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NAME
sigtimedwait, sigwaitinfo - wait for queued signals (REALTIME)
SYNOPSIS
#include <signal.h>
int sigtimedwait(const sigset_t *restrict set,
siginfo_t *restrict info,
const struct timespec *restrict timeout);
int sigwaitinfo(const sigset_t *restrict set,
siginfo_t *restrict info);
DESCRIPTION
The sigtimedwait() function shall be equivalent to sigwaitinfo() except that if none of the signals
specified by set are pending, sigtimedwait() shall wait for the time interval specified in the timespec
structure referenced by timeout. If the timespec structure pointed to by timeout is zero-valued and if
none of the signals specified by set are pending, then sigtimedwait() shall return immediately with an
error. If timeout is the NULL pointer, the behavior is unspecified. If the Monotonic Clock option is
supported, the CLOCK_MONOTONIC clock shall be used to measure the time interval specified by the timeout
argument.
The sigwaitinfo() function selects the pending signal from the set specified by set. Should any of
multiple pending signals in the range SIGRTMIN to SIGRTMAX be selected, it shall be the lowest numbered
one. The selection order between realtime and non-realtime signals, or between multiple pending non-
realtime signals, is unspecified. If no signal in set is pending at the time of the call, the calling
thread shall be suspended until one or more signals in set become pending or until it is interrupted by
an unblocked, caught signal.
The sigwaitinfo() function shall be equivalent to the sigwait() function if the info argument is NULL. If
the info argument is non-NULL, the sigwaitinfo() function shall be equivalent to sigwait(), except that
the selected signal number shall be stored in the si_signo member, and the cause of the signal shall be
stored in the si_code member. If any value is queued to the selected signal, the first such queued value
shall be dequeued and, if the info argument is non-NULL, the value shall be stored in the si_value member
of info. The system resource used to queue the signal shall be released and returned to the system for
other use. If no value is queued, the content of the si_value member is undefined. If no further signals
are queued for the selected signal, the pending indication for that signal shall be reset.
RETURN VALUE
Upon successful completion (that is, one of the signals specified by set is pending or is generated)
sigwaitinfo() and sigtimedwait() shall return the selected signal number. Otherwise, the function shall
return a value of -1 and set errno to indicate the error.
ERRORS
The sigtimedwait() function shall fail if:
EAGAIN No signal specified by set was generated within the specified timeout period.
The sigtimedwait() and sigwaitinfo() functions may fail if:
EINTR The wait was interrupted by an unblocked, caught signal. It shall be documented in system
documentation whether this error causes these functions to fail.
The sigtimedwait() function may also fail if:
EINVAL The timeout argument specified a tv_nsec value less than zero or greater than or equal to 1000
million.
An implementation only checks for this error if no signal is pending in set and it is necessary to wait.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
The sigtimedwait() function times out and returns an [EAGAIN] error. Application writers should note that
this is inconsistent with other functions such as pthread_cond_timedwait() that return [ETIMEDOUT].
RATIONALE
Existing programming practice on realtime systems uses the ability to pause waiting for a selected set of
events and handle the first event that occurs in-line instead of in a signal-handling function. This
allows applications to be written in an event-directed style similar to a state machine. This style of
programming is useful for largescale transaction processing in which the overall throughput of an
application and the ability to clearly track states are more important than the ability to minimize the
response time of individual event handling.
It is possible to construct a signal-waiting macro function out of the realtime signal function mechanism
defined in this volume of IEEE Std 1003.1-2001. However, such a macro has to include the definition of a
generalized handler for all signals to be waited on. A significant portion of the overhead of handler
processing can be avoided if the signal-waiting function is provided by the kernel. This volume of
IEEE Std 1003.1-2001 therefore provides two signal-waiting functions-one that waits indefinitely and one
with a timeout-as part of the overall realtime signal function specification.
The specification of a function with a timeout allows an application to be written that can be broken out
of a wait after a set period of time if no event has occurred. It was argued that setting a timer event
before the wait and recognizing the timer event in the wait would also implement the same functionality,
but at a lower performance level. Because of the performance degradation associated with the user-level
specification of a timer event and the subsequent cancellation of that timer event after the wait
completes for a valid event, and the complexity associated with handling potential race conditions
associated with the user-level method, the separate function has been included.
Note that the semantics of the sigwaitinfo() function are nearly identical to that of the sigwait()
function defined by this volume of IEEE Std 1003.1-2001. The only difference is that sigwaitinfo()
returns the queued signal value in the value argument. The return of the queued value is required so that
applications can differentiate between multiple events queued to the same signal number.
The two distinct functions are being maintained because some implementations may choose to implement the
POSIX Threads Extension functions and not implement the queued signals extensions. Note, though, that
sigwaitinfo() does not return the queued value if the value argument is NULL, so the POSIX Threads
Extension sigwait() function can be implemented as a macro on sigwaitinfo().
The sigtimedwait() function was separated from the sigwaitinfo() function to address concerns regarding
the overloading of the timeout pointer to indicate indefinite wait (no timeout), timed wait, and
immediate return, and concerns regarding consistency with other functions where the conditional and timed
waits were separate functions from the pure blocking function. The semantics of sigtimedwait() are
specified such that sigwaitinfo() could be implemented as a macro with a NULL pointer for timeout.
The sigwait functions provide a synchronous mechanism for threads to wait for asynchronously-generated
signals. One important question was how many threads that are suspended in a call to a sigwait() function
for a signal should return from the call when the signal is sent. Four choices were considered:
1. Return an error for multiple simultaneous calls to sigwait functions for the same signal.
2. One or more threads return.
3. All waiting threads return.
4. Exactly one thread returns.
Prohibiting multiple calls to sigwait() for the same signal was felt to be overly restrictive. The "one
or more" behavior made implementation of conforming packages easy at the expense of forcing POSIX threads
clients to protect against multiple simultaneous calls to sigwait() in application code in order to
achieve predictable behavior. There was concern that the "all waiting threads" behavior would result in
"signal broadcast storms", consuming excessive CPU resources by replicating the signals in the general
case. Furthermore, no convincing examples could be presented that delivery to all was either simpler or
more powerful than delivery to one.
Thus, the consensus was that exactly one thread that was suspended in a call to a sigwait function for a
signal should return when that signal occurs. This is not an onerous restriction as:
* A multi-way signal wait can be built from the single-way wait.
* Signals should only be handled by application-level code, as library routines cannot guess what the
application wants to do with signals generated for the entire process.
* Applications can thus arrange for a single thread to wait for any given signal and call any needed
routines upon its arrival.
In an application that is using signals for interprocess communication, signal processing is typically
done in one place. Alternatively, if the signal is being caught so that process cleanup can be done, the
signal handler thread can call separate process cleanup routines for each portion of the application.
Since the application main line started each portion of the application, it is at the right abstraction
level to tell each portion of the application to clean up.
Certainly, there exist programming styles where it is logical to consider waiting for a single signal in
multiple threads. A simple sigwait_multiple() routine can be constructed to achieve this goal. A possible
implementation would be to have each sigwait_multiple() caller registered as having expressed interest in
a set of signals. The caller then waits on a thread-specific condition variable. A single server thread
calls a sigwait() function on the union of all registered signals. When the sigwait() function returns,
the appropriate state is set and condition variables are broadcast. New sigwait_multiple() callers may
cause the pending sigwait() call to be canceled and reissued in order to update the set of signals being
waited for.
FUTURE DIRECTIONS
None.
SEE ALSO
Realtime Signals , pause() , pthread_sigmask() , sigaction() , sigpending() , sigsuspend() , sigwait() ,
the Base Definitions volume of IEEE Std 1003.1-2001, <signal.h>, <time.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition,
Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The
original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 SIGTIMEDWAIT(P)