NAME¶

sigwaitinfo, sigtimedwait, rt_sigtimedwait - synchronously wait for queued signals

LIBRARY¶

Standard C library (libc, -lc)

SYNOPSIS¶

#include <signal.h>

int sigwaitinfo(const sigset_t *restrict set,
                siginfo_t *_Nullable restrict info);
int sigtimedwait(const sigset_t *restrict set,
                siginfo_t *_Nullable restrict info,
                const struct timespec *restrict timeout);

sigwaitinfo(), sigtimedwait():

    _POSIX_C_SOURCE >= 199309L

DESCRIPTION¶

sigwaitinfo() suspends execution of the calling thread until one of the signals in set is pending (If one of the signals in set is already pending for the calling thread, sigwaitinfo() will return immediately.)

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 a timespec(3) structure.

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.

RETURN VALUE¶

On success, both sigwaitinfo() and sigtimedwait() return a signal number (i.e., a value greater than zero). On failure both calls return -1, with errno set to indicate the error.

ERRORS¶

EAGAIN

No signal in set became pending within the timeout period specified to sigtimedwait().

EINTR

The wait was interrupted by a signal handler; see signal(7). (This handler was for a signal other than one of those in set.)

EINVAL

timeout was invalid.

STANDARDS¶

POSIX.1-2001, POSIX.1-2008.

NOTES¶

In normal usage, the calling program blocks the signals in set via a prior call to sigprocmask(2) (so that the default disposition for these signals does not occur if they become pending between successive calls to sigwaitinfo() or sigtimedwait()) and does not establish handlers for these signals. In a multithreaded program, the signal should be blocked in all threads, in order to prevent the signal being treated according to its default disposition in a thread other than the one calling sigwaitinfo() or sigtimedwait()).

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.

sigwaitinfo() or sigtimedwait(), can't be used to receive signals that are synchronously generated, such as the SIGSEGV signal that results from accessing an invalid memory address or the SIGFPE signal that results from an arithmetic error. Such signals can be caught only via signal handler.

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.

SEE ALSO¶

kill(2), sigaction(2), signal(2), signalfd(2), sigpending(2), sigprocmask(2), sigqueue(3), sigsetops(3), sigwait(3), timespec(3), signal(7), time(7)