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.

We call this preamble.js because Emscriptens output JS, at a high level, contains the preamble (from src/preamble.js), then the compiled code, then the postamble. (In slightly more detail, the preamble contains utility functions and setup, while the postamble connects things and handles running the application.)

The preamble code is included in the output JS, which is then optimized all together by the compiler, together with any --pre-js and --post-js files you added and code from any JavaScript libraries (--js-library). That means that you can call methods from the preamble directly, and the compiler will see that you need them, and not remove them as being unused.

If you want to call preamble methods from somewhere the compiler cant see, like another script tag on the HTML, you need to export them. To do so, add them to EXPORTED_RUNTIME_METHODS (for example, -sEXPORTED_RUNTIME_METHODS=ccall,cwrap will export ccall and cwrap). Once exported, you can access them on the Module object (as Module.ccall, for example).

Note

If you try to use Module.ccall or another runtime method without exporting it, you will get an error. In a build with -sASSERTIONS, the compiler emits code to show you a useful error message, which will explain that you need to export it. In general, if you see something odd, its useful to build with assertions.

Table of Contents

Calling compiled C functions from JavaScript
Accessing memory
Conversion functions strings, pointers and arrays
Run dependencies
Stack trace
Type accessors for the memory model

Calling compiled C functions from JavaScript

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

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".

returnType and argTypes let you specify the types of parameters and the return value. The possible types are "number", "string", "array", or "boolean", which correspond to the appropriate JavaScript types. Use "number" for any numeric type or C pointer, string for C char* that represent strings, "boolean" for a boolean type, "array" for JavaScript arrays and typed arrays, containing 8-bit integer data - that is, the data is written into a C array of 8-bit integers; and in particular if you provide a typed array here, it must be a Uint8Array or Int8Array. If you want to receive an array of another type of data, you can manually allocate memory and write to it, then provide a pointer here (as a "number", as pointers are just numbers).

// 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 stringToUTF8(). 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:
```
-sEXPORTED_FUNCTIONS=_main,_myfunc"
```
(Note that we also export main - if we didnt, the compiler would assume we dont need it.) Exported functions can then 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. Note that array is not supported as there is no way for us to know the length of the array. For a void function this can be null (note: the JavaScript null value, not a string containing the word null).

Note

64-bit integers become two 32-bit parameters, for the low and high bits (since 64-bit integers cannot be represented in JavaScript numbers).

Arguments:

argTypes An array of the types of arguments for the function (if there are no arguments, this can be omitted).
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) or, if the async option is set, a JavaScript Promise of the result.

Opts:

An optional options object. It can contain the following properties:

async: If true, implies that the ccall will perform an async operation. This assumes you are build with asyncify support.

Note

Async calls currently dont support promise error handling.

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 times 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 stringToUTF8(). However, you must later delete it manually!
To wrap a function it must be exported by adding it to the EXPORTED_FUNCTIONS list when you invoke emcc. If a function is not exported, optimizations may remove it, and cwrap will not be able to find it at runtime. (In builds with ASSERTIONS enabled, cwrap will show an error in such a situation; in release builds without assertions, trying to wrap a non-existent function will error, either by returning undefined or by returning a function that will error when actually called, depending on how cwrap optimizes.)
cwrap does not actually call compiled code (only calling the wrapper it returns does that). That means that it is safe to call cwrap early, before the runtime is fully initialized (but calling the returned wrapped function must wait for the runtime, of course, like calling compiled code in general).
```
-sEXPORTED_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 can be "number", "string" or "array", which correspond to the appropriate JavaScript types (use "number" for any C pointer, and "array" for JavaScript arrays and typed arrays; note that arrays are 8-bit), or for a void function it can be null (note: the JavaScript null value, not a string containing the word null).
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).
opts An optional options object, see ccall().

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 only used in SAFE_HEAP compilation mode, where it can help avoid infinite recursion in some specialist use cases.

Returns:

The value stored at the specified memory address.

Conversion functions strings, pointers and arrays

UTF8ToString(ptr[, maxBytesToRead][, ignoreNul])

Given a pointer ptr to a null-terminated UTF8-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 UTF8-encoded string in the Emscripten HEAP.
maxBytesToRead An optional length that specifies the maximum number of bytes to read. You can omit this parameter to scan the string until the first 0 byte. If maxBytesToRead is passed, and the string at [ptr, ptr+maxBytesToReadr) contains a null byte in the middle, then the string will cut short at that byte index. N.B. mixing frequent uses of this function with and without maxBytesToRead may throw JS JIT optimizations off, so it is worth to consider consistently using one style or the other.
ignoreNul If true, the function will not stop on a NUL character, but will read the entire string up to maxBytesToRead.

Returns:

A JavaScript String object

stringToUTF8(str, outPtr, maxBytesToWrite)

Copies the given JavaScript String object str to the Emscripten HEAP at address outPtr, null-terminated and encoded in UTF8 form.

The copy will require at most str.length*4+1 bytes of space in the HEAP. You can use the function lengthBytesUTF8() to compute the exact amount of bytes (excluding the null terminator) needed to encode the string.

Arguments:

str (String()) A JavaScript String object.
outPtr Pointer to data copied from str, encoded in UTF8 format and null-terminated.
maxBytesToWrite A limit on the number of bytes that this function can at most write out. If the string is longer than this, the output is truncated. The outputted string will always be null terminated, even if truncation occurred, as long as maxBytesToWrite > 0.

UTF16ToString(ptr[, maxBytesToRead][, ignoreNul])

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.
maxBytesToRead An optional length that specifies the maximum number of bytes to read. You can omit this parameter to scan the string until the first 0 byte. If maxBytesToRead is passed, and the string at [ptr, ptr+maxBytesToReadr) contains a null byte in the middle, then the string will cut short at that byte index. N.B. mixing frequent uses of this function with and without maxBytesToRead may throw JS JIT optimizations off, so it is worth to consider consistently using one style or the other.
ignoreNul If true, the function will not stop on a NUL character, but will read the entire string up to maxBytesToRead.

Returns:

A JavaScript String object

stringToUTF16(str, outPtr, maxBytesToWrite)

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 exactly (str.length+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.
maxBytesToWrite A limit on the number of bytes that this function can at most write out. If the string is longer than this, the output is truncated. The outputted string will always be null terminated, even if truncation occurred, as long as maxBytesToWrite >= 2 so that there is space for the null terminator.

UTF32ToString(ptr[, maxBytesToRead][, ignoreNul])

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.
maxBytesToRead An optional length that specifies the maximum number of bytes to read. You can omit this parameter to scan the string until the first 0 byte. If maxBytesToRead is passed, and the string at [ptr, ptr+maxBytesToReadr) contains a null byte in the middle, then the string will cut short at that byte index. N.B. mixing frequent uses of this function with and without maxBytesToRead may throw JS JIT optimizations off, so it is worth to consider consistently using one style or the other.
ignoreNul If true, the function will not stop on a NUL character, but will read the entire string up to maxBytesToRead.

Returns:

A JavaScript String object.

stringToUTF32(str, outPtr, maxBytesToWrite)

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. You can use the function lengthBytesUTF32() to compute the exact amount of bytes (excluding the null terminator) needed to encode 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.
maxBytesToWrite A limit on the number of bytes that this function can at most write out. If the string is longer than this, the output is truncated. The outputted string will always be null terminated, even if truncation occurred, as long as maxBytesToWrite >= 4 so that there is space for the null terminator.

AsciiToString(ptr)

Converts an ASCII or Latin-1 encoded string to a JavaScript String object.

Arguments:

ptr The pointer to be converted to a String.

Returns:

A JavaScript String containing the data from ptr.

Return type:

String

intArrayFromString(stringy, dontAddNull[, length])

This converts a JavaScript string into a C-like 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-like array of numbers.

Arguments:

array The array to convert.

Returns:

A String, containing the content of array.

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.

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 the memory model

The Emscripten memory representation uses a typed array buffer (ArrayBuffer) 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 8-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.

On this page

preamble.js

Calling compiled C functions from JavaScript

Accessing memory

Conversion functions strings, pointers and arrays

Run dependencies

Stack trace

Type accessors for the memory model