What are signals and how do processes handle them?

Deep dive

Signals are used for control and lifecycle events:

• SIGTERM requests graceful shutdown; SIGKILL forces termination.
• Handlers must be fast and async-signal-safe.
• Use flags/pipe to notify the main loop safely.
• Child processes report via SIGCHLD.

Examples

Graceful shutdown pattern:

SIGTERM -> set flag -> stop accepting traffic -> flush -> exit

Common pitfalls

• Doing non-reentrant work inside a handler.
• Ignoring SIGTERM and forcing SIGKILL.
• Missing SIGCHLD handling (zombie processes).

Interview follow-ups

• What’s the difference between SIGTERM and SIGKILL?
• How do you avoid zombies?
• How do signals work with threads?