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 Emscripten’s 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 can’t see, like another script tag on the HTML, you need to export them. To do so, add them to EXTRA_EXPORTED_RUNTIME_METHODS (for example, -s 'EXTRA_EXPORTED_RUNTIME_METHODS=["ccall", "cwrap"]' will export call and cwrap). Once exported, you can access them on the Module object (as Module.ccall, for example).
注解
If you try to use Module.ccall or another runtime method without exporting it, you will get an error. In a build with -s ASSERTIONS=1, 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, it’s useful to build with assertions.
目录
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
注解
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:
-s EXPORTED_FUNCTIONS="['_main', '_myfunc']"
(Note that we also export main - if we didn’t, the compiler would assume we don’t need it.) Exported functions can then be called as normal:
a_result = Module.ccall('myfunc', 'number', ['number'], [10])
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注解
64-bit integers become two 32-bit parameters, for the low and high bits (since 64-bit integers cannot be represented in JavaScript numbers).
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| 返回: | The result of the function call as a native JavaScript value (as in |
| Opts: | An optional options object. It can contain the following properties:
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注解
Async calls currently don’t 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 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
注解
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!
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 :
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).
-s EXPORTED_FUNCTIONS="['_main', '_myfunc']"
Exported functions can be called as normal:
my_func = Module.cwrap('myfunc', 'number', ['number'])
my_func(12)
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| 返回: | A JavaScript function that can be used for running the C function. |
setValue(ptr, value, type[, noSafe])¶Sets a value at a specific memory address at run-time.
注解
setValue() and getValue() only do aligned writes and reads.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.| 参数: |
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getValue(ptr, type[, noSafe])¶Gets a value at a specific memory address at run-time.
注解
setValue() and getValue() only do aligned writes and reads!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.| 参数: |
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| 返回: | The value stored at the specified memory address. |
UTF8ToString(ptr[, maxBytesToRead])¶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.
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| 返回: | A JavaScript |
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.
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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.
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| 返回: | A JavaScript |
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.
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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.
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| 返回: | A JavaScript |
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.
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AsciiToString(ptr)¶Converts an ASCII or Latin-1 encoded string to a JavaScript String object.
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| 返回: | A JavaScript |
| 返回类型: | String |
intArrayFromString(stringy, dontAddNull[, length])¶This converts a JavaScript string into a C-line array of numbers, 0-terminated.
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| 返回: | The array created from |
intArrayToString(array)¶This creates a JavaScript string from a zero-terminated C-line array of numbers.
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| 返回: | A |
writeStringToMemory(string, buffer, dontAddNull)¶Writes a JavaScript string to a specified address in the heap.
警告
This function is deprecated, you should call the function stringToUTF8 instead, which provides a secure bounded version of the same functionality instead.
// 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
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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.
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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);
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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.
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removeRunDependency(id)¶Removes a specified id from the list of run dependencies.
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stackTrace()¶Returns the current stack track.
注解
The stack trace is not available at least on IE10 and Safari 6.
| 返回: | The current stack trace, if available. |
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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.