我对 C 语言不是很流利，但我在 Java、C#、Python、Rust 等方面有经验。我目前正在尝试将 fd_set 包装在 Rust 中，但我完全不知道如何阅读这段代码。
我只对这部分感兴趣：
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;
到目前为止，我明白这__fd_mask只是一个长整数。__NFDBITS只是给我一个 long int 的大小并将其乘以 8。
但我不知道这是在做什么#define FD_SETSIZE __FD_SETSIZE，因为我不知道这是在做什么_fd_mask __fds_bits[__FD_SETSIZE / __NFDBITS];对我来说FD_SETSIZE根本没有价值。我完全不知道# define __FDS_BITS(set) ((set)->__fds_bits)在做什么。
我真的很想看看这段代码在 java 或 c# 等中的样子
完整的头文件：
/* `fd_set' type and related macros, and `select'/`pselect' declarations.
Copyright (C) 1996-2003, 2009, 2011 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/sigset.h>
#ifndef __sigset_t_defined
# define __sigset_t_defined
typedef __sigset_t sigset_t;
#endif
/* Get definition of timer specification structures. */
#define __need_time_t
#define __need_timespec
#include <time.h>
#define __need_timeval
#include <bits/time.h>
#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) 1 << ((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 */