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Module sys/signals

sys/signals
Stability: unstable — and not merely by convention: the table is known to be wrong on macOS for four of its 27 entries, so freezing it would freeze a bug. It is also a raw numeric table with one Yo consumer, the `std/sys/signal` tests; the supported surface is `std/signal.yo`'s `Signal` enum, which is where a new signal should be added. Freezing follows fixing the four values and deciding whether this module should exist at all rather than being folded into `std/signal.yo`'s table. — stable modules only change additively; this one may still change.

POSIX signal numbers, as plain integers for std/sys/signal's on_signal / off_signal / kill.

Signal numbers are NOT uniform across Unix. Numbers 1-15 agree between Linux and macOS except for three, and everything above 15 was renumbered wholesale by 4.4BSD, so a portable table has to branch per platform. Four members here do (SIGCHLD, SIGCONT, SIGSTOP, SIGTSTP, via cond on process.platform); four more differ on macOS and do NOT branch, and are therefore wrong there — see the warnings on SIGBUS, SIGURG, SIGUSR1 and SIGUSR2, and issues/sys-signals-macos-numbers-are-linux-values.md. Non-Unix targets take the Linux arm.

std/signal.yo is the portable surface — its Signal enum carries its own per-platform number table (_signal_num) with the macOS values right, and deliberately does not read this module.

Stability

unstable — and not merely by convention: the table is known to be wrong on macOS for four of its 27 entries, so freezing it would freeze a bug. It is also a raw numeric table with one Yo consumer, the std/sys/signal tests; the supported surface is std/signal.yo's Signal enum, which is where a new signal should be added. Freezing follows fixing the four values and deciding whether this module should exist at all rather than being folded into std/signal.yo's table.

Constants

SIGHUP constant i32

Hangup — the controlling terminal closed, or the session leader exited. Conventionally also "reload your configuration" for a daemon.

Value: 1

SIGINT constant i32

Interrupt from the keyboard (Ctrl-C).

Value: 2

SIGQUIT constant i32

Quit from the keyboard (Ctrl-\) — like SIGINT, but the default action also dumps core.

Value: 3

SIGILL constant i32

Illegal instruction.

Value: 4

SIGTRAP constant i32

Trace/breakpoint trap — what a debugger's breakpoints raise.

Value: 5

SIGABRT constant i32

Abort — what C's abort() raises, and so what a Yo panic ends up as. This is process termination, and has nothing to do with the unwind keyword (see AGENTS.md).

Value: 6

SIGBUS constant i32

Bus error — an unaligned or otherwise unmappable access.

WRONG ON macOS. 7 is Linux's number; macOS's SIGBUS is 10 and its 7 is SIGEMT. This constant does not branch on platform, so on macOS it names the wrong signal (issues/sys-signals-macos-numbers-are-linux-values.md).

Value: 7

SIGFPE constant i32

Arithmetic exception — integer divide by zero, not floating point despite the name (IEEE arithmetic does not trap by default).

Value: 8

SIGKILL constant i32

Kill. Cannot be caught, blocked or ignored, so on_signal for it is pointless; kill(pid, SIGKILL) is the unconditional stop.

Value: 9

SIGUSR1 constant i32

User-defined signal 1.

WRONG ON macOS. 10 is Linux's number; macOS's SIGUSR1 is 30, and 10 there is SIGBUS — so sending this on macOS crashes the target instead of poking it (issues/sys-signals-macos-numbers-are-linux-values.md). std/signal.yo's Signal.User1 has the right value.

Value: 10

SIGSEGV constant i32

Segmentation fault. A -O0 Yo binary that takes this on deep recursion is out of stack, not corrupting the heap — see the YO_MAIN_STACK_MB note in AGENTS.md before reaching for a sanitizer.

Value: 11

SIGUSR2 constant i32

User-defined signal 2.

WRONG ON macOS. 12 is Linux's number; macOS's SIGUSR2 is 31, and 12 there is SIGSYS, whose default action kills the process (issues/sys-signals-macos-numbers-are-linux-values.md). std/signal.yo's Signal.User2 has the right value.

Value: 12

SIGPIPE constant i32

Wrote to a pipe or socket with no reader. Terminates the process by default, which is why servers normally ignore it and read the EPIPE from the write instead.

Value: 13

SIGALRM constant i32

Timer expired (alarm()/setitimer() real time).

Value: 14

SIGTERM constant i32

Polite termination request — the default of kill(1), and the one a process is expected to catch in order to shut down cleanly.

Value: 15

SIGCHLD constant i32

A child process stopped or exited. macOS 20, Linux 17.

Value: 17

SIGCONT constant i32

Continue a stopped process. macOS 19, Linux 18.

Value: 18

SIGSTOP constant i32

Stop the process; like SIGKILL it cannot be caught or ignored. macOS 17, Linux 19.

Value: 19

SIGTSTP constant i32

Stop typed at the terminal (Ctrl-Z) — catchable, unlike SIGSTOP. macOS 18, Linux 20.

Value: 20

SIGTTIN constant i32

A background process tried to read from the terminal.

Value: 21

SIGTTOU constant i32

A background process tried to write to the terminal (only when the tty has TOSTOP set).

Value: 22

SIGURG constant i32

Urgent (out-of-band) data arrived on a socket.

WRONG ON macOS. 23 is Linux's number; macOS's SIGURG is 16 and its 23 is SIGIO (issues/sys-signals-macos-numbers-are-linux-values.md).

Value: 23

SIGXCPU constant i32

CPU time limit exceeded (RLIMIT_CPU).

Value: 24

SIGXFSZ constant i32

File size limit exceeded (RLIMIT_FSIZE).

Value: 25

SIGVTALRM constant i32

Virtual (process CPU time) timer expired.

Value: 26

SIGPROF constant i32

Profiling timer expired.

Value: 27

SIGWINCH constant i32

The terminal window was resized — the cue to re-read the size from std/term.

Value: 28