c - 了解 unix sys/select.h 中的 fd_set

Question

我不是很擅长 C，但我有 Java、C#、Python、Rust 等方面的经验。我目前正在尝试用 Rust 包装 fd_set，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，我了解到 __fd_mask 只是一个 long int。 __NFDBITS 只是给我一个 long int 的大小并将它乘以 8。

但我不知道它在做什么#define FD_SETSIZE __FD_SETSIZE 而且我不知道它在做什么_fd_mask __fds_bits[__FD_SETSIZE/__NFDBITS];对我来说，FD_SETSIZE 没有任何值(value)。而且我完全不知道 #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 */

Answer 1

fd_set 用来表示文件描述符集合。例如，我需要 select() 来处理 1024 个文件描述符，但是 long 只有 8 字节，所以是 64 (8 * 8) 位，所以 fd_set 自然应该表示为一个 long 数组。

 typedef struct
 {
     long fds_bits[1024 / 64];
 } fd_set;

请注意，这只是一个演示示例，但您明白了，将其与真实的 fd_set 进行比较，您就会明白。