Signalfdsiginfo

Signalfdsiginfo, often referred to by the type name signalfd_siginfo, is the user-space data structure used by the Linux signalfd API to convey information about delivered signals when reading from a signalfd file descriptor. Signalfd provides a mechanism to receive POSIX signals through a file descriptor instead of traditional signal handlers, enabling event-driven or polling I/O workflows.

Definition and origin: The structure is defined in the Linux kernel header linux/signalfd.h and is exposed to user space, typically via C headers on Linux systems. It mirrors key parts of the traditional siginfo_t payload but is formatted for consumption through a signalfd read operation.

Purpose and usage: When a program reads from a signalfd, it obtains a signalfd_siginfo object that contains metadata about the signal that was delivered. This information enables the program to identify the exact signal (ssi_signo) and, in many cases, the sender’s identity and context. The data is designed to be read atomically in a single read operation, simplifying integration with non-blocking I/O loops, select/epoll, or other event-driven models.

Typical fields: The structure includes a signal number field (ssi_signo) and fields such as ssi_code (the signal code), ssi_pid (the sender process ID), ssi_uid (the sender user ID), and ssi_fd (an associated file descriptor for certain signal sources like SIGIO). Some kernel versions also include ssi_band or additional padding. The exact fields and types can vary slightly by architecture and kernel version, but the general purpose remains the same: to convey signal identity and sender context in a machine-readable form.

See also: signalfd, siginfo_t, linux/signalfd.h.