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The Signal Belfry

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Living collection · SIG-001 to SIG-026

The bells the system has rung since the teletype, one per page.

POSIX signals of termination, stopping, faults, clocks and resources: every bell gets its card, its label, and the answer to the question that matters — how to ring it. Some announce, some alarm, one brings down the tower.

Climb the tower Why this way Where it came from

Drawers

The six drawers these bells were hung in.

The full carillon

  1. SIG-001 SIGHUP The phone went dead — and daemons took it to mean “reread your configuration”. Termination common
  2. SIG-002 SIGINT Ctrl+C: the interruption that leaves the keyboard and arrives as a signal. Termination common
  3. SIG-003 SIGQUIT Ctrl+\ — ends things like its sibling, but leaves a core dump as a keepsake. Termination uncommon
  4. SIG-004 SIGTERM The polite request: tidy up, close the doors, turn off the lights. Termination common
  5. SIG-005 SIGKILL The bullet: the process never hears this ring — the kernel acts alone. Termination common
  6. SIG-006 SIGSTOP Freezes the process wherever it stands — and takes no refusal. Stop and resume uncommon
  7. SIG-007 SIGTSTP Ctrl+Z: the pause that comes from the keyboard and can be refused. Stop and resume common
  8. SIG-008 SIGCONT The resume ring: it wakes whatever was frozen. Stop and resume uncommon
  9. SIG-009 SIGCHLD A child has finished — the one notice parents may ignore guilt-free. Stop and resume uncommon
  10. SIG-010 SIGSEGV The segmentation fault: the program touched memory that wasn't its own. Program faults common
  11. SIG-011 SIGBUS Bus error: the address does exist — it's the access that cannot happen. Program faults uncommon
  12. SIG-012 SIGFPE The “floating-point” exception that is almost never about floating point. Program faults uncommon
  13. SIG-013 SIGILL Illegal instruction: the CPU read something it doesn't recognize as an order. Program faults forgotten
  14. SIG-014 SIGABRT The program gives up on itself: abort() rings its own bell. Program faults uncommon
  15. SIG-015 SIGTRAP The debugger's bell: every breakpoint is a pre-arranged ring. Program faults forgotten
  16. SIG-016 SIGSYS A forbidden system call — nowadays it's seccomp that rings it. Program faults forgotten
  17. SIG-017 SIGALRM The alarm clock: alarm() winds it up and the kernel rings on schedule. Clocks uncommon
  18. SIG-018 SIGVTALRM The alarm clock that only counts the time the process spent on the CPU. Clocks forgotten
  19. SIG-019 SIGPROF The profilers' metronome: a steady tick in CPU time, user plus system. Clocks forgotten
  20. SIG-020 SIGPIPE A write into a pipe with nobody on the other end — which is why head ends the conversation. Resources uncommon
  21. SIG-021 SIGXCPU The CPU limit burst — a warning before the final cut. Resources forgotten
  22. SIG-022 SIGXFSZ The file grew past its allowance; the bell rings at the write. Resources forgotten
  23. SIG-023 SIGUSR1 A bell with no factory ring: each program agrees on its own meaning. The user's uncommon
  24. SIG-024 SIGUSR2 The second free bell — the one that usually remains without an agreed ring. The user's forgotten
  25. SIG-025 SIGWINCH The window changed size — the bell that makes the whole terminal redraw. The user's uncommon
  26. SIG-026 SIGINFO Ctrl+T on the BSDs and macOS: it tells you what the process is doing, interrupting nothing. The user's forgotten