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SIGWAITINFO(2)                                                          Linux Programmer's Manual                                                         SIGWAITINFO(2)

NAME
       sigwaitinfo, sigtimedwait, rt_sigtimedwait - synchronously wait for queued signals

SYNOPSIS
       #include <signal.h>

       int sigwaitinfo(const sigset_t *restrict set,
                       siginfo_t *restrict info);
       int sigtimedwait(const sigset_t *restrict set,
                       siginfo_t *restrict info,
                       const struct timespec *restrict 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 call‐
       ing 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 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 oc‐
       cur  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 re‐
       ceive 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 in‐
       valid 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 sup‐
       ported  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)

Linux                                                                          2021-03-22                                                                 SIGWAITINFO(2)