rt_sigtimedwait(2)

Purpose

rt_sigtimedwait() is the raw kernel call behind sigtimedwait(3)/sigwaitinfo(3): it suspends the calling thread until one of the signals in set becomes pending, and – crucially – delivers it synchronously: the signal is consumed from the pending set and returned as a siginfo_t, without ever running a signal handler (rt_sigtimedwait.2, DESCRIPTION). With a timeout it becomes a bounded wait (sigtimedwait); with a NULL timeout it waits indefinitely (sigwaitinfo).

Synopsis

#include <signal.h>
int sigtimedwait(const sigset_t *restrict set,
                 siginfo_t *_Nullable restrict info,
                 const struct timespec *restrict timeout);
/* raw: syscall(SYS_rt_sigtimedwait, set, info, timeout, sigsetsize) */

Arguments and behavior

set selects the signals to wait for; info (optional) receives the siginfo of the consumed signal; timeout bounds the wait (NULL on the sigwaitinfo path). The raw call takes a fourth argument, sigsetsize, required to be sizeof(sigset_t) – EINVAL otherwise (rt_sigtimedwait.2, C library/kernel differences). glibc wrappers silently ignore attempts to wait for the two real-time signals NPTL uses internally (rt_sigtimedwait.2, C library/kernel differences; nptl(7)). The time64 twin rt_sigtimedwait_time64 (i386 421) swaps the timespec for its 64-bit form; semantics unchanged.

Return value and errors

Returns the signal number (> 0) on success; -1 with errno on failure (rt_sigtimedwait.2, RETURN VALUE). Errors: EAGAIN (no signal in set became pending within the timeout), EINTR (interrupted by a handler for a signal not in set), EINVAL (invalid timeout, or on the raw call a wrong sigsetsize) (rt_sigtimedwait.2, ERRORS).

Flags and constants

None (sigset built with sigset(3) macros; SA_SIGINFO-style siginfo payload returned).

Security and notes

The signals in set should be blocked before the wait, or the call can race an unblocked delivery into the handler (the classic synchronous-wait pattern from sigwaitinfo.2 DESCRIPTION). No capability is involved. EINTR semantics matter for cancellation points: POSIX makes sigtimedwait a cancellation point in threads. Interview angle: sigwaitinfo() vs sigtimedwait() vs handler-based delivery is the standard “synchronous vs asynchronous signal handling” question, and EAGAIN is the timeout answer that distinguishes it from pause(2). Cross-reference: rt_sigaction, rt_sigprocmask, rt_sigpending, pause, rt_sigqueueinfo.

References

• https://man7.org/linux/man-pages/man2/rt_sigtimedwait.2.html
• man-pages syscalls(2) version table (2.2)
• worklog: wrapper symbols sigtimedwait W @@GLIBC_2.2.5 verified with nm -D against glibc 2.41 (2026-10-17)