How do I use multi threading in TCL?

Question

I'm trying to run two procedures in parallel. As TCL is the interpreter, it will process procedures one by one. Can someone explain with an example how I can use multi-threading in TCL?

Answer 1

These days, the usual way to do multi-threading in Tcl is to use its Thread extension — it's being developed along with the Tcl's core, but on certain platforms (such as various Linux-based OSes) you might need to install a separate package to get this extension available.

The threading model the Thread extension implements is "one thread per interpreter". This means, each thread can "host" just one Tcl interpreter (and an unlimited number of its child interpreters), but no code executed by any thread may access interpreters hosted in other threads. This, in turn, means that when you work with threads in Tcl, you have to master the idea of multiple interpreters.

The classical approach to exchanging data between interpreters running in different threads is message passing: you post scripts to the input queue of the target interpreter running in different thread and then wait for reply. On the other hand, thread-shared variables (implementing sharing memory by locking) is also available. Another available feature is support for thread pools.

Read the "Tcl and threads" wiki page, the Thread's extension manual pages.

The code examples are on the wiki. Here's just one of them.

Please note that if your procedures which, you think, have to be run in parrallel, are mostly I/O bound (that is, they read something from the network and/or send something there) and not CPU-bound (doing heavy computations), you might have better results with the event-based approach to processing: the Tcl has built-in support for the event loop, and you are able to make Tcl execute your code when the next chunk of data can be read from a channel (such as a network socket) or written to a channel.