linuxsignalfdh

Linux signalfd is a Linux kernel interface that delivers POSIX signals to a process through a file descriptor rather than via traditional signal handlers. Implemented by the signalfd family of system calls, it enables integrating signal handling into event-driven I/O loops using mechanisms such as select, poll, or epoll. The interface is exposed to user space through headers such as linux/signalfd.h.

How it works: A process first blocks the signals it wishes to receive via signalfd (for example with sigprocmask). It then creates a signalfd with a mask specifying which signals should be delivered through the file descriptor. When a signal in the mask is generated, the corresponding signalfd file descriptor becomes readable. Reading from the descriptor yields information about the signal, typically in a signalfd_siginfo structure, including the signal number and, in some cases, the sender's process ID and user ID.

API and flags: The signalfd API includes both signalfd and signalfd4 variants. The latter adds support for flags such as SFD_NONBLOCK and SFD_CLOEXEC, enabling non-blocking I/O and close-on-exec behavior. The additional options support more flexible integration into modern event-driven programs.

Usage scenarios: Signalfd is commonly used in daemons and network servers that rely on asynchronous I/O loops. It avoids the complexity of asynchronous signal handlers and race conditions by consolidating signal processing into a single, well-defined read path.

Limitations and notes: Signalfd requires that the targeted signals be blocked in the process to ensure they are delivered via the descriptor. It may not be suitable for all signaling needs and is one of several approaches to signal handling in Linux.

See also: signalfd, signalfd4, linux/signalfd.h, signals, epoll, sigprocmask.