sigwait(2)sigwait(2)NAMEsigwait(), sigwaitinfo(), sigtimedwait() - synchronously accept a sig‐
The function atomically selects and clears a pending signal from set
and returns the signal number in the location pointed to by sig. If
none of the signals in set is pending at the time of the call, the
calling thread will be suspended until one or more signals become pend‐
ing or the thread is interrupted by an unblocked, caught signal. The
signals in set should be blocked at the time of the call to Otherwise,
the behavior is undefined.
If there are multiple signals queued for the selected signal number,
will return with the first queued signal and the remainder will remain
queued. If any of multiple pending signals in the range to is
selected, the lowest numbered signal will be returned. The selection
order between realtime and nonrealtime signals, or between multiple
pending nonrealtime signals, is unspecified.
If more than one thread in a process is in for the same signal, only
one thread will return from with the signal number; which thread
returns is undefined.
has the same behavior as if the info parameter is If the info parameter
is not has the same behavior as except that the selected signal number
is returned in the si_signo field of the info parameter and the cause
of the signal is returned in the si_code field. If any value is queued
to the selected signal, the first such queued value will be dequeued
and stored in the si_value member of info and the system resource used
to queue the signal will be released and made available to queue other
signals. If no value is queued, the contents of the si_value member is
undefined. If no further signals are queued for the selected signal,
the pending indication for that signal will be reset.
has the same behavior as except that will only wait for the time inter‐
val specified by the timeout parameter if none of the signals specified
by set are pending at the time of the call. If the timeout parameter
specifies a zero-valued time interval, then will return immediately
with an error if no signals in set are pending at the time of the call.
If the timeout parameter is the behavior is undefined.
For a given signal number, the sigwait family of routines should not be
used in conjunction with or any other functions which change signal
action. If they are used together, the results are undefined.
The sigwait family of routines enable a thread to synchronously wait
for signals. This makes the sigwait routines ideal for handling sig‐
nals in a multithreaded process. The suggested method for signal han‐
dling in a multithreaded process is to have all threads block the sig‐
nals of interest and dedicate one thread to call a sigwait function to
wait for the signals. When a signal causes a sigwait function to
return, the code to handle the signal can be placed immediately after
the return from the sigwait routine. After the signal is handled, a
sigwait function can again be called to wait for another signal.
In order to ensure that the dedicated thread handles the signal, it is
essential that all threads, including the thread issuing the sigwait
call, block the signals of interest. Otherwise, the signal could be
delivered to a thread other than the dedicated signal handling thread.
This could result in the default action being carried out for the sig‐
nal. It is important that the thread issuing the sigwait call also
block the signal. This will prevent signals from carrying out the
default signal action while the dedicated signal handling thread is
between calls to a sigwait function.
Upon successful completion, stores the signal number selected in the
location pointed to by sig and returns with a value of (zero). Other‐
wise, it returns an error number to indicate the error. The variable
is NOT set if an error occurs.
Upon successful completion, and will return the selected signal number.
Otherwise a value of -1 is returned and is set to indicate the error.
If any of the following conditions occur, the sigwait family of rou‐
tines will return the following error number:
[EAGAIN] was called and no signal in the set parameter was
delivered within the time interval specified by
the timeout parameter.
If any of the following conditions occur and the condition is detected,
the sigwait family of routines will fail and return the following error
[EINVAL] set contains an invalid or unsupported signal
[EINVAL] was called and the timeout parameter specified a
tv_nsec value less than zero or greater than or
equal to 1000 million, or a tv_sec value less
than zero or greater than or equal to 2147483648
(that is, a value too large to be represented as
a signed 32-bit integer).
[EINTR] The wait was interrupted by an unblocked, caught
[EFAULT] At least one of the set, sig, info, or timeout
parameters references an illegal address.
AUTHORsigwaitinfo() and sigtimedwait() were derived from the IEEE POSIX
sigwait() was derived from the IEEE POSIX P1003.1c standard.
SEE ALSOpause(2), sigaction(2), sigpending(2), sigsuspend(2), pthread_sig‐