Runtime Signals

The runtime uses fiber signals for internal coordination. These are distinct from user-level error handling.

Built-in signals

Signal     Bit   Purpose
───────────────────────────────────────────
:error      0    Error propagation
:yield      1    Coroutine yield
:debug      2    Debugger breakpoints
:ffi        4    FFI callbacks
:halt       8    Fiber termination
:io         9    Async I/O completion
:exec      11    Subprocess completion
:fuel      12    Instruction budget exhaustion
:switch    13    Context switch
:wait      14    Blocking wait

User-defined signals (via (signal :keyword)) get bits 16–31.

Fuel budgets

Fuel limits instruction execution on a fiber. When fuel runs out, the fiber pauses with a :fuel signal.

(def f (fiber/new (fn [] (while true (yield :tick))) |:fuel :yield|))
(fiber/set-fuel f 1000)    # instruction budget
(fiber/resume f nil)       # runs until fuel exhausted
(fiber/fuel f)             # => 0 (exhausted)
(fiber/set-fuel f 10000)   # refuel
(fiber/resume f nil)       # resume execution
(fiber/clear-fuel f)       # remove budget, unlimited execution

SIG_QUERY

SIG_QUERY requests introspection from a fiber without disrupting its execution. Used by arena/count, arena/stats, and other introspection primitives.

SIG_EXEC

Signals subprocess completion. Used by the async scheduler when a subprocess/exec process finishes.

See also

• signals — signal system design
• fibers — fiber architecture
• scheduler.md — async event loop
• processes.md — fuel-based preemptive scheduling of Erlang-style processes