NAME

       sigwaitinfo, sigtimedwait, rt_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 pending (If one
       of the signals in set is already pending for the calling thread, sigwaitinfo() will return immediately.)

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

       If multiple signals in set are pending for the caller, the signal that is retrieved by  sigwaitinfo()  is
       determined according to the usual ordering rules; see signal(7) for further details.

       sigtimedwait()  operates  in  exactly  the  same  way  as  sigwaitinfo() except that it has an additional
       argument, timeout, which specifies the interval for which the thread is suspended waiting for  a  signal.
       (This interval will be rounded up to the system clock granularity, and kernel scheduling delays mean that
       the interval may overrun by a small amount.)  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 became pending 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, POSIX.1-2008.

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 become pending 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, in order to prevent the signal being
       treated according to its default disposition in 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)).

       Attempts to wait for SIGKILL and SIGSTOP are silently ignored.

       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  becomes  pending
       for the process as a whole; which of the threads receives the signal is indeterminate.

       sigwaitinfo()  or sigtimedwait(), can't be used to receive signals that are synchronously generated, such
       as the SIGSEGV signal that results from accessing an invalid memory address or  the  SIGFPE  signal  that
       results from an arithmetic error.  Such signals can be caught only via signal handler.

       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.

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

       The glibc wrapper functions for sigwaitinfo() and sigtimedwait() silently ignore attempts to wait for the
       two real-time signals that are used internally by the NPTL threading  implementation.   See  nptl(7)  for
       details.

       The original Linux system call was named sigtimedwait().  However, with the addition of real-time signals
       in Linux 2.2, the fixed-size, 32-bit sigset_t type supported by that system call was no  longer  fit  for
       purpose.   Consequently,  a new system call, rt_sigtimedwait(), was added to support an enlarged sigset_t
       type.  The new system call takes a fourth argument, size_t sigsetsize, which specifies the size in  bytes
       of the signal set in set.  This argument is currently required to have the value sizeof(sigset_t) (or the
       error EINVAL  results).   The  glibc  sigtimedwait()  wrapper  function  hides  these  details  from  us,
       transparently calling rt_sigtimedwait() when the kernel provides it.

SEE ALSO

       kill(2),  sigaction(2), signal(2), signalfd(2), sigpending(2), sigprocmask(2), sigqueue(3), sigsetops(3),
       sigwait(3), signal(7), time(7)

COLOPHON

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