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libext2fs: add a Windows implementation of the IO manager
authorPaulo Antonio Alvarez <pauloaalvarez@gmail.com>
Tue, 22 Dec 2020 18:15:50 +0000 (15:15 -0300)
committerTheodore Ts'o <tytso@mit.edu>
Wed, 31 Mar 2021 20:08:52 +0000 (16:08 -0400)
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
lib/ext2fs/windows_io.c [new file with mode: 0644]

diff --git a/lib/ext2fs/windows_io.c b/lib/ext2fs/windows_io.c
new file mode 100644 (file)
index 0000000..68b5571
--- /dev/null
@@ -0,0 +1,1041 @@
+/*
+ * windows_io.c --- This is the Windows implementation of the I/O manager.
+ *
+ * Implements a one-block write-through cache.
+ *
+ * Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001,
+ *     2002 by Theodore Ts'o.
+ *
+ * %Begin-Header%
+ * This file may be redistributed under the terms of the GNU Library
+ * General Public License, version 2.
+ * %End-Header%
+ */
+
+#include <windows.h>
+#include <winioctl.h>
+#include <io.h>
+
+#if !defined(__FreeBSD__) && !defined(__NetBSD__) && !defined(__OpenBSD__)
+#define _XOPEN_SOURCE 600
+#define _DARWIN_C_SOURCE
+#define _FILE_OFFSET_BITS 64
+#ifndef _LARGEFILE_SOURCE
+#define _LARGEFILE_SOURCE
+#endif
+#ifndef _LARGEFILE64_SOURCE
+#define _LARGEFILE64_SOURCE
+#endif
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+#endif
+
+#include "config.h"
+#include <stdio.h>
+#include <string.h>
+#if HAVE_UNISTD_H
+#include <unistd.h>
+#endif
+#if HAVE_ERRNO_H
+#include <errno.h>
+#endif
+#include <fcntl.h>
+#include <time.h>
+#if HAVE_SYS_TYPES_H
+#include <sys/types.h>
+#endif
+#define PR_GET_DUMPABLE 3
+#if HAVE_SYS_STAT_H
+#include <sys/stat.h>
+#endif
+#include <fcntl.h>
+
+#undef ALIGN_DEBUG
+
+//#define DEBUG
+#ifdef DEBUG
+#define TRACE(...) {\
+                   char __log[256];\
+                   snprintf(__log, sizeof(__log), __VA_ARGS__);\
+                   __log[sizeof(__log)-1] = 0;\
+                   OutputDebugString(__log);\
+               }
+#else
+#define TRACE(...) do { } while (0);
+#endif
+
+#include "ext2_fs.h"
+#include "ext2fs.h"
+#include "ext2fsP.h"
+
+/*
+ * For checking structure magic numbers...
+ */
+
+#define EXT2_CHECK_MAGIC(struct, code) \
+         if ((struct)->magic != (code)) return (code)
+#define EXT2_CHECK_MAGIC_RETURN(struct, code, ret) \
+         if ((struct)->magic != (code)) return (ret)
+
+#define EXT2_ET_MAGIC_WINDOWS_IO_CHANNEL  0x10ed
+
+struct windows_cache {
+       char                    *buf;
+       unsigned long long      block;
+       int                     access_time;
+       unsigned                dirty:1;
+       unsigned                in_use:1;
+};
+
+#define CACHE_SIZE 8
+#define WRITE_DIRECT_SIZE 4    /* Must be smaller than CACHE_SIZE */
+#define READ_DIRECT_SIZE 4     /* Should be smaller than CACHE_SIZE */
+
+struct windows_private_data {
+       int     magic;
+       char    name[MAX_PATH];
+       HANDLE  handle;
+       char    dos_device[MAX_PATH];
+       char    cf_device[MAX_PATH];
+       int     dev;
+       int     flags;
+       int     align;
+       int     access_time;
+       ext2_loff_t offset;
+       struct windows_cache cache[CACHE_SIZE];
+       void    *bounce;
+       struct struct_io_stats io_stats;
+};
+
+#define IS_ALIGNED(n, align) ((((uintptr_t) n) & \
+                              ((uintptr_t) ((align)-1))) == 0)
+
+static int fake_dos_name_for_device(struct windows_private_data *data)
+{
+       if (strncmp(data->name, "\\\\", 2) == 0) {
+               data->dos_device[0] = 0;
+               strcpy(data->cf_device, data->name);
+               return 0;
+       }
+
+       _snprintf(data->dos_device, MAX_PATH, "fakedevice%lu", GetCurrentProcessId());
+
+       if (!DefineDosDevice(DDD_RAW_TARGET_PATH, data->dos_device, data->name))
+               return 1;
+
+       _snprintf(data->cf_device, MAX_PATH, "\\\\.\\%s", data->dos_device);
+       TRACE("e2fsprogs::fake_dos_name_for_device::DefineDosDevice(\"%s\")", data->dos_device);
+
+       return 0;
+}
+
+static void remove_fake_dos_name(struct windows_private_data *data)
+{
+       if (*data->dos_device) {
+               TRACE("e2fsprogs::remove_fake_dos_name::DefineDosDevice(\"%s\")", data->dos_device);
+               DefineDosDevice(DDD_RAW_TARGET_PATH | DDD_EXACT_MATCH_ON_REMOVE | DDD_REMOVE_DEFINITION, data->dos_device, data->name);
+       }
+}
+
+static errcode_t windows_get_stats(io_channel channel, io_stats *stats)
+{
+       errcode_t       retval = 0;
+
+       struct windows_private_data *data;
+
+       EXT2_CHECK_MAGIC(channel, EXT2_ET_MAGIC_IO_CHANNEL);
+       data = (struct windows_private_data *) channel->private_data;
+       EXT2_CHECK_MAGIC(data, EXT2_ET_MAGIC_WINDOWS_IO_CHANNEL);
+
+       if (stats)
+               *stats = &data->io_stats;
+
+       return retval;
+}
+
+/*
+ * Here are the raw I/O functions
+ */
+static errcode_t raw_read_blk(io_channel channel,
+                             struct windows_private_data *data,
+                             unsigned long long block,
+                             int count, void *bufv)
+{
+       errcode_t       retval;
+       ssize_t size;
+       ext2_loff_t     location;
+       DWORD           actual = 0;
+       unsigned char   *buf = bufv;
+       ssize_t         really_read = 0;
+
+       size = (count < 0) ? -count : count * channel->block_size;
+       data->io_stats.bytes_read += size;
+       location = ((ext2_loff_t) block * channel->block_size) + data->offset;
+
+       if (data->flags & IO_FLAG_FORCE_BOUNCE) {
+               if (SetFilePointer(data->handle, location, NULL, FILE_BEGIN) == INVALID_SET_FILE_POINTER) {
+                       retval = GetLastError();
+                       goto error_out;
+               }
+               goto bounce_read;
+       }
+
+       if (SetFilePointer(data->handle, location, NULL, FILE_BEGIN) == INVALID_SET_FILE_POINTER) {
+               retval = GetLastError();
+               goto error_out;
+       }
+       if ((channel->align == 0) || (IS_ALIGNED(buf, channel->align) && IS_ALIGNED(size, channel->align))) {
+               if (!ReadFile(data->handle, buf, size, &actual, NULL)) {
+                       retval = GetLastError();
+                       goto error_out;
+               }
+               if (actual != size) {
+       short_read:
+               if (actual < 0) {
+                       retval = GetLastError();
+                       actual = 0;
+               } else
+                       retval = EXT2_ET_SHORT_READ;
+               goto error_out;
+               }
+       return 0;
+       }
+
+       /*
+        * The buffer or size which we're trying to read isn't aligned
+        * to the O_DIRECT rules, so we need to do this the hard way...
+        */
+bounce_read:
+       while (size > 0) {
+               if (!ReadFile(data->handle, data->bounce, channel->block_size, &actual, NULL)) {
+                       retval = GetLastError();
+                       goto error_out;
+               }
+               if (actual != channel->block_size) {
+                       actual = really_read;
+                       buf -= really_read;
+                       size += really_read;
+                       goto short_read;
+               }
+               actual = size;
+               if (size > channel->block_size)
+                       actual = channel->block_size;
+               memcpy(buf, data->bounce, actual);
+               really_read += actual;
+               size -= actual;
+               buf += actual;
+       }
+       return 0;
+
+error_out:
+       if (actual >= 0 && actual < size)
+               memset((char *) buf+actual, 0, size-actual);
+       if (channel->read_error)
+               retval = (channel->read_error)(channel, block, count, buf,
+                                              size, actual, retval);
+       return retval;
+}
+
+static errcode_t raw_write_blk(io_channel channel,
+                              struct windows_private_data *data,
+                              unsigned long long block,
+                              int count, const void *bufv)
+{
+       ssize_t         size;
+       ext2_loff_t     location;
+       DWORD           actual = 0;
+       errcode_t       retval;
+       const unsigned char *buf = bufv;
+
+       if (count == 1)
+               size = channel->block_size;
+       else {
+               if (count < 0)
+                       size = -count;
+               else
+                       size = count * channel->block_size;
+       }
+       data->io_stats.bytes_written += size;
+
+       location = ((ext2_loff_t) block * channel->block_size) + data->offset;
+
+       if (data->flags & IO_FLAG_FORCE_BOUNCE) {
+               if (SetFilePointer(data->handle, location, NULL, FILE_BEGIN) == INVALID_SET_FILE_POINTER) {
+                       retval = GetLastError();
+                       goto error_out;
+               }
+       goto bounce_write;
+       }
+
+               if (SetFilePointer(data->handle, location, NULL, FILE_BEGIN) == INVALID_SET_FILE_POINTER) {
+                       retval = GetLastError();
+               goto error_out;
+       }
+
+       SetLastError(0);
+
+       if ((channel->align == 0) || (IS_ALIGNED(buf, channel->align) && IS_ALIGNED(size, channel->align))) {
+               if (!WriteFile(data->handle, buf, size, &actual, NULL)) {
+                       retval = GetLastError();
+                       goto error_out;
+               }
+
+               if (actual != size) {
+               short_write:
+                       retval = EXT2_ET_SHORT_WRITE;
+                       goto error_out;
+               }
+               return 0;
+       }
+
+       /*
+        * The buffer or size which we're trying to write isn't aligned
+        * to the O_DIRECT rules, so we need to do this the hard way...
+        */
+bounce_write:
+       while (size > 0) {
+               if (size < channel->block_size) {
+                       if (!ReadFile(data->handle, data->bounce, channel->block_size, &actual, NULL)) {
+                               retval = GetLastError();
+                               goto error_out;
+                       }
+                       if (actual != channel->block_size) {
+                               if (actual < 0) {
+                                       retval = GetLastError();
+                                       goto error_out;
+                               }
+                               memset((char *) data->bounce + actual, 0,
+                                      channel->block_size - actual);
+                       }
+               }
+               actual = size;
+               if (size > channel->block_size)
+                       actual = channel->block_size;
+               memcpy(data->bounce, buf, actual);
+               if (SetFilePointer(data->handle, location, NULL, FILE_BEGIN) == INVALID_SET_FILE_POINTER) {
+                       retval = GetLastError();
+                       goto error_out;
+               }
+               if (!WriteFile(data->handle, data->bounce, channel->block_size, &actual, NULL)) {
+                       retval = GetLastError();
+                       goto error_out;
+               }
+
+               if (actual != channel->block_size)
+                       goto short_write;
+               size -= actual;
+               buf += actual;
+               location += actual;
+       }
+       return 0;
+
+error_out:
+       if (channel->write_error)
+               retval = (channel->write_error)(channel, block, count, buf,
+                                               size, actual, retval);
+       return retval;
+}
+
+
+/*
+ * Here we implement the cache functions
+ */
+
+/* Allocate the cache buffers */
+static errcode_t alloc_cache(io_channel channel,
+                            struct windows_private_data *data)
+{
+       errcode_t               retval;
+       struct windows_cache    *cache;
+       int                     i;
+
+       data->access_time = 0;
+       for (i=0, cache = data->cache; i < CACHE_SIZE; i++, cache++) {
+               cache->block = 0;
+               cache->access_time = 0;
+               cache->dirty = 0;
+               cache->in_use = 0;
+               if (cache->buf)
+                       ext2fs_free_mem(&cache->buf);
+               retval = io_channel_alloc_buf(channel, 0, &cache->buf);
+               if (retval)
+                       return retval;
+       }
+       if (channel->align || data->flags & IO_FLAG_FORCE_BOUNCE) {
+               if (data->bounce)
+                       ext2fs_free_mem(&data->bounce);
+               retval = io_channel_alloc_buf(channel, 0, &data->bounce);
+       }
+       return retval;
+}
+
+/* Free the cache buffers */
+static void free_cache(struct windows_private_data *data)
+{
+       struct windows_cache    *cache;
+       int                     i;
+
+       data->access_time = 0;
+       for (i=0, cache = data->cache; i < CACHE_SIZE; i++, cache++) {
+               cache->block = 0;
+               cache->access_time = 0;
+               cache->dirty = 0;
+               cache->in_use = 0;
+               if (cache->buf)
+                       ext2fs_free_mem(&cache->buf);
+       }
+       if (data->bounce)
+               ext2fs_free_mem(&data->bounce);
+}
+
+#ifndef NO_IO_CACHE
+/*
+ * Try to find a block in the cache.  If the block is not found, and
+ * eldest is a non-zero pointer, then fill in eldest with the cache
+ * entry to that should be reused.
+ */
+static struct windows_cache *find_cached_block(struct windows_private_data *data,
+                                           unsigned long long block,
+                                           struct windows_cache **eldest)
+{
+       struct windows_cache    *cache, *unused_cache, *oldest_cache;
+       int                     i;
+
+       unused_cache = oldest_cache = 0;
+       for (i=0, cache = data->cache; i < CACHE_SIZE; i++, cache++) {
+               if (!cache->in_use) {
+                       if (!unused_cache)
+                               unused_cache = cache;
+                       continue;
+               }
+               if (cache->block == block) {
+                       cache->access_time = ++data->access_time;
+                       return cache;
+               }
+               if (!oldest_cache ||
+                  (cache->access_time < oldest_cache->access_time))
+                       oldest_cache = cache;
+       }
+       if (eldest)
+               *eldest = (unused_cache) ? unused_cache : oldest_cache;
+       return 0;
+}
+
+/*
+ * Reuse a particular cache entry for another block.
+ */
+static void reuse_cache(io_channel channel, struct windows_private_data *data,
+                struct windows_cache *cache, unsigned long long block)
+{
+       if (cache->dirty && cache->in_use)
+               raw_write_blk(channel, data, cache->block, 1, cache->buf);
+
+       cache->in_use = 1;
+       cache->dirty = 0;
+       cache->block = block;
+       cache->access_time = ++data->access_time;
+}
+
+/*
+ * Flush all of the blocks in the cache
+ */
+static errcode_t flush_cached_blocks(io_channel channel,
+                                    struct windows_private_data *data,
+                                    int invalidate)
+{
+       struct windows_cache    *cache;
+       errcode_t       retval, retval2;
+       int                     i;
+
+       retval2 = 0;
+       for (i=0, cache = data->cache; i < CACHE_SIZE; i++, cache++) {
+               if (!cache->in_use)
+                       continue;
+
+               if (invalidate)
+                       cache->in_use = 0;
+
+               if (!cache->dirty)
+                       continue;
+
+               retval = raw_write_blk(channel, data,
+                                      cache->block, 1, cache->buf);
+               if (retval)
+                       retval2 = retval;
+               else
+                       cache->dirty = 0;
+       }
+       return retval2;
+}
+#endif /* NO_IO_CACHE */
+
+static errcode_t windows_open_channel(struct windows_private_data *data,
+                                  int flags, io_channel *channel,
+                                  io_manager io_mgr)
+{
+       io_channel      io = NULL;
+       errcode_t       retval;
+       ext2fs_struct_stat st;
+
+       retval = ext2fs_get_mem(sizeof(struct struct_io_channel), &io);
+       if (retval)
+               goto cleanup;
+       memset(io, 0, sizeof(struct struct_io_channel));
+       io->magic = EXT2_ET_MAGIC_IO_CHANNEL;
+
+       io->manager = io_mgr;
+       retval = ext2fs_get_mem(strlen(data->name)+1, &io->name);
+       if (retval)
+               goto cleanup;
+
+       strcpy(io->name, data->name);
+       io->private_data = data;
+       io->block_size = 1024;
+       io->read_error = 0;
+       io->write_error = 0;
+       io->refcount = 1;
+
+#if defined(O_DIRECT)
+       if (flags & IO_FLAG_DIRECT_IO)
+               io->align = ext2fs_get_dio_alignment(data->dev);
+#elif defined(F_NOCACHE)
+       if (flags & IO_FLAG_DIRECT_IO)
+               io->align = 4096;
+#endif
+
+       /*
+        * If the device is really a block device, then set the
+        * appropriate flag, otherwise we can set DISCARD_ZEROES flag
+        * because we are going to use punch hole instead of discard
+        * and if it succeed, subsequent read from sparse area returns
+        * zero.
+        */
+       if (ext2fs_fstat(data->dev, &st) == 0) {
+               if (ext2fsP_is_disk_device(st.st_mode))
+                       io->flags |= CHANNEL_FLAGS_BLOCK_DEVICE;
+               else
+                       io->flags |= CHANNEL_FLAGS_DISCARD_ZEROES;
+       }
+
+       if ((retval = alloc_cache(io, data)))
+               goto cleanup;
+
+#ifdef BLKROGET
+       if (flags & IO_FLAG_RW) {
+               int error;
+               int readonly = 0;
+
+               /* Is the block device actually writable? */
+               error = ioctl(data->dev, BLKROGET, &readonly);
+               if (!error && readonly) {
+                       retval = EPERM;
+                       goto cleanup;
+               }
+       }
+#endif
+
+       *channel = io;
+       return 0;
+
+cleanup:
+       if (data) {
+               if (data->dev >= 0)
+                       close(data->dev);
+               free_cache(data);
+               ext2fs_free_mem(&data);
+       }
+       if (io) {
+               if (io->name) {
+                       ext2fs_free_mem(&io->name);
+               }
+               ext2fs_free_mem(&io);
+       }
+       return retval;
+}
+
+static DWORD windows_open_device(struct windows_private_data *data, int open_flags)
+{
+       DWORD ret = 0;
+
+       if (*data->name != '\\')
+               strcpy(data->cf_device, data->name);
+       else if (fake_dos_name_for_device(data))
+               return -1;
+
+       DWORD desired_access = GENERIC_READ | ((open_flags & O_RDWR) ? GENERIC_WRITE : 0);
+       DWORD share_mode = (open_flags & O_EXCL) ? 0 : FILE_SHARE_READ | ((open_flags & O_RDWR) ? FILE_SHARE_WRITE : 0);
+       DWORD flags_and_attributes =
+#if defined(O_DIRECT)
+       (open_flags & O_DIRECT) ? (FILE_FLAG_NO_BUFFERING | FILE_FLAG_WRITE_THROUGH) : FILE_ATTRIBUTE_NORMAL;
+#else
+       FILE_FLAG_NO_BUFFERING | FILE_FLAG_WRITE_THROUGH;
+#endif
+
+       data->handle = CreateFile(data->cf_device, desired_access, share_mode, NULL, OPEN_EXISTING,
+                                 flags_and_attributes, NULL);
+
+       if (data->handle == INVALID_HANDLE_VALUE) {
+               ret = GetLastError();
+               goto invalid_handle;
+       }
+
+       TRACE("e2fsprogs::windows_open_device::CreateFile(\"%s\") = %p", data->cf_device, data->handle);
+
+       data->dev = _open_osfhandle((intptr_t)data->handle, 0);
+       if (data->dev < 0) {
+               ret = GetLastError() ? GetLastError() : 9999;
+               goto osfhandle_error;
+       }
+
+       TRACE("e2fsprogs::windows_open_device::_open_osfhandle(%p) = %d", data->handle, data->dev);
+
+       return 0;
+
+osfhandle_error:
+       TRACE("e2fsprogs::windows_open_device::CloseHandle(%p)", data->handle);
+       CloseHandle(data->handle);
+invalid_handle:
+       remove_fake_dos_name(data);
+       TRACE("e2fsprogs::windows_open_device() = %lu, errno = %d", ret, errno);
+       return ret;
+}
+
+static struct windows_private_data *init_private_data(const char *name, int flags)
+{
+       struct windows_private_data *data = NULL;
+
+       if (ext2fs_get_mem(sizeof(struct windows_private_data), &data))
+               return NULL;
+
+       memset(data, 0, sizeof(struct windows_private_data));
+       strncpy(data->name, name, sizeof(data->name));
+       data->magic = EXT2_ET_MAGIC_WINDOWS_IO_CHANNEL;
+       data->io_stats.num_fields = 2;
+       data->flags = flags;
+       data->handle = INVALID_HANDLE_VALUE;
+
+       return data;
+}
+
+static errcode_t windows_open(const char *name, int flags, io_channel *channel)
+{
+       int fd = -1;
+       int open_flags;
+       struct windows_private_data *data;
+
+       if (name == 0)
+               return EXT2_ET_BAD_DEVICE_NAME;
+
+       data = init_private_data(name, flags);
+       if (!data)
+               return EXT2_ET_NO_MEMORY;
+
+       open_flags = (flags & IO_FLAG_RW) ? O_RDWR : O_RDONLY;
+       if (flags & IO_FLAG_EXCLUSIVE)
+               open_flags |= O_EXCL;
+#if defined(O_DIRECT)
+       if (flags & IO_FLAG_DIRECT_IO)
+               open_flags |= O_DIRECT;
+#endif
+
+       if (windows_open_device(data, open_flags)) {
+               ext2fs_free_mem(&data);
+               return EXT2_ET_BAD_DEVICE_NAME;
+       }
+
+#if defined(F_NOCACHE) && !defined(IO_DIRECT)
+       if (flags & IO_FLAG_DIRECT_IO) {
+               if (fcntl(fd, F_NOCACHE, 1) < 0)
+                       return errno;
+       }
+#endif
+       return windows_open_channel(data, flags, channel, windows_io_manager);
+}
+
+static errcode_t windows_close(io_channel channel)
+{
+       struct windows_private_data *data;
+       errcode_t       retval = 0;
+
+       EXT2_CHECK_MAGIC(channel, EXT2_ET_MAGIC_IO_CHANNEL);
+       data = (struct windows_private_data *) channel->private_data;
+       EXT2_CHECK_MAGIC(data, EXT2_ET_MAGIC_WINDOWS_IO_CHANNEL);
+
+       if (--channel->refcount > 0)
+               return 0;
+
+#ifndef NO_IO_CACHE
+       retval = flush_cached_blocks(channel, data, 0);
+#endif
+
+       remove_fake_dos_name(data);
+
+       if (_close(data->dev) != 0)
+               retval = errno;
+
+       TRACE("e2fsprogs::windows_close::_close(%d)", data->dev);
+
+       free_cache(data);
+
+       ext2fs_free_mem(&channel->private_data);
+       if (channel->name)
+               ext2fs_free_mem(&channel->name);
+       ext2fs_free_mem(&channel);
+       return retval;
+}
+
+static errcode_t windows_set_blksize(io_channel channel, int blksize)
+{
+       struct windows_private_data *data;
+       errcode_t               retval;
+
+       EXT2_CHECK_MAGIC(channel, EXT2_ET_MAGIC_IO_CHANNEL);
+       data = (struct windows_private_data *) channel->private_data;
+       EXT2_CHECK_MAGIC(data, EXT2_ET_MAGIC_WINDOWS_IO_CHANNEL);
+
+       if (channel->block_size != blksize) {
+#ifndef NO_IO_CACHE
+               if ((retval = flush_cached_blocks(channel, data, 0)))
+                       return retval;
+#endif
+
+               channel->block_size = blksize;
+               free_cache(data);
+               if ((retval = alloc_cache(channel, data)))
+                       return retval;
+       }
+       return 0;
+}
+
+static errcode_t windows_read_blk64(io_channel channel, unsigned long long block, int count, void *buf)
+{
+       struct windows_private_data *data;
+       struct windows_cache *cache, *reuse[READ_DIRECT_SIZE];
+       errcode_t       retval;
+       char            *cp;
+       int             i, j;
+
+       EXT2_CHECK_MAGIC(channel, EXT2_ET_MAGIC_IO_CHANNEL);
+       data = (struct windows_private_data *) channel->private_data;
+       EXT2_CHECK_MAGIC(data, EXT2_ET_MAGIC_WINDOWS_IO_CHANNEL);
+
+#ifdef NO_IO_CACHE
+       return raw_read_blk(channel, data, block, count, buf);
+#else
+       /*
+        * If we're doing an odd-sized read or a very large read,
+        * flush out the cache and then do a direct read.
+        */
+       if (count < 0 || count > WRITE_DIRECT_SIZE) {
+               if ((retval = flush_cached_blocks(channel, data, 0)))
+                       return retval;
+               return raw_read_blk(channel, data, block, count, buf);
+       }
+
+       cp = buf;
+       while (count > 0) {
+               /* If it's in the cache, use it! */
+               if ((cache = find_cached_block(data, block, &reuse[0]))) {
+#ifdef DEBUG
+                       printf("Using cached block %lu\n", block);
+#endif
+                       memcpy(cp, cache->buf, channel->block_size);
+                       count--;
+                       block++;
+                       cp += channel->block_size;
+                       continue;
+               }
+               if (count == 1) {
+                       /*
+                        * Special case where we read directly into the
+                        * cache buffer; important in the O_DIRECT case
+                        */
+                       cache = reuse[0];
+                       reuse_cache(channel, data, cache, block);
+                       if ((retval = raw_read_blk(channel, data, block, 1,
+                                                  cache->buf))) {
+                               cache->in_use = 0;
+                               return retval;
+                       }
+                       memcpy(cp, cache->buf, channel->block_size);
+                       return 0;
+               }
+
+               /*
+                * Find the number of uncached blocks so we can do a
+                * single read request
+                */
+               for (i=1; i < count; i++)
+                       if (find_cached_block(data, block+i, &reuse[i]))
+                               break;
+#ifdef DEBUG
+               printf("Reading %d blocks starting at %lu\n", i, block);
+#endif
+               if ((retval = raw_read_blk(channel, data, block, i, cp)))
+                       return retval;
+
+               /* Save the results in the cache */
+               for (j=0; j < i; j++) {
+                       count--;
+                       cache = reuse[j];
+                       reuse_cache(channel, data, cache, block++);
+                       memcpy(cache->buf, cp, channel->block_size);
+                       cp += channel->block_size;
+               }
+       }
+       return 0;
+#endif /* NO_IO_CACHE */
+}
+
+static errcode_t windows_read_blk(io_channel channel, unsigned long block,
+                              int count, void *buf)
+{
+       return windows_read_blk64(channel, block, count, buf);
+}
+
+static errcode_t windows_write_blk64(io_channel channel,
+                               unsigned long long block,
+                               int count, const void *buf)
+{
+       struct windows_private_data *data;
+       struct windows_cache *cache, *reuse;
+       errcode_t       retval = 0;
+       const char      *cp;
+       int             writethrough;
+
+       EXT2_CHECK_MAGIC(channel, EXT2_ET_MAGIC_IO_CHANNEL);
+       data = (struct windows_private_data *) channel->private_data;
+       EXT2_CHECK_MAGIC(data, EXT2_ET_MAGIC_WINDOWS_IO_CHANNEL);
+
+#ifdef NO_IO_CACHE
+       return raw_write_blk(channel, data, block, count, buf);
+#else
+       /*
+        * If we're doing an odd-sized write or a very large write,
+        * flush out the cache completely and then do a direct write.
+        */
+       if (count < 0 || count > WRITE_DIRECT_SIZE) {
+               if ((retval = flush_cached_blocks(channel, data, 1)))
+                       return retval;
+               return raw_write_blk(channel, data, block, count, buf);
+       }
+
+       /*
+        * For a moderate-sized multi-block write, first force a write
+        * if we're in write-through cache mode, and then fill the
+        * cache with the blocks.
+        */
+       writethrough = channel->flags & CHANNEL_FLAGS_WRITETHROUGH;
+       if (writethrough)
+               retval = raw_write_blk(channel, data, block, count, buf);
+
+       cp = buf;
+       while (count > 0) {
+               cache = find_cached_block(data, block, &reuse);
+               if (!cache) {
+                       cache = reuse;
+                       reuse_cache(channel, data, cache, block);
+               }
+               if (cache->buf != cp)
+                       memcpy(cache->buf, cp, channel->block_size);
+               cache->dirty = !writethrough;
+               count--;
+               block++;
+               cp += channel->block_size;
+       }
+       return retval;
+#endif /* NO_IO_CACHE */
+}
+
+static errcode_t windows_cache_readahead(io_channel channel,
+                                     unsigned long long block,
+                                     unsigned long long count)
+{
+       return EXT2_ET_OP_NOT_SUPPORTED;
+}
+
+static errcode_t windows_write_blk(io_channel channel, unsigned long block,
+                               int count, const void *buf)
+{
+       return windows_write_blk64(channel, block, count, buf);
+}
+
+static errcode_t windows_write_byte(io_channel channel, unsigned long offset,
+                                int size, const void *buf)
+{
+       return EXT2_ET_UNIMPLEMENTED;
+}
+
+HANDLE windows_get_handle(io_channel channel)
+{
+       struct windows_private_data *data;
+
+       EXT2_CHECK_MAGIC_RETURN(channel, EXT2_ET_MAGIC_IO_CHANNEL, INVALID_HANDLE_VALUE);
+       data = (struct windows_private_data *) channel->private_data;
+       EXT2_CHECK_MAGIC_RETURN(data, EXT2_ET_MAGIC_WINDOWS_IO_CHANNEL, INVALID_HANDLE_VALUE);
+
+       return data->handle;
+}
+
+/*
+ * Flush data buffers to disk.
+ */
+static errcode_t windows_flush(io_channel channel)
+{
+       struct windows_private_data *data;
+       errcode_t retval = 0;
+
+       EXT2_CHECK_MAGIC(channel, EXT2_ET_MAGIC_IO_CHANNEL);
+       data = (struct windows_private_data *) channel->private_data;
+       EXT2_CHECK_MAGIC(data, EXT2_ET_MAGIC_WINDOWS_IO_CHANNEL);
+
+#ifndef NO_IO_CACHE
+       retval = flush_cached_blocks(channel, data, 0);
+#endif
+
+       return retval;
+}
+
+static errcode_t windows_set_option(io_channel channel, const char *option,
+                                const char *arg)
+{
+       struct windows_private_data *data;
+       unsigned long long tmp;
+       char *end;
+
+       EXT2_CHECK_MAGIC(channel, EXT2_ET_MAGIC_IO_CHANNEL);
+       data = (struct windows_private_data *) channel->private_data;
+       EXT2_CHECK_MAGIC(data, EXT2_ET_MAGIC_WINDOWS_IO_CHANNEL);
+
+       if (!strcmp(option, "offset")) {
+               if (!arg)
+                       return EXT2_ET_INVALID_ARGUMENT;
+
+               tmp = strtoull(arg, &end, 0);
+               if (*end)
+                       return EXT2_ET_INVALID_ARGUMENT;
+               data->offset = tmp;
+               if (data->offset < 0)
+                       return EXT2_ET_INVALID_ARGUMENT;
+               return 0;
+       }
+       return EXT2_ET_INVALID_ARGUMENT;
+}
+
+static errcode_t windows_discard(io_channel channel, unsigned long long block,
+                                unsigned long long count)
+{
+       TRACE("e2fsprogs::windows_discard::EXT2_ET_UNIMPLEMENTED");
+       return EXT2_ET_UNIMPLEMENTED;
+}
+
+/* parameters might not be used if OS doesn't support zeroout */
+#if __GNUC_PREREQ (4, 6)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wunused-parameter"
+#endif
+static errcode_t windows_zeroout(io_channel channel, unsigned long long block,
+                             unsigned long long count)
+{
+       struct windows_private_data *data;
+       int             ret;
+
+       EXT2_CHECK_MAGIC(channel, EXT2_ET_MAGIC_IO_CHANNEL);
+       data = (struct windows_private_data *) channel->private_data;
+       EXT2_CHECK_MAGIC(data, EXT2_ET_MAGIC_WINDOWS_IO_CHANNEL);
+
+       if (channel->flags & CHANNEL_FLAGS_BLOCK_DEVICE) {
+               /* Not implemented until the BLKZEROOUT mess is fixed */
+               goto unimplemented;
+       } else {
+               /* Regular file, try to use truncate/punch/zero. */
+               struct stat statbuf;
+
+               if (count == 0)
+                       return 0;
+               /*
+                * If we're trying to zero a range past the end of the file,
+                * extend the file size, then truncate everything.
+                */
+               ret = fstat(data->dev, &statbuf);
+               if (ret)
+                       goto err;
+               if ((unsigned long long) statbuf.st_size <
+                       (block + count) * channel->block_size + data->offset) {
+                       ret = ftruncate(data->dev,
+                                       (block + count) * channel->block_size + data->offset);
+                       if (ret)
+                               goto err;
+               }
+               goto unimplemented;
+       }
+err:
+       if (ret < 0) {
+               if (errno == EOPNOTSUPP)
+                       goto unimplemented;
+               return errno;
+       }
+       return 0;
+unimplemented:
+       return EXT2_ET_UNIMPLEMENTED;
+}
+
+int ext2fs_open_file(const char *pathname, int flags, mode_t mode)
+{
+       flags |= O_BINARY;
+
+       if (mode)
+#if defined(HAVE_OPEN64) && !defined(__OSX_AVAILABLE_BUT_DEPRECATED)
+               return open64(pathname, flags, mode);
+       else
+               return open64(pathname, flags);
+#else
+               return open(pathname, flags, mode);
+       else
+               return open(pathname, flags);
+#endif
+}
+
+int ext2fs_stat(const char *path, ext2fs_struct_stat *buf)
+{
+#if defined(HAVE_FSTAT64) && !defined(__OSX_AVAILABLE_BUT_DEPRECATED)
+       return stat64(path, buf);
+#else
+       return stat(path, buf);
+#endif
+}
+
+int ext2fs_fstat(int fd, ext2fs_struct_stat *buf)
+{
+#if defined(HAVE_FSTAT64) && !defined(__OSX_AVAILABLE_BUT_DEPRECATED)
+       return fstat64(fd, buf);
+#else
+       return fstat(fd, buf);
+#endif
+}
+
+#if __GNUC_PREREQ (4, 6)
+#pragma GCC diagnostic pop
+#endif
+
+static struct struct_io_manager struct_windows_manager = {
+       .magic          = EXT2_ET_MAGIC_IO_MANAGER,
+       .name           = "Windows I/O Manager",
+       .open           = windows_open,
+       .close          = windows_close,
+       .set_blksize    = windows_set_blksize,
+       .read_blk       = windows_read_blk,
+       .write_blk      = windows_write_blk,
+       .flush          = windows_flush,
+       .write_byte     = windows_write_byte,
+       .set_option     = windows_set_option,
+       .get_stats      = windows_get_stats,
+       .read_blk64     = windows_read_blk64,
+       .write_blk64    = windows_write_blk64,
+       .discard        = windows_discard,
+       .cache_readahead        = windows_cache_readahead,
+       .zeroout        = windows_zeroout,
+};
+
+io_manager windows_io_manager = &struct_windows_manager;