v8+: Node.js addon C++ to C boundary layer

This layer offers a way to write at least simple Node.js addons in C without all the horrible C++ goop you'd otherwise be expected to use. That goop still exists, but you don't have to write it. More importantly, you can write your module in a sane programming environment, avoiding the confusing and error-prone C++ semantics.

Usage

Unlike most Node.js modules, v8+ does nothing by itself. It is intended to be used as a build-time dependency of your native addon, providing you with an alternate programming environment.

For full docs, read the source code.

Node.js Support

v8+ works with, and has been tested to some extent with, Node.js 0.6.18, 0.8.1, 0.8.26, 0.10.24, and 0.11.10. It most likely works with other micro versions in the 0.6 and 0.8 series as well; if you are using 0.10, you will need 0.10.24 or later so that you have headers to build against. Note that this does not mean you can necessarily expect an addon built against a particular minor release of Node.js to work with any other minor release of Node.js.

Node 0.11.10 and later are also supported, and contain a new module API that v8plus can leverage to provide an entirely new model for building and using C modules.

Building and Installing

The v8+ source code is compiled into your module directly along with your code. There is no separate v8+ library or node module, so the v8+ source, tools, and makefiles are required to be present at the time your module is built. They are not required at runtime.

Normally, your addon module will depend on the v8plus package and install it using npm. The v8+ makefiles are set up to accommodate the installation of v8+ anywhere node(1) would be able to find it using require() if it were a normal JavaScript module, so simply including it as a dependency in your package.json will work correctly. In addition, you will need to create a (normally trivial) makefile for your module that includes the makefiles distributed as part of v8+. Once you have done so, it is sufficient to run gmake to generate the native loadable module used by Node.js.

The overall outline for creating a v8+ module looks something like this:

1. Write the C code that does whatever your module does. Be sure to #include "v8plus_glue.h". Do not include any other v8+ headers.

2. Create an appropriate package.json file. See below for details.

3. Create a skeleton makefile. See below for details.

You should not (and need not) modify either of the delivered makefiles; override the definitions in Makefile.v8plus.defs in your makefile as appropriate.

Packaging Considerations

There are two essential properties your package.json must contain in order to use v8+ with npm:

1. A dependency on v8plus.

2. An appropriate script entry for building your module. It is strongly recommended that you use something like the following:

    "postinstall": "gmake $(eval echo ${MAKE_OVERRIDES})"
   

This will allow someone building your module to set make variables by adding them to the MAKE_OVERRIDES environment variable; e.g.,

    $ MAKE_OVERRIDES="CTFCONVERT=/bin/true CTFMERGE=/bin/true" npm install

Tying into the Makefiles

The makefiles shipped with v8+ do the great majority of the heavy lifting for you. A minimally functional makefile for your addon must contain four things:

1. Variable definitions for V8PLUS and PREFIX_NODE. Alternately, you may choose to provide these on the command line or via the environment. It is recommended that these assignments be made exactly as follows, which will cause the addon to be built against the node that is found first in your path:

    PREFIX_NODE := $(shell dirname `bash -c 'hash node; hash -t node'`)/..
    V8PLUS :=      $(shell $(PREFIX_NODE)/bin/node -e 'require("v8plus");')
   

   Note that the mechanism for finding node will not work correctly if yours is a symlink. This invocation of node(1) uses a v8+ mechanism to locate v8+ sources anywhere that node(1) can find them and should not be modified unless you want to test an alternate v8+.

2. The exact line:

    include $(V8PLUS)/Makefile.v8plus.defs
   

3. Variable assignments specific to your module. In particular, you must define SRCS and MODULE Note that ERRNO_JSON is no longer required nor used in v8plus 0.3 and later. Additional customisation is optional.

4. The exact line:

    include $(V8PLUS)/Makefile.v8plus.targ
   

Additional arbitrary customisation is possible using standard makefile syntax; most things that are useful to change already have variables defined in Makefile.v8plus.defs whose values you may append to or override. For example, you may cause additional system libraries to be linked in by appending -lname to the LIBS variable. By default, the makefiles assume that your sources are located in the src subdirectory of your module, and that you want the sole output of the build process to be called $(MODULE).node and located in the lib subdirectory. This can be changed by overriding the MODULE_DIR variable.

A simple example makefile may be found in the examples/ subdirectory, and additional examples may be found in existing consumers; see Consumers below. The GNU people also provide a good manual for make if you get really stuck; see http://www.gnu.org/software/make/manual/make.html. In general, writing the necessary makefile fragment is expected to be as easy as or easier than the equivalent task using node-waf or node-gyp, so if you're finding it unexpectedly difficult or complicated there's probably an easier way.

The makefiles follow GNU make syntax; other makes may not work but patches that correct this are generally welcome (in particular, Sun make and GNU make have different and incompatible ways to set a variable from the output of a shell command, and there is no way I know to accommodate both).

Binary Interface

By default, the resulting object is linked with the -zdefs option, which will cause the build to fail if any unresolved symbols remain. In order to accommodate this, a mapfile specifying the available global symbols in your node binary is automatically generated as part of the build process. This makes it much easier to debug missing libraries; otherwise, a module with unresolved symbols will fail to load at runtime with no useful explanation. Mapfile generation probably works only on illumos-derived systems. Patches that add support for other linkers are welcome.

Your module will have all symbols (other than init, which is used directly by Node.js) reduced to local visibility, which is strongly recommended. If for some reason you want your module's symbols to be visible to Node.js or to other modules, you will have to modify the script that generates the mapfile. See the $(MAPFILE) target in Makefile.v8plus.targ.

API

Your module is an object factory that instantiates and returns native objects, to which a fixed set of methods is attached as properties. The constructor, destructor, and methods all correspond 1-1 with C functions. In addition, you may create additional class methods associated with the native module itself, each of which will also have a 1-1 relationship to a set of C functions.

This functionality is generally sufficient to interface with the system in useful ways, but it is by no means exhaustive. Architectural limitations are noted throughout the documentation.

Subsequent sections describe the API in greater detail, along with most of the C functions that v8+ provides. Some utility functions may not be listed here; see v8plus_glue.h for additional commentary and functions that are available to you.

Constructors, Methods, and Functions

The interface between your module and v8+ consists of a handful of objects with fixed types and names. These are:

const v8plus_c_ctor_f v8plus_ctor = my_ctor;
const v8plus_c_dtor_f v8plus_dtor = my_dtor;
const char *v8plus_js_factory_name = "_new";
const char *v8plus_js_class_name = "MyObjectBinding";
const v8plus_method_descr_t v8plus_methods[] = {
        {
                md_name: "_my_method",
                md_c_func: my_method
        },
	...
};
const uint_t v8plus_method_count =
    sizeof (v8plus_methods) / sizeof (v8plus_methods[0]);

const v8plus_static_descr_t v8plus_static_methods[] = {
        {
                sd_name: "_my_function",
                sd_c_func: my_function
        },
	...
};
const uint_t v8plus_static_method_count =
    sizeof (v8plus_static_methods) / sizeof (v8plus_static_methods[0]);

All of these must be present even if they have zero length or are NULL. The prototypes and semantics of each function type are as follows:

nvlist_t *v8plus_c_ctor_f(const nvlist_t *ap, void **opp)

The constructor is responsible for creating the C object corresponding to the native JavaScript object being created. It is not a true constructor in that you are actually an object factory; the C++ function associated with the JavaScript constructor is called for you. Your encoded arguments are in ap. Allocate and populate a C object, stuff it into *opp, and return v8plus_void(). If you need to throw an exception you can do so by calling v8plus_throw_exception() or any of its wrappers. As of v8plus 0.3, you may no longer return an nvlist with an err member to throw an exception, and the _v8plus_errno global variable is no longer available.

void v8plus_c_dtor_f(void *op)

Free the C object op and anything else associated with it. Your object is going away. This function may be empty if the constructor did not allocate any memory (i.e., op is not a pointer to dynamically allocated memory).

nvlist_t *v8plus_c_method_f(void *op, const nvlist_t *ap)

When the JavaScript method is called in the context of your object, the corresponding C function is invoked. op is the C object associated with the JavaScript object, and ap is the encoded list of arguments to the function. Return an encoded object with a res member, or use one of the error/exception patterns.

nvlist_t *v8plus_c_static_method_f(const nvlist_t *ap)

In addition to methods on the native objects returned by your constructor, you can also provide a set of functions on the native binding object itself. This may be useful for providing bindings to libraries for which no object representation makes sense, or that have functions that operate outside the context of any particular object. Your arguments are once again encoded in ap, and your return values are an object containing res or an error.

Argument Handling

When JavaScript objects cross the boundary from C++ to C, they are converted from v8 C++ objects into C nvlists. This means that they are effectively passed by value, unlike in JavaScript or in native addons written in C++. The arguments to the JavaScript function are treated as an array and marshalled into a single nvlist whose properties are named "0", "1", and so on. Each such property is encoded as follows:

• numbers and Number objects (regardless of size): double
• strings and String objects: UTF-8 encoded C string
• booleans and Boolean objects: boolean_value
• undefined: boolean
• null: byte, value 0
• Objects, including Arrays: nvlist with own properties as members and the member ".__v8plus_type" set to the object's JavaScript type name. Note that the member name itself begins with a . to reduce the likelihood of a collision with an actual JavaScript member name.
• JavaScript Functions are passed in a format suitable for use with nvlist_lookup_jsfunc() and v8plus_args() with the V8PLUS_TYPE_JSFUNC token. This type is restricted; see below.

Because JavaScript arrays may be sparse, we cannot use the libnvpair array types. Consider them reserved for internal use. JavaScript Arrays are represented as they really are in JavaScript: objects with properties whose names happen to be integers.

Other data types cannot be represented and will result in a TypeError being thrown. If your object has methods that need other argument types, you cannot use v8+.

Side effects within the VM, including modification of the arguments, are not supported. If you need them, you cannot use v8+.

While the standard libnvpair functions may be used to inspect the arguments to a method or function, v8+ also provides the v8plus_args() and v8plus_typeof() convenience functions, which simplify checking the types and obtaining the values of arguments.

int v8plus_args(const nvlist_t *lp, uint_t flags, v8plus_type_t t, ...)

This function checks lp for the exact sequence of arguments specified by the list of types provided in the parameter list. If V8PLUS_ARG_F_NOEXTRA is set in flags, the list of arguments must match exactly, with no additional arguments. The parameter list must be terminated by V8PLUS_TYPE_NONE.

Following flags is a list of argument data types and, for most data types, pointers to locations at which the native C value of that argument should be stored. The following JavaScript argument data types are supported; for each, the parameter immediately following the data type parameter must be of the indicated C type. This parameter may be NULL, in which case the value will not be stored anywhere.

• V8PLUS_TYPE_NONE: used to terminate the parameter list only
• V8PLUS_TYPE_STRING: char **
• V8PLUS_TYPE_NUMBER: double *
• V8PLUS_TYPE_BOOLEAN: boolean_t *
• V8PLUS_TYPE_JSFUNC: v8plus_jsfunc_t *
• V8PLUS_TYPE_OBJECT: nvlist_t **
• V8PLUS_TYPE_NULL: no parameter
• V8PLUS_TYPE_UNDEFINED: no parameter
• V8PLUS_TYPE_INVALID: data_type_t (see below)
• V8PLUS_TYPE_ANY: nvpair_t **
• V8PLUS_TYPE_STRNUMBER64: uint64_t *
• V8PLUS_TYPE_INL_OBJECT: illegal

In most cases, the behaviour is straightforward: the value pointer parameter provides a location into which the C value of the specified argument should be stored. If the entire argument list matches the template, each argument's C value is stored in its respective location. If not, no values are stored, in the return value locations, an exception is set pending, and -1 is returned.

Three data types warrant further explanation: an argument of type V8PLUS_TYPE_INVALID is any argument that may or may not match one of the acceptable types. Its nvpair data type tag is stored and the argument treated as matching. The value is ignored. V8PLUS_TYPE_STRNUMBER64 is used with strings that should be interpreted as 64-bit unsigned integers. If the argument is not a string, or is not parseable as a 64-bit unsigned integer, the argument will be treated as a mismatch. Finally, V8PLUS_TYPE_INL_OBJECT is not supported with v8plus_args(); JavaScript objects in the argument list must be individually inspected as nvlists.

A simple example:

double_t d;
boolean_t b;
char *s;
v8plus_jsfunc_t f;

/*
 * This function requires exactly four arguments: a number, a
 * boolean, a string, and a callback function.  It is not acceptable
 * to pass superfluous arguments to it.
 */
if (v8plus_args(ap, V8PLUS_ARG_F_NOEXTRA,
    V8PLUS_TYPE_NUMBER, &d,
    V8PLUS_TYPE_BOOLEAN, &b,
    V8PLUS_TYPE_STRING, &s,
    V8PLUS_TYPE_JSFUNC, &f,
    V8PLUS_TYPE_NONE) != 0)
	return (NULL);

v8plus_type_t v8plus_typeof(const nvpair_t *pp)

This function simply returns the v8+ data type corresponding to the name/value pair pp. If the value's type does not match the v8+ encoding rules, V8PLUS_TYPE_INVALID is returned. This function cannot fail and does not set pending any exceptions.

Returning Values

Similarly, when returning data across the boundary from C to C++, a pointer to an nvlist must be returned. This object will be decoded in the same manner as described above and returned to the JavaScript caller of your method. Note that booleans, strings, and numbers will be encoded as their primitive types, not objects. If you need to return something containing these object types, you cannot use v8+. Other data types cannot be represented. If you need to return them, you cannot use v8+.

The nvlist being returned must have a single member named: "res", an nvpair containing the result of the call to be returned. The use of "err" to decorate an exception is no longer supported as of v8plus 0.3. You may return a value of any decodable type.

For convenience, you may return v8plus_void() instead of an nvlist, which indicates successful execution of a function that returns nothing.

In addition, the v8plus_obj() routine is available for instantiating JavaScript objects to return.

nvlist_t *v8plus_void(void)

This function clears any pending exception and returns NULL. This is used to indicate to internal v8+ code that the method or function should not return a value.

nvlist_t *v8plus_obj(v8plus_type_t t, ...)

This function creates and populates an nvlist conforming to the encoding rules of v8+ for returning a value or creating an exception. It can be used to create anything from a single encoded value to arbitrarily nested objects. It is essentially the inverse of v8plus_args() above, with a few differences:

• It cannot be used to encode invalid or illegal data types.
• It accepts native C values, not pointers to them.
• Each value must be named.
• It can be used to encode nested objects inline using V8PLUS_TYPE_INL_OBJECT, followed by type, name, value triples, terminated with V8PLUS_TYPE_NONE.

This function can fail due to out-of-memory conditions, invalid or unsupported data types, or, most commonly, programmer error in casting the arguments to the correct type. It is extremely important that data values, particularly integers, be cast to the appropriate type (double) when passed into this function!

Following is a list of types and the C data types corresponding to their values:

• V8PLUS_TYPE_NONE: used to terminate the parameter list only
• V8PLUS_TYPE_STRING: char *
• V8PLUS_TYPE_NUMBER: double
• V8PLUS_TYPE_BOOLEAN: boolean_t
• V8PLUS_TYPE_JSFUNC: v8plus_jsfunc_t
• V8PLUS_TYPE_OBJECT: nvlist_t *
• V8PLUS_TYPE_NULL: no parameter
• V8PLUS_TYPE_UNDEFINED: no parameter
• V8PLUS_TYPE_ANY: nvpair_t *
• V8PLUS_TYPE_STRNUMBER64: uint64_t
• V8PLUS_TYPE_INL_OBJECT: NONE-terminated type/value list

A simple example, in which we return a JavaScript object with two members, one number and one embedded object with a 64-bit integer property. Note that if this function fails, we will return NULL with an exception pending.

int x;
const char *s;

...
return (v8plus_obj(
    V8PLUS_TYPE_INL_OBJECT, "res",
	V8PLUS_TYPE_NUMBER, "value", (double)x,
	V8PLUS_TYPE_INL_OBJECT, "detail",
	    V8PLUS_TYPE_STRNUMBER64, "value64", s,
	    V8PLUS_TYPE_NONE,
	V8PLUS_TYPE_NONE,
    V8PLUS_TYPE_NONE));

The JSON representation of this object would be:

{
	"res": {
		"value": <x>,
		"detail": {
			"value64": "<s>"
		}
	}
}

v8plus_obj_setprops(nvlist_t *lp, v8plus_type_t t, ...)

You can also add or replace the values of properties in an existing nvlist, whether created using nvlist_alloc() directly or via v8plus_obj(). The effect is very similar to nvlist_merge(), where the second list is created on the fly from your argument list. The interpretation of the argument list is the same as for v8plus_obj(), and the two functions are implemented using the same logic.

Exceptions and Errors

Prior to v8plus 0.3.0, the v8plus_errno_t enumerated type was controlled by a consumer-supplied JSON file, allowing the consumer to specify the set of error values. In v8plus 0.3.0 and newer, this type is fixed and contains only a small set of basic errors that can be used with the v8plus_error() routine for compatibility with previous versions. In v8plus 0.3.0 and later, consumers should explicitly throw exceptions instead.

In v8plus 0.3.0 and later, the _v8plus_errno global no longer exists. If your code examined this variable, there are two alternatives:

• If you were comparing against V8PLUSERR_NOERROR, instead use `v8plus_exception_pending() to determine whether a previously invoked function failed.
• If you wish to inspect the previous error state, v8plus_pending_exception() will provide an nvlist-encoded representation of the pending exception object.

A survey of consumers indicated that custom error codes, _v8plus_errno, and nontrivial uses of _v8plus_error() did not exist in consumers; therefore this functionality has been removed.

All exceptions are generated and made pending by v8plus_throw_exception() or its wrappers, identified below. Only one exception may be pending at one time, and a call to v8plus_throw_exception() or its wrappers with an exception already pending has no effect. Functions are provided for clearing any pending exceptions, testing for the existence of a pending exception, and obtaining (to inspect or modify) the current pending exception; see API details below.

A pending exception will be ignored and will not be thrown if any of the following occurs prior to your C function (method, static method, or constructor) returning:

• v8plus_clear_exception() is invoked, or
• You return v8plus_void(), or
• Your method or static method routine returns non-NULL, or
• Your constructor sets its object pointer to a non-NULL value

It is programmer error for a constructor to set its object pointer to NULL (or to not set it at all) and return without an exception pending.

Because a common source of exceptions is out-of-memory conditions, the space used by exceptions is obtained statically and is limited in size. This allows for exceptions to be thrown into V8 reliably, with enough information to debug the original failure even if that failure was, or was caused by, an out of memory condition. V8 may or may not provide a similar mechanism for ensuring that the C++ representation of exceptions is reliable.

Exceptions may be raised in any context; however, raising an exception in a context other than the V8 event thread will not by itself cause any JavaScript exception to be thrown; it is the consumer's responsibility to provide for an exception to be set pending in the event thread if it is to be made visible from JavaScript. Functions used to inspect or alter the state of the pending exception, if any, also work in any context.

nvlist_t *v8plus_error(v8plus_errno_t e, const char *fmt, ...)

This function generates and makes pending a default exception based on the value of e and a message based on the formatted string fmt using the argument list that follows. The format string and arguments are interpreted as by vsnprintf(3c). NULL is returned, suitable for returning directly from a C function that provides a method if no exception decoration is required.

If fmt is NULL, a generic default message is used.

This function is a wrapper for v8plus_throw_exception().

nvlist_t *v8plus_nverr(int err, const char *propname)

This function generates and makes pending an exception based on the system error code err and sets the error message to a non-localised explanation of the problem. The string propname, if non-NULL, is indicated in the message as the name of the nvlist property being manipulated when the error occurred. NULL is returned.

This function is a wrapper for v8plus_throw_exception().

nvlist_t *v8plus_syserr(int err, const char *fmt, ...)

Analogous to v8plus_error(), this function instead generates and sets pending an exception derived from the system error code err. Not all error codes can be mapped; those that are not known are mapped onto an unknown error string. The generated exception will contain additional properties similar to those provided by node.js's ErrnoException() routine. See also v8plus_throw_errno_exception().

This function is a wrapper for v8plus_throw_exception().

nvlist_t *v8plus_throw_exception(const char *type, const char *msg, v8plus_type_t t, ...)

Generate and set pending an exception whose JavaScript type is type, with message msg (or the empty string, if msg is NULL), and optionally additional properties as specified by a series type, name, value triples. These triples have the same syntax as the arguments to v8plus_obj() and are likewise terminated by V8PLUS_TYPE_NONE.

The generated JavaScript exception will be thrown upon return from the current constructor or method, unless v8plus_clear_exception() is invoked first, or v8plus_void() is returned. The exception may be obtained via v8plus_pending_exception() and its presence or absence tested via v8plus_exception_pending().

nvlist_t *v8plus_throw_errno_exception(int err, const char *syscall, const char *msg, const char *path, v8plus_type_t t, ...)

Generate and set pending an exception with type Error and message msg (if msg is NULL, the message will be automatically generated from your system's strerror() value for this error number). The exception will further be decorated with properties indicating the relevant system call and path, if the syscall and path arguments, respectively, are non-NULL, and any additional properties specified as in v8plus_throw_exception().

This function is a wrapper for v8plus_throw_exception().

boolean_t v8plus_exception_pending(void)

This function returns B_TRUE if and only if an exception is pending.

nvlist_t *v8plus_pending_exception(void)

This function returns a pointer to the nvlist-encoded pending exception, if any exists; NULL, otherwise. This object may be inspected and properties added to or removed from it.

void v8plus_clear_exception(void)

Clear the pending exception, if any.

void v8plus_rethrow_pending_exception(void)

Immediately throw the pending exception. This is appropriate only in the context of an asynchronous callback, in which there is no return value; in all other cases, return the exception as the err member of the function's return value. Note that this is slightly different from node::FatalException() in that it is still possible for a JavaScript caller to catch and handle it. If it is absolutely essential that the process terminate immediately, use v8plus_panic() instead.

The main purpose of this facility is to allow re-throwing an exception generated by a JavaScript callback invoked from an asynchronous completion routine. The completion routine has no way to return a value, so this is the only way to propagate the exception out of the native completion routine.

This function may be called only on the main event loop thread.

void v8plus_panic(const char *fmt, ...) __NORETURN

This function indicates a fatal runtime error. The format string fmt and subsequent arguments are interpreted as by vsnprintf(3c) and written to standard error, which is then flushed. abort(3c) or similar is then invoked to terminate the Node.js process in which the addon is running. Use of this function should be limited to those circumstances in which an internal inconsistency has been detected that renders further progress hazardous to user data or impossible.

Asynchrony

There are two main types of asynchrony supported by v8+. The first is the deferred work model (using uv_queue_work() or the deprecated eio_custom() mechanisms) frequently written about and demonstrated by various practitioners around the world. In this model, your method or function takes a callback argument and returns immediately after enqueuing a task to run on one of the threads in the Node.js worker thread pool. That task consists of a C function to be run on the worker thread, which may not use any V8 (or v8+) state, and a function to be run in the main event loop thread when that task has completed. The latter function is normally expected to invoke the caller's original callback. In v8+, this takes the following form:

void *
async_worker(void *cop, void *ctxp)
{
	my_object_t *op = cop;
	my_context_t *cp = ctxp;
	my_result_t *rp = ...;

	/*
	 * In thread pool context -- do not call any of the
	 * following functions:
	 * v8plus_obj_hold()
	 * v8plus_obj_rele_direct()
	 * v8plus_jsfunc_hold()
	 * v8plus_jsfunc_rele_direct()
	 * v8plus_call_direct()
	 * v8plus_method_call_direct()
	 * v8plus_defer()
	 *
	 * If you touch anything inside op, you may need locking to
	 * protect against functions called in the main thread.
	 */
	...

	return (rp);
}

void
async_completion(void *cop, void *ctxp, void *resp)
{
	my_object_t *op = cop;
	my_context_t *cp = ctxp;
	my_result_t *rp = resp;
	nvlist_t *cbap;
	nvlist_t *cbrp;

	...
	cbap = v8plus_obj(
	    V8PLUS_TYPE_WHATEVER, "0", rp->mr_value,
	    V8PLUS_TYPE_NONE);

	if (cbap != NULL) {
		cbrp = v8plus_call(cp->mc_callback, cbap);
		nvlist_free(cbap);
		nvlist_free(cbrp);
	}

	v8plus_jsfunc_rele(cp->mc_callback);
	free(cp);
	free(rp);
}

nvlist_t *
async(void *cop, const nvlist_t *ap)
{
	my_object_t *op = cop;
	v8plus_jsfunc_t cb;
	my_context_t *cp = malloc(sizeof (my_context_t));
	...
	if (v8plus_args(ap, 0, V8PLUS_TYPE_JSFUNC, &cb,
	    V8PLUS_TYPE_NONE) != 0) {
		free(cp);
		return (NULL);
	}

	v8plus_jsfunc_hold(cb);
	cp->mc_callback = cb;
	v8plus_defer(op, cp, async_worker, async_completion);

	return (v8plus_void());
}

This mechanism uses uv_queue_work() and as such will tie up one of the worker threads in the pool for as long as async_worker is running.

The other asynchronous mechanism is the Node.js EventEmitter model. This model requires some assistance from JavaScript code, because v8+ native objects do not inherit from EventEmitter. To make this work, you will need to create a JavaScript object (the object your consumers actually use) that inherits from EventEmitter, hang your native object off this object, and populate the native object with an appropriate method that will cause the JavaScript object to emit events when the native object invokes that method. A simple example might look like this:

var util = require('util');
var binding = require('./native_binding');
var events = require('events');

function
MyObjectWrapper()
{
	var self = this;

	events.EventEmitter.call(this);
	this._native = binding._create.apply(this,
	    Array.prototype.slice.call(arguments));
	this._native._emit = function () {
		var args = Array.prototype.slice.call(arguments);
		self.emit.apply(self, args);
	};
}
util.inherits(MyObjectWrapper, events.EventEmitter);

Then, in C code, you must arrange for libuv to call a C function in the context of the main event loop. The function v8plus_method_call() is safe to call from any thread: depending on the context in which it is invoked, it will either make the call directly or queue the call in the main event loop and block on a reply. Simply arrange to call back into your JavaScript object when you wish to post an event:

nvlist_t *eap;
nvlist_t *erp;
my_object_t *op = ...;
...
eap = v8plus_obj(
    V8PLUS_TYPE_STRING, "0", "my_event",
    ...,
    V8PLUS_TYPE_NONE);

if (eap != NULL) {
	erp = v8plus_method_call(op, "_emit", eap);
	nvlist_free(eap);
	nvlist_free(erp);
}

This example will generate an event named "my_event" and propagate it to listeners registered with the MyObjectWrapper instance. If additional arguments are associated with the event, they may be added to eap and will also be passed along to listeners as arguments to their callbacks.

void v8plus_obj_hold(const void *op)

Places a hold on the V8 representation of the specified C object. This is rarely necessary; v8plus_defer() performs this action for you, but other asynchronous mechanisms may require it. If you are returning from a method call but have stashed a reference to the object somewhere and are not calling v8plus_defer(), you must call this first. Holds and releases must be balanced. Use of the object within a thread after releasing is a bug. This hold includes an implicit event loop hold, as if v8plus_eventloop_hold() was called.

void v8plus_obj_rele(const void *op)

Releases a hold placed by v8plus_obj_hold(). This function may be called safely from any thread; releases from threads other than the main event loop are non-blocking and will occur some time in the future. Releases the implicit event loop hold obtained by v8plus_obj_hold().

void v8plus_jsfunc_hold(v8plus_jsfunc_t f)

Places a hold on the V8 representation of the specified JavaScript function. This is required when returning from a C function that has stashed a reference to the function, typically to use it asynchronously as a callback. All holds must be balanced with a release. Because a single hold is placed on such objects when passed to you in an argument list (and released for you when you return), it is legal to reference and even to invoke such a function without first placing an additional hold on it. This hold includes an implicit event loop hold, as if v8plus_eventloop_hold() was called.

void v8plus_jsfunc_rele(v8plus_jsfunc_t f)

Releases a hold placed by v8plus_jsfunc_hold(). This function may be called safely from any thread; releases from threads other than the main event loop thread are non-blocking and will occur some time in the future. Releases the implicit event loop hold obtained by v8plus_jsfunc_hold().

void v8plus_defer(void *op, void *ctx, worker, completion)

Enqueues work to be performed in the Node.js shared thread pool. The object op and context ctx are passed as arguments to worker executing in a thread from that pool. The same two arguments, along with the worker's return value, are passed to completion executing in the main event loop thread. See example above.

void v8plus_eventloop_hold(void)

Places a hold on the V8 event loop. V8 will terminate when it detects that there is no more work to do. This liveliness check includes things like open sockets or file descriptors, but only if they are tracked by the event loop itself. If you are using multiple threads, some of which may blocking waiting for input (e.g. a message subscription thread) then you will need to prevent V8 from terminating prematurely. This function must be called from within the main event loop thread. Each hold must be balanced with a release. Note that holds on objects or functions obtained via v8plus_obj_hold() or v8plus_jsfunc_hold() will implicitly hold the event loop for you.

void v8plus_eventloop_rele(void)

Release a hold on the V8 event loop. If there are no more pending events or input sources, then V8 will generally terminate the process shortly afterward. This function may be called safely from any thread; releases from threads other than the main event loop thread are non-blocking and will occur some time in the future.

nvlist_t *v8plus_call(v8plus_jsfunc_t f, const nvlist_t *ap)

Calls the JavaScript function referred to by f with encoded arguments ap. The return value is the encoded return value of the function. The argument and return value encoding match the encodings that are used by C functions that provide methods.

As JavaScript functions must be called from the event loop thread, v8plus_call() contains logic to determine whether we are in the correct context or not. If we are running on some other thread we will queue the request and sleep, waiting for the event loop thread to make the call. In the simple case, where we are already in the correct thread, we make the call directly.

Note that when passing JavaScript functions around as callbacks, you must use first use v8plus_jsfunc_hold() from within the main event loop thread. Once finished with the function, you may pass it to v8plus_jsfunc_rele() from any thread to clean up.

nvlist_t *v8plus_method_call(void *op, const char *name, const nvlist_t *ap)

Calls the method named by name in the native object op with encoded argument list ap. The method must exist and must be a JavaScript function. Such functions may be attached by JavaScript code as in the event emitter example above. The effects of using this function to call a native method are undefined.

When called from threads other than the main event loop thread, v8plus_method_call() uses the same queue-and-block logic as described above in v8plus_call().

FAQ

• Why?

Because C++ is garbage. Writing good software is challenging enough without trying to understand a bunch of implicit side effects or typing templated identifiers that can't fit in 80 columns without falling afoul of the language's ambiguous grammar. Don't get me started.

• Why not use FFI?

FFI is really cool; it offers us the ability to use C libraries without writing bindings at all. However, it also exposes a lot of C nastiness to JavaScript code, essentially placing the interface boundary in consuming code itself. This pretty much breaks the JavaScript interface model -- for example, you can't really have a function that inspects the types of its arguments -- and requires you to write an additional C library anyway if you want or need to do something natively that's not quite what the C library already does. Of course, one could use it to write "bindings" in JavaScript that actually look like a JavaScript interface, which may end up being the best answer, especially if those are autogenerated from CTF! In short, v8+ and FFI are different approaches to the problem. Use whichever fits your need, and note that they're not mutually exclusive, either.

• What systems can I use this on?

illumos distributions, or possibly other platforms with a working libnvpair. I'm sorry if your system doesn't have it; it's open source and pretty easy to port.

There is an OSX port; see the ZFS port's implementation. Unfortunately this port lacks the requisite support for floating-point data (DATA_TYPE_DOUBLE) but you could easily add that from the illumos sources.

• What about node-waf and node-gyp?

Fuck python, fuck WAF, and fuck all the hipster douchebags for whom make is too hard, too old, or "too Unixy". Make is simple, easy to use, and extremely reliable. It was building big, important pieces of software when your parents were young, and it Just Works. If you don't like using make here, you probably don't want to use v8+ either, so just go away. Write your CoffeeScript VM in something else, and gyp-scons-waf-rake your way to an Instagram retirement in Bali with all your hipster douchebag friends. Just don't bother me about it, because I don't care.

• Why is Node failing in dlopen()?

Most likely, your module has a typo or needs to be linked with a library. Normally, shared objects like Node addons should be linked with -zdefs so that these problems are found at build time, but Node doesn't deliver a mapfile specifying its API so you're left with a bunch of undefined symbols you just have to hope are defined somewhere in your node process's address space. If they aren't, you're boned. LD_DEBUG=all will help you find the missing symbol(s).

As of 0.0.2, v8+ builds a mapfile for your node binary at the time you build your addon. It does not attempt to restrict the visibility of any symbols, so you will not be warned if your addon is using private or deprecated functionality in V8 or Node.js. Your build will, however, fail if you've neglected to link in any required libraries, typo'd a symbol name, etc.

• Why can't I see my exception's decorative properties in JavaScript?

Be careful when decorating exceptions. There are several built-in hidden properties; if you decorate the exception with a property with the same name, you will change the hidden property's value but it will still be hidden. This almost certainly is not what you want, so you should prefix the decorative property names with something unique to your module to avoid stepping on V8's (or JavaScript's) property namespace.

• What if the factory model doesn't work for me?

See "License" below. Note also that one can export plain functions as well.

• Why do I always die with "invalid property type -3621" (or other garbage)?

You are passing an object with the wrong C type to v8plus_obj(). Like all varargs functions, it cannot tell the correct size or type of the objects you have passed it; they must match the preceding type argument or it will not work correctly. In this particular case, you've most likely done something like:

int foo = 0xdead;

v8plus_obj(V8PLUS_TYPE_NUMBER, "foo", foo, V8PLUS_TYPE_NONE);

An 'int' is 4 bytes in size, and the compiler reserves 4 bytes on the stack and sticks the value of foo there. When v8plus_obj goes to read it, it sees that the type is V8PLUS_TYPE_NUMBER, casts the address of the next argument slot to a double *, and dereferences it, then moves the argument list pointer ahead by the size of a double. Unfortunately, a double is usually 8 bytes long, meaning that (a) the value of the property is going to be comprised of the integer-encoded foo appended to the next data type, and (b) the next data type is going to be read from either undefined memory or from part of the address of the name of the next property. To cure this, always make sure that you cast your integral arguments properly when using V8PLUS_TYPE_NUMBER:

v8plus_obj(V8PLUS_TYPE_NUMBER, "foo", (double)foo, V8PLUS_TYPE_NONE);

License

MIT.

Bugs

See https://github.com/joyent/v8plus/issues.

Consumers

This is an incomplete list of native addons known to be using v8+. If your addon uses v8+, please let me know and I will include it here.

• https://github.com/joyent/node-contract