Janet 1.17.1-e1c4fc2 Documentation
Starting in version 1.6.0, Janet has the ability to do true, non-cooperative
multithreading with the
thread/ functions. Janet threads correspond to
native threads on the host operating system, which may be either pthreads on
POSIX systems or Windows threads. Each thread has its own Janet heap, which
means threads behave more like processes that communicate by message passing.
However, this does not prevent native code from sharing memory across these
threads. Without native extensions, however, the only way for two Janet threads
to communicate directly is through message passing.
(thread/new func &opt capacity) to create a new thread. This thread
will start and wait for a message containing a function that it will run as the
main body. This function must also be able to take 1 parameter, the parent
thread. Each thread has its own mailbox, which can asynchronously receive
messages. These messages are added to a queue, which has a configurable maximum
capacity according to the second optional argument passed to
If unspecified, the mailbox will have an initial capacity of 10.
(defn worker [parent] (print "New thread started!")) # New thread's mailbox has capacity for 32 messages. (def thread (thread/new worker 32))
Sending and receiving messages
Threads in Janet do not share a memory heap and must communicate via message
thread/send to send a message to a thread, and
thread/receive to get messages sent to the current thread.
(defn worker [parent] (print "waiting for message...") (def msg (thread/receive)) (print "got message: " msg)) (def thread (thread/new worker)) # Thread objects support the :send method as an alias for thread/send. (:send thread "Hello!")
Limitations of messages
Since threads do not share Janet heaps, all values sent as messages are first
marshalled to a byte sequence by the sending thread, and then unmarshalled by
the receiving thread. For marshalling and unmarshalling, threads use the two
load-image-dict. This means you can
send over core bindings, even if the underlying value cannot be marshalled.
Semantically, messages sent with
(thread/send to msg) are first converted
to a buffer via
(marshal msg make-image-dict mailbox-buf), and then
unmarshalled by the receiving thread via
Values that cannot be marshalled, including thread values, cannot be sent as messages to other threads, or even as part of a message to another thread. For example, the following will not work because open file handles cannot be marshalled.
(def file (file/open "myfile.txt" :w)) (defn worker [parent] (with-dyns [:out file] (print "Writing to file."))) # Will throw an error, as worker contains a reference to file, which cannot be marshalled. (thread/new worker)
The fix here is to move the file creation inside the worker function, since every worker thread is going to have its own copy of the file handle.
(defn worker [parent] (def file (file/open "myfile.txt" :w)) (with-dyns [:out file] (print "Writing to file."))) # No error (thread/new worker)
This limitation has been removed for some values (C functions and raw pointers)
in version 1.9.0, to make use of threads with native modules more seamless,
although certain abstract types like open files can still not be sent to threads
Blocking and non-blocking sends and receives
All sends and receives can be blocking or non-blocking by providing an optional
timeout value. A timeout of
(thread/send to msg &opt timeout) and
(thread/receive &opt timeout) non-blocking, while a positive timeout
value indicates a blocking send or receive that will resume the current thread
by throwing an error after timeout seconds. Timeouts are limited to 30 days,
with exception the using
math/inf for a timeout means that a timeout
will never occur.
(defn worker [parent] (for i 0 10 (os/sleep 0.5) (:send parent :ping)) # sleep a long while to make the parent timeout (os/sleep 2) (:send parent :done)) # Will print the first 10 pings, but timeout and throw an error before :done (try (let [t (thread/new worker)] (for i 0 11 (print "got message: " (thread/receive 1)))) ([err] (print "error: " err))) # Flush :done message (thread/receive math/inf) # Will not error and work successfully (let [t (thread/new worker)] (for i 0 11 (print "got message: " (thread/receive math/inf))))