sysdeps/unix/sysv/linux/getdents.c

   1 /* Copyright (C) 1993, 1995, 1996, 1997 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Library General Public License as
   6    published by the Free Software Foundation; either version 2 of the
   7    License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Library General Public License for more details.
  13
  14    You should have received a copy of the GNU Library General Public
  15    License along with the GNU C Library; see the file COPYING.LIB.  If not,
  16    write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  17    Boston, MA 02111-1307, USA.  */
  18
  19 #include <alloca.h>
  20 #include <dirent.h>
  21 #include <stddef.h>
  22 #include <string.h>
  23 #include <unistd.h>
  24 #include <sys/param.h>
  25 #include <sys/types.h>
  26
  27 #include <linux/posix_types.h>
  28
  29 #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
  30
  31
  32 extern int __getdents __P ((int fd, char *buf, size_t nbytes));
  33
  34 /* For Linux we need a special version of this file since the
  35    definition of `struct dirent' is not the same for the kernel and
  36    the libc.  There is one additional field which might be introduced
  37    in the kernel structure in the future.
  38
  39    He is the kernel definition of `struct dirent' as of 2.1.20:  */
  40
  41 struct kernel_dirent
  42   {
  43     long int d_ino;
  44     __kernel_off_t d_off;
  45     unsigned short int d_reclen;
  46     char d_name[256];
  47   };
  48
  49 #ifdef GETDENTS64
  50 #define __getdirentries __getdirentries64
  51 #define dirent dirent64
  52 #endif
  53
  54 /* The problem here is that we cannot simply read the next NBYTES
  55    bytes.  We need to take the additional field into account.  We use
  56    some heuristic.  Assuming the directory contains names with 14
  57    characters on average we can compute an estimate number of entries
  58    which fit in the buffer.  Taking this number allows us to specify a
  59    correct number of bytes to read.  If we should be wrong, we can reset
  60    the file descriptor.  */
  61 ssize_t
  62 __getdirentries (fd, buf, nbytes, basep)
  63      int fd;
  64      char *buf;
  65      size_t nbytes;
  66      off_t *basep;
  67 {
  68   off_t base = __lseek (fd, (off_t) 0, SEEK_CUR);
  69   off_t last_offset = base;
  70   size_t red_nbytes;
  71   struct kernel_dirent *skdp, *kdp;
  72   struct dirent *dp;
  73   int retval;
  74   const size_t size_diff = (offsetof (struct dirent, d_name)
  75                             - offsetof (struct kernel_dirent, d_name));
  76
  77   red_nbytes = nbytes - ((nbytes / (offsetof (struct dirent, d_name) + 14))
  78                          * size_diff);
  79
  80   dp = (struct dirent *) buf;
  81   skdp = kdp = __alloca (red_nbytes);
  82
  83   retval = __getdents (fd, (char *) kdp, red_nbytes);
  84
  85   while ((char *) kdp < (char *) skdp + retval)
  86     {
  87       const size_t alignment = __alignof__ (struct dirent);
  88       /* Since kdp->d_reclen is already aligned for the kernel structure
  89          this may compute a value that is bigger than necessary.  */
  90       size_t new_reclen = ((kdp->d_reclen + size_diff + alignment - 1)
  91                            & ~(alignment - 1));
  92       if ((char *) dp + new_reclen > buf + nbytes)
  93         {
  94           /* Our heuristic failed.  We read too many entries.  Reset
  95              the stream.  */
  96           __lseek (fd, last_offset, SEEK_SET);
  97           break;
  98         }
  99
 100       last_offset = kdp->d_off;
 101       dp->d_ino = kdp->d_ino;
 102       dp->d_off = kdp->d_off;
 103       dp->d_reclen = new_reclen;
 104       dp->d_type = DT_UNKNOWN;
 105       memcpy (dp->d_name, kdp->d_name,
 106               kdp->d_reclen - offsetof (struct kernel_dirent, d_name));
 107
 108       dp = (struct dirent *) ((char *) dp + new_reclen);
 109       kdp = (struct kernel_dirent *) (((char *) kdp) + kdp->d_reclen);
 110     }
 111
 112   if (basep)
 113     *basep = base;
 114
 115   return (char *) dp - buf;
 116 }
 117
 118 #ifndef GETDENTS64
 119 weak_alias (__getdirentries, getdirentries)
 120 #endif