title: Understanding fd_set in unix sys/select.h
body: <p>I am not really fluent in C but I have experience in Java, C#, Python, Rust etc. I am currently trying to wrap fd_set in Rust but I have absolutely no idea how to read this code.</p>
  
  <p>I am only interested in this part:</p>
  
  <pre><code>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)-&gt;fds_bits)
  #else
      __fd_mask __fds_bits[__FD_SETSIZE / __NFDBITS];
  # define __FDS_BITS(set) ((set)-&gt;__fds_bits)
  #endif
    } fd_set;
  </code></pre>
  
  <p>So far I understood that <code>__fd_mask</code> is just a long int. <code>__NFDBITS</code> just gives me the size of a long int and multiplies it by 8. </p>
  
  <p>But I have no idea what this is doing <code>#define  FD_SETSIZE      __FD_SETSIZE</code> and there for I don't know what this is doing <code>_fd_mask __fds_bits[__FD_SETSIZE / __NFDBITS];</code> For me <code>FD_SETSIZE</code> has no value at all. And I have absolutely no idea what <code># define __FDS_BITS(set) ((set)-&gt;__fds_bits)</code> is doing.</p>
  
  <p>I really would like to see how this code would look in java or c# etc</p>
  
  <p>Full header file:</p>
  
  <pre><code>/* `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
     &lt;http://www.gnu.org/licenses/&gt;.  */
  
  /*  POSIX 1003.1g: 6.2 Select from File Descriptor Sets &lt;sys/select.h&gt;  */
  
  #ifndef _SYS_SELECT_H
  #define _SYS_SELECT_H   1
  
  #include &lt;features.h&gt;
  
  /* Get definition of needed basic types.  */
  #include &lt;bits/types.h&gt;
  
  /* Get __FD_* definitions.  */
  #include &lt;bits/select.h&gt;
  
  /* Get __sigset_t.  */
  #include &lt;bits/sigset.h&gt;
  
  #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 &lt;time.h&gt;
  #define __need_timeval
  #include &lt;bits/time.h&gt;
  
  #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 &lt;linux/posix_types.h&gt; 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 &lt;&lt; ((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)-&gt;fds_bits)
  #else
      __fd_mask __fds_bits[__FD_SETSIZE / __NFDBITS];
  # define __FDS_BITS(set) ((set)-&gt;__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 &gt; 0 &amp;&amp; defined __GNUC__
  # include &lt;bits/select2.h&gt;
  #endif
  
  __END_DECLS
  
  #endif /* sys/select.h */
  </code></pre>