sysdeps/unix/sysv/linux/getdents.c

   1 /* Copyright (C) 1993, 95, 96, 97, 98, 99, 2000 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 <assert.h>
  21 #include <errno.h>
  22 #include <dirent.h>
  23 #include <stddef.h>
  24 #include <string.h>
  25 #include <unistd.h>
  26 #include <sys/param.h>
  27 #include <sys/types.h>
  28
  29 #include <sysdep.h>
  30 #include <sys/syscall.h>
  31
  32 #include <linux/posix_types.h>
  33
  34 #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
  35
  36
  37 extern int __syscall_getdents (int fd, char *buf, size_t nbytes);
  38
  39 /* For Linux we need a special version of this file since the
  40    definition of `struct dirent' is not the same for the kernel and
  41    the libc.  There is one additional field which might be introduced
  42    in the kernel structure in the future.
  43
  44    Here is the kernel definition of `struct dirent' as of 2.1.20:  */
  45
  46 struct kernel_dirent
  47   {
  48     long int d_ino;
  49     __kernel_off_t d_off;
  50     unsigned short int d_reclen;
  51     char d_name[256];
  52   };
  53
  54 #ifdef GETDENTS64
  55 # define __getdents __getdents64
  56 # define dirent dirent64
  57 #endif
  58
  59 /* The problem here is that we cannot simply read the next NBYTES
  60    bytes.  We need to take the additional field into account.  We use
  61    some heuristic.  Assuming the directory contains names with 14
  62    characters on average we can compute an estimated number of entries
  63    which fit in the buffer.  Taking this number allows us to specify a
  64    reasonable number of bytes to read.  If we should be wrong, we can
  65    reset the file descriptor.  In practice the kernel is limiting the
  66    amount of data returned much more then the reduced buffer size.  */
  67 ssize_t
  68 internal_function
  69 __getdents (int fd, char *buf, size_t nbytes)
  70 {
  71   off_t last_offset = -1;
  72   size_t red_nbytes;
  73   struct kernel_dirent *skdp, *kdp;
  74   struct dirent *dp;
  75   int retval;
  76   const size_t size_diff = (offsetof (struct dirent, d_name)
  77                             - offsetof (struct kernel_dirent, d_name));
  78
  79   red_nbytes = MIN (nbytes
  80                     - ((nbytes / (offsetof (struct dirent, d_name) + 14))
  81                        * size_diff),
  82                     nbytes - size_diff);
  83
  84   dp = (struct dirent *) buf;
  85   skdp = kdp = __alloca (red_nbytes);
  86
  87   retval = INLINE_SYSCALL (getdents, 3, fd, (char *) kdp, red_nbytes);
  88
  89   if (retval == -1)
  90     return -1;
  91
  92   while ((char *) kdp < (char *) skdp + retval)
  93     {
  94       const size_t alignment = __alignof__ (struct dirent);
  95       /* Since kdp->d_reclen is already aligned for the kernel structure
  96          this may compute a value that is bigger than necessary.  */
  97       size_t new_reclen = ((kdp->d_reclen + size_diff + alignment - 1)
  98                            & ~(alignment - 1));
  99       if ((char *) dp + new_reclen > buf + nbytes)
 100         {
 101           /* Our heuristic failed.  We read too many entries.  Reset
 102              the stream.  */
 103           assert (last_offset != -1);
 104           __lseek (fd, last_offset, SEEK_SET);
 105
 106           if ((char *) dp == buf)
 107             {
 108               /* The buffer the user passed in is too small to hold even
 109                  one entry.  */
 110               __set_errno (EINVAL);
 111               return -1;
 112             }
 113
 114           break;
 115         }
 116
 117       last_offset = kdp->d_off;
 118       dp->d_ino = kdp->d_ino;
 119       dp->d_off = kdp->d_off;
 120       dp->d_reclen = new_reclen;
 121       dp->d_type = DT_UNKNOWN;
 122       memcpy (dp->d_name, kdp->d_name,
 123               kdp->d_reclen - offsetof (struct kernel_dirent, d_name));
 124
 125       dp = (struct dirent *) ((char *) dp + new_reclen);
 126       kdp = (struct kernel_dirent *) (((char *) kdp) + kdp->d_reclen);
 127     }
 128
 129   return (char *) dp - buf;
 130 }