zZzzZ. — /usr/include/bits

Editing: select.h

/* Copyright (C) 1997-2021 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <https://www.gnu.org/licenses/>.  */

#ifndef _SYS_SELECT_H
# error "Never use <bits/select.h> directly; include <sys/select.h> instead."
#endif

/* We don't use `memset' because this would require a prototype and
   the array isn't too big.  */
#define __FD_ZERO(s) \
  do {									      \
    unsigned int __i;							      \
    fd_set *__arr = (s);						      \
    for (__i = 0; __i < sizeof (fd_set) / sizeof (__fd_mask); ++__i)	      \
      __FDS_BITS (__arr)[__i] = 0;					      \
  } while (0)
#define __FD_SET(d, s) \
  ((void) (__FDS_BITS (s)[__FD_ELT(d)] |= __FD_MASK(d)))
#define __FD_CLR(d, s) \
  ((void) (__FDS_BITS (s)[__FD_ELT(d)] &= ~__FD_MASK(d)))
#define __FD_ISSET(d, s) \
  ((__FDS_BITS (s)[__FD_ELT (d)] & __FD_MASK (d)) != 0)