systemsignals

System signals are a mechanism used by Unix-like operating systems to deliver asynchronous notifications to a running process. They are generated by the kernel in response to events such as user actions, program errors, or timer expirations, and are delivered to a thread within the target process. Each signal is identified by a number and a symbolic name defined by the POSIX standard (for example, SIGHUP, SIGINT, SIGTERM, SIGKILL, SIGSEGV). Signals can indicate events like a terminal hangup, an interrupt from the keyboard, a request to terminate, or a segmentation fault.

Signals have default actions, which may terminate the process, ignore the signal, stop or continue its execution, or suspend and resume. Some signals, notably SIGKILL and SIGSTOP, cannot be caught, blocked, or ignored. Others can be intercepted by installing a signal handler with facilities such as sigaction or signal, allowing custom behavior when the signal is received. Signals can also be generated synchronously within a process (for example, by calling raise) or asynchronously by the kernel or other processes (for example, via the kill command or timer facilities).

Threaded programs can block or mask signals, and signals may be delivered to any thread in the process depending on the threading model. Real-time signals provide queuing and priority semantics beyond standard signals, enabling more reliable inter-process signaling in some scenarios. Timers and alarms can generate signals to implement time-based events.

Common usage patterns include graceful shutdown with SIGTERM, configuration reloads with SIGHUP, and user-defined actions with SIGUSR1 or SIGUSR2. Signals are a fundamental but sometimes tricky tool, offering simple asynchronous communication with constraints related to safety and portability across platforms.

---