Overview
Overview
Libdex is a GNOME library that provides support for futures and fibers in GObject-based applications.
The library attempts to follow well established patterns and terminology around programming with futures.
Libdex depends on GLib 2.68 or newer.
Building
To build a program that uses libdex, you can use the following command to get the CFLAGS and libraries necessary to compile and link.
cc hello.c `pkg-config --cflags --libs libdex-1` -o hello
Use #include <libdex.h> to include support for libdex.
Platform Support
Libdex, including Fiber support, has been known to work on Linux, FreeBSD, Solaris, Illumos, macOS, and Windows.
Support for io_uring is limited to Linux.
Futures and Promises
A DexFuture is the primary object representing work that will
complete now or at some point in the future. There are various types of
futures, such as a DexPromise or DexTimeout.
See the class hierarchy for other types of futures.
Fibers
Libdex has support for fibers which run on a separate stack from your thread
stack. They are represented with the DexFiber type which is also a
DexFuture.
To create a fiber, use dex_scheduler_spawn() with the appropriate
scheduler. For thread pool usage, [func@Dex.ThreadPoolScheduler.get_default()]
will get you the default thread pool.
Fibers are scheduled from a GSource in a GMainContext
which allows them to run cooperatively with other work.
Awaiting Futures
If you are running on a fiber, you may await completion of a future. This will suspend your fiber stack and yield back to the main thread until that future has completed, rejected, or the fiber has been canceled.
Use dex_await() or it’s variants such as
dex_await_object() to await completion of any future. If you are
not on a fiber, then the await will fail and the error argument will be set.
Reference Counting
Everything that is a DexObject, include DexFuture may be
referenced using dex_ref() and unref’d using dex_unref() or dex_clear().
You may also use g_autoptr(DexFuture) to automate this.
Callbacks
When using fibers is not desired you can manually use callbacks based on the completion or rejection of futures. This is faster than using fibers and does not require the use of a special stack. However, it can be more cumbersome to write when in C.
Use dex_future_then(), dex_future_catch(), and
dex_future_finally() to do typical try/catch/finally semantics. Each
of these return a new DexFuture that may be further acted upon.
Disowning Futures
When a future is no longer needed and unref’d, it will notify the futures it was depending on that they are no longer requiring their completion. Some future types may use this to cascade work cancellation.
If this is not a behavior you want (e.g. you want the dependent task to always
complete), then you may use dex_future_disown() to disown a future
ensuring it will always complete or reject without cancellation.
Futures as Sets of Work
A common use of libdex is to await for multiple futures to complete before
taking further action. Use dex_future_all(), dex_future_all_race(),
dex_future_any(), and dex_future_first() to await multiple futures
with different semantics about when the new future will complete.
Work Queues
In addition to fibers, DexScheduler can also schedule general work
items to be run. Use dex_scheduler_push() to push a work item onto that scheduler.
On systems that support double-wide compare-and-swap, this will avoid an allocation for the work item and data pair. They will be placed into the wait-free work queue atomically.
Thread Pools and Schedulers
In many thread pool implementations, the worker thread is only executing
work items. In libdex, thread pools can both service short work items and
fibers along with longer running timeouts and generalized GSource on the
GMainContext. This allows a great deal of flexibility in using futures on
the thread pool.
The thread pool is implemented as a global work queue and a series of lock-free work queues per worker thread. Work stealing is also used between thread pools for general purpose work items. However, fibers will never be migrated between threads in order to ensure runtime safety.
Because of the expectation that you will await futures on the thread pool
threads, should you need to do long blocking work you may consider using
dex_thread_spawn(). This thread may not await like fibers as it is a
real operating system thread. You may however block on a future using
dex_thread_wait_for().