SIGWAITINFO

Section: Linux Programmer's Manual (2)
Updated: 2008-10-04
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NAME

sigwaitinfo, sigtimedwait - synchronously wait for queued signals

SYNOPSIS

#include <signal.h>

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

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

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

sigwaitinfo(), sigtimedwait(): _POSIX_C_SOURCE >= 199309L

DESCRIPTION

sigwaitinfo() suspends execution of the calling thread until one of the signals in set is delivered. (If one of the signals in set is already pending for the calling thread, sigwaitinfo() will return immediately with information about that signal.)

sigwaitinfo() removes the delivered signal from the set of pending signals and returns the signal number as its function result. If the info argument is not NULL, then it returns a structure of type siginfo_t (see sigaction(2)) containing information about the signal.

Signals returned via sigwaitinfo() are delivered in the usual order; see signal(7) for further details.

sigtimedwait() operates in exactly the same way as sigwaitinfo() except that it has an additional argument, timeout, which enables an upper bound to be placed on the time for which the thread is suspended. This argument is of the following type:

struct timespec {
    long    tv_sec;         /* seconds */
    long    tv_nsec;        /* nanoseconds */
}

If both fields of this structure are specified as 0, a poll is performed: sigtimedwait() returns immediately, either with information about a signal that was pending for the caller, or with an error if none of the signals in set was pending.

RETURN VALUE

On success, both sigwaitinfo() and sigtimedwait() return a signal number (i.e., a value greater than zero). On failure both calls return -1, with errno set to indicate the error.

ERRORS

EAGAIN: No signal in set was delivered within the timeout period specified to sigtimedwait().
EINTR: The wait was interrupted by a signal handler; see signal(7). (This handler was for a signal other than one of those in set.)
EINVAL: timeout was invalid.

CONFORMING TO

POSIX.1-2001.

NOTES

In normal usage, the calling program blocks the signals in set via a prior call to sigprocmask(2) (so that the default disposition for these signals does not occur if they are delivered between successive calls to sigwaitinfo() or sigtimedwait()) and does not establish handlers for these signals. In a multithreaded program, the signal should be blocked in all threads to prevent the signal being delivered to a thread other than the one calling sigwaitinfo() or sigtimedwait()).

The set of signals that is pending for a given thread is the union of the set of signals that is pending specifically for that thread and the set of signals that is pending for the process as a whole (see signal(7)).

If multiple threads of a process are blocked waiting for the same signal(s) in sigwaitinfo() or sigtimedwait(), then exactly one of the threads will actually receive the signal if it is delivered to the process as a whole; which of the threads receives the signal is indeterminate.

POSIX leaves the meaning of a NULL value for the timeout argument of sigtimedwait() unspecified, permitting the possibility that this has the same meaning as a call to sigwaitinfo(), and indeed this is what is done on Linux.

On Linux, sigwaitinfo() is a library function implemented on top of sigtimedwait().

COLOPHON

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