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Node.js native add-ons in C
https://github.com/joyent/v8plus

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Node.js native add-ons in C

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README 

        
# 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 .  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": ,

			"detail": {

				"value64": ""

			}

		}

	}



### 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](https://github.com/rbranson/node-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](http://illumos.org) 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](http://code.google.com/p/maczfs/source/browse/#git%2Fusr%2Fsrc%2Flib%2Flibnvpair).

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 .



## 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.



-