sysdeps/unix/bsd/telldir.c

   1 /* Copyright (C) 1994-2013 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 Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the 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    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <http://www.gnu.org/licenses/>.  */
  17
  18 #include <errno.h>
  19 #include <stddef.h>
  20 #include <dirent.h>
  21 #include <unistd.h>
  22 #include <sys/types.h>
  23 #include <stdlib.h>
  24 #include "dirstream.h"
  25
  26 /* Internal data structure for telldir and seekdir.  */
  27 struct record
  28   {
  29     struct record *next; /* Link in chain.  */
  30     off_t cookie;               /* Value returned by `telldir'.  */
  31     off_t pos;
  32     size_t offset;
  33   };
  34 #define NBUCKETS 32
  35 static struct record *records[32];
  36 static off_t lastpos;
  37 __libc_lock_define_initialized(static, lock) /* Locks above data.  */
  38
  39
  40 /* Return the current position of DIRP.  */
  41 long int
  42 telldir (dirp)
  43      DIR *dirp;
  44 {
  45   struct record *new;
  46   long int pos;
  47
  48   new = malloc (sizeof *new);
  49   if (new == NULL)
  50     return -1l;
  51
  52   __libc_lock_lock (lock);
  53
  54   new->pos = dirp->filepos;
  55   new->offset = dirp->offset;
  56   new->cookie = ++lastpos;
  57   new->next = records[new->cookie % NBUCKETS];
  58   records[new->cookie % NBUCKETS] = new;
  59
  60   pos = new->cookie;
  61
  62   __libc_lock_unlock (lock);
  63
  64   return pos;
  65 }
  66
  67 \f
  68
  69 /* Seek to position POS in DIRP.  */
  70 void
  71 seekdir (dirp, pos)
  72      DIR *dirp;
  73      long int pos;
  74 {
  75   struct record *r, **prevr;
  76
  77   __libc_lock_lock (lock);
  78
  79   for (prevr = &records[pos % NBUCKETS], r = *prevr;
  80        r != NULL;
  81        prevr = &r->next, r = r->next)
  82     if (r->cookie == pos)
  83       {
  84         __libc_lock_lock (dirp->__lock);
  85         if (dirp->filepos != r->pos || dirp->offset != r->offset)
  86           {
  87             dirp->size = 0;     /* Must read a fresh buffer.  */
  88             /* Move to the saved position.  */
  89             __lseek (dirp->fd, r->pos, SEEK_SET);
  90             dirp->filepos = r->pos;
  91             dirp->offset = 0;
  92             /* Read entries until we reach the saved offset.  */
  93             while (dirp->offset < r->offset)
  94               {
  95                 struct dirent *scan;
  96                 __libc_lock_unlock (dirp->__lock);
  97                 scan = readdir (dirp);
  98                 __libc_lock_lock (dirp->__lock);
  99                 if (! scan)
 100                   break;
 101               }
 102           }
 103         __libc_lock_unlock (dirp->__lock);
 104
 105         /* To prevent leaking memory, cookies returned from telldir
 106            can only be used once.  So free this one's record now.  */
 107         *prevr = r->next;
 108         free (r);
 109         break;
 110       }
 111
 112   __libc_lock_unlock (lock);
 113
 114   /* If we lost there is no way to indicate it.  Oh well.  */
 115 }