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NAME
sigwaitinfo, sigtimedwait, rt_sigtimedwait - synchronously wait for queued signals
SYNOPSIS
#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
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 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.
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 was 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.
CONFORMING TO
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.
C library/kernel differences
On Linux, sigwaitinfo() is a library function implemented on top of sigtimedwait().
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.
SEE ALSO
kill(2), sigaction(2), signal(2), signalfd(2), sigpending(2), sigprocmask(2), sigqueue(3),
sigsetops(3), sigwait(3), signal(7), time(7)
COLOPHON
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