both linking and compilation

Default value: 0

.. _side_module:

SIDE_MODULE
===========

Corresponds to MAIN_MODULE (also supports modes 1 and 2)

.. note:: Applicable during both linking and compilation

Default value: 0

.. _runtime_linked_libs:

RUNTIME_LINKED_LIBS
===================

Deprecated, list shared libraries directly on the command line instead.

.. note:: This setting is deprecated

Default value: []

.. _build_as_worker:

BUILD_AS_WORKER
===============

If set to 1, this is a worker library, a special kind of library that is run
in a worker. See emscripten.h

Default value: false

.. _proxy_to_pthread:

PROXY_TO_PTHREAD
================

If set to 1, compiles in a small stub main() in between the real main() which
calls pthread_create() to run the application main() in a pthread.  This is
something that applications can do manually as well if they wish, this option
is provided as convenience.

The pthread that main() runs on is a normal pthread in all ways, with the one
difference that its stack size is the same as the main thread would normally
have, that is, STACK_SIZE. This makes it easy to flip between
PROXY_TO_PTHREAD and non-PROXY_TO_PTHREAD modes with main() always getting
the same amount of stack.

This proxies Module['canvas'], if present, and if OFFSCREENCANVAS_SUPPORT
is enabled. This has to happen because this is the only chance - this browser
main thread does the only pthread_create call that happens on
that thread, so it's the only chance to transfer the canvas from there.

Default value: false

.. _linkable:

LINKABLE
========

If set to 1, this file can be linked with others, either as a shared library
or as the main file that calls a shared library. To enable that, we will not
internalize all symbols and cull the unused ones, in other words, we will not
remove unused functions and globals, which might be used by another module we
are linked with.

MAIN_MODULE and SIDE_MODULE both imply this, so it not normally necessary
to set this explicitly. Note that MAIN_MODULE and SIDE_MODULE mode 2 do
*not* set this, so that we still do normal DCE on them, and in that case
you must keep relevant things alive yourself using exporting.

.. note:: Applicable during both linking and compilation

.. note:: This setting is deprecated

Default value: false

.. _strict:

STRICT
======

Emscripten 'strict' build mode: Drop supporting any deprecated build options.
Set the environment variable EMCC_STRICT=1 or pass -sSTRICT to test that a
codebase builds nicely in forward compatible manner.
Changes enabled by this:

  - STRICT_JS is enabled.
  - IGNORE_MISSING_MAIN is disabled.
  - AUTO_JS_LIBRARIES is disabled.
  - AUTO_NATIVE_LIBRARIES is disabled.
  - DEFAULT_TO_CXX is disabled.
  - ALLOW_UNIMPLEMENTED_SYSCALLS is disabled.
  - INCOMING_MODULE_JS_API is set to empty by default.

.. note:: Applicable during both linking and compilation

Default value: false

.. _ignore_missing_main:

IGNORE_MISSING_MAIN
===================

Allow program to link with or without ``main`` symbol.
If this is disabled then one must provide a ``main`` symbol or explicitly
opt out by passing ``--no-entry`` or an EXPORTED_FUNCTIONS list that doesn't
include ``_main``.

Default value: true

.. _strict_js:

STRICT_JS
=========

Add ``"use strict;"`` to generated JS

Default value: false

.. _warn_on_undefined_symbols:

WARN_ON_UNDEFINED_SYMBOLS
=========================

If set to 1, we will warn on any undefined symbols that are not resolved by
the ``library_*.js`` files. Note that it is common in large projects to not
implement everything, when you know what is not going to actually be called
(and don't want to mess with the existing buildsystem), and functions might
be implemented later on, say in --pre-js, so you may want to build with -s
WARN_ON_UNDEFINED_SYMBOLS=0 to disable the warnings if they annoy you.  See
also ERROR_ON_UNDEFINED_SYMBOLS.  Any undefined symbols that are listed in
EXPORTED_FUNCTIONS will also be reported.

Default value: true

.. _error_on_undefined_symbols:

ERROR_ON_UNDEFINED_SYMBOLS
==========================

If set to 1, we will give a link-time error on any undefined symbols (see
WARN_ON_UNDEFINED_SYMBOLS). To allow undefined symbols at link time set this
to 0, in which case if an undefined function is called a runtime error will
occur.  Any undefined symbols that are listed in EXPORTED_FUNCTIONS will also
be reported.

Default value: true

.. _small_xhr_chunks:

SMALL_XHR_CHUNKS
================

Use small chunk size for binary synchronous XHR's in Web Workers.  Used for
testing.  See test_chunked_synchronous_xhr in runner.py and library.js.

Default value: false

.. _modularize:

MODULARIZE
==========

By default we emit all code in a straightforward way into the output
.js file. That means that if you load that in a script tag in a web
page, it will use the global scope. With ``MODULARIZE`` set, we instead emit
the code wrapped in an async function. This function returns a promise that
resolves to a module instance once it is safe to run the compiled code
(similar to the ``onRuntimeInitialized`` callback).

The default name of the function is ``Module``, but can be changed using the
``EXPORT_NAME`` option. We recommend renaming it to a more typical name for a
factory function, e.g. ``createModule``.

You use the factory function like so::

  const module = await EXPORT_NAME();

or::

  let module;
  EXPORT_NAME().then(instance => {
    module = instance;
  });


The factory function accepts 1 parameter, an object with default values for
the module instance::

  const module = await EXPORT_NAME({ option: value, ... });

Note the parentheses - we are calling EXPORT_NAME in order to instantiate
the module. This allows you to create multiple instances of the module.

Note that in MODULARIZE mode we do *not* look for a global ``Module`` object
for default values. Default values must be passed as a parameter to the
factory function.

The default .html shell file provided in MINIMAL_RUNTIME mode will create
a singleton instance automatically, to run the application on the page.
(Note that it does so without using the Promise API mentioned earlier, and
so code for the Promise is not even emitted in the .js file if you tell
emcc to emit an .html output.)
The default .html shell file provided by traditional runtime mode is only
compatible with MODULARIZE=0 mode, so when building with traditional
runtime, you should provided your own html shell file to perform the
instantiation when building with MODULARIZE=1. (For more details, see
https://github.com/emscripten-core/emscripten/issues/7950)

If you add --pre-js or --post-js files, they will be included inside
the factory function with the rest of the emitted code in order to be
optimized together with it.

If you want to include code outside all of the generated code, including the
factory function, you can use --extern-pre-js or --extern-post-js. While
--pre-js and --post-js happen to do that in non-MODULARIZE mode, their
intended usage is to add code that is optimized with the rest of the emitted
code, allowing better dead code elimination and minification.

Experimental Feature - Instance ES Modules:

Note this feature is still under active development and is subject to change!

To enable this feature use -sMODULARIZE=instance. Enabling this mode will
produce an ES module that is a singleton with ES module exports. The
module will export a default value that is an async init function and will
also export named values that correspond to the Wasm exports and runtime
exports. The init function must be called before any of the exports can be
used. An example of using the module is below.

  import init, { foo, bar } from "./my_module.mjs"
  await init(optionalArguments);
  foo();
  bar();

The ``init`` function exists so the caller can configure the instance (via
``moduleArg``) before it starts. When there is nothing to configure, see
``AUTO_INIT`` to have the module self-initialize on import.

Default value: false

.. _auto_init:

AUTO_INIT
=========

When set, an instance ES module (``MODULARIZE=instance`` or
``WASM_ESM_INTEGRATION``) initializes itself via top-level await on import
rather than exporting an ``init`` function to be called by the consumer. The
named Wasm/runtime exports are ready to use as soon as the module is
imported::

  import { foo, bar } from "./my_module.mjs"
  foo();
  bar();

Since the module initializes without any caller involvement, there is no
opportunity for module-level configuration: ``moduleArg`` cannot be passed
and the entire ``INCOMING_MODULE_JS_API`` is disabled (passing a non-empty
``INCOMING_MODULE_JS_API`` is an error).

Because no default ``init`` export is emitted, this also frees up the
``default`` export name for the program's own use.

Requires ``MODULARIZE=instance`` or ``WASM_ESM_INTEGRATION``.

Default value: false

.. _export_es6:

EXPORT_ES6
==========

Export using an ES6 Module export rather than a UMD export.  MODULARIZE must
be enabled for ES6 exports and is implicitly enabled if not already set.

This is implicitly enabled if the output suffix is set to 'mjs'.

Default value: false

.. _export_name:

EXPORT_NAME
===========

Global variable to export the module as for environments without a
standardized module loading system (e.g. the browser and SM shell).

Default value: 'Module'

.. _dynamic_execution:

DYNAMIC_EXECUTION
=================

When set to 0, we do not emit eval() and new Function(), which disables some
functionality (causing runtime errors if attempted to be used), but allows
the emitted code to be acceptable in places that disallow dynamic code
execution (chrome packaged app, privileged firefox app, etc.). Pass this flag
when developing an Emscripten application that is targeting a privileged or a
certified execution environment, see Firefox Content Security Policy (CSP)
webpage for details:
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/script-src
in particular the 'unsafe-eval' and 'wasm-unsafe-eval' policies.

When this flag is set, the following features (linker flags) are unavailable:

 - RELOCATABLE: the function loadDynamicLibrary would need to eval().

and some features may fall back to slower code paths when they need to:
Embind: uses eval() to jit functions for speed.

Additionally, the following Emscripten runtime functions are unavailable when
DYNAMIC_EXECUTION=0 is set, and an attempt to call them will throw an exception:

- emscripten_run_script(),
- emscripten_run_script_int(),
- emscripten_run_script_string(),
- dlopen(),
- the functions ccall() and cwrap() are still available, but they are
  restricted to only being able to call functions that have been exported in
  the Module object in advance.

When -sDYNAMIC_EXECUTION=2 is set, attempts to call to eval() are demoted to
warnings instead of throwing an exception.

Default value: 1

.. _bootstrapping_struct_info:

BOOTSTRAPPING_STRUCT_INFO
=========================

whether we are in the generate struct_info bootstrap phase

Default value: false

.. _emscripten_tracing:

EMSCRIPTEN_TRACING
==================

Add some calls to emscripten tracing APIs

.. note:: Applicable during both linking and compilation

Default value: false

.. _use_glfw:

USE_GLFW
========

Specify the GLFW version that is being linked against.  Only relevant, if you
are linking against the GLFW library.  Valid options are 2 for GLFW2 and 3
for GLFW3.

Default value: 0

.. _wasm:

WASM
====

Whether to use compile code to WebAssembly. Set this to 0 to compile to JS
instead of wasm.

Specify -sWASM=2 to target both WebAssembly and JavaScript at the same time.
In that build mode, two files a.wasm and a.wasm.js are produced, and at runtime
the WebAssembly file is loaded if browser/shell supports it. Otherwise the
.wasm.js fallback will be used.

If WASM=2 is enabled and the browser fails to compile the WebAssembly module,
the page will be reloaded in Wasm2JS mode.

Default value: 1

.. _standalone_wasm:

STANDALONE_WASM
===============

Indicates that we want to emit a wasm file that can run without JavaScript.
The file will use standard APIs such as wasi as much as possible to achieve
that.

This option does not guarantee that the wasm can be used by itself - if you
use APIs with no non-JS alternative, we will still use those (e.g., OpenGL
at the time of writing this). This gives you the option to see which APIs
are missing, and if you are compiling for a custom wasi embedding, to add
those to your embedding.

We may still emit JS with this flag, but the JS should only be a convenient
way to run the wasm on the Web or in Node.js, and you can run the wasm by
itself without that JS (again, unless you use APIs for which there is no
non-JS alternative) in a wasm runtime like wasmer or wasmtime.

Note that even without this option we try to use wasi etc. syscalls as much
as possible. What this option changes is that we do so even when it means
a tradeoff with JS size. For example, when this option is set we do not
import the Memory - importing it is useful for JS, so that JS can start to
use it before the wasm is even loaded, but in wasi and other wasm-only
environments the expectation is to create the memory in the wasm itself.
Doing so prevents some possible JS optimizations, so we only do it behind
this flag.

When this flag is set we do not legalize the JS interface, since the wasm is
meant to run in a wasm VM, which can handle i64s directly. If we legalized it
the wasm VM would not recognize the API. However, this means that the
optional JS emitted won't run if you use a JS API with an i64. You can use
the WASM_BIGINT option to avoid that problem by using BigInts for i64s which
means we don't need to legalize for JS (but this requires a new enough JS
VM).

Standalone builds require a ``main`` entry point by default.  If you want to
build a library (also known as a reactor) instead you can pass ``--no-entry``.

Default value: false

.. _binaryen_ignore_implicit_traps:

BINARYEN_IGNORE_IMPLICIT_TRAPS
==============================

Whether to ignore implicit traps when optimizing in binaryen.  Implicit
traps are the traps that happen in a load that is out of bounds, or
div/rem of 0, etc. With this option set, the optimizer assumes that loads
cannot trap, and therefore that they have no side effects at all. This
is *not* safe in general, as you may have a load behind a condition which
ensures it it is safe; but if the load is assumed to not have side effects it
could be executed unconditionally. For that reason this option is generally
not useful on large and complex projects, but in a small and simple enough
codebase it may help reduce code size a little bit.

Default value: false

.. _binaryen_extra_passes:

BINARYEN_EXTRA_PASSES
=====================

A comma-separated list of extra passes to run in the binaryen optimizer.
Setting this does not override/replace the default passes. It is appended at
the end of the list of passes.

Default value: ""

.. _wasm_async_compilation:

WASM_ASYNC_COMPILATION
======================

Whether to compile the wasm asynchronously, which is more efficient and does
not block the main thread. This is currently required for all but the
smallest modules to run in chrome.

(This option was formerly called BINARYEN_ASYNC_COMPILATION)

Default value: true

.. _dyncalls:

DYNCALLS
========

If set to 1, the dynCall() and dynCall_sig() API is made available
to caller.

Default value: false

.. _wasm_bigint:

WASM_BIGINT
===========

WebAssembly integration with JavaScript BigInt. When enabled we don't need to
legalize i64s into pairs of i32s, as the wasm VM will use a BigInt where an
i64 is used.

Default value: true

.. _emit_producers_section:

EMIT_PRODUCERS_SECTION
======================

WebAssembly defines a "producers section" which compilers and tools can
annotate themselves in, and LLVM emits this by default.
Emscripten will strip that out so that it is *not* emitted because it
increases code size, and also some users may not want information
about their tools to be included in their builds for privacy or security
reasons, see
https://github.com/WebAssembly/tool-conventions/issues/93.

Default value: false

.. _emit_emscripten_license:

EMIT_EMSCRIPTEN_LICENSE
=======================

Emits emscripten license info in the JS output.

Default value: false

.. _legalize_js_ffi:

LEGALIZE_JS_FFI
===============

Whether to legalize the JS FFI interfaces (imports/exports) by wrapping them
to automatically demote i64 to i32 and promote f32 to f64. This is necessary
in order to interface with JavaScript.  For non-web/non-JS embeddings,
setting this to 0 may be desirable.

.. note:: This setting is deprecated

Default value: true

.. _use_sdl:

USE_SDL
=======

Specify the SDL version that is being linked against.
1, the default, is 1.3, which is implemented in JS
2 is a port of the SDL C code on emscripten-ports
When AUTO_JS_LIBRARIES is set to 0 this defaults to 0 and SDL
is not linked in.
Alternate syntax for using the port: --use-port=sdl2

.. note:: Applicable during both linking and compilation

Default value: 0

.. _use_sdl_gfx:

USE_SDL_GFX
===========

Specify the SDL_gfx version that is being linked against. Must match USE_SDL

.. note:: Applicable during both linking and compilation

Default value: 0

.. _use_sdl_image:

USE_SDL_IMAGE
=============

Specify the SDL_image version that is being linked against. Must match USE_SDL

.. note:: Applicable during both linking and compilation

Default value: 1

.. _use_sdl_ttf:

USE_SDL_TTF
===========

Specify the SDL_ttf version that is being linked against. Must match USE_SDL

.. note:: Applicable during both linking and compilation

Default value: 1

.. _use_sdl_net:

USE_SDL_NET
===========

Specify the SDL_net version that is being linked against. Must match USE_SDL

.. note:: Applicable during both linking and compilation

Default value: 1

.. _use_icu:

USE_ICU
=======

1 = use icu from emscripten-ports
Alternate syntax: --use-port=icu

.. note:: Applicable during both linking and compilation

Default value: false

.. _use_zlib:

USE_ZLIB
========

1 = use zlib from emscripten-ports
Alternate syntax: --use-port=zlib

.. note:: Applicable during both linking and compilation

Default value: false

.. _use_bzip2:

USE_BZIP2
=========

1 = use bzip2 from emscripten-ports
Alternate syntax: --use-port=bzip2

.. note:: Applicable during both linking and compilation

Default value: false

.. _use_giflib:

USE_GIFLIB
==========

1 = use giflib from emscripten-ports
Alternate syntax: --use-port=giflib

.. note:: Applicable during both linking and compilation

Default value: false

.. _use_libjpeg:

USE_LIBJPEG
===========

1 = use libjpeg from emscripten-ports
Alternate syntax: --use-port=libjpeg

.. note:: Applicable during both linking and compilation

Default value: false

.. _use_libpng:

USE_LIBPNG
==========

1 = use libpng from emscripten-ports
Alternate syntax: --use-port=libpng

.. note:: Applicable during both linking and compilation

Default value: false

.. _use_regal:

USE_REGAL
=========

1 = use Regal from emscripten-ports
Alternate syntax: --use-port=regal

.. note:: Applicable during both linking and compilation

Default value: false

.. _use_boost_headers:

USE_BOOST_HEADERS
=================

1 = use Boost headers from emscripten-ports
Alternate syntax: --use-port=boost_headers

.. note:: Applicable during both linking and compilation

Default value: false

.. _use_bullet:

USE_BULLET
==========

1 = use bullet from emscripten-ports
Alternate syntax: --use-port=bullet

.. note:: Applicable during both linking and compilation

Default value: false

.. _use_vorbis:

USE_VORBIS
==========

1 = use vorbis from emscripten-ports
Alternate syntax: --use-port=vorbis

.. note:: Applicable during both linking and compilation

Default value: false

.. _use_ogg:

USE_OGG
=======

1 = use ogg from emscripten-ports
Alternate syntax: --use-port=ogg

.. note:: Applicable during both linking and compilation

Default value: false

.. _use_mpg123:

USE_MPG123
==========

1 = use mpg123 from emscripten-ports
Alternate syntax: --use-port=mpg123

.. note:: Applicable during both linking and compilation

Default value: false

.. _use_freetype:

USE_FREETYPE
============

1 = use freetype from emscripten-ports
Alternate syntax: --use-port=freetype

.. note:: Applicable during both linking and compilation

Default value: false

.. _use_sdl_mixer:

USE_SDL_MIXER
=============

Specify the SDL_mixer version that is being linked against.
Doesn't *have* to match USE_SDL, but a good idea.

.. note:: Applicable during both linking and compilation

Default value: 1

.. _use_harfbuzz:

USE_HARFBUZZ
============

1 = use harfbuzz from harfbuzz upstream
Alternate syntax: --use-port=harfbuzz

.. note:: Applicable during both linking and compilation

Default value: false

.. _use_cocos2d:

USE_COCOS2D
===========

3 = use cocos2d v3 from emscripten-ports
Alternate syntax: --use-port=cocos2d

.. note:: Applicable during both linking and compilation

Default value: 0

.. _use_modplug:

USE_MODPLUG
===========

1 = use libmodplug from emscripten-ports
Alternate syntax: --use-port=libmodplug

.. note:: Applicable during both linking and compilation

Default value: false

.. _sdl2_image_formats:

SDL2_IMAGE_FORMATS
==================

Formats to support in SDL2_image. Valid values: bmp, gif, lbm, pcx, png, pnm,
tga, xcf, xpm, xv

.. note:: Applicable during both linking and compilation

Default value: []

.. _sdl2_mixer_formats:

SDL2_MIXER_FORMATS
==================

Formats to support in SDL2_mixer. Valid values: ogg, mp3, mod, mid

.. note:: Applicable during both linking and compilation

Default value: ["ogg"]

.. _use_sqlite3:

USE_SQLITE3
===========

1 = use sqlite3 from emscripten-ports
Alternate syntax: --use-port=sqlite3

.. note:: Applicable during both linking and compilation

Default value: false

.. _shared_memory:

SHARED_MEMORY
=============

If 1, target compiling a shared Wasm Memory.

.. note:: Applicable during both linking and compilation

Default value: false

.. _wasm_workers:

WASM_WORKERS
============

Enables support for Wasm Workers.  Wasm Workers enable applications
to create threads using a lightweight web-specific API that builds on top
of Wasm SharedArrayBuffer + Atomics API.

.. note:: Applicable during both linking and compilation

Default value: 0

.. _audio_worklet:

AUDIO_WORKLET
=============

If true, enables targeting Wasm Web Audio AudioWorklets. Check out the
full documentation in site/source/docs/api_reference/wasm_audio_worklets.rst

Note: The setting will implicitly add ``worklet`` to the :ref:`ENVIRONMENT`,
(i.e. the resulting code and run in a worklet environment) but additionaly
depends on ``WASM_WORKERS`` and Wasm SharedArrayBuffer to run new Audio
Worklets.

Default value: 0

.. _audio_worklet_support_audio_params:

AUDIO_WORKLET_SUPPORT_AUDIO_PARAMS
==================================

If true, enables utilizing k- and a-rate AudioParams based properties in
Wasm Audio Worklet code. If false, AudioParams are not used. Set to false
for a tiny improvement to code size and AudioWorklet CPU performance when
audio synthesis is synchronized using custom WebAssembly Memory-based means.

Default value: true

.. _webaudio_debug:

WEBAUDIO_DEBUG
==============

If true, enables deep debugging of Web Audio backend.

Default value: 0

.. _pthread_pool_size:

PTHREAD_POOL_SIZE
=================

In web browsers, Workers cannot be created while the main browser thread
is executing JS/Wasm code, but the main thread must regularly yield back
to the browser event loop for Worker initialization to occur.
This means that pthread_create() is essentially an asynchronous operation
when called from the main browser thread, and the main thread must
repeatedly yield back to the JS event loop in order for the thread to
actually start.
If your application needs to be able to synchronously create new threads,
you can pre-create a pthread pool by specifying -sPTHREAD_POOL_SIZE=x,
in which case the specified number of Workers will be preloaded into a pool
before the application starts, and that many threads can then be available
for synchronous creation.
Note that this setting is a string, and will be emitted in the JS code
(directly, with no extra quotes) so that if you set it to '5' then 5 workers
will be used in the pool, and so forth. The benefit of this being a string
is that you can set it to something like
'navigator.hardwareConcurrency' (which will use the number of cores the
browser reports, and is how you can get exactly enough workers for a
threadpool equal to the number of cores).
[link] - affects generated JS runtime code at link time

Default value: 0

.. _pthread_pool_size_strict:

PTHREAD_POOL_SIZE_STRICT
========================

Normally, applications can create new threads even when the pool is empty.
When application breaks out to the JS event loop before trying to block on
the thread via ``pthread_join`` or any other blocking primitive,
an extra Worker will be created and the thread callback will be executed.
However, breaking out to the event loop requires custom modifications to
the code to adapt it to the Web, and not something that works for
off-the-shelf apps. Those apps without any modifications are most likely
to deadlock. This setting ensures that, instead of a risking a deadlock,
they get a runtime EAGAIN error instead that can at least be gracefully
handled from the C / C++ side.
Values:

- ``0`` - disable warnings on thread pool exhaustion
- ``1`` - enable warnings on thread pool exhaustion (default)
- ``2`` - make thread pool exhaustion a hard error

Default value: 1

.. _pthread_pool_delay_load:

PTHREAD_POOL_DELAY_LOAD
=======================

If your application does not need the ability to synchronously create
threads, but it would still like to opportunistically speed up initial thread
startup time by prewarming a pool of Workers, you can specify the size of
the pool with -sPTHREAD_POOL_SIZE=x, but then also specify
-sPTHREAD_POOL_DELAY_LOAD, which will cause the runtime to not wait up at
startup for the Worker pool to finish loading. Instead, the runtime will
immediately start up and the Worker pool will asynchronously spin up in
parallel on the background. This can shorten the time that pthread_create()
calls take to actually start a thread, but without actually slowing down
main application startup speed. If PTHREAD_POOL_DELAY_LOAD=0 (default),
then the runtime will wait for the pool to start up before running main().
If you do need to synchronously wait on the created threads
(e.g. via pthread_join), you must wait on the Module.pthreadPoolReady
promise before doing so or you're very likely to run into deadlocks.
[link] - affects generated JS runtime code at link time

Default value: false

.. _default_pthread_stack_size:

DEFAULT_PTHREAD_STACK_SIZE
==========================

Default stack size to use for newly created pthreads.  When not set, this
defaults to STACK_SIZE (which in turn defaults to 64k).  Can also be set at
runtime using pthread_attr_setstacksize().  Note that the wasm control flow
stack is separate from this stack.  This stack only contains certain function
local variables, such as those that have their addresses taken, or ones that
are too large to fit as local vars in wasm code.

Default value: 0

.. _pthreads_profiling:

PTHREADS_PROFILING
==================

True when building with --threadprofiler

Default value: false

.. _allow_blocking_on_main_thread:

ALLOW_BLOCKING_ON_MAIN_THREAD
=============================

It is dangerous to call pthread_join or pthread_cond_wait
on the main thread, as doing so can cause deadlocks on the Web (and also
it works using a busy-wait which is expensive). See
https://emscripten.org/docs/porting/pthreads.html#blocking-on-the-main-browser-thread
This may become set to 0 by default in the future; for now, this just
warns in the console.

Default value: true

.. _pthreads_debug:

PTHREADS_DEBUG
==============

If true, add in debug traces for diagnosing pthreads related issues.

Default value: false

.. _eval_ctors:

EVAL_CTORS
==========

This tries to evaluate code at compile time. The main use case is to eval
global ctor functions, which are those that run before main(), but main()
itself or parts of it can also be evalled. Evaluating code this way can avoid
work at runtime, as it applies the results of the execution to memory and
globals and so forth, "snapshotting" the wasm and then just running it from
there when it is loaded.

This will stop when it sees something it cannot eval at compile time, like a
call to an import. When running with this option you will see logging that
indicates what is evalled and where it stops.

This optimization can either reduce or increase code size. If a small amount
of code generates many changes in memory, for example, then overall size may
increase.

LLVM's GlobalOpt *almost* does this operation. It does in simple cases, where
LLVM IR is not too complex for its logic to evaluate, but it isn't powerful
enough for e.g. libc++ iostream ctors. It is just hard to do at the LLVM IR
level - LLVM IR is complex and getting more complex, so this would require
GlobalOpt to have a full interpreter, plus a way to write back into LLVM IR
global objects.  At the wasm level, however, everything has been lowered
into a simple low level, and we also just need to write bytes into an array,
so this is easy for us to do. A further issue for LLVM is that it doesn't
know that we will not link in further code, so it only tries to optimize
ctors with lowest priority (while we do know explicitly if dynamic linking is
enabled or not).

If set to a value of 2, this also makes some "unsafe" assumptions,
specifically that there is no input received while evalling ctors. That means
we ignore args to main() as well as assume no environment vars are readable.
This allows more programs to be optimized, but you need to make sure your
program does not depend on those features - even just checking the value of
argc can lead to problems.

Default value: 0

.. _textdecoder:

TEXTDECODER
===========

The default value or 1 means the generated code will use TextDecoder if
available and fall back to custom decoder code when not available.
If set to 2, we assume TextDecoder is present and usable, and do not emit
any JS code to fall back if it is missing. Setting this zero to avoid even
conditional usage of TextDecoder is no longer supported.
Note: In -Oz builds, the default value of TEXTDECODER is set to 2, to save on
code size (except when AUDIO_WORKLET is specified, or when `shell` is part
of ENVIRONMENT since TextDecoder is not available in those environments).

Default value: 1

.. _embind_std_string_is_utf8:

EMBIND_STD_STRING_IS_UTF8
=========================

Embind specific: If enabled, assume UTF-8 encoded data in std::string binding.
Disable this to support binary data transfer.

Default value: true

.. _embind_aot:

EMBIND_AOT
==========

Embind specific: If enabled, generate Embind's JavaScript invoker functions
at compile time and include them in the JS output file. When used with
DYNAMIC_EXECUTION=0 this allows exported bindings to be just as fast as
DYNAMIC_EXECUTION=1 mode, but without the need for eval(). If there are many
bindings the JS output size may be larger though.

Default value: false

.. _offscreencanvas_support:

OFFSCREENCANVAS_SUPPORT
=======================

If set to 1, enables support for transferring canvases to pthreads and
creating WebGL contexts in them, as well as explicit swap control for GL
contexts. This needs browser support for the OffscreenCanvas specification.

Default value: false

.. _offscreencanvases_to_pthread:

OFFSCREENCANVASES_TO_PTHREAD
============================

If you are using PROXY_TO_PTHREAD with OFFSCREENCANVAS_SUPPORT, then specify
here a comma separated list of CSS ID selectors to canvases to proxy over
to the pthread at program startup, e.g. '#canvas1, #canvas2'.

Default value: "#canvas"

.. _offscreen_framebuffer:

OFFSCREEN_FRAMEBUFFER
=====================

If set to 1, enables support for WebGL contexts to render to an offscreen
render target, to avoid the implicit swap behavior of WebGL where exiting any
event callback would automatically perform a "flip" to present rendered
content on screen. When an Emscripten GL context has Offscreen Framebuffer
enabled, a single frame can be composited from multiple event callbacks, and
the swap function emscripten_webgl_commit_frame() is then explicitly called
to present the rendered content on screen.

The OffscreenCanvas feature also enables explicit GL frame swapping support,
and also, -sOFFSCREEN_FRAMEBUFFER feature can be used to polyfill support
for accessing WebGL in multiple threads in the absence of OffscreenCanvas
support in browser, at the cost of some performance and latency.
OffscreenCanvas and Offscreen Framebuffer support can be enabled at the same
time, and allows one to utilize OffscreenCanvas where available, and to fall
back to Offscreen Framebuffer otherwise.

Default value: false

.. _fetch_support_indexeddb:

FETCH_SUPPORT_INDEXEDDB
=======================

If nonzero, Fetch API supports backing to IndexedDB. If 0, IndexedDB is not
utilized. Set to 0 if IndexedDB support is not interesting for target
application, to save a few kBytes.

Default value: true

.. _fetch_debug:

FETCH_DEBUG
===========

If nonzero, prints out debugging information in library_fetch.js

Default value: false

.. _fetch:

FETCH
=====

If nonzero, enables emscripten_fetch API.

Default value: false

.. _fetch_streaming:

FETCH_STREAMING
===============

Enables streaming fetched data when the fetch attribute
EMSCRIPTEN_FETCH_STREAM_DATA is used. For streaming requests, the DOM Fetch
API is used otherwise XMLHttpRequest is used.
Both modes generally support the same API, but there are some key
differences:

 - XHR supports synchronous requests
 - XHR supports overriding mime types
 - Fetch supports streaming data using the 'onprogress' callback

If set to a value of 2, only the DOM Fetch backend will be used. This should
only be used in testing.

Default value: 0

.. _wasmfs:

WASMFS
======

ATTENTION [WIP]: Experimental feature. Please use at your own risk.
This will eventually replace the current JS file system implementation.
If set to 1, uses new filesystem implementation.

.. note:: This is an experimental setting

Default value: false

.. _single_file:

SINGLE_FILE
===========

If set to 1, embeds all subresources in the emitted file as base64 string
literals. Embedded subresources may include (but aren't limited to) wasm,
asm.js, and static memory initialization code.

When using code that depends on this option, your Content Security Policy may
need to be updated. Specifically, embedding asm.js requires the script-src
directive to allow 'unsafe-inline', and using a Worker requires the
child-src directive to allow blob:. If you aren't using Content Security
Policy, or your CSP header doesn't include either script-src or child-src,
then you can safely ignore this warning.

Note that SINGLE_FILE with binary encoding requires the HTML/JS files to be
served with UTF-8 encoding. See the details on SINGLE_FILE_BINARY_ENCODE.

Default value: false

.. _single_file_binary_encode:

SINGLE_FILE_BINARY_ENCODE
=========================

If true, binary Wasm content is encoded using a custom UTF-8 embedding
instead of base64. This generates a smaller binary that compresses well.
Set this to false to revert back to earlier base64 encoding if you run into
issues with the binary encoding. (and please let us know of any such issues)
If no issues arise, this option will permanently become the default in the
future.

NOTE: Binary encoding requires that the HTML/JS files are served with UTF-8
encoding, and will not work with the default legacy Windows-1252 encoding
that browsers might use on Windows. To enable UTF-8 encoding in a
hand-crafted index.html file, apply any of:

1. Add `<meta charset="utf-8">` inside the <head> section of HTML, or
2. Add `<meta http-equiv="content-type" content="text/html; charset=UTF-8" />`` inside <head>, or
3. Add `<meta http-equiv="content-type" content="application/json; charset=utf-8" />` inside <head>
   (if using -o foo.js with SINGLE_FILE mode to build HTML+JS), or
4. pass the header `Content-Type: text/html; charset=utf-8` and/or header
   `Content-Type: application/javascript; charset=utf-8` when serving the
   relevant files that contain binary encoded content.

If none of these are possible, disable binary encoding with
-sSINGLE_FILE_BINARY_ENCODE=0 to fall back to base64 encoding.

Default value: true

.. _auto_js_libraries:

AUTO_JS_LIBRARIES
=================

If set to 1, all JS libraries will be automatically available at link time.
This gets set to 0 in STRICT mode (or with MINIMAL_RUNTIME) which mean you
need to explicitly specify -lfoo.js in at link time in order to access
library function in library_foo.js.

Default value: true

.. _auto_native_libraries:

AUTO_NATIVE_LIBRARIES
=====================

Like AUTO_JS_LIBRARIES but for the native libraries such as libgl, libal
and libhtml5.   If this is disabled it is necessary to explicitly add
e.g. -lhtml5 and also to first build the library using ``embuilder``.

Default value: true

.. _min_firefox_version:

MIN_FIREFOX_VERSION
===================

Specifies the oldest major version of Firefox to target. I.e. all Firefox
versions >= MIN_FIREFOX_VERSION
are desired to work. Pass -sMIN_FIREFOX_VERSION=majorVersion to drop support
for Firefox versions older than majorVersion.
Firefox 79 was released on 2020-07-28.
MAX_INT (0x7FFFFFFF, or -1) specifies that target is not supported.
Minimum supported value is 68 which was released on 2019-07-09 (see
feature_matrix.py)

Default value: 79

.. _min_safari_version:

MIN_SAFARI_VERSION
==================

Specifies the oldest version of desktop Safari to target. Version is encoded
in MMmmVV, e.g. 160101 denotes Safari 16.1.1.
Safari 15 was released on September 20, 2021, bundled with macOS 12.0
Monterey and iOS 15.
NOTE: Emscripten is unable to produce code that would work in iOS 9.3.5 and
older, i.e. iPhone 4s, iPad 2, iPad 3, iPad Mini 1, Pod Touch 5 and older,
see https://github.com/emscripten-core/emscripten/pull/7191.
MAX_INT (0x7FFFFFFF, or -1) specifies that target is not supported.
Minimum supported value is 140100 which was released on 2021-04-26 (see
feature_matrix.py).

Default value: 150000

.. _min_chrome_version:

MIN_CHROME_VERSION
==================

Specifies the oldest version of Chrome. E.g. pass -sMIN_CHROME_VERSION=100 to
drop support for Chrome 99 and older.
This setting also applies to modern Chromium-based Edge, which shares version
numbers with Chrome.
Chrome 85 was released on 2020-08-25.
MAX_INT (0x7FFFFFFF, or -1) specifies that target is not supported.
Minimum supported value is 85, which was released on 2020-08-25 (see
feature_matrix.py).

Default value: 85

.. _min_node_version:

MIN_NODE_VERSION
================

Specifies minimum node version to target for the generated code.  This is
distinct from the minimum version required to run the emscripten compiler.
Version is encoded in MMmmVV, e.g. 181401 denotes Node 18.14.01.
Minimum supported value is 180300, which was released 2022-05-18 (see
feature_matrix.py). This version aligns with the version available in
debian/stable (bookworm).

Default value: 180300

.. _minimal_runtime:

MINIMAL_RUNTIME
===============

If true, uses minimal sized runtime without POSIX features, Module,
preRun/preInit/etc., Emscripten built-in XHR loading or library_browser.js.
Enable this setting to target the smallest code size possible.  Set
MINIMAL_RUNTIME=2 to further enable even more code size optimizations. These
opts are quite hacky, and work around limitations in Closure and other parts
of the build system, so they may not work in all generated programs (But can
be useful for really small programs).

By default, no symbols will be exported on the ``Module`` object. In order
to export kept alive symbols, please use ``-sEXPORT_KEEPALIVE=1``.

Default value: 0

.. _minimal_runtime_streaming_wasm_compilation:

MINIMAL_RUNTIME_STREAMING_WASM_COMPILATION
==========================================

If set to 1, MINIMAL_RUNTIME will utilize streaming WebAssembly compilation,
where WebAssembly module is compiled already while it is being downloaded.
In order for this to work, the web server MUST properly serve the .wasm file
with a HTTP response header "Content-Type: application/wasm". If this HTTP
header is not present, e.g. Firefox 73 will fail with an error message
``TypeError: Response has unsupported MIME type``
and Chrome 78 will fail with an error message
`Uncaught (in promise) TypeError: Failed to execute 'compile' on
'WebAssembly': Incorrect response MIME type. Expected 'application/wasm'`.
If set to 0 (default), streaming WebAssembly compilation is disabled, which
means that the WebAssembly Module will first be downloaded fully, and only
then compilation starts.
For large .wasm modules and production environments, this should be set to 1
for faster startup speeds. However this setting is disabled by default
since it requires server side configuration and for really small pages there
is no observable difference (also has a ~100 byte impact to code size)
This setting is only compatible with html output.

Default value: false

.. _minimal_runtime_streaming_wasm_instantiation:

MINIMAL_RUNTIME_STREAMING_WASM_INSTANTIATION
============================================

If set to 1, MINIMAL_RUNTIME will utilize streaming WebAssembly instantiation,
where WebAssembly module is compiled+instantiated already while it is being
downloaded. Same restrictions/requirements apply as with
MINIMAL_RUNTIME_STREAMING_WASM_COMPILATION.
MINIMAL_RUNTIME_STREAMING_WASM_COMPILATION and
MINIMAL_RUNTIME_STREAMING_WASM_INSTANTIATION cannot be simultaneously active.
Which one of these two is faster depends on the size of the wasm module,
the size of the JS runtime file, and the size of the preloaded data file
to download, and the browser in question.

Default value: false

.. _support_longjmp:

SUPPORT_LONGJMP
===============

If set to 'emscripten' or 'wasm', compiler supports setjmp() and longjmp().
If set to 0, these APIs are not available.  If you are using C++ exceptions,
but do not need setjmp()+longjmp() API, then you can set this to 0 to save a
little bit of code size and performance when catching exceptions.

'emscripten': (default) Emscripten setjmp/longjmp handling using JavaScript
'wasm': setjmp/longjmp handling using Wasm EH instructions (experimental)

- 0: No setjmp/longjmp handling
- 1: Default setjmp/longjmp/handling, depending on the mode of exceptions.
  'wasm' if '-fwasm-exceptions' is used, 'emscripten' otherwise.

At compile time this enables the transformations needed for longjmp support
at codegen time, while at link it allows linking in the library support.

.. note:: Applicable during both linking and compilation

Default value: true

.. _disable_deprecated_find_event_target_behavior:

DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR
=============================================

If set to 1, disables old deprecated HTML5 API event target lookup behavior.
When enabled, there is no "Module.canvas" object, no magic "null" default
handling, and DOM element 'target' parameters are taken to refer to CSS
selectors, instead of referring to DOM IDs.

Default value: true

.. _html5_support_deferring_user_sensitive_requests:

HTML5_SUPPORT_DEFERRING_USER_SENSITIVE_REQUESTS
===============================================

Certain browser DOM API operations, such as requesting fullscreen mode
transition or pointer lock require that the request originates from within
a user initiated event, such as mouse click or keyboard press. Refactoring
an application to follow this kind of program structure can be difficult, so
HTML5_SUPPORT_DEFERRING_USER_SENSITIVE_REQUESTS allows transparent emulation
of this by deferring such requests until a suitable event callback is
generated. Set this to 0 to disable support for deferring to save code
size if your application does not need support for deferred calls.

Default value: true

.. _minify_html:

MINIFY_HTML
===========

Specifies whether the generated .html file is run through html-minifier. The
set of optimization passes run by html-minifier depends on debug and
optimization levels. In -g2 and higher, no minification is performed. In -g1,
minification is done, but whitespace is retained. Minification requires at
least -O1 or -Os to be used. Pass -sMINIFY_HTML=0 to explicitly choose to
disable HTML minification altogether.

Default value: true

.. _asan_shadow_size:

ASAN_SHADOW_SIZE
================

This option is no longer used. The appropriate shadow memory size is now
calculated from INITIAL_MEMORY and MAXIMUM_MEMORY. Will be removed in a
future release.

Default value: -1

.. _source_map_prefixes:

SOURCE_MAP_PREFIXES
===================

List of path substitutions to apply in the "sources" field of the source map.
Corresponds to the ``--prefix`` option used in ``tools/wasm-sourcemap.py``.
Must be used with ``-gsource-map``.

This setting allows to map path prefixes to the proper ones so that the final
(possibly relative) URLs point to the correct locations :
``-sSOURCE_MAP_PREFIXES=/old/path=/new/path``

Default value: []

.. _default_to_cxx:

DEFAULT_TO_CXX
==============

Default to c++ mode even when run as ``emcc`` rather than ``emc++``.
When this is disabled ``em++`` is required when linking C++ programs.
Disabling this will match the behaviour of gcc/g++ and clang/clang++.

Default value: true

.. _printf_long_double:

PRINTF_LONG_DOUBLE
==================

While LLVM's wasm32 has long double = float128, we don't support printing
that at full precision by default. Instead we print as 64-bit doubles, which
saves libc code size. You can flip this option on to get a libc with full
long double printing precision.

Default value: false

.. _separate_dwarf_url:

SEPARATE_DWARF_URL
==================

Setting this affects the path emitted in the wasm that refers to the DWARF
file, in -gseparate-dwarf mode. This allows the debugging file to be hosted
in a custom location.

Default value: ''

.. _error_on_wasm_changes_after_link:

ERROR_ON_WASM_CHANGES_AFTER_LINK
================================

Emscripten runs wasm-ld to link, and in some cases will do further changes to
the wasm afterwards, like running wasm-opt to optimize the binary in
optimized builds. However, in some builds no wasm changes are necessary after
link. This can make the entire link step faster, and can also be important
for other reasons, like in debugging if the wasm is not modified then the
DWARF info from LLVM is preserved (wasm-opt can rewrite it in some cases, but
not in others like split-dwarf).
When this flag is turned on, we error at link time if the build requires any
changes to the wasm after link. This can be useful in testing, for example.
Some example of features that require post-link wasm changes are:

- Lowering i64 to i32 pairs at the JS boundary (See WASM_BIGINT)
- Lowering nontrapping-float-to-int operations when targeting older browsers.

Default value: false

.. _abort_on_wasm_exceptions:

ABORT_ON_WASM_EXCEPTIONS
========================

Abort on unhandled exceptions that occur when calling exported WebAssembly
functions. This makes the program behave more like a native program where the
OS would terminate the process and no further code can be executed when an
unhandled exception (e.g. out-of-bounds memory access) happens.
This will instrument all exported functions to catch thrown exceptions and
call abort() when they happen. Once the program aborts any exported function
calls will fail with a "program has already aborted" exception to prevent
calls into code with a potentially corrupted program state.
This adds a small fixed amount to code size in optimized builds and a slight
overhead for the extra instrumented function indirection.  Enable this if you
want Emscripten to handle unhandled exceptions nicely at the cost of a few
bytes extra.
Exceptions that occur within the ``main`` function are already handled via an
alternative mechanism.

Default value: false

.. _pure_wasi:

PURE_WASI
=========

Build binaries that use as many WASI APIs as possible, and include additional
JS support libraries for those APIs.  This allows emscripten to produce
binaries that are more WASI compliant and also allows it to process and
execute WASI binaries built with other SDKs (e.g.  wasi-sdk).
This setting is experimental and subject to change or removal.
Implies :ref:`STANDALONE_WASM`.

.. note:: This is an experimental setting

Default value: false

.. _imported_memory:

IMPORTED_MEMORY
===============

Set to 1 to define the WebAssembly.Memory object outside of the wasm
module.  By default the wasm module defines the memory and exports
it to JavaScript.
Use of the following settings will enable this settings since they
depend on being able to define the memory in JavaScript:

- -pthread
- RELOCATABLE
- ASYNCIFY_LAZY_LOAD_CODE
- WASM2JS (WASM=0)

Default value: false

.. _split_module:

SPLIT_MODULE
============

Generate code to load split wasm modules.
This option will automatically generate two wasm files as output, one
with the ``.orig`` suffix and one without.  The default file (without
the suffix) when run will generate instrumentation data that can later be
fed into wasm-split (the binaryen tool).
As well as this the generated JS code will contain helper functions
to load split modules.

.. note:: This is an experimental setting

Default value: false

.. _autoload_dylibs:

AUTOLOAD_DYLIBS
===============

For MAIN_MODULE builds, automatically load any dynamic library dependencies
on startup, before loading the main module.

Default value: true

.. _allow_unimplemented_syscalls:

ALLOW_UNIMPLEMENTED_SYSCALLS
============================

Link against stub implementations of unsupported/unimplemented syscalls. This
allows programs that depend on these syscalls to be compiled, even though
these functions will fail (or do nothing) at runtime.

Default value: true

.. _trusted_types:

TRUSTED_TYPES
=============

Allow calls to Worker(...) and importScripts(...) to be Trusted Types
compatible. Trusted Types is a Web Platform feature designed to mitigate DOM
XSS by restricting the usage of DOM sink APIs.
See https://www.w3.org/TR/trusted-types/.

Default value: false

.. _polyfill:

POLYFILL
========

When targeting older browsers emscripten will sometimes require that
polyfills be included in the output.  If you would prefer to take care of
polyfilling yourself via some other mechanism you can prevent emscripten
from generating these by passing ``-sNO_POLYFILL`` or ``-sPOLYFILL=0``
With default browser targets emscripten does not need any polyfills so this
settings is *only* needed when also explicitly targeting older browsers.

Default value: true

.. _runtime_debug:

RUNTIME_DEBUG
=============

If non-zero, add tracing to core runtime functions.  Can be set to 2 for
extra tracing (for example, tracing that occurs on each turn of the event
loop or each user callback, which can flood the console).
This setting is enabled by default if any of the following debugging settings
are enabled:

- PTHREADS_DEBUG
- DYLINK_DEBUG
- LIBRARY_DEBUG
- GL_DEBUG
- OPENAL_DEBUG
- EXCEPTION_DEBUG
- SYSCALL_DEBUG
- WEBSOCKET_DEBUG
- SOCKET_DEBUG
- FETCH_DEBUG

Default value: 0

.. _legacy_runtime:

LEGACY_RUNTIME
==============

Include JS library symbols that were previously part of the default runtime.
Without this, such symbols can be made available by adding them to
:ref:`DEFAULT_LIBRARY_FUNCS_TO_INCLUDE`, or via the dependencies of another
JS library symbol.

Default value: false

.. _signature_conversions:

SIGNATURE_CONVERSIONS
=====================

User-defined functions to wrap with signature conversion, which take or
return pointer arguments. Only affects ``MEMORY64=1`` builds, see
``create_pointer_conversion_wrappers`` in ``emscripten.py`` for details.
Use ``_`` for non-pointer arguments, ``p`` for pointer/i53 arguments, and
``P`` for optional pointer/i53 values.
Example use ``-sSIGNATURE_CONVERSIONS=someFunction:_p,anotherFunction:p``

Default value: []

.. _wasm_bindgen:

WASM_BINDGEN
============

Run wasm-bindgen and integrate the rust-exported symbols into the rest of Emscripten's JS output.

Default value: 0

.. _source_phase_imports:

SOURCE_PHASE_IMPORTS
====================

Experimental support for wasm source phase imports.
This is only currently implemented in the pre-release/nightly version of
node, and not yet supported by browsers.
Requires EXPORT_ES6

.. note:: This is an experimental setting

Default value: false

.. _wasm_esm_integration:

WASM_ESM_INTEGRATION
====================

Experimental support for wasm ESM integration.
Requires :ref:`EXPORT_ES6` and ``MODULARIZE=instance``

.. note:: This is an experimental setting

Default value: false

.. _js_base64_api:

JS_BASE64_API
=============

Enable use of the JS arraybuffer-base64 API:
https://github.com/tc39/proposal-arraybuffer-base64
To run the resulting code currently requires passing `--js_base_64` to node
or chrome.

.. note:: This is an experimental setting

Default value: false

.. _growable_arraybuffers:

GROWABLE_ARRAYBUFFERS
=====================

Enable support for growable views of Wasm memory. This is a recent Web
platform feature that can make growing the Wasm memory more efficient,
especially in multi-threaded builds.
Setting this to 1 will auto-detect the presence of this API and use
it when available.
Setting this to 2 will unconditionally require it. This is the only way
to completely remove the overhead of growable memory + pthreads.
This settings does nothing unless ALLOW_MEMORY_GROWTH is set.

Default value: 0

.. _cross_origin:

CROSS_ORIGIN
============

If the emscripten-generated program is hosted on separate origin then
starting new pthread worker can violate CSP rules.  Enabling
CROSS_ORIGIN uses an inline worker to instead load the worker script
indirectly using `importScripts`

Default value: false

.. _cross_origin_storage:

CROSS_ORIGIN_STORAGE
====================

Enables Cross-Origin Storage (COS) API support for Wasm loading on the
Web target. At link time Emscripten computes the SHA-256 hash of the
final ``.wasm`` binary and embeds it in the generated JS. At runtime the
COS API is used as a progressive enhancement: the binary is fetched from
the shared cross-origin cache on a hit, or stored there after a network
fetch on a miss; when the API is absent or errors the runtime falls
through to the standard fetch path.

Requires the Web environment; using it without ``-sENVIRONMENT=web`` is a
hard link-time error. Incompatible with SINGLE_FILE and
WASM_ASYNC_COMPILATION=0 (both produce hard link-time errors).

See :ref:`CrossOriginStorage` for the full guide.

.. note:: This is an experimental setting

Default value: false

.. _cross_origin_storage_origins:

CROSS_ORIGIN_STORAGE_ORIGINS
============================

Controls which origins may read the Wasm binary from the COS cache. Only
meaningful when ``-sCROSS_ORIGIN_STORAGE`` is set. Applied only during the
write (cache-miss) path, not the read (cache-hit) path.

``['*']`` (default) — any origin can retrieve the file.
Explicit HTTPS origin list — restricted to those origins only::

  -sCROSS_ORIGIN_STORAGE_ORIGINS=https://app.example.com,https://api.example.com

``[]`` — same-site only (omits the ``origins`` field entirely).

Mixing ``'*'`` with explicit origins is a link-time error.

Default value: ['*']

.. _fake_dylibs:

FAKE_DYLIBS
===========

This setting changes the behaviour of the ``-shared`` flag.  When set to true
you get the old emscripten behaviour where the ``-shared`` flag actually
produces a normal object file (i.e. ``ld -r``).  When set to true (the
default) the ``-shared`` flag is equivelent to :ref:`SIDE_MODULE` and will
produce a Wasn dynamic library.

Default value: false

.. _executable:

EXECUTABLE
==========

Add a #! line to generated JS file and make it executable.  This is useful
for building command line tools that run under node.
This setting can also be set to a string value, in which case that string
will be used as the #! command to embed in the generated file.

Default value: false

.. _deprecated-settings:

===================
Deprecated Settings
===================

The following settings have been proposed for removal from emscripten.  These settings
still function but may be removed in a future version.  If your project is using one of
these settings please open a bug (or reply to one of the existing bugs).

 - ``RUNTIME_LINKED_LIBS``: you can simply list the libraries directly on the commandline now
 - ``CLOSURE_WARNINGS``: use -Wclosure/-Wno-closure instead
 - ``LEGALIZE_JS_FFI``: to disable JS type legalization use `-sWASM_BIGINT` or `-sSTANDALONE_WASM`
 - ``ASYNCIFY_EXPORTS``: please use JSPI_EXPORTS instead
 - ``LINKABLE``: under consideration for removal (https://github.com/emscripten-core/emscripten/issues/25262)
 - ``EXPORT_EXCEPTION_HANDLING_HELPERS``: getExceptionMessage is exported anyway when ASSERTIONS or EXCEPTION_STACK_TRACES is set, which are set by default at -O0. At -O1 or above, you can export it separately by -sEXPORTED_RUNTIME_METHODS=getExceptionMessage,decrementExceptionRefcount.
 - ``DETERMINISTIC``: under consideration for removal (https://github.com/emscripten-core/emscripten/issues/26647)
 - ``USE_PTHREADS``: prefer the standard -pthread flag
 - ``MEMORY64``: prefer the standard -m64 or --target=wasm64 flags
 - ``SOCKET_WEBRTC``: under consideration for removal (https://github.com/emscripten-core/emscripten/issues/27366)

.. _legacy-settings:

===============
Legacy Settings
===============

The following settings no longer have any effect but are still accepted by emscripten
for backwards compatibility with older versions:

 - ``BINARYEN``: Valid values: WASM
 - ``TOTAL_STACK``: Valid values: STACK_SIZE
 - ``BINARYEN_ASYNC_COMPILATION``: Valid values: WASM_ASYNC_COMPILATION
 - ``UNALIGNED_MEMORY``: forced unaligned memory not supported in fastcomp (Valid values: [0])
 - ``FORCE_ALIGNED_MEMORY``: forced aligned memory is not supported in fastcomp (Valid values: [0])
 - ``PGO``: pgo no longer supported (Valid values: [0])
 - ``QUANTUM_SIZE``: altering the QUANTUM_SIZE is not supported (Valid values: [4])
 - ``FUNCTION_POINTER_ALIGNMENT``: Starting from Emscripten 1.37.29, no longer available (https://github.com/emscripten-core/emscripten/pull/6091) (Valid values: [2])
 - ``RESERVED_FUNCTION_POINTERS``: Valid values: ALLOW_TABLE_GROWTH
 - ``BUILD_AS_SHARED_LIB``: Starting from Emscripten 1.38.16, no longer available (https://github.com/emscripten-core/emscripten/pull/7433) (Valid values: [0])
 - ``SAFE_SPLIT_MEMORY``: Starting from Emscripten 1.38.19, SAFE_SPLIT_MEMORY codegen is no longer available (https://github.com/emscripten-core/emscripten/pull/7465) (Valid values: [0])
 - ``SPLIT_MEMORY``: Starting from Emscripten 1.38.19, SPLIT_MEMORY codegen is no longer available (https://github.com/emscripten-core/emscripten/pull/7465) (Valid values: [0])
 - ``BINARYEN_METHOD``: Starting from Emscripten 1.38.23, Emscripten now always builds either to Wasm (-sWASM - default), or to JavaScript (-sWASM=0), other methods are not supported (https://github.com/emscripten-core/emscripten/pull/7836) (Valid values: ['native-wasm'])
 - ``BINARYEN_TRAP_MODE``: The wasm backend does not support a trap mode (it always clamps, in effect) (Valid values: [-1])
 - ``PRECISE_I64_MATH``: Starting from Emscripten 1.38.26, PRECISE_I64_MATH is always enabled (https://github.com/emscripten-core/emscripten/pull/7935) (Valid values: [1, 2])
 - ``MEMFS_APPEND_TO_TYPED_ARRAYS``: Starting from Emscripten 1.38.26, MEMFS_APPEND_TO_TYPED_ARRAYS=0 is no longer supported. MEMFS no longer supports using JS arrays for file data (https://github.com/emscripten-core/emscripten/pull/7918) (Valid values: [1])
 - ``ERROR_ON_MISSING_LIBRARIES``: missing libraries are always an error now (Valid values: [1])
 - ``EMITTING_JS``: The new STANDALONE_WASM flag replaces this (replace EMITTING_JS=0 with STANDALONE_WASM=1) (Valid values: [1])
 - ``SKIP_STACK_IN_SMALL``: SKIP_STACK_IN_SMALL is no longer needed as the backend can optimize it directly (Valid values: [0, 1])
 - ``SAFE_STACK``: Replace SAFE_STACK=1 with STACK_OVERFLOW_CHECK=2 (Valid values: [0])
 - ``MEMORY_GROWTH_STEP``: Valid values: MEMORY_GROWTH_LINEAR_STEP
 - ``ELIMINATE_DUPLICATE_FUNCTIONS``: Duplicate function elimination for wasm is handled automatically by binaryen (Valid values: [0, 1])
 - ``ELIMINATE_DUPLICATE_FUNCTIONS_DUMP_EQUIVALENT_FUNCTIONS``: Duplicate function elimination for wasm is handled automatically by binaryen (Valid values: [0])
 - ``ELIMINATE_DUPLICATE_FUNCTIONS_PASSES``: Duplicate function elimination for wasm is handled automatically by binaryen (Valid values: [5])
 - ``WASM_OBJECT_FILES``: For LTO, use -flto or -fto=thin instead. Otherwise, Wasm object files are the default (Valid values: [1])
 - ``TOTAL_MEMORY``: Valid values: INITIAL_MEMORY
 - ``WASM_MEM_MAX``: Valid values: MAXIMUM_MEMORY
 - ``BINARYEN_MEM_MAX``: Valid values: MAXIMUM_MEMORY
 - ``BINARYEN_PASSES``: Use BINARYEN_EXTRA_PASSES to add additional passes (Valid values: [''])
 - ``SWAPPABLE_ASM_MODULE``: Fully swappable asm modules are no longer supported (Valid values: [0])
 - ``ASM_JS``: asm.js output is not supported anymore (Valid values: [1])
 - ``FINALIZE_ASM_JS``: asm.js output is not supported anymore (Valid values: [0, 1])
 - ``ASYNCIFY_WHITELIST``: Valid values: ASYNCIFY_ONLY
 - ``ASYNCIFY_BLACKLIST``: Valid values: ASYNCIFY_REMOVE
 - ``EXCEPTION_CATCHING_WHITELIST``: Valid values: EXCEPTION_CATCHING_ALLOWED
 - ``SEPARATE_ASM``: Separate asm.js only made sense for fastcomp with asm.js output (Valid values: [0])
 - ``SEPARATE_ASM_MODULE_NAME``: Separate asm.js only made sense for fastcomp with asm.js output (Valid values: [''])
 - ``FAST_UNROLLED_MEMCPY_AND_MEMSET``: The wasm backend implements memcpy/memset in C (Valid values: [0, 1])
 - ``DOUBLE_MODE``: The wasm backend always implements doubles normally (Valid values: [0, 1])
 - ``PRECISE_F32``: The wasm backend always implements floats normally (Valid values: [0, 1, 2])
 - ``ALIASING_FUNCTION_POINTERS``: The wasm backend always uses a single index space for function pointers, in a single Table (Valid values: [0, 1])
 - ``AGGRESSIVE_VARIABLE_ELIMINATION``: Wasm ignores asm.js-specific optimization flags (Valid values: [0, 1])
 - ``SIMPLIFY_IFS``: Wasm ignores asm.js-specific optimization flags (Valid values: [1])
 - ``DEAD_FUNCTIONS``: The wasm backend does not support dead function removal (Valid values: [[]])
 - ``WASM_BACKEND``: Only the wasm backend is now supported (note that setting it as -s has never been allowed anyhow) (Valid values: [-1])
 - ``EXPORT_BINDINGS``: No longer needed (Valid values: [0, 1])
 - ``RUNNING_JS_OPTS``: Fastcomp cared about running JS which could alter asm.js validation, but not upstream (Valid values: [0])
 - ``EXPORT_FUNCTION_TABLES``: No longer needed (Valid values: [0])
 - ``BINARYEN_SCRIPTS``: No longer needed (Valid values: [''])
 - ``WARN_UNALIGNED``: No longer needed (Valid values: [0, 1])
 - ``ASM_PRIMITIVE_VARS``: No longer needed (Valid values: [[]])
 - ``WORKAROUND_IOS_9_RIGHT_SHIFT_BUG``: Wasm2JS does not support iPhone 4s, iPad 2, iPad 3, iPad Mini 1, Pod Touch 5 (devices with end-of-life at iOS 9.3.5) and older (Valid values: [0])
 - ``RUNTIME_FUNCS_TO_IMPORT``: No longer needed (Valid values: [[]])
 - ``LIBRARY_DEPS_TO_AUTOEXPORT``: No longer needed (Valid values: [[]])
 - ``EMIT_EMSCRIPTEN_METADATA``: No longer supported (Valid values: [0])
 - ``SHELL_FILE``: No longer supported (Valid values: [''])
 - ``LLD_REPORT_UNDEFINED``: Disabling is no longer supported (Valid values: [1])
 - ``MEM_INIT_METHOD``: No longer supported (Valid values: [0])
 - ``USE_PTHREADS``: No longer needed. Use -pthread instead (Valid values: [0, 1])
 - ``USES_DYNAMIC_ALLOC``: No longer supported. Use -sMALLOC=none (Valid values: [1])
 - ``REVERSE_DEPS``: No longer needed (Valid values: ['auto', 'all', 'none'])
 - ``RUNTIME_LOGGING``: Valid values: RUNTIME_DEBUG
 - ``MIN_EDGE_VERSION``: No longer supported (Valid values: [2147483647])
 - ``MIN_IE_VERSION``: No longer supported (Valid values: [2147483647])
 - ``WORKAROUND_OLD_WEBGL_UNIFORM_UPLOAD_IGNORED_OFFSET_BUG``: No longer supported (Valid values: [0])
 - ``AUTO_ARCHIVE_INDEXES``: No longer needed (Valid values: [0, 1])
 - ``USE_ES6_IMPORT_META``: Disabling is no longer supported (Valid values: [1])
 - ``EXTRA_EXPORTED_RUNTIME_METHODS``: No longer supported, use EXPORTED_RUNTIME_METHODS (Valid values: [[]])
 - ``SUPPORT_ERRNO``: No longer supported (Valid values: [0])
 - ``DEMANGLE_SUPPORT``: No longer supported (Valid values: [0])
 - ``MAYBE_WASM2JS``: No longer supported (use -sWASM=2) (Valid values: [0])
 - ``HEADLESS``: No longer supported, use headless browsers or Node.js with JSDOM (Valid values: [0])
 - ``USE_OFFSET_COVERTER``: No longer supported, not needed with modern v8 versions (Valid values: [0])
 - ``ASYNCIFY_LAZY_LOAD_CODE``: No longer supported (Valid values: [0])
 - ``USE_WEBGPU``: No longer supported; replaced by --use-port=emdawnwebgpu, which implements a newer (but incompatible) version of webgpu.h - see tools/ports/emdawnwebgpu.py (Valid values: [0])
 - ``PROXY_TO_WORKER``: No longer supported (Valid values: [0])
 - ``NODEJS_CATCH_EXIT``: No longer supported (Valid values: [0])
 - ``NODEJS_CATCH_REJECTION``: No longer supported (Valid values: [0])
 - ``POLYFILL_OLD_MATH_FUNCTIONS``: No longer supported (Valid values: [0])
 - ``RELOCATABLE``: No longer supported (Valid values: [0])
 - ``WASM_JS_TYPES``: No longer supported (Valid values: [0])
 - ``DETERMINISTIC``: No longer supported (Valid values: [0])
