#include <signal.h> int sigwaitinfo(const sigset_t *set, siginfo_t *info); int sigtimedwait(const sigset_t *set, siginfo_t *info, const struct timespec *timeout);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
sigwaitinfo(), sigtimedwait(): _POSIX_C_SOURCE >= 199309L
sigwaitinfo() removes the signal from the set of pending signals and returns the signal number as its function result. If the info argument is not NULL, then the buffer that it points to is used to return a structure of type siginfo_t (see sigaction(2)) containing information about the signal.
If multiple signals in set are pending for the caller, the signal that is retrieved by sigwaitinfo() is determined according to the usual ordering rules; see signal(7) for further details.
sigtimedwait() operates in exactly the same way as sigwaitinfo() except that it has an additional argument, timeout, which specifies the interval for which the thread is suspended waiting for a signal. (This interval will be rounded up to the system clock granularity, and kernel scheduling delays mean that the interval may overrun by a small amount.) This argument is of the following type:
struct timespec {
long tv_sec; /* seconds */
long tv_nsec; /* nanoseconds */
}
If both fields of this structure are specified as 0, a poll is performed: sigtimedwait() returns immediately, either with information about a signal that was pending for the caller, or with an error if none of the signals in set was pending.
The set of signals that is pending for a given thread is the union of the set of signals that is pending specifically for that thread and the set of signals that is pending for the process as a whole (see signal(7)).
Attempts to wait for SIGKILL and SIGSTOP are silently ignored.
If multiple threads of a process are blocked waiting for the same signal(s) in sigwaitinfo() or sigtimedwait(), then exactly one of the threads will actually receive the signal if it becomes pending for the process as a whole; which of the threads receives the signal is indeterminate.
POSIX leaves the meaning of a NULL value for the timeout argument of sigtimedwait() unspecified, permitting the possibility that this has the same meaning as a call to sigwaitinfo(), and indeed this is what is done on Linux.
The glibc wrapper functions for sigwaitinfo() and sigtimedwait() silently ignore attempts to wait for the two real-time signals that are used internally by the NPTL threading implementation. See nptl(7) for details.
The original Linux system call was named sigtimedwait(). However, with the addition of real-time signals in Linux 2.2, the fixed-size, 32-bit sigset_t type supported by that system call was no longer fit for purpose. Consequently, a new system call, rt_sigtimedwait(), was added to support an enlarged sigset_t type. The new system call takes a fourth argument, size_t sigsetsize, which specifies the size in bytes of the signal set in set. This argument is currently required to have the value sizeof(sigset_t) (or the error EINVAL results). The glibc sigtimedwait() wrapper function hides these details from us, transparently calling rt_sigtimedwait() when the kernel provides it.