threading.rst

Duktape threading

Overview

This document describes native threading options that can be used with Duktape.

The basic threading rules are as follows.

Only one active native thread at a time per Duktape heap

When user code calls e.g. duk_call(), control flow is transferred to Duktape. Duktape executes with that native thread until the original call returns or errors out. While Duktape has control, it may execute multiple Duktape threads (coroutines, not to be confused with native threads). The caller must not interact with the Duktape heap with any other native thread until the original call returns.

After the original call returns, further calls into Duktape may use a different native thread, but never at the same time.

Multiple native threads can execute code on separate Duktape heaps

Multiple threads can use different Duktape heaps: each heap is entirely independent.

Although Duktape itself is fully re-entrant, there are some situations (discussed separately below) which can limit re-entrancy and prevent running multiple threads at the same time, even when they run code in separate Duktape heaps.

Platform limitations on re-entrancy

Lack of variadic preprocessor macros

When variadic preprocessor macros are not available, Duktape currently passes some fields through globals in that case. This creates a race condition where error information (__FILE__ and __LINE__, for instance) can be corrupted for errors thrown in different Duktape heaps.

This should not cause unsafe behavior but may corrupt error messages or tracebacks.

Note

This limitation can be fixed by passing these values through duk_context (duk_hthread) instead of globals, but at the moment Duktape public API macros don't access duk_hthread directly. There is no longer a reason for this limitation because duktape.h sees the internal structures (even when compiling application code). A fix is scheduled for Duktape 1.1 release.

Non-re-entrant system calls

Duktape uses some system calls which don't always have re-entrant variants (or perhaps the re-entrant variants don't work). This mainly impacts the Date built-in, which uses gmtime_r() and localtime_r() on UNIX when they are available, but falls back to gmtime() and localtime() if the platform doesn't support them.

The impact on multithreading behavior depends on the non-re-entrant system calls in question.

A few workarounds:

• Implement your own re-entrant native functions (e.g. date/time functions) for those not provided by your platform. You'll need to change Duktape internals to make Duktape use the replacements.
• Replace the built-ins (such as Date) entirely with a replacement written specifically for your platform. This approach may allow you to avoid changes to Duktape internals as the non-re-entrant calls won't be used.