preamble.js — Emscripten 1.20.0 documentation

# preamble.js

The JavaScript APIs in preamble.js provide programmatic access for interacting with the compiled C code, including: calling compiled C functions, accessing memory, converting pointers to JavaScript `Strings` and `Strings` to pointers (with different encodings/formats), and other convenience functions.

Note

All functions should be called though the `Module` object (for example: `Module.functionName`). At optimisation `-O2` (and higher) function names are minified by the closure compiler, and calling them directly will fail.

Table of Contents

- Calling compiled C functions from JavaScript
- Accessing memory
- Conversion functions — strings, pointers and arrays
- Run dependencies
- Stack trace
- Type accessors for Typed Arrays Mode 2

## Calling compiled C functions from JavaScript

`ccall`(ident, returnType, argTypes, args)

  Call a compiled C function from JavaScript.

  The function executes a compiled C function from JavaScript and returns the result. C++ name mangling means that “normal” C++ functions cannot be called; the function must either be defined in a .c file or be a C++ function defined with `extern "C"`.

      // Call C from JavaScript
      var result = Module.ccall('c_add', // name of C function
              'number', // return type
              ['number', 'number'], // argument types
              [10, 20]); // arguments

      // result is 30

  Note

  - `ccall` uses the C stack for temporary values. If you pass a string then it is only “alive” until the call is complete. If the code being called saves the pointer to be used later, it may point to invalid data.

  - If you need a string to live forever, you can create it, for example, using `_malloc` and `writeStringToMemory()`. However, you must later delete it manually!

  - LLVM optimizations can inline and remove functions, after which you will not be able to call them. Similarly, function names minified by the Closure Compiler are inaccessible. In either case, the solution is to add the functions to the `EXPORTED_FUNCTIONS` list when you invoke emcc :

    >     -s EXPORTED_FUNCTIONS='["_main", "_myfunc"]'

    > Exported functions can be called as normal:

    >     a_result = Module.ccall('myfunc', 'number', ['number'], 10)

  Arguments: | ident – The name of the C function to be called. returnType – The return type of the function. This will be one of the JavaScript types `number`, `string` or `array` (use `number` for any C pointer, and `array` for JavaScript arrays and typed arrays; note that arrays are 8-bit). argTypes – An array of the types of arguments for the function (if there are no arguments, this can be omitted). Types are as in `returnType`, except that `array` is not supported as there is no way for us to know the length of the array). args – An array of the arguments to the function, as native JavaScript values (as in `returnType`). Note that string arguments will be stored on the stack (the JavaScript string will become a C string on the stack).
  Returns: | The result of the function call as a native JavaScript value (as in `returnType`).

`cwrap`(ident, returnType, argTypes)

  Returns a native JavaScript wrapper for a C function.

  This is similar to `ccall()`, but returns a JavaScript function that can be reused as many time as needed. The C function can be defined in a C file, or be a C-compatible C++ function defined using `extern "C"` (to prevent name mangling).

      // Call C from JavaScript
      var c_javascript_add = Module.cwrap('c_add', // name of C function
              'number', // return type
              ['number', 'number']); // argument types

      // Call c_javascript_add normally
      console.log(c_javascript_add(10, 20)); // 30
      console.log(c_javascript_add(20, 30)); // 50

  Note

  - `cwrap` uses the C stack for temporary values. If you pass a string then it is only “alive” until the call is complete. If the code being called saves the pointer to be used later, it may point to invalid data.

  - If you need a string to live forever, you can create it, for example, using `_malloc` and `writeStringToMemory()`. However, you must later delete it manually!

  - LLVM optimizations can inline and remove functions, after which you will not be able to “wrap” them. Similarly, function names minified by the Closure Compiler are inaccessible. In either case, the solution is to add the functions to the `EXPORTED_FUNCTIONS` list when you invoke emcc :

    >     -s EXPORTED_FUNCTIONS='["_main", "_myfunc"]'

    > Exported functions can be called as normal:

    >     my_func = Module.cwrap('myfunc', 'number', ['number'])
    >     my_func(12)

  Arguments: | ident – The name of the C function to be called. returnType – The return type of the function. This will be one of the JavaScript types `number`, `string` or `array` (use `number` for any C pointer, and `array` for JavaScript arrays and typed arrays; note that arrays are 8-bit). argTypes – An array of the types of arguments for the function (if there are no arguments, this can be omitted). Types are as in `returnType`, except that `array` is not supported as there is no way for us to know the length of the array).
  Returns: | A JavaScript function that can be used for running the C function.

## Accessing memory

`setValue`(ptr, value, type[, noSafe])

  Sets a value at a specific memory address at run-time.

  Note

  - `setValue()` and `getValue()` only do aligned writes and reads.
  - The `type` is an LLVM IR type (one of `i8`, `i16`, `i32`, `i64`, `float`, `double`, or a pointer type like `i8*` or just `*`), not JavaScript types as used in `ccall()` or `cwrap()`. This is a lower-level operation, and we do need to care what specific type is being used.

  Arguments: | ptr – A pointer (number) representing the memory address. value – The value to be stored type – An LLVM IR type as a string (see “note” above). noSafe (bool) – Developers should ignore this variable. It is only used in `SAFE_HEAP` compilation mode, where it can help avoid infinite recursion in some specialist use cases.

`getValue`(ptr, type[, noSafe])

  Gets a value at a specific memory address at run-time.

  Note

  - `setValue()` and `getValue()` only do aligned writes and reads!
  - The `type` is an LLVM IR type (one of `i8`, `i16`, `i32`, `i64`, `float`, `double`, or a pointer type like `i8*` or just `*`), not JavaScript types as used in `ccall()` or `cwrap()`. This is a lower-level operation, and we do need to care what specific type is being used.

  Arguments: | ptr – A pointer (number) representing the memory address. type – An LLVM IR type as a string (see “note” above). noSafe (bool) – Developers should ignore this variable. It is on used in `SAFE_HEAP` compilation mode, where it can be avoid infinite recursion in some specialist use cases.
  Returns: | The value stored at the specified memory address.

## Conversion functions — strings, pointers and arrays

`Pointer_stringify`(ptr[, length])

  Returns a JavaScript String from a pointer, for use in compiled code.

  Arguments: | ptr – The pointer to be converted to a `String`. length – The length of the data in the pointer (optional).
  Returns: | A JavaScript `String` containing the data from `ptr`.
  Return type: | String

`UTF16ToString`(ptr)

  Given a pointer `ptr` to a null-terminated UTF16LE-encoded string in the Emscripten HEAP, returns a copy of that string as a Javascript `String` object.

  Arguments: | ptr – A pointer to a null-terminated UTF16LE-encoded string in the Emscripten HEAP.
  Returns: | A Javascript `String` object

`stringToUTF16`(str, outPtr)

  Copies the given JavaScript `String` object `str` to the Emscripten HEAP at address `outPtr`, null-terminated and encoded in UTF16LE form.

  The copy will require at most `(str.length*2+1)*2` bytes of space in the HEAP.

  Arguments: | str (String) – A JavaScript `String` object. outPtr – Pointer to data copied from `str`, encoded in UTF16LE format and null-terminated.

`UTF32ToString`(ptr)

  Given a pointer `ptr` to a null-terminated UTF32LE-encoded string in the Emscripten HEAP, returns a copy of that string as a JavaScript `String` object.

  Arguments: | ptr – A pointer to a null-terminated UTF32LE-encoded string in the Emscripten HEAP.
  Returns: | A Javascript `String` object.

`stringToUTF32`(str, outPtr)

  Copies the given JavaScript `String` object `str` to the Emscripten HEAP at address `outPtr`, null-terminated and encoded in UTF32LE form.

  The copy will require at most `(str.length+1)*4` bytes of space in the HEAP, but can use less, since `str.length` does not return the number of characters in the string, but the number of UTF-16 code units in the string.

  Arguments: | str (String) – A JavaScript `String` object. outPtr – Pointer to data copied from `str`, encoded in encoded in UTF32LE format and null-terminated.

`intArrayFromString`(stringy, dontAddNull[, length])

  This converts a JavaScript string into a C-line array of numbers, 0-terminated.

  Arguments: | stringy (String) – The string to be converted. dontAddNull (bool) – If `true`, the new array is not zero-terminated. length – The length of the array (optional).
  Returns: | The array created from `stringy`.

`intArrayToString`(array)

  This creates a JavaScript string from a zero-terminated C-line array of numbers.

  Arguments: | array – The array to convert.
  Returns: | A `String`, containing the content of `array`.

`writeStringToMemory`(string, buffer, dontAddNull)

  Writes a JavaScript string to a specified address in the heap.

      // Allocate space for string and extra '0' at the end
      var buffer = Module._malloc(myString.length+1);

      // Write the string to memory
      Module.writeStringToMemory(myString, buffer);

      // We can now send buffer into a C function, it is just a normal char* pointer

  Arguments: | string (String) – The string to write into memory. buffer (Number) – The address (number) where `string` is to be written. dontAddNull (bool) – If `true`, the new array is not zero-terminated.

`writeArrayToMemory`(array, buffer)

  Writes an array to a specified address in the heap. Note that memory should to be allocated for the array before it is written.

  Arguments: | array – The array to write to memory. buffer (Number) – The address (number) where `array` is to be written.

`writeAsciiToMemory`(str, buffer, dontAddNull)

  Writes an ASCII string to a specified address in the heap. Note that memory should to be allocated for the string before it is written.

  The string is assumed to only have characters in the ASCII character set. If ASSERTIONS are enabled and this is not the case, it will fail.

      // Allocate space for string
      var buffer = Module._malloc(myString.length);

      // Write the string to memory
      Module.writeStringToMemory(myString, buffer);

  Arguments: | string – The string to write into memory. buffer – The address where `string` is to be written. dontAddNull (bool) – If `true`, the new string is not zero-terminated.

## Run dependencies

Note that generally run dependencies are managed by the file packager and other parts of the system. It is rare for developers to use this API directly.

`addRunDependency`(id)

  Adds an `id` to the list of run dependencies.

  This adds a run dependency and increments the run dependency counter.

  Arguments: | id (String) – An arbitrary id representing the operation.

`removeRunDependency`(id)

  Removes a specified `id` from the list of run dependencies.

  Arguments: | id (String) – The identifier for the specific dependency to be removed (added with `addRunDependency()`)

## Stack trace

`stackTrace`()

  Returns the current stack track.

  > Note

  > The stack trace is not available at least on IE10 and Safari 6.

  Returns: | The current stack trace, if available.

## Type accessors for Typed Arrays Mode 2

When using Typed Arrays Mode 2 a typed array buffer (`ArrayBuffer`) is used to represent memory, with different views into it giving access to the different types. The views for accessing different types of memory are listed below.

`HEAP8`

  View for 8-bit signed memory.

`HEAP16`

  View for 16-bit signed memory.

`HEAP32`

  View for 32-bit signed memory.

`HEAPU8`

  View for 32-bit unsigned memory.

`HEAPU8`

  View for 32-bit unsigned memory.

`HEAPU16`

  View for 16-bit unsigned memory.

`HEAPU32`

  View for 32-bit unsigned memory.

`HEAPF32`

  View for 32-bit float memory.

`HEAPF64`

  View for 64-bit float memory.