Question

我不是很精通C，但我有Java，C＃，Python，Rust等方面的经验。我目前正在尝试在Rust中包装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，我理解__fd_mask只是一个长整数。 __NFDBITS只给我一个long int的大小并将它乘以8.

但我不知道这是做什么#define FD_SETSIZE __FD_SETSIZE因为我不知道这是做什么_fd_mask __fds_bits[__FD_SETSIZE / __NFDBITS];对我而言FD_SETSIZE根本没有任何价值。我完全不知道# define __FDS_BITS(set) ((set)->__fds_bits)正在做什么。

我真的很想看看这个代码在java或c＃etc中的外观如何

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 */

Answer 1

fd_set用于表示文件描述符集。例如，我需要select()来处理1024个文件描述符，但long只有8个字节，因此64（8 * 8}位，所以自然fd_set应该表示为long的数组。

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

请注意，这只是一个示例，但您明白了，将其与真实的fd_set进行比较，您会看到。

了解unix sys / select.h中的fd_set

1 个答案: