Signalfd

Signalfd is a Linux-specific interface that provides a file descriptor for receiving signals. It allows a process to deliver signals through normal I/O multiplexing mechanisms (such as poll, epoll, or select) rather than through traditional asynchronous signal handlers. By creating a signalfd, a program can read signal information from a file descriptor just like reading data from a socket or pipe.

Operation and usage: A process typically blocks the signals it wants to receive through a signalfd using sigprocmask, then creates a signalfd (or signalfd4) with a mask that specifies the signals to trap. The resulting file descriptor becomes readable when a signal in that set is generated for the process. The program reads signalfd_siginfo data from the descriptor, which contains information about the delivered signal (for example, the signal number) and may include sender details such as the originating process ID and user ID. After reading, the signal event is consumed and can be processed by the program in its event loop.

Variants and flags: The signalfd interface has wrappers such as signalfd and signalfd4. signalfd4 supports additional flags (for example, non-blocking and close-on-exec). The file descriptor can be monitored with standard IO multiplexing calls, and it can be used alongside other event sources in multi-threaded or multi-process programs.

Portability and scope: signalfd is available only on Linux; it is not part of the POSIX standard. It is commonly used in event-driven software, daemons, and servers where unified handling of IO and signals is desirable.

See also: sigprocmask, signalfd4, sigaction.