# os/on-signal Register a handler closure for an OS signal. Catchable set: `:sigint`, `:sigterm`, `:sighup`, `:sigquit`, `:sigusr1`, `:sigusr2`, `:sigalrm` — names are keywords or strings, case-insensitive, with or without the `sig` prefix. Handlers fire only while the event loop pumps (`[vwait]`, `[update]`, REPL, servers). ## Syntax

[os/on-signal $signal $handler]

## Parameters - `$signal` — signal name (`:sigusr1`, `"USR1"`, `:usr1`, …) - `$handler` — closure `{|ev| ...}` receiving `%{ :type :signal :signal :sigusr1 :code N :pid N :uid N }` (`:pid`/`:uid` are the sender's; `:code` is the siginfo code) ## Examples

set got nil
[os/on-signal :sigusr1 {|ev| set got $ev~signal}]
[os/kill [os/pid] :sigusr1]
[update]                     # pumps the loop once → handler fires
echo $got                    # → :sigusr1
[os/off-signal :sigusr1]     # → 1

## Returns `true` when registered. `%Err{:message}` for an unknown or uncatchable signal (`sigkill`/`sigstop`), a non-closure handler, a full table (32 handlers), a non-Linux build, or when another runtime in the process already owns signal delivery. Registering a signal intercepts it: while any handler exists, Ctrl-C no longer kills. ## See Also `os/off-signal`, `os/signals`, `os/kill`, `os/pid`