sigwait(2)

NAME

sigwait(), sigwaitinfo(), sigtimedwait() — synchronously accept a signal

SYNOPSIS

#include <signal.h>

int sigwait(const sigset_t *set, int *sig);

int sigwaitinfo(const sigset_t *set, siginfo_t *info);

int sigtimedwait(const sigset_t *set, siginfo_t *info,
const struct timespec *timeout);

DESCRIPTION

The sigwait() 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 pending or the thread is interrupted by an unblocked, caught signal. The signals in set should be blocked at the time of the call to sigwait(). Otherwise, the behavior is undefined.

If there are multiple signals queued for the selected signal number, sigwait() will return with the first queued signal and the remainder will remain queued. If any of multiple pending signals in the range SIGRTMIN to SIGRTMAX 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 sigwait() for the same signal, only one thread will return from sigwait() with the signal number; which thread returns is undefined.

sigwaitinfo() has the same behavior as sigwait() if the info parameter is NULL. If the info parameter is not NULL, sigwaitinfo() has the same behavior as sigwait(), 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.

sigtimedwait() has the same behavior as sigwaitinfo() except that sigtimedwait() will only wait for the time interval 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 sigtimedwait() will return immediately with an error if no signals in set are pending at the time of the call. If the timeout parameter is NULL, the behavior is undefined.

APPLICATION USAGE

For a given signal number, the sigwait family of routines should not be used in conjunction with sigaction() or any other functions which change signal action. If they are used together, the results are undefined.

Threads Considerations

The sigwait family of routines enable a thread to synchronously wait for signals. This makes the sigwait routines ideal for handling signals in a multithreaded process. The suggested method for signal handling in a multithreaded process is to have all threads block the signals 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 signal. 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.

RETURN VALUE

Upon successful completion, sigwait() stores the signal number selected in the location pointed to by sig and returns with a value of 0 (zero). Otherwise, it returns an error number to indicate the error. The errno variable is NOT set if an error occurs.

Upon successful completion, sigwaitinfo() and sigtimedwait() will return the selected signal number. Otherwise a value of -1 is returned and errno is set to indicate the error.

ERRORS

If any of the following conditions occur, the sigwait family of routines will return the following error number:

[EAGAIN]: sigtimedwait() 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 number:

[EINVAL]: set contains an invalid or unsupported signal number.
[EINVAL]: sigtimedwait() 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 signal.
[EFAULT]: At least one of the set, sig, info, or timeout parameters references an illegal address.

AUTHOR

sigwaitinfo() and sigtimedwait() were derived from the IEEE POSIX P1003.1b standard.

sigwait() was derived from the IEEE POSIX P1003.1c standard.

STANDARDS CONFORMANCE

sigwait(): POSIX.1c

sigwaitinfo(): POSIX.1b

sigtimedwait(): POSIX.1b