sigwaitinfo(2) System Calls Manual sigwaitinfo(2)
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);
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 pend-
ing for the calling thread, sigwaitinfo() will return immediately.)
sigwaitinfo() removes the signal from the set of pending signals and re-
turns 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 struc-
ture 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 or-
dering 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 inter-
val for which the thread is suspended waiting for a signal. (This in-
terval 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 per-
formed: 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 speci-
fied 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.
VERSIONS
C library/kernel differences
On Linux, sigwaitinfo() is a library function implemented on top of sig-
timedwait().
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 pur-
pose. 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 sig-
timedwait() wrapper function hides these details from us, transparently
calling rt_sigtimedwait() when the kernel provides it.
STANDARDS
POSIX.1-2008.
HISTORY
POSIX.1-2001.
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 sig-
waitinfo() 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 sig-
nal(7)).
Attempts to wait for SIGKILL and SIGSTOP are silently ignored.
If multiple threads of a process are blocked waiting for the same sig-
nal(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 indeter-
minate.
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 re-
sults 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), sigproc-
mask(2), sigqueue(3), sigsetops(3), sigwait(3), timespec(3), signal(7),
time(7)
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