.. _emscripten-h:

============ emscripten.h

This page documents the public C++ APIs provided by emscripten.h <https://github.com/emscripten-core/emscripten/blob/main/system/include/emscripten/emscripten.h>_.

Emscripten uses existing/familiar APIs where possible (for example: :term:SDL). This API provides C++ support for capabilities that are specific to JavaScript or the browser environment, or for which there is no existing API.

.. contents:: Table of Contents :local: :depth: 1

Inline assembly/JavaScript

Guide material for the following APIs can be found in :ref:interacting-with-code-call-javascript-from-native.

Defines

.. c:macro:: EM_JS(return_type, function_name, arguments, code)

Convenient syntax for JavaScript library functions.

This allows you to declare JavaScript in your C code as a function, which can be called like a normal C function. For example, the following C program would display two alerts if it was compiled with Emscripten and run in the browser:

.. code-block:: none

EM_JS(void, two_alerts, (), {
  alert('hai');
  alert('bai');
});

int main() {
  two_alerts();
  return 0;
}

Arguments can be passed as normal C arguments, and have the same name in the JavaScript code. These arguments can either be of type int32_t or double.

.. code-block:: none

EM_JS(void, take_args, (int x, float y), {
  console.log('I received: ' + [x, y]);
});

int main() {
  take_args(100, 35.5);
  return 0;
}

Null-terminated C strings can also be passed into EM_JS functions, but to operate on them, they need to be copied out from the heap to convert to high-level JavaScript strings.

.. code-block:: none

EM_JS(void, say_hello, (const char* str), {
  console.log('hello ' + UTF8ToString(str));
}

In the same manner, pointers to any type (including void *) can be passed inside EM_JS code, where they appear as integers like char * pointers above did. Accessing the data can be managed by reading the heap directly.

.. code-block:: none

EM_JS(void, read_data, (int* data), {
  console.log('Data: ' + HEAP32[data>>2] + ', ' + HEAP32[(data+4)>>2]);
});

int main() {
  int arr[2] = { 30, 45 };
  read_data(arr);
  return 0;
}

In addition, EM_JS functions can return a value back to C code. The output value is passed back with a return statement:

.. code-block:: none

EM_JS(int, add_forty_two, (int n), {
  return n + 42;
});

EM_JS(int, get_memory_size, (), {
  return HEAP8.length;
});

int main() {
  int x = add_forty_two(100);
  int y = get_memory_size();
  // ...
}

Strings can be returned back to C from JavaScript, but one needs to be careful about memory management.

.. code-block:: none

EM_JS(char*, get_unicode_str, (), {
  var jsString = 'Hello with some exotic Unicode characters: Tässä on yksi lumiukko: ☃, ole hyvä.';
  // 'jsString.length' would return the length of the string as UTF-16
  // units, but Emscripten C strings operate as UTF-8.
  return stringToNewUTF8(jsString);
});

int main() {
  char* str = get_unicode_str();
  printf("UTF8 string says: %s\n", str);
  // Each call to _malloc() must be paired with free(), or heap memory will leak!
  free(str);
  return 0;
}

.. c:macro:: EM_ASM(...)

Convenient syntax for inline assembly/JavaScript.

This allows you to declare JavaScript in your C code "inline", which is then executed when your compiled code is run in the browser. For example, the following C code would display two alerts if it was compiled with Emscripten and run in the browser:

.. code-block:: none

EM_ASM(alert('hai'); alert('bai'));

Arguments can be passed inside the JavaScript code block, where they arrive as variables $0, $1 etc. These arguments can either be of type int32_t or double.

.. code-block:: none

EM_ASM({
  console.log('I received: ' + [$0, $1]);
}, 100, 35.5);

Note the { and }.

Null-terminated C strings can also be passed into EM_ASM blocks, but to operate on them, they need to be copied out from the heap to convert to high-level JavaScript strings.

.. code-block:: none

EM_ASM(console.log('hello ' + UTF8ToString($0)), "world!");

In the same manner, pointers to any type (including void *) can be passed inside EM_ASM code, where they appear as integers like char * pointers above did. Accessing the data can be managed by reading the heap directly.

.. code-block:: none

int arr[2] = { 30, 45 };
EM_ASM({
  console.log('Data: ' + HEAP32[$0>>2] + ', ' + HEAP32[($0+4)>>2]);
}, arr);

.. note:: - As of Emscripten 1.30.4, the contents of EM_ASM code blocks appear inside the normal JS file, and as result, Closure compiler and other JavaScript minifiers will be able to operate on them. You may need to use safety quotes in some places (a['b'] instead of a.b) to avoid minification from occurring. - The C preprocessor does not have an understanding of JavaScript tokens, and as a result, if the code block contains a comma character ,, it may be necessary to wrap the code block inside parentheses. For example, code EM_ASM(return [1,2,3].length); will not compile, but EM_ASM((return [1,2,3].length)); does.

.. c:macro:: EM_ASM_INT(code, ...)

This macro, as well as :c:macro:EM_ASM_DOUBLE and :c:macro:EM_ASM_PTR, behave like :c:macro:EM_ASM, but in addition they also return a value back to C code. The output value is passed back with a return statement:

.. code-block:: none

int x = EM_ASM_INT({
  return $0 + 42;
}, 100);

int y = EM_ASM_INT(return HEAP8.length);

.. c:macro:: EM_ASM_PTR(code, ...)

Similar to :c:macro:EM_ASM_INT but for a pointer-sized return values. When building with -sMEMORY64 this results in i64 return value, otherwise it results in an i32 return value.

Strings can be returned back to C from JavaScript, but one needs to be careful about memory management.

.. code-block:: none

char *str = (char*)EM_ASM_PTR({
  var jsString = 'Hello with some exotic Unicode characters: Tässä on yksi lumiukko: ☃, ole hyvä.';
  var lengthBytes = lengthBytesUTF8(jsString)+1;
  // 'jsString.length' would return the length of the string as UTF-16
  // units, but Emscripten C strings operate as UTF-8.
  return stringToNewUTF8(jsString);
});
printf("UTF8 string says: %s\n", str);
free(str); // Each call to _malloc() must be paired with free(), or heap memory will leak!

.. c:macro:: EM_ASM_DOUBLE(code, ...)

Similar to :c:macro:EM_ASM_INT but for a double return value.

.. c:macro:: MAIN_THREAD_EM_ASM(code, ...)

This behaves like :c:macro:EM_ASM, but does the call on the main thread. This is useful in a pthreads build, when you want to interact with the DOM from a pthread; this basically proxies the call for you.

This call is proxied in a synchronous way to the main thread, that is, execution will resume after the main thread has finished running the JS. Synchronous proxying also makes it possible to return a value, see the next two variants.

.. c:macro:: MAIN_THREAD_EM_ASM_INT(code, ...)

Similar to :c:macro:MAIN_THREAD_EM_ASM but returns an int value.

.. c:macro:: MAIN_THREAD_EM_ASM_DOUBLE(code, ...)

Similar to :c:macro:MAIN_THREAD_EM_ASM but returns a double value.

.. c:macro:: MAIN_THREAD_EM_ASM_PTR(code, ...)

Similar to :c:macro:MAIN_THREAD_EM_ASM but returns a pointer value.

.. c:macro:: MAIN_THREAD_ASYNC_EM_ASM(code, ...)

Similar to :c:macro:MAIN_THREAD_EM_ASM but is proxied in an asynchronous way, that is, the main thread will receive a request to run the code, and will run it when it can; the worker will not wait for that. (Note that if this is called on the main thread, then there is nothing to proxy, and the JS is executed immediately and synchronously.)

Calling JavaScript From C/C++

Guide material for the following APIs can be found in :ref:interacting-with-code-call-javascript-from-native.

Function pointer types for callbacks

The following types are used to define function callback signatures used in a number of functions in this file.

.. c:type:: em_callback_func

General function pointer type for use in callbacks with no parameters.

Defined as: ::

typedef void (*em_callback_func)(void)

.. c:type:: em_arg_callback_func

Generic function pointer type for use in callbacks with a single void* parameter.

This type is used to define function callbacks that need to pass arbitrary data. For example, :c:func:emscripten_set_main_loop_arg sets user-defined data, and passes it to a callback of this type on completion.

Defined as: ::

typedef void (*em_arg_callback_func)(void*)

.. c:type:: em_str_callback_func

General function pointer type for use in callbacks with a C string (const char *) parameter.

This type is used for function callbacks that need to be passed a C string. For example, it is used in :c:func:emscripten_async_wget to pass the name of a file that has been asynchronously loaded.

Defined as: ::

typedef void (*em_str_callback_func)(const char *)

Functions

.. c:function:: void emscripten_run_script(const char *script)

Interface to the underlying JavaScript engine. This function will eval() the given script. Note: If -sDYNAMIC_EXECUTION=0 is set, this function will not be available.

This function can be called from a pthread, and it is executed in the scope of the Web Worker that is hosting the pthread. To evaluate a function in the scope of the main runtime thread, see the function emscripten_sync_run_in_main_runtime_thread().

:param script: The script to evaluate. :type script: const char* :rtype: void

.. c:function:: int emscripten_run_script_int(const char *script)

Interface to the underlying JavaScript engine. This function will eval() the given script. Note: If -sDYNAMIC_EXECUTION=0 is set, this function will not be available.

:param script: The script to evaluate. :type script: const char* :return: The result of the evaluation, as an integer. :rtype: int

.. c:function:: char *emscripten_run_script_string(const char *script)

Interface to the underlying JavaScript engine. This function will eval() the given script. Note that this overload uses a single buffer shared between calls. Note: If -sDYNAMIC_EXECUTION=0 is set, this function will not be available.

:param script: The script to evaluate. :type script: const char* :return: The result of the evaluation, as a string. :rtype: char*

.. c:function:: void emscripten_async_run_script(const char *script, int millis)

Asynchronously run a script, after a specified amount of time.

:param script: The script to evaluate. :type script: const char* :param int millis: The amount of time before the script is run, in milliseconds. :rtype: void

.. c:function:: void emscripten_async_load_script(const char *script, em_callback_func onload, em_callback_func onerror)

Asynchronously loads a script from a URL.

This integrates with the run dependencies system, so your script can call addRunDependency multiple times, prepare various asynchronous tasks, and call removeRunDependency on them; when all are complete (or if there were no run dependencies to begin with), onload is called. An example use for this is to load an asset module, that is, the output of the file packager.

This function is currently only available in main browser thread, and it will immediately fail by calling the supplied onerror() handler if called in a pthread.

:param script: The script to evaluate. :type script: const char* :param em_callback_func onload: A callback function, with no parameters, that is executed when the script has fully loaded. :param em_callback_func onerror: A callback function, with no parameters, that is executed if there is an error in loading. :rtype: void

.. _emscripten-h-browser-execution-environment:

Browser Execution Environment

Guide material for the following APIs can be found in :ref:emscripten-runtime-environment.

Functions

.. c:function:: void emscripten_set_main_loop(em_callback_func func, int fps, bool simulate_infinite_loop)

Set a C function as the main event loop for the calling thread.

If the main loop function needs to receive user-defined data, use :c:func:emscripten_set_main_loop_arg instead.

The JavaScript environment will call that function at a specified number of frames per second. If called on the main browser thread, setting 0 or a negative value as the fps will use the browser’s requestAnimationFrame mechanism to call the main loop function. This is HIGHLY recommended if you are doing rendering, as the browser’s requestAnimationFrame will make sure you render at a proper smooth rate that lines up properly with the browser and monitor. If you do not render at all in your application, then you should pick a specific frame rate that makes sense for your code.

If simulate_infinite_loop is true, the function will throw an exception in order to stop execution of the caller. This will lead to the main loop being entered instead of code after the call to :c:func:emscripten_set_main_loop being run, which is the closest we can get to simulating an infinite loop (we do something similar in glutMainLoop <https://github.com/emscripten-core/emscripten/blob/1.29.12/system/include/GL/freeglut_std.h#L400>_ in GLUT <http://www.opengl.org/resources/libraries/glut/>_). If this parameter is false, then the behavior is the same as it was before this parameter was added to the API, which is that execution continues normally. Note that in both cases we do not run global destructors, atexit, etc., since we know the main loop will still be running, but if we do not simulate an infinite loop then the stack will be unwound. That means that if simulate_infinite_loop is false, and you created an object on the stack, it will be cleaned up before the main loop is called for the first time.

This function can be called in a pthread, in which case the callback loop will be set up to be called in the context of the calling thread. In order for the loop to work, the calling thread must regularly "yield back" to the browser by exiting from its pthread main function, since the callback will be able to execute only when the calling thread is not executing any other code. This means that running a synchronously blocking main loop is not compatible with the emscripten_set_main_loop() function.

Since requestAnimationFrame() API is not available in web workers, when called emscripten_set_main_loop() in a pthread with fps <= 0, the effect of syncing up to the display's refresh rate is emulated, and generally will not precisely line up with vsync intervals.

.. tip:: There can be only one main loop function at a time, per thread. To change the main loop function, first :c:func:cancel <emscripten_cancel_main_loop> the current loop, and then call this function to set another.

.. note:: See :c:func:emscripten_set_main_loop_expected_blockers, :c:func:emscripten_pause_main_loop, :c:func:emscripten_resume_main_loop and :c:func:emscripten_cancel_main_loop for information about blocking, pausing, and resuming the main loop of the calling thread.

.. note:: Calling this function overrides the effect of any previous calls to :c:func:emscripten_set_main_loop_timing in the calling thread by applying the timing mode specified by the parameter fps. To specify a different timing mode for the current thread, call the function :c:func:emscripten_set_main_loop_timing after setting up the main loop.

.. note:: Currently, using the new Wasm exception handling <https://emscripten.org/docs/porting/exceptions.html#webassembly-exception-handling-proposal>_ and simulate_infinite_loop == true at the same time does not work yet in C++ projects that have objects with destructors on the stack at the time of the call.

:param em_callback_func func: C function to set as main event loop for the calling thread. :param int fps: Number of frames per second that the JavaScript will call the function. Setting int <=0 (recommended) uses the browser’s requestAnimationFrame mechanism to call the function. :param bool simulate_infinite_loop: If true, this function will throw an exception in order to stop execution of the caller.

.. c:function:: void emscripten_set_main_loop_arg(em_arg_callback_func func, void *arg, int fps, bool simulate_infinite_loop)

Set a C function as the main event loop for the calling thread, passing it user-defined data.

.. seealso:: The information in :c:func:emscripten_set_main_loop also applies to this function.

:param em_arg_callback_func func: C function to set as main event loop. The function signature must have a void* parameter for passing the arg value. :param void* arg: User-defined data passed to the main loop function, untouched by the API itself. :param int fps: Number of frames per second at which the JavaScript will call the function. Setting int <=0 (recommended) uses the browser’s requestAnimationFrame mechanism to call the function. :param bool simulate_infinite_loop: If true, this function will throw an exception in order to stop execution of the caller.

.. c:function:: void emscripten_push_main_loop_blocker(em_arg_callback_func func, void *arg) void emscripten_push_uncounted_main_loop_blocker(em_arg_callback_func func, void *arg)

Add a function that blocks the main loop for the calling thread.

The function is added to the back of a queue of events to be blocked; the main loop will not run until all blockers in the queue complete.

In the "counted" version, blockers are counted (internally) and Module.setStatus is called with some text to report progress (setStatus is a general hook that a program can define in order to show processing updates).

.. note:: - Main loop blockers block the main loop from running, and can be counted to show progress. In contrast, :c:func:emscripten_async_call is not counted, does not block the main loop, and can fire at a specific time in the future.

:param em_arg_callback_func func: The main loop blocker function. The function signature must have a void* parameter for passing the arg value. :param void* arg: User-defined arguments to pass to the blocker function. :rtype: void

.. c:function:: void emscripten_pause_main_loop(void) void emscripten_resume_main_loop(void)

Pause and resume the main loop for the calling thread.

Pausing and resuming the main loop is useful if your app needs to perform some synchronous operation, for example to load a file from the network. It might be wrong to run the main loop before that finishes (the original code assumes that), so you can break the code up into asynchronous callbacks, but you must pause the main loop until they complete.

.. note:: These are fairly low-level functions. :c:func:emscripten_push_main_loop_blocker (and friends) provide more convenient alternatives.

.. c:function:: void emscripten_cancel_main_loop(void)

Cancels the main event loop for the calling thread.

See also :c:func:emscripten_set_main_loop and :c:func:emscripten_set_main_loop_arg for information about setting and using the main loop.

.. note:: This function cancels the main loop, which means that it will no longer be called. No other changes occur to control flow. In particular, if you started the main loop with the simulate_infinite_loop option, you can still cancel the main loop, but execution will not continue in the code right after setting the main loop (we do not actually run an infinite loop there - that's not possible in JavaScript, so to simulate an infinite loop we halt execution at that stage, and then the next thing that runs is the main loop itself, so it seems like an infinite loop has begun there; canceling the main loop sort of breaks the metaphor).

.. c:function:: int emscripten_set_main_loop_timing(int mode, int value)

Specifies the scheduling mode that the main loop tick function of the calling thread will be called with.

This function can be used to interactively control the rate at which Emscripten runtime drives the main loop specified by calling the function :c:func:emscripten_set_main_loop. In native development, this corresponds with the "swap interval" or the "presentation interval" for 3D rendering. The new tick interval specified by this function takes effect immediately on the existing main loop, and this function must be called only after setting up a main loop via :c:func:emscripten_set_main_loop.

:param int mode: The timing mode to use. Allowed values are EM_TIMING_SETTIMEOUT, EM_TIMING_RAF and EM_TIMING_SETIMMEDIATE.

:param int value: The timing value to activate for the main loop. This value interpreted differently according to the mode parameter:

 - If ``mode`` is EM_TIMING_SETTIMEOUT, then ``value`` specifies the number
   of milliseconds to wait between subsequent ticks to the main loop and
   updates occur independent of the vsync rate of the display (vsync off).
   This method uses the JavaScript ``setTimeout`` function to drive the
   animation.
 - If ``mode`` is EM_TIMING_RAF, then updates are performed using the
   ``requestAnimationFrame`` function (with vsync enabled), and this value
   is interpreted as a "swap interval" rate for the main loop. The value of
   ``1`` specifies the runtime that it should render at every vsync
   (typically 60fps), whereas the value ``2`` means that the main loop
   callback should be called only every second vsync (30fps). As a general
   formula, the value ``n`` means that the main loop is updated at every
   n'th vsync, or at a rate of ``60/n`` for 60Hz displays, and ``120/n`` for
   120Hz displays.
 - If ``mode`` is EM_TIMING_SETIMMEDIATE, then updates are performed using
   the ``setImmediate`` function, or if not available, emulated via
   ``postMessage``. See `setImmediate on MDN
   <https://developer.mozilla.org/en-US/docs/Web/API/Window/setImmediate>`
   for more information. Note that this mode is **strongly not recommended**
   to be used when deploying Emscripten output to the web, since it depends
   on an unstable web extension that is in draft status, browsers other than
   IE do not currently support it, and its implementation has been
   considered controversial in review.

:rtype: int :return: The value 0 is returned on success, and a nonzero value is returned on failure. A failure occurs if there is no main loop active before calling this function.

.. note:: Browsers heavily optimize towards using requestAnimationFrame for animation instead of the other provided modes. Because of that, for best experience across browsers, calling this function with mode=EM_TIMING_RAF and value=1 will yield best results. Using the JavaScript setTimeout function is known to cause stutter and generally worse experience than using the requestAnimationFrame function.

.. note:: There is a functional difference between setTimeout and requestAnimationFrame: If the user minimizes the browser window or hides your application tab, browsers will typically stop calling requestAnimationFrame callbacks, but setTimeout-based main loop will continue to be run, although with heavily throttled intervals. See setTimeout on MDN <https://developer.mozilla.org/en-US/docs/Web/API/WindowTimers.setTimeout#Inactive_tabs> for more information.

.. c:function:: void emscripten_get_main_loop_timing(int *mode, int *value)

Returns the current main loop timing mode that is in effect. For interpretation of the values, see the documentation of the function :c:func:emscripten_set_main_loop_timing. The timing mode is controlled by calling the functions :c:func:emscripten_set_main_loop_timing and :c:func:emscripten_set_main_loop.

:param mode: If not null, the used timing mode is returned here.
:type mode: int*
:param value: If not null, the used timing value is returned here.
:type value: int*

.. c:function:: void emscripten_set_main_loop_expected_blockers(int num)

Sets the number of blockers that are about to be pushed.

The number is used for reporting the relative progress through a set of blockers, after which the main loop will continue.

For example, a game might have to run 10 blockers before starting a new level. The operation would first set this value as '10' and then push the 10 blockers. When the 3\ :sup:rd blocker (say) completes, progress is displayed as 3/10.

:param int num: The number of blockers that are about to be pushed.

.. c:function:: void emscripten_async_call(em_arg_callback_func func, void *arg, int millis)

Call a C function asynchronously, that is, after returning control to the JavaScript event loop.

This is done by a setTimeout.

When building natively this becomes a simple direct call, after SDL_Delay (you must include SDL.h for that).

If millis is negative, the browser's requestAnimationFrame mechanism is used. (Note that 0 means that setTimeout is still used, which basically means "run asynchronously as soon as possible".)

:param em_arg_callback_func func: The C function to call asynchronously. The function signature must have a void* parameter for passing the arg value. :param void* arg: User-defined argument to pass to the C function. :param int millis: Timeout before function is called.

.. c:function:: void emscripten_exit_with_live_runtime(void)

Stops the current thread of execution, but leaves the runtime alive so that you can continue to run code later (so global destructors etc., are not run). Note that the runtime is kept alive automatically when you do an asynchronous operation like :c:func:emscripten_async_call, so you don't need to call this function for those cases.

In a multithreaded application, this just exits the current thread (and allows running code later in the Web Worker in which it runs).

.. c:function:: void emscripten_force_exit(int status)

Shuts down the runtime and exits (terminates) the program, as if you called exit().

The difference is that emscripten_force_exit will shut down the runtime even if you previously called :c:func:emscripten_exit_with_live_runtime or otherwise kept the runtime alive. In other words, this method gives you the option to completely shut down the runtime after it was kept alive beyond the completion of main().

Note that if EXIT_RUNTIME is not set (which is the case by default) then the runtime cannot be shut down, as we do not include the code to do so. Build with -sEXIT_RUNTIME if you want to be able to exit the runtime.

:param int status: The same as for the libc function exit() <http://linux.die.net/man/3/exit>_.

.. c:function:: double emscripten_get_device_pixel_ratio(void)

Returns the value of window.devicePixelRatio.

:rtype: double :return: The pixel ratio or 1.0 if not supported.

.. c:function:: char *emscripten_get_window_title()

Returns the window title.

The returned string will be valid until the next call of the function

.. c:function:: void emscripten_set_window_title(char *title)

Sets the window title.

.. c:function:: void emscripten_get_screen_size(int *width, int *height)

Returns the width and height of the screen.

.. c:function:: void emscripten_hide_mouse(void)

Hide the OS mouse cursor over the canvas.

Note that SDL’s SDL_ShowCursor command shows and hides the SDL cursor, not the OS one. This command is useful to hide the OS cursor if your app draws its own cursor.

.. c:function:: double emscripten_get_now(void)

Returns the highest-precision representation of the current time that the browser provides.

This uses either Date.now or performance.now. The result is not an absolute time, and is only meaningful in comparison to other calls to this function.

:rtype: double :return: The current time, in milliseconds (ms).

.. c:function:: float emscripten_random(void)

Generates a random number in the range 0-1. This maps to Math.random().

:rtype: float :return: A random number.

.. _emscripten-h-asynchronous-file-system-api:

Asynchronous File System API

Typedefs

.. c:type:: em_async_wget_onload_func

Function pointer type for the onload callback of :c:func:emscripten_async_wget_data (specific values of the parameters documented in that method).

Defined as: ::

typedef void (*em_async_wget_onload_func)(void*, void*, int)

.. c:type:: em_async_wget2_onload_func

Function pointer type for the onload callback of :c:func:emscripten_async_wget2 (specific values of the parameters documented in that method).

Defined as: ::

typedef void (*em_async_wget2_onload_func)(void*, const char*)

.. c:type:: em_async_wget2_onstatus_func

Function pointer type for the onerror and onprogress callbacks of :c:func:emscripten_async_wget2 (specific values of the parameters documented in that method).

Defined as: ::

typedef void (*em_async_wget2_onstatus_func)(void*, int)

.. c:type:: em_async_wget2_data_onload_func

Function pointer type for the onload callback of :c:func:emscripten_async_wget2_data (specific values of the parameters documented in that method).

Defined as: ::

typedef void (*em_async_wget2_data_onload_func)(unsigned, void*, void *, unsigned)

.. c:type:: em_async_wget2_data_onerror_func

Function pointer type for the onerror callback of :c:func:emscripten_async_wget2_data (specific values of the parameters documented in that method).

Defined as: ::

typedef void (*em_async_wget2_data_onerror_func)(unsigned, void*, int, const char*)

.. c:type:: em_async_wget2_data_onprogress_func

Function pointer type for the onprogress callback of :c:func:emscripten_async_wget2_data (specific values of the parameters documented in that method).

Defined as: ::

typedef void (*em_async_wget2_data_onprogress_func)(unsigned, void*, int, int)

.. c:type:: em_run_preload_plugins_data_onload_func

Function pointer type for the onload callback of :c:func:emscripten_run_preload_plugins_data (specific values of the parameters documented in that method).

Defined as: ::

typedef void (*em_run_preload_plugins_data_onload_func)(void*, const char*)

Functions

.. c:function:: void emscripten_async_wget(const char* url, const char* file, em_str_callback_func onload, em_str_callback_func onerror)

Loads a file from a URL asynchronously.

In addition to fetching the URL from the network, preload plugins are executed so that the data is usable in IMG_Load and so forth (we asynchronously do the work to make the browser decode the image or audio etc.). See :ref:preloading-files for more information on preloading files.

When the file is ready the onload callback will be called. If any error occurs onerror will be called. The callbacks are called with the file as their argument.

:param const char* url: The URL to load. :param const char* file: The name of the file created and loaded from the URL. If the file already exists it will be overwritten. If the destination directory for the file does not exist on the filesystem, it will be created. A relative pathname may be passed, which will be interpreted relative to the current working directory at the time of the call to this function. :param em_str_callback_func onload: Callback on successful load of the file. The callback function parameter value is:

- *(const char*)* : The name of the ``file`` that was loaded from the URL.

:param em_str_callback_func onerror: Callback in the event of failure. The callback function parameter value is:

- *(const char*)* : The name of the ``file`` that failed to load from the URL.

.. c:function:: void emscripten_async_wget_data(const char* url, void *arg, em_async_wget_onload_func onload, em_arg_callback_func onerror)

Loads a buffer from a URL asynchronously.

This is the "data" version of :c:func:emscripten_async_wget.

Instead of writing to a file, this function writes to a buffer directly in memory. This avoids the overhead of using the emulated file system; note however that since files are not used, it cannot run preload plugins to set things up for IMG_Load and so forth (IMG_Load etc. work on files).

When the file is ready then the onload callback will be called. If any error occurred onerror will be called.

:param url: The URL of the file to load. :type url: const char* :param void* arg: User-defined data that is passed to the callbacks, untouched by the API itself. This may be used by a callback to identify the associated call. :param em_async_wget_onload_func onload: Callback on successful load of the URL into the buffer. The callback function parameter values are:

- *(void*)* : Equal to ``arg`` (user defined data).
- *(void*)* : A pointer to a buffer with the data. Note that, as with the worker API, the data buffer only lives during the callback; it must be used or copied during that time.
- *(int)* : The size of the buffer, in bytes.

:param em_arg_callback_func onerror: Callback in the event of failure. The callback function parameter values are:

- *(void*)* : Equal to ``arg`` (user defined data).

.. c:function:: int emscripten_async_wget2(const char* url, const char* file, const char* requesttype, const char* param, void *arg, em_async_wget2_onload_func onload, em_async_wget2_onstatus_func onerror, em_async_wget2_onstatus_func onprogress)

Loads a file from a URL asynchronously.

This is an experimental "more feature-complete" version of :c:func:emscripten_async_wget.

Preload plug-ins are at this time not executed on the downloaded data. You may want to call :c:func:emscripten_run_preload_plugins in the onload callback if you want to be able to use the downloaded file with IMG_Load and such.

When the file is ready the onload callback will be called with the object pointers given in arg and file. During the download the onprogress callback is called.

:param url: The URL of the file to load. :type url: const char* :param file: The name of the file created and loaded from the URL. If the file already exists it will be overwritten. If the destination directory for the file does not exist on the filesystem, it will be created. A relative pathname may be passed, which will be interpreted relative to the current working directory at the time of the call to this function. :type file: const char* :param requesttype: 'GET' or 'POST'. :type requesttype: const char* :param param: Request parameters for POST requests (see requesttype). The parameters are specified in the same way as they would be in the URL for an equivalent GET request: e.g. key=value&key2=value2. :type param: const char* :param void* arg: User-defined data that is passed to the callbacks, untouched by the API itself. This may be used by a callback to identify the associated call. :param em_async_wget2_onload_func onload: Callback on successful load of the file. The callback function parameter values are:

- *(void*)* : Equal to ``arg`` (user defined data).
- *(const char*)* : The ``file`` passed to the original call.

:param em_async_wget2_onstatus_func onerror: Callback in the event of failure. The callback function parameter values are:

- *(void*)* : Equal to ``arg`` (user defined data).
- *(int)* : The HTTP status code.

:param em_async_wget2_onstatus_func onprogress: Callback during load of the file. The callback function parameter values are:

- *(void*)* : Equal to ``arg`` (user defined data).
- *(int)* : The progress (percentage completed).

:returns: A handle to request (int) that can be used to :c:func:abort <emscripten_async_wget2_abort> the request.

.. c:function:: int emscripten_async_wget2_data(const char* url, const char* requesttype, const char* param, void *arg, int free, em_async_wget2_data_onload_func onload, em_async_wget2_data_onerror_func onerror, em_async_wget2_data_onprogress_func onprogress)

Loads a buffer from a URL asynchronously.

This is the "data" version of :c:func:emscripten_async_wget2. It is an experimental "more feature complete" version of :c:func:emscripten_async_wget_data.

When the file is ready the onload callback will be called with the object pointers given in arg, a pointer to the buffer in memory, and an unsigned integer containing the size of the buffer. During the download the onprogress callback is called with progress information. If an error occurs, onerror will be called with the HTTP status code and a string containing the status description.

:param url: The URL of the file to load. :type url: const char* :param requesttype: 'GET' or 'POST'. :type requesttype: const char* :param param: Request parameters for POST requests (see requesttype). The parameters are specified in the same way as they would be in the URL for an equivalent GET request: e.g. key=value&key2=value2. :type param: const char* :param void* arg: User-defined data that is passed to the callbacks, untouched by the API itself. This may be used by a callback to identify the associated call. :param int free: Tells the runtime whether to free the returned buffer after onload is complete. If false freeing the buffer is the receiver's responsibility. :type free: int :param em_async_wget2_data_onload_func onload: Callback on successful load of the file. The callback function parameter values are:

- *(unsigned)* : Handle to the request
- *(void*)* : Equal to ``arg`` (user defined data).
- *(void*)* : A pointer to the buffer in memory.
- *(unsigned)* : The size of the buffer (in bytes).

:param em_async_wget2_data_onerror_func onerror: Callback in the event of failure. The callback function parameter values are:

- *(unsigned)* : Handle to the request
- *(void*)* : Equal to ``arg`` (user defined data).
- *(int)* : The HTTP error code.
- *(const char*)* : A string with the status description.

:param em_async_wget2_data_onprogress_func onprogress: Callback called (regularly) during load of the file to update progress. The callback function parameter values are:

- *(unsigned)* : Handle to the request
- *(void*)* : Equal to ``arg`` (user defined data).
- *(int)* : The number of bytes loaded.
- *(int)* : The total size of the data in bytes, or zero if the size is unavailable.

:returns: A handle to request (int) that can be used to :c:func:abort <emscripten_async_wget2_abort> the request.

.. c:function:: void emscripten_async_wget2_abort(int handle)

Abort an asynchronous request raised using :c:func:emscripten_async_wget2 or :c:func:emscripten_async_wget2_data.

:param int handle: A handle to request to be aborted.

.. c:function:: void emscripten_run_preload_plugins_data(char* data, int size, const char *suffix, void *arg, em_run_preload_plugins_data_onload_func onload, em_arg_callback_func onerror)

Runs preload plugins on a buffer of data asynchronously. This is a "data" version of :c:func:emscripten_run_preload_plugins, which receives raw data as input instead of a filename (this can prevent the need to write data to a file first). See :ref:preloading-files for more information on preload plugins.

When file is loaded then the onload callback will be called. If any error occurs onerror will be called.

onload also receives a second parameter, which is a 'fake' filename which you can pass into IMG_Load (it is not an actual file, but it identifies this image for IMG_Load to be able to process it). Note that the user of this API is responsible for free() ing the memory allocated for the fake filename.

:param char* data: The buffer of data to process. :param suffix: The file suffix, e.g. 'png' or 'jpg'. :type suffix: const char* :param void* arg: User-defined data that is passed to the callbacks, untouched by the API itself. This may be used by a callback to identify the associated call. :param em_run_preload_plugins_data_onload_func onload: Callback on successful processing of the data. The callback function parameter values are:

- *(void*)* : Equal to ``arg`` (user defined data).
- *(const char*)* : A 'fake' filename which you can pass into ``IMG_Load``. See above for more information.

:param em_arg_callback_func onerror: Callback in the event of failure. The callback function parameter value is:

- *(void*)* : Equal to ``arg`` (user defined data).

.. c:function:: void emscripten_dlopen(const char filename, int flags, void user_data, em_dlopen_callback onsuccess, em_arg_callback_func onerror);

Starts an asynchronous dlopen operation to load a shared library from a filename or URL. Returns immediately and requires the caller to return to the event loop. The onsuccess and onerror callbacks are used to signal success or failure of the request. Upon onerror callback the normal dlerror C function can be used get the error details. The flags are the same as those used in the normal dlopen C function.

:param const char* filename: The filename (or URLs) of the shared library to load. :param int flags: See dlopen flags. :param void* user_data: User data passed to onsuccess, and onerror callbacks. :param em_dlopen_callback onsuccess: Called if the library was loaded successfully. :param em_arg_callback_func onerror: Called if there is an error loading the library.

Asynchronous IndexedDB API

IndexedDB is a browser API that lets you store data persistently, that is, you can save data there and load it later when the user re-visits the web page. IDBFS provides one way to use IndexedDB, through the Emscripten filesystem layer. The emscripten_idb_* methods listed here provide an alternative API, directly to IndexedDB, thereby avoiding the overhead of the filesystem layer.

.. c:function:: void emscripten_idb_async_load(const char *db_name, const char file_id, void arg, em_async_wget_onload_func onload, em_arg_callback_func onerror)

Loads data from local IndexedDB storage asynchronously. This allows use of persistent data, without the overhead of the filesystem layer.

When the data is ready then the onload callback will be called. If any error occurred onerror will be called.

:param db_name: The IndexedDB database from which to load. :param file_id: The identifier of the data to load. :param void* arg: User-defined data that is passed to the callbacks, untouched by the API itself. This may be used by a callback to identify the associated call. :param em_async_wget_onload_func onload: Callback on successful load of the URL into the buffer. The callback function parameter values are:

- *(void*)* : Equal to ``arg`` (user defined data).
- *(void*)* : A pointer to a buffer with the data. Note that, as with the worker API, the data buffer only lives during the callback; it must be used or copied during that time.
- *(int)* : The size of the buffer, in bytes.

:param em_arg_callback_func onerror: Callback in the event of failure. The callback function parameter values are:

- *(void*)* : Equal to ``arg`` (user defined data).

.. c:function:: void emscripten_idb_async_store(const char db_name, const char file_id, void ptr, int num, void arg, em_arg_callback_func onstore, em_arg_callback_func onerror);

Stores data to local IndexedDB storage asynchronously. This allows use of persistent data, without the overhead of the filesystem layer.

When the data has been stored then the onstore callback will be called. If any error occurred onerror will be called.

:param db_name: The IndexedDB database from which to load. :param file_id: The identifier of the data to load. :param ptr: A pointer to the data to store. :param num: How many bytes to store. :param void* arg: User-defined data that is passed to the callbacks, untouched by the API itself. This may be used by a callback to identify the associated call. :param em_arg_callback_func onstore: Callback on successful store of the data buffer to the URL. The callback function parameter values are:

- *(void*)* : Equal to ``arg`` (user defined data).

:param em_arg_callback_func onerror: Callback in the event of failure. The callback function parameter values are:

- *(void*)* : Equal to ``arg`` (user defined data).

.. c:function:: void emscripten_idb_async_delete(const char *db_name, const char file_id, void arg, em_arg_callback_func ondelete, em_arg_callback_func onerror)

Deletes data from local IndexedDB storage asynchronously.

When the data has been deleted then the ondelete callback will be called. If any error occurred onerror will be called.

:param db_name: The IndexedDB database. :param file_id: The identifier of the data. :param void* arg: User-defined data that is passed to the callbacks, untouched by the API itself. This may be used by a callback to identify the associated call. :param em_arg_callback_func ondelete: Callback on successful delete

- *(void*)* : Equal to ``arg`` (user defined data).

:param em_arg_callback_func onerror: Callback in the event of failure. The callback function parameter values are:

- *(void*)* : Equal to ``arg`` (user defined data).

.. c:function:: void emscripten_idb_async_exists(const char *db_name, const char file_id, void arg, em_idb_exists_func oncheck, em_arg_callback_func onerror)

Checks if data with a certain ID exists in the local IndexedDB storage asynchronously.

When the data has been checked then the oncheck callback will be called. If any error occurred onerror will be called.

:param db_name: The IndexedDB database. :param file_id: The identifier of the data. :param void* arg: User-defined data that is passed to the callbacks, untouched by the API itself. This may be used by a callback to identify the associated call. :param em_idb_exists_func oncheck: Callback on successful check, with arguments

- *(void*)* : Equal to ``arg`` (user defined data).
- *int* : Whether the file exists or not.

:param em_arg_callback_func onerror: Callback in the event of failure. The callback function parameter values are:

- *(void*)* : Equal to ``arg`` (user defined data).

.. c:function:: void emscripten_idb_async_clear(const char db_name, void arg, em_arg_callback_func onclear, em_arg_callback_func onerror)

Clears all data from local IndexedDB storage asynchronously.

When the storage has been cleared then the onclear callback will be called. If any error occurred onerror will be called.

:param db_name: The IndexedDB database. :param void* arg: User-defined data that is passed to the callbacks, untouched by the API itself. This may be used by a callback to identify the associated call. :param em_arg_callback_func onclear: Callback on successful clear. The callback function parameter is:

- *(void*)* : Equal to ``arg`` (user defined data).

:param em_arg_callback_func onerror: Callback in the event of failure. The callback function parameter is:

- *(void*)* : Equal to ``arg`` (user defined data).

.. c:function:: int emscripten_run_preload_plugins(const char* file, em_str_callback_func onload, em_str_callback_func onerror)

Runs preload plugins on a file asynchronously. It works on file data already present and performs any required asynchronous operations available as preload plugins, such as decoding images for use in IMG_Load, or decoding audio for use in Mix_LoadWAV. See :ref:preloading-files for more information on preloading plugins.

Once the operations are complete, the onload callback will be called. If any error occurs onerror will be called. The callbacks are called with the file as their argument.

:param file: The name of the file to process. :type file: const char* :param em_str_callback_func onload: Callback on successful processing of the file. The callback function parameter value is:

- *(const char*)* : The name of the ``file`` that was processed.

:param em_str_callback_func onerror: Callback in the event of failure. The callback function parameter value is:

- *(const char*)* : The name of the ``file`` for which the operation failed.

:return: 0 if successful, -1 if the file does not exist :rtype: int

Compiling

.. c:macro:: EMSCRIPTEN_KEEPALIVE

Tells the compiler and linker to preserve a symbol, and export it, as if you added it to :ref:EXPORTED_FUNCTIONS <faq-dead-code-elimination>.

For example: ::

void EMSCRIPTEN_KEEPALIVE my_function() { printf("I am being kept alive\n"); }

Note that this will only work if the object file in which the symbol is defined is otherwise included by the linker. If the object file is part of an archive, and is not otherwise referenced the linker will not include it at all and any symbols in the object file will not be included or exported. One way to work around this limitation is to use the -Wl,--whole-archive / -Wl,--no-whole-archive flags on either side of the archive file.

Worker API

Typedefs

.. c:var:: int worker_handle

A wrapper around web workers that lets you create workers and communicate with them.

Note that the current API is mainly focused on a main thread that sends jobs to workers and waits for responses, i.e., in an asymmetrical manner, there is no current API to send a message without being asked for it from a worker to the main thread.

.. c:type:: em_worker_callback_func

Function pointer type for the callback from :c:func:emscripten_call_worker (specific values of the parameters documented in that method).

Defined as: ::

typedef void (*em_worker_callback_func)(char*, int, void*)

Functions

.. c:function:: worker_handle emscripten_create_worker(const char * url)

Creates a worker.

A worker must be compiled separately from the main program, and with the BUILD_AS_WORKER flag set to 1.

That worker must not be compiled with the -pthread flag as the POSIX threads implementation and this Worker API are incompatible.

:param url: The URL of the worker script. :type url: const char* :return: A handle to the newly created worker. :rtype: worker_handle

.. c:function:: void emscripten_destroy_worker(worker_handle worker)

Destroys a worker. See :c:func:emscripten_create_worker

:param worker_handle worker: A handle to the worker to be destroyed.

.. c:function:: void emscripten_call_worker(worker_handle worker, const char *funcname, char *data, int size, em_worker_callback_func callback, void *arg)

Asynchronously calls a worker.

The worker function will be called with two parameters: a data pointer, and a size. The data block defined by the pointer and size exists only during the callback: it cannot be relied upon afterwards. If you need to keep some of that information outside the callback, then it needs to be copied to a safe location.

The called worker function can return data, by calling :c:func:emscripten_worker_respond. When the worker is called, if a callback was given it will be called with three arguments: a data pointer, a size, and an argument that was provided when calling :c:func:emscripten_call_worker (to more easily associate callbacks to calls). The data block defined by the data pointer and size behave like the data block in the worker function — it exists only during the callback.

:param worker_handle worker: A handle to the worker to be called. :param funcname: The name of the function in the worker. The function must be a C function (so no C++ name mangling), and must be exported (:ref:EXPORTED_FUNCTIONS <faq-dead-code-elimination>). :type funcname: const char* :param char* data: The address of a block of memory to copy over. :param int size: The size of the block of memory. :param em_worker_callback_func callback: Worker callback with the response. This can be null. The callback function parameter values are:

- *(char*)* : The ``data`` pointer provided in :c:func:`emscripten_call_worker`.
- *(int)* : The ``size`` of the block of data.
- *(void*)* : Equal to ``arg`` (user defined data).

:param void* arg: An argument (user data) to be passed to the callback

.. c:function:: void emscripten_worker_respond(char *data, int size) void emscripten_worker_respond_provisionally(char *data, int size)

Sends a response when in a worker call (that is, when called by the main thread using :c:func:emscripten_call_worker).

Both functions post a message back to the thread which called the worker. The :c:func:emscripten_worker_respond_provisionally variant can be invoked multiple times, which will queue up messages to be posted to the worker’s creator. Eventually, the _respond variant must be invoked, which will disallow further messages and free framework resources previously allocated for this worker call.

.. note:: Calling the provisional version is optional, but you must call the non-provisional version to avoid leaks.

:param char* data: The message to be posted. :param int size: The size of the message, in bytes.

.. c:function:: int emscripten_get_worker_queue_size(worker_handle worker)

Checks how many responses are being waited for from a worker.

This only counts calls to :c:func:emscripten_call_worker that had a callback (calls with null callbacks are ignored), and where the response has not yet been received. It is a simple way to check on the status of the worker to see how busy it is, and do basic decisions about throttling.

:param worker_handle worker: The handle to the relevant worker. :return: The number of responses waited on from a worker. :rtype: int

Logging utilities

Defines

.. c:macro:: EM_LOG_CONSOLE

If specified, logs directly to the browser console/inspector window. If not specified, logs via the application Module.

.. c:macro:: EM_LOG_WARN

If specified, prints a warning message (combined with :c:data:EM_LOG_CONSOLE).

.. c:macro:: EM_LOG_INFO

If specified, prints an info message to console (combined with :c:data:EM_LOG_CONSOLE).

.. c:macro:: EM_LOG_DEBUG

If specified, prints a debug message to console (combined with :c:data:EM_LOG_CONSOLE).

.. c:macro:: EM_LOG_ERROR

If specified, prints an error message (combined with :c:data:EM_LOG_CONSOLE). If neither :c:data:EM_LOG_WARN, :c:data:EM_LOG_ERROR, :c:data:EM_LOG_INFO nor :c:data:EM_LOG_DEBUG is specified, a log message is printed. :c:data:EM_LOG_WARN, :c:data:EM_LOG_INFO, :c:data:EM_LOG_DEBUG and :c:data:EM_LOG_ERROR are mutually exclusive. If :c:data:EM_LOG_CONSOLE is not specified then the message will be outputted via err() (for :c:data:EM_LOG_ERROR or :c:data:EM_LOG_WARN) or out() otherwise.

.. c:macro:: EM_LOG_C_STACK

If specified, prints a call stack that contains file names referring to original C sources using source map information.

.. c:macro:: EM_LOG_JS_STACK

If specified, prints a call stack that contains file names referring to lines in the built .js/.html file along with the message. The flags :c:data:EM_LOG_C_STACK and :c:data:EM_LOG_JS_STACK can be combined to output both untranslated and translated file and line information.

.. c:macro:: EM_LOG_NO_PATHS

If specified, the pathnames of the file information in the call stack will be omitted.

Functions

.. c:function:: long emscripten_get_compiler_setting(const char *name)

Returns the value of a compiler setting.

For example, to return the integer representing the value of INITIAL_MEMORY during compilation: ::

emscripten_get_compiler_setting("INITIAL_MEMORY")

For values containing anything other than an integer, a string is returned (you will need to cast the long return value to a char*).

Some useful things this can do is provide the version of Emscripten (“EMSCRIPTEN_VERSION”), the optimization level (“OPT_LEVEL”), debug level (“DEBUG_LEVEL”), etc.

For this command to work, you must build with the following compiler option (as we do not want to increase the build size with this metadata): ::

-sRETAIN_COMPILER_SETTINGS

:param name: The compiler setting to return. :type name: const char* :returns: The value of the specified setting. Note that for values other than an integer, a string is returned (cast the int return value to a char*). :rtype: int

.. c:function:: int emscripten_has_asyncify()

Returns whether pseudo-synchronous functions can be used.

:rtype: int :returns: 1 if program was compiled with -sASYNCIFY, 0 otherwise.

.. c:function:: void emscripten_debugger()

Emits debugger.

This is inline in the code, which tells the JavaScript engine to invoke the debugger if it gets there.

.. c:function:: void emscripten_log(int flags, const char* format, ...)

Prints out a message to the console, optionally with the callstack information.

:param int flags: A binary OR of items from the list of :c:data:EM_LOG_xxx <EM_LOG_CONSOLE> flags that specify printing options. :param const char* format: A printf-style format string. :param ...: A printf-style "..." parameter list that is parsed according to the printf formatting rules.

.. c:function:: int emscripten_get_callstack(int flags, char *out, int maxbytes)

Programmatically obtains the current callstack.

To query the amount of bytes needed for a callstack without writing it, pass 0 to out and maxbytes, in which case the function will return the number of bytes (including the terminating zero) that will be needed to hold the full callstack. Note that this might be fully accurate since subsequent calls will carry different line numbers, so it is best to allocate a few bytes extra to be safe.

:param int flags: A binary OR of items from the list of :c:data:EM_LOG_xxx <EM_LOG_CONSOLE> flags that specify printing options. The flags :c:data:EM_LOG_CONSOLE, :c:data:EM_LOG_WARN and :c:data:EM_LOG_ERROR do not apply in this function and are ignored. :param char* out: A pointer to a memory region where the callstack string will be written to. The string outputted by this function will always be null-terminated. :param int maxbytes: The maximum number of bytes that this function can write to the memory pointed to by out. If there is not enough space, the output will be truncated (but always null-terminated). :returns: The number of bytes written (not number of characters, so this will also include the terminating zero). :rtype: int

.. c:function:: char *emscripten_get_preloaded_image_data(const char *path, int *w, int *h)

Gets preloaded image data and the size of the image.

The function returns pointer to loaded image or NULL — the pointer should be free()'d. The width/height of the image are written to the w and h parameters if the data is valid.

:param path: Full path/filename to the file containing the preloaded image. :type path: const char* :param int* w: Width of the image (if data is valid). :param int* h: Height of the image (if data is valid). :returns: A pointer to the preloaded image or NULL. :rtype: char*

.. c:function:: char *emscripten_get_preloaded_image_data_from_FILE(FILE *file, int *w, int *h)

Gets preloaded image data from a C FILE*.

:param file: The FILE containing the preloaded image. :type file: FILE* :param int* w: Width of the image (if data is valid). :param int* h: Height of the image (if data is valid). :returns: A pointer to the preloaded image or NULL. :rtype: char*

.. c:function:: int emscripten_print_double(double x, char *to, signed max)

Prints a double as a string, including a null terminator. This is useful because JS engines have good support for printing out a double in a way that takes the least possible size, but preserves all the information in the double, i.e., it can then be parsed back in a perfectly reversible manner (snprintf etc. do not do so, sadly).

:param double x: The double. :param char* to: A pre-allocated buffer of sufficient size, or NULL if no output is requested (useful to get the necessary size). :param signed max: The maximum number of bytes that can be written to the output pointer 'to' (including the null terminator). :rtype: The number of necessary bytes, not including the null terminator (actually written, if to is not NULL).

.. _emscripten-api-reference-sockets:

Socket event registration

The functions in this section register callback functions for receiving socket events. These events are analogous to WebSocket <https://developer.mozilla.org/en/docs/WebSockets>_ events but are emitted after the internal Emscripten socket processing has occurred. This means, for example, that the message callback will be triggered after the data has been added to the recv_queue, so that an application receiving this callback can simply read the data using the file descriptor passed as a parameter to the callback. All of the callbacks are passed a file descriptor (fd) representing the socket that the notified activity took place on. The error callback also takes an int representing the socket error number (errno) and a char* that represents the error message (msg).

Only a single callback function may be registered to handle any given event, so calling a given registration function more than once will cause the first callback to be replaced. Similarly, passing a NULL callback function to any emscripten_set_socket_*_callback call will de-register the callback registered for that event.

The userData pointer allows arbitrary data specified during event registration to be passed to the callback, this is particularly useful for passing this pointers around in Object Oriented code.

In addition to being able to register network callbacks from C it is also possible for native JavaScript code to directly use the underlying mechanism used to implement the callback registration. For example, the following code shows simple logging callbacks that are registered by default when SOCKET_DEBUG is enabled:

.. code-block:: javascript

Module['websocket']['on']('error', function(error) {console.log('Socket error ' + error);}); Module['websocket']['on']('open', function(fd) {console.log('Socket open fd = ' + fd);}); Module['websocket']['on']('listen', function(fd) {console.log('Socket listen fd = ' + fd);}); Module['websocket']['on']('connection', function(fd) {console.log('Socket connection fd = ' + fd);}); Module['websocket']['on']('message', function(fd) {console.log('Socket message fd = ' + fd);}); Module['websocket']['on']('close', function(fd) {console.log('Socket close fd = ' + fd);});

Most of the JavaScript callback functions above get passed the file descriptor of the socket that triggered the callback, the on error callback however gets passed an array that contains the file descriptor, the error code and an error message.

.. note:: The underlying JavaScript implementation doesn't pass userData. This is mostly of use to C/C++ code and the emscripten_set_socket_*_callback calls simply create a closure containing the userData and pass that as the callback to the underlying JavaScript event registration mechanism.

Callback functions

.. c:type:: em_socket_callback

Function pointer for :c:func:emscripten_set_socket_open_callback, and the other socket functions (except :c:func:emscripten_set_socket_error_callback). This is defined as:

.. code-block:: cpp

typedef void (*em_socket_callback)(int fd, void *userData);

:param int fd: The file descriptor of the socket that triggered the callback. :param void* userData: The userData originally passed to the event registration function.

.. c:type:: em_socket_error_callback

Function pointer for the :c:func:emscripten_set_socket_error_callback, defined as:

.. code-block:: cpp

typedef void (*em_socket_error_callback)(int fd, int err, const char* msg, void *userData);

:param int fd: The file descriptor of the socket that triggered the callback. :param int err: The code for the error that occurred. :param int msg: The message for the error that occurred. :param void* userData: The userData originally passed to the event registration function.

Functions

.. c:function:: void emscripten_set_socket_error_callback(void *userData, em_socket_error_callback callback)

Triggered by a WebSocket error.