Introduction

Threads

To enable thread support the --threads:on{.interpreted-text role="option"} command-line switch needs to be used. The system module then contains several threading primitives. See the channels modules for the low-level thread API. There are also high-level parallelism constructs available. See spawn for further details.

Nim\'s memory model for threads is quite different than that of other common programming languages (C, Pascal, Java): Each thread has its own (garbage collected) heap, and sharing of memory is restricted to global variables. This helps to prevent race conditions. GC efficiency is improved quite a lot, because the GC never has to stop other threads and see what they reference.

The only way to create a thread is via spawn or createThread. The invoked proc must not use var parameters nor must any of its parameters contain a ref or closure type. This enforces the no heap sharing restriction.

Threads and exceptions

The interaction between threads and exceptions is simple: A handled exception in one thread cannot affect any other thread. However, an unhandled exception in one thread terminates the whole process.