NAME

       sigwaitinfo, sigtimedwait, rt_sigtimedwait - synchronously wait for queued signals

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <signal.h>

       int sigwaitinfo(const sigset_t *restrict set,
                       siginfo_t *_Nullable restrict info);
       int sigtimedwait(const sigset_t *restrict set,
                       siginfo_t *_Nullable 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 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 a timespec(3) structure.

       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.

VERSIONS

   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.

STANDARDS

       POSIX.1-2008.

HISTORY

       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 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.

SEE ALSO

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

Linux man-pages 6.7                                2023-10-31                                     sigwaitinfo(2)