✏ EDIT: select.h

/* `fd_set' type and related macros, and `select'/`pselect' declarations.
   Copyright (C) 1996-2018 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
   <http://www.gnu.org/licenses/>.  */

/*	POSIX 1003.1g: 6.2 Select from File Descriptor Sets <sys/select.h>  */

#ifndef _SYS_SELECT_H
#define _SYS_SELECT_H	1

#include <features.h>

/* Get definition of needed basic types.  */
#include <bits/types.h>

/* Get __FD_* definitions.  */
#include <bits/select.h>

/* Get sigset_t.  */
#include <bits/types/sigset_t.h>

/* Get definition of timer specification structures.  */
#include <bits/types/time_t.h>
#include <bits/types/struct_timeval.h>
#ifdef __USE_XOPEN2K
# include <bits/types/struct_timespec.h>
#endif

#ifndef __suseconds_t_defined
typedef __suseconds_t suseconds_t;
# define __suseconds_t_defined
#endif

/* The fd_set member is required to be an array of longs.  */
typedef long int __fd_mask;

/* Some versions of <linux/posix_types.h> define this macros.  */
#undef	__NFDBITS
/* It's easier to assume 8-bit bytes than to get CHAR_BIT.  */
#define __NFDBITS	(8 * (int) sizeof (__fd_mask))
#define	__FD_ELT(d)	((d) / __NFDBITS)
#define	__FD_MASK(d)	((__fd_mask) (1UL << ((d) % __NFDBITS)))

/* fd_set for select and pselect.  */
typedef struct
  {
    /* XPG4.2 requires this member name.  Otherwise avoid the name
       from the global namespace.  */
#ifdef __USE_XOPEN
    __fd_mask fds_bits[__FD_SETSIZE / __NFDBITS];
# define __FDS_BITS(set) ((set)->fds_bits)
#else
    __fd_mask __fds_bits[__FD_SETSIZE / __NFDBITS];
# define __FDS_BITS(set) ((set)->__fds_bits)
#endif
  } fd_set;

/* Maximum number of file descriptors in `fd_set'.  */
#define	FD_SETSIZE		__FD_SETSIZE

#ifdef __USE_MISC
/* Sometimes the fd_set member is assumed to have this type.  */
typedef __fd_mask fd_mask;

/* Number of bits per word of `fd_set' (some code assumes this is 32).  */
# define NFDBITS		__NFDBITS
#endif

/* Access macros for `fd_set'.  */
#define	FD_SET(fd, fdsetp)	__FD_SET (fd, fdsetp)
#define	FD_CLR(fd, fdsetp)	__FD_CLR (fd, fdsetp)
#define	FD_ISSET(fd, fdsetp)	__FD_ISSET (fd, fdsetp)
#define	FD_ZERO(fdsetp)		__FD_ZERO (fdsetp)

__BEGIN_DECLS

/* Check the first NFDS descriptors each in READFDS (if not NULL) for read
   readiness, in WRITEFDS (if not NULL) for write readiness, and in EXCEPTFDS
   (if not NULL) for exceptional conditions.  If TIMEOUT is not NULL, time out
   after waiting the interval specified therein.  Returns the number of ready
   descriptors, or -1 for errors.

This function is a cancellation point and therefore not marked with
   __THROW.  */
extern int select (int __nfds, fd_set *__restrict __readfds,
		   fd_set *__restrict __writefds,
		   fd_set *__restrict __exceptfds,
		   struct timeval *__restrict __timeout);

#ifdef __USE_XOPEN2K
/* Same as above only that the TIMEOUT value is given with higher
   resolution and a sigmask which is been set temporarily.  This version
   should be used.

This function is a cancellation point and therefore not marked with
   __THROW.  */
extern int pselect (int __nfds, fd_set *__restrict __readfds,
		    fd_set *__restrict __writefds,
		    fd_set *__restrict __exceptfds,
		    const struct timespec *__restrict __timeout,
		    const __sigset_t *__restrict __sigmask);
#endif

/* Define some inlines helping to catch common problems.  */
#if __USE_FORTIFY_LEVEL > 0 && defined __GNUC__
# include <bits/select2.h>
#endif

__END_DECLS

#endif /* sys/select.h */

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