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Duktape C module convention

Overview

This document provides a recommended convention for writing an init function for a C module. The convention allows modules to be used with both static linking and DLL loading, and either as part of Duktape's CommonJS module loading or outside of it.

The convention is in no way mandatory and it's perfectly fine to use a different module loader convention for your project. However, modules following this convention will be easier to share between projects.

Module init function

The init function for a module my_module should have the following form:

duk_ret_t dukopen_my_module(duk_context *ctx) {
    /* Initialize module in whatever way is most appropriate.
     * Called as a Duktape/C function.
     *
     * Push the module result (e.g. an object with exported symbols or
     * a function) on top of the value stack and return 1 to indicate
     * there's a return value.  Temporary values can be left below
     * the return value like in normal Duktape/C functions.
     */

    duk_push_object(ctx);  /* module result */

    duk_put_function_list(ctx, -1, my_module_funcs);

    duk_push_int(ctx, 42);
    duk_put_prop_string(ctx, -2, "meaningOfLife");

    return 1;  /* return module value */
}

The init function is called as a Duktape/C function. When initializing the module manually, you should use:

duk_push_c_function(ctx, dukopen_my_module, 0 /*nargs*/);
duk_call(ctx, 0);  /* or duk_pcall() if you want to catch errors */

/* Stack top contains module value */

A DLL loader should use the same convention to call the init function after figuring out the init function name and locating it from the DLL symbol table.

DLL name

When a C module is compiled into a DLL, the DLL filename should include the module name (my_module in the running example) with any platform specific prefix and suffix. For example:

my_module.so   # Linux
my_module.dll  # Windows

A DLL loader should assume that the init function name is dukopen_ followed by the module name part extracted from the DLL filename (here, dukopen_my_module()).

Module name

To avoid case conversion and special character issues, module names should have the form:

[a-zA-Z_][0-9a-zA-Z_-]*

This should minimize platform issues.

Mixed ECMAScript / C modules

When a module is being initialized by a CommonJS aware module loader, the loader can support mixed modules containing both C and ECMAScript code. For example:

my_module.so   # C module
my_module.js   # ECMAScript module (CommonJS)

To support mixed modules, a Duktape 1.x modSearch() function should:

  • First load the C module normally, yielding a return value RET.
  • If RET is an object, copy the own properties of RET into the exports value created by Duktape. It should then return the source code of the ECMAScript module; when executed, further symbols get added to the same exports value.
  • If RET is not an object, ignore it and load the ECMAScript module normally. (Alternatively, write RET to a fixed export name to make it accessible, e.g. exports.value.)

The algorithm for Duktape 2.0 is still under design, but at a high level:

  • First load the C module normally, yielding a return value RET.
  • If RET is an object, use it to initialize the CommonJS exports value before loading the ECMAScript module. The ECMAScript module can then use whatever symbols the C modules registered, and add further symbols to the same exports value.
  • If RET is not an object, ignore it and load the ECMAScript module normally. (Alternatively, expose RET with a fixed name, e.g. initialize exports as { value: RET }.)

Since Duktape 1.3 the modSearch() function can overwrite module.exports which allows you to implement the Duktape 2.0 approach in Duktape 1.x too.

Duktape 1.x CommonJS notes

Duktape 1.3 and above

In Duktape 1.3 you can replace module.exports with the object returned by the native module initialization function. That value then becomes the result of the original require() call.

For an example, see: https://github.com/svaarala/duktape/blob/master/tests/api/test-dev-cmodule-guide.c.

Prior to Duktape 1.3

Prior to Duktape 1.3 the module exports value is always an object created by Duktape, and cannot be replaced by modSearch(). modSearch() can only add symbols to the pre-created object. This has two implications for implementing modSearch():

  • When a C module returns an object, the symbols from the object must be copied to the pre-created exports value manually by the modSearch() function.
  • When a C module returns a non-object, there are several alternatives:
    • The modSearch() function can ignore the module value. This will make the module value inaccessible (unless the C module init function registered symbols directly to the global object or similar).
    • The modSearch() function can copy the module value into a fixed name in the exports table. Suggested name is exports.value.

Limitations

  • The convention may not work on all platforms where Duktape itself ports to. For instance, a platform might have no DLL support or have filename restrictions that don't allow DLLs to be named as specified above.
  • The convention is not "CommonJS native": a C module doesn't get an exports table and cannot load sub-modules (at least relative to its own CommonJS identifier). This trade-off is intentional to keep the C module convention as simple as possible.
  • CommonJS module loading prior to Duktape 1.3 doesn't support modules with a non-object return value (i.e. all modules return an exports table). This module convention is not limited to object return values so that non-object modules can be supported in Duktape 1.3 and above.