Signals
Signals are special information which the kernel passes to running processes. Each application decides what to do when a signal arrives.
Signal Handlers are regular userspace code. They don't need special privileges.
Handling Signals
Signals are processed in two different ways by the kernel.
-
Process currently in user mode - In this case, the signal is received by the kernel, and it forwards this to the process by writing it to the process's process structure.
-
Process already in kernel mode - If a signal arrives during a system call, the kernel handles it differently. The application is already in the kernel mode so the kernel will handle it immediately. It's handled by scheduling the handler for execution.
Default Signal Handling
If no handlers are registered, the kernel uses default handling based on signal type.
In such cases, the kernel will either terminate or stop the process or ignore the message depending on the type of the signal.