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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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SIGTIMEDWAIT(3P) POSIX Programmer's Manual SIGTIMEDWAIT(3P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
sigtimedwait, sigwaitinfo — wait for queued signals
#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);
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, except that the return value and the error reporting
method are different (see RETURN VALUE), and that if the info
argument is non-NULL, 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.
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.
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 should only check for this error if no signal is
pending in set and it is necessary to wait.
The following sections are informative.
None.
The sigtimedwait() function times out and returns an [EAGAIN]
error. Application developers should note that this is
inconsistent with other functions such as pthread_cond_timedwait()
that return [ETIMEDOUT].
Note that in order to ensure that generated signals are queued and
signal values passed to sigqueue() are available in si_value,
applications which use sigwaitinfo() or sigtimedwait() need to set
the SA_SIGINFO flag for each signal in the set (see Section 2.4,
Signal Concepts). This means setting each signal to be handled by
a three-argument signal-catching function, even if the handler
will never be called. It is not possible (portably) to set a
signal handler to SIG_DFL while setting the SA_SIGINFO flag,
because assigning to the sa_handler member of struct sigaction
instead of the sa_sigaction member would result in undefined
behavior, and SIG_DFL need not be assignment-compatible with
sa_sigaction. Even if an assignment of SIG_DFL to sa_sigaction is
accepted by the compiler, the implementation need not treat this
value as special—it could just be taken as the address of a
signal-catching function.
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
POSIX.1‐2017. 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 POSIX.1‐2017 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 POSIX.1‐2017. 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.
None.
Section 2.4, Signal Concepts, Section 2.8.1, Realtime Signals,
pause(3p), pthread_sigmask(3p), sigaction(3p), sigpending(3p),
sigsuspend(3p), sigwait(3p)
The Base Definitions volume of POSIX.1‐2017, signal.h(0p),
time.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 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 .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 SIGTIMEDWAIT(3P)
Pages that refer to this page: signal.h(0p), sigwait(3p), sigwaitinfo(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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