sigwaitinfo, sigtimedwait, rt_sigtimedwait - synchronously wait for queued
signals
#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() 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.
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.
- 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.
POSIX.1-2001, POSIX.1-2008.
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.
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.
kill(2),
sigaction(2),
signal(2),
signalfd(2),
sigpending(2),
sigprocmask(2),
sigqueue(3),
sigsetops(3),
sigwait(3),
signal(7),
time(7)