signalfd4

Signalfd4 is a Linux system call that creates a file descriptor for receiving signals delivered to a process, enabling asynchronous signal handling through I/O multiplexing rather than traditional signal handlers. It is part of the signalfd family, which also includes signalfd (the earlier interface). The signalfd4 variant adds an explicit parameter to control the size of the signal information payload returned to userspace.

The prototype is int signalfd4(int fd, const sigset_t *mask, int flags, size_t siginfo_size). If fd is -1, a new signalfd is created; otherwise, the provided file descriptor is reused. The mask argument points to a set of signals to be delivered via the file descriptor. The flags argument accepts SFD_CLOEXEC to set the close-on-exec flag and SFD_NONBLOCK to make reads non-blocking. The siginfo_size parameter specifies the size, in bytes, of the signal information structure that will be written to the read buffer (for example, a signalfd_siginfo struct).

Usage typically requires blocking the signals in the set with sigprocmask or pthread_sigmask before creating the signalfd, so that signals are delivered to the file descriptor rather than a traditional signal handler. When a blocked signal in the mask is delivered, the signalfd file descriptor becomes readable; a read returns the corresponding signal information. This enables integration of signal handling into event loops using poll, select, or epoll.

Notes: signalfd4 provides a mechanism to tailor the size of the returned signal information payload and coexists with the standard signalfd interface. The API is defined in sys/signalfd.h, and users typically work with the signalfd_siginfo structure returned by reads.