#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 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.
The sigtimedwait() and sigwaitinfo() functions may fail if:
The sigtimedwait() function may also fail if:
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.
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.
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:
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:
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.
The Base Definitions volume of POSIX.1-2017, <signal.h>, <time.h>
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