SIGUSR1

SIGUSR1 is a POSIX signal designated as user-defined signal 1. It is one of two signals reserved for application-defined use (the other being SIGUSR2). The system does not assign a fixed meaning to SIGUSR1; the receiving process determines its interpretation. Signals are delivered asynchronously, allowing a process to be notified of events without exchanging data.

On most Unix-like systems, the default action for SIGUSR1 is to terminate the process, with no automatic core dump. This default can be changed by installing a signal handler. The signal can be sent with commands such as kill or raise, and, in some implementations, sigqueue can deliver data alongside the signal. In multi-threaded programs, signals are delivered to one thread according to specific rules, and handlers are typically installed with sigaction rather than the older signal interface to ensure portability and reliable behavior.

In practice, SIGUSR1 is used to implement custom notifications that are meaningful to the application, such as triggering a configuration reload, rotating logs, toggling verbose output, or signaling to dump internal state. Because signal handlers must be async-signal-safe, developers usually set a small, atomic flag in the handler and perform the actual work in the main loop, or use inter-thread communication mechanisms (like a pipe or signalfd) to handle the event safely.

Notes: exact signal numbers and some delivery semantics are platform-dependent. Programs intended to be portable should avoid relying on a particular interpretation of SIGUSR1 beyond its role as a user-defined signal, and should document the intended meaning within the application.