resource.c: Cleanup and refactor
[wimlib] / src / resource.c
index 69ccf38..b38c65d 100644 (file)
@@ -1,11 +1,11 @@
 /*
  * resource.c
  *
- * Read uncompressed and compressed metadata and file resources.
+ * Read uncompressed and compressed metadata and file resources from a WIM file.
  */
 
 /*
- * Copyright (C) 2012 Eric Biggers
+ * Copyright (C) 2012, 2013 Eric Biggers
  *
  * This file is part of wimlib, a library for working with WIM files.
  *
  * wimlib; if not, see http://www.gnu.org/licenses/.
  */
 
-#include "config.h"
+#ifdef HAVE_CONFIG_H
+#  include "config.h"
+#endif
 
-#include <stdlib.h>
-#include <stdarg.h>
+#include "wimlib.h"
+#include "wimlib/endianness.h"
+#include "wimlib/error.h"
+#include "wimlib/file_io.h"
+#include "wimlib/lookup_table.h"
+#include "wimlib/resource.h"
+#include "wimlib/sha1.h"
+
+#ifdef __WIN32__
+/* for read_win32_file_prefix(), read_win32_encrypted_file_prefix() */
+#  include "wimlib/win32.h"
+#endif
 
 #ifdef WITH_NTFS_3G
-#include <ntfs-3g/attrib.h>
-#include <ntfs-3g/inode.h>
-#include <ntfs-3g/dir.h>
+/* for read_ntfs_file_prefix() */
+#  include "wimlib/ntfs_3g.h"
 #endif
 
-#include "wimlib_internal.h"
-#include "lookup_table.h"
-#include "io.h"
-#include "lzx.h"
-#include "xpress.h"
-#include "sha1.h"
-#include "dentry.h"
-#include <unistd.h>
-#include <errno.h>
 #ifdef HAVE_ALLOCA_H
-#include <alloca.h>
+#  include <alloca.h>
 #endif
-
+#include <errno.h>
+#include <fcntl.h>
+#include <stdlib.h>
+#include <unistd.h>
 
 /*
- * Reads all or part of a compressed resource into an in-memory buffer.
+ *                         Compressed WIM resources
+ *
+ * A compressed resource in a WIM consists of a number of compressed chunks,
+ * each of which decompresses to a fixed chunk size (given in the WIM header;
+ * usually 32768) except possibly the last, which always decompresses to any
+ * remaining bytes.  In addition, immediately before the chunks, a table (the
+ * "chunk table") provides the offset, in bytes relative to the end of the chunk
+ * table, of the start of each compressed chunk, except for the first chunk
+ * which is omitted as it always has an offset of 0.  Therefore, a compressed
+ * resource with N chunks will have a chunk table with N - 1 entries.
  *
- * @fp:                The FILE* for the WIM file.
- * @resource_compressed_size:           The compressed size of the resource.
- * @resource_uncompressed_size:  The uncompressed size of the resource.
- * @resource_offset:            The offset of the start of the resource from
- *                                     the start of the stream @fp.
- * @resource_ctype:    The compression type of the resource.
- * @len:               The number of bytes of uncompressed data to read from
- *                             the resource.
- * @offset:            The offset of the bytes to read within the uncompressed
- *                             resource.
- * @contents_len:      An array into which the uncompressed data is written.
- *                             It must be at least @len bytes long.
+ * Additional information:
  *
- * Returns zero on success, nonzero on failure.
+ * - Entries in the chunk table are 4 bytes each, except if the uncompressed
+ *   size of the resource is greater than 4 GiB, in which case the entries in
+ *   the chunk table are 8 bytes each.  In either case, the entries are unsigned
+ *   little-endian integers.
+ *
+ * - The chunk table is included in the compressed size of the resource provided
+ *   in the corresponding entry in the WIM's stream lookup table.
+ *
+ * - The compressed size of a chunk is never greater than the uncompressed size.
+ *   From the compressor's point of view, chunks that would have compressed to a
+ *   size greater than or equal to their original size are in fact stored
+ *   uncompressed.  From the decompresser's point of view, chunks with
+ *   compressed size equal to their uncompressed size are in fact uncompressed.
+ *
+ * Furthermore, wimlib supports its own "pipable" WIM format, and for this the
+ * structure of compressed resources was modified to allow piped reading and
+ * writing.  To make sequential writing possible, the chunk table is placed
+ * after the chunks rather than before the chunks, and to make sequential
+ * reading possible, each chunk is prefixed with a 4-byte header giving its
+ * compressed size as a 32-bit, unsigned, little-endian integer.  Otherwise the
+ * details are the same.
  */
-static int read_compressed_resource(FILE *fp, u64 resource_compressed_size,
-                                   u64 resource_uncompressed_size,
-                                   u64 resource_offset, int resource_ctype,
-                                   u64 len, u64 offset, u8  contents_ret[])
-{
 
-       DEBUG2("comp size = %"PRIu64", uncomp size = %"PRIu64", "
-              "res offset = %"PRIu64"",
-              resource_compressed_size,
-              resource_uncompressed_size,
-              resource_offset);
-       DEBUG2("resource_ctype = %s, len = %"PRIu64", offset = %"PRIu64"",
-              wimlib_get_compression_type_string(resource_ctype), len, offset);
-       /* Trivial case */
-       if (len == 0)
-               return 0;
 
-       int (*decompress)(const void *, uint, void *, uint);
-       /* Set the appropriate decompress function. */
-       if (resource_ctype == WIM_COMPRESSION_TYPE_LZX)
-               decompress = lzx_decompress;
-       else
-               decompress = xpress_decompress;
-
-       /* The structure of a compressed resource consists of a table of chunk
-        * offsets followed by the chunks themselves.  Each chunk consists of
-        * compressed data, and there is one chunk for each WIM_CHUNK_SIZE =
-        * 32768 bytes of the uncompressed file, with the last chunk having any
-        * remaining bytes.
-        *
-        * The chunk offsets are measured relative to the end of the chunk
-        * table.  The first chunk is omitted from the table in the WIM file
-        * because its offset is implicitly given by the fact that it directly
-        * follows the chunk table and therefore must have an offset of 0.
-        */
-
-       /* Calculate how many chunks the resource conists of in its entirety. */
-       u64 num_chunks = (resource_uncompressed_size + WIM_CHUNK_SIZE - 1) /
-                                                               WIM_CHUNK_SIZE;
-       /* As mentioned, the first chunk has no entry in the chunk table. */
-       u64 num_chunk_entries = num_chunks - 1;
-
-
-       /* The index of the chunk that the read starts at. */
-       u64 start_chunk = offset / WIM_CHUNK_SIZE;
-       /* The byte offset at which the read starts, within the start chunk. */
-       u64 start_chunk_offset = offset % WIM_CHUNK_SIZE;
-
-       /* The index of the chunk that contains the last byte of the read. */
-       u64 end_chunk   = (offset + len - 1) / WIM_CHUNK_SIZE;
-       /* The byte offset of the last byte of the read, within the end chunk */
-       u64 end_chunk_offset = (offset + len - 1) % WIM_CHUNK_SIZE;
-
-       /* Number of chunks that are actually needed to read the requested part
-        * of the file. */
-       u64 num_needed_chunks = end_chunk - start_chunk + 1;
-
-       /* If the end chunk is not the last chunk, an extra chunk entry is
-        * needed because we need to know the offset of the chunk after the last
-        * chunk read to figure out the size of the last read chunk. */
-       if (end_chunk != num_chunks - 1)
-               num_needed_chunks++;
-
-       /* Declare the chunk table.  It will only contain offsets for the chunks
-        * that are actually needed for this read. */
-       u64 chunk_offsets[num_needed_chunks];
-
-       /* Set the implicit offset of the first chunk if it is included in the
-        * needed chunks.
-        *
-        * Note: M$'s documentation includes a picture that shows the first
-        * chunk starting right after the chunk entry table, labeled as offset
-        * 0x10.  However, in the actual file format, the offset is measured
-        * from the end of the chunk entry table, so the first chunk has an
-        * offset of 0. */
-       if (start_chunk == 0)
-               chunk_offsets[0] = 0;
-
-       /* According to M$'s documentation, if the uncompressed size of
-        * the file is greater than 4 GB, the chunk entries are 8-byte
-        * integers.  Otherwise, they are 4-byte integers. */
-       u64 chunk_entry_size = (resource_uncompressed_size >= (u64)1 << 32) ?
-                                                                       8 : 4;
-
-       /* Size of the full chunk table in the WIM file. */
-       u64 chunk_table_size = chunk_entry_size * num_chunk_entries;
-
-       /* Read the needed chunk offsets from the table in the WIM file. */
-
-       /* Index, in the WIM file, of the first needed entry in the
-        * chunk table. */
-       u64 start_table_idx = (start_chunk == 0) ? 0 : start_chunk - 1;
-
-       /* Number of entries we need to actually read from the chunk
-        * table (excludes the implicit first chunk). */
-       u64 num_needed_chunk_entries = (start_chunk == 0) ?
-                               num_needed_chunks - 1 : num_needed_chunks;
-
-       /* Skip over unneeded chunk table entries. */
-       u64 file_offset_of_needed_chunk_entries = resource_offset +
-                               start_table_idx * chunk_entry_size;
-       if (fseeko(fp, file_offset_of_needed_chunk_entries, SEEK_SET) != 0) {
-               ERROR_WITH_ERRNO("Failed to seek to byte %"PRIu64" to read "
-                                "chunk table of compressed resource",
-                                file_offset_of_needed_chunk_entries);
-               return WIMLIB_ERR_READ;
+/* Decompress the specified chunk that uses the specified compression type
+ * @ctype, part of a WIM with default chunk size @wim_chunk_size.  For LZX the
+ * separate @wim_chunk_size is needed because it determines the window size used
+ * for LZX compression.  */
+static int
+decompress(const void *cchunk, unsigned clen,
+          void *uchunk, unsigned ulen,
+          int ctype, u32 wim_chunk_size)
+{
+       switch (ctype) {
+       case WIMLIB_COMPRESSION_TYPE_XPRESS:
+               return wimlib_xpress_decompress(cchunk,
+                                               clen,
+                                               uchunk,
+                                               ulen);
+       case WIMLIB_COMPRESSION_TYPE_LZX:
+               return wimlib_lzx_decompress2(cchunk,
+                                             clen,
+                                             uchunk,
+                                             ulen,
+                                             wim_chunk_size);
+       default:
+               wimlib_assert(0);
+               return -1;
        }
+}
 
-       /* Number of bytes we need to read from the chunk table. */
-       size_t size = num_needed_chunk_entries * chunk_entry_size;
+/* Read data from a compressed WIM resource.  Assumes parameters were already
+ * verified by read_partial_wim_resource().  */
+static int
+read_compressed_wim_resource(const struct wim_lookup_table_entry * const lte,
+                            const u64 size, const consume_data_callback_t cb,
+                            const u32 cb_chunk_size, void * const ctx_or_buf,
+                            const int flags, const u64 offset)
+{
+       int ret;
+       int errno_save;
 
-       u8 chunk_tab_buf[size];
+       const u32 orig_chunk_size = wim_resource_chunk_size(lte);
+       const u32 orig_chunk_order = bsr32(orig_chunk_size);
 
-       if (fread(chunk_tab_buf, 1, size, fp) != size)
-               goto err;
+       wimlib_assert(is_power_of_2(orig_chunk_size));
 
-       /* Now fill in chunk_offsets from the entries we have read in
-        * chunk_tab_buf. */
+       /* Handle the trivial case.  */
+       if (size == 0)
+               return 0;
 
-       u64 *chunk_tab_p = chunk_offsets;
-       if (start_chunk == 0)
-               chunk_tab_p++;
+       u64 *chunk_offsets = NULL;
+       u8 *out_buf = NULL;
+       u8 *tmp_buf = NULL;
+       void *compressed_buf = NULL;
+       bool chunk_offsets_malloced = false;
+       bool out_buf_malloced = false;
+       bool tmp_buf_malloced = false;
+       bool compressed_buf_malloced = false;
 
-       if (chunk_entry_size == 4) {
-               u32 *entries = (u32*)chunk_tab_buf;
-               while (num_needed_chunk_entries--)
-                       *chunk_tab_p++ = le32_to_cpu(*entries++);
-       } else {
-               u64 *entries = (u64*)chunk_tab_buf;
-               while (num_needed_chunk_entries--)
-                       *chunk_tab_p++ = le64_to_cpu(*entries++);
-       }
+       /* Get the file descriptor for the WIM.  */
+       struct filedes * const in_fd = &lte->wim->in_fd;
 
-       /* Done with the chunk table now.  We must now seek to the first chunk
-        * that is needed for the read. */
+       /* Determine if we're reading a pipable resource from a pipe or not.  */
+       const bool is_pipe_read = !filedes_is_seekable(in_fd);
 
-       u64 file_offset_of_first_needed_chunk = resource_offset +
-                               chunk_table_size + chunk_offsets[0];
-       if (fseeko(fp, file_offset_of_first_needed_chunk, SEEK_SET) != 0) {
-               ERROR_WITH_ERRNO("Failed to seek to byte %"PRIu64" to read "
-                                "first chunk of compressed resource",
-                                file_offset_of_first_needed_chunk);
-               return WIMLIB_ERR_READ;
-       }
+       /* Calculate the number of chunks the resource is divided into.  */
+       const u64 num_chunks = wim_resource_chunks(lte);
 
-       /* Pointer to current position in the output buffer for uncompressed
-        * data. */
-       u8 *out_p = (u8*)contents_ret;
-
-       /* Buffer for compressed data.  While most compressed chunks will have a
-        * size much less than WIM_CHUNK_SIZE, WIM_CHUNK_SIZE - 1 is the maximum
-        * size in the worst-case.  This assumption is valid only if chunks that
-        * happen to compress to more than the uncompressed size (i.e. a
-        * sequence of random bytes) are always stored uncompressed. But this seems
-        * to be the case in M$'s WIM files, even though it is undocumented. */
-       u8 compressed_buf[WIM_CHUNK_SIZE - 1];
-
-
-       /* Decompress all the chunks. */
-       for (u64 i = start_chunk; i <= end_chunk; i++) {
-
-               DEBUG2("Chunk %"PRIu64" (start %"PRIu64", end %"PRIu64").",
-                      i, start_chunk, end_chunk);
-
-               /* Calculate the sizes of the compressed chunk and of the
-                * uncompressed chunk. */
-               uint compressed_chunk_size, uncompressed_chunk_size;
-               if (i != num_chunks - 1) {
-                       /* All the chunks except the last one in the resource
-                        * expand to WIM_CHUNK_SIZE uncompressed, and the amount
-                        * of compressed data for the chunk is given by the
-                        * difference of offsets in the chunk offset table. */
-                       compressed_chunk_size = chunk_offsets[i + 1 - start_chunk] -
-                                               chunk_offsets[i - start_chunk];
-                       uncompressed_chunk_size = WIM_CHUNK_SIZE;
+       /* Calculate the 0-based index of the chunk at which the read starts.
+        */
+       const u64 start_chunk = offset >> orig_chunk_order;
+
+       /* For pipe reads, we always must start from the 0th chunk.  */
+       const u64 actual_start_chunk = (is_pipe_read ? 0 : start_chunk);
+
+       /* Calculate the offset, within the start chunk, of the first byte of
+        * the read.  */
+       const u32 start_offset_in_chunk = offset & (orig_chunk_size - 1);
+
+       /* Calculate the index of the chunk that contains the last byte of the
+        * read.  */
+       const u64 end_chunk = (offset + size - 1) >> orig_chunk_order;
+
+       /* Calculate the offset, within the end chunk, of the last byte of the
+        * read.  */
+       const u32 end_offset_in_chunk = (offset + size - 1) & (orig_chunk_size - 1);
+
+       /* Calculate the number of entries in the chunk table; it's one less
+        * than the number of chunks, since the first chunk has no entry.  */
+       const u64 num_chunk_entries = num_chunks - 1;
+
+       /* Set the size of each chunk table entry based on the resource's
+        * uncompressed size.  */
+       const u64 chunk_entry_size = (wim_resource_size(lte) > (1ULL << 32)) ? 8 : 4;
+
+       /* Calculate the size, in bytes, of the full chunk table.  */
+       const u64 chunk_table_size = num_chunk_entries * chunk_entry_size;
+
+       /* Current offset to read from.  */
+       u64 cur_read_offset = lte->resource_entry.offset;
+       if (!is_pipe_read) {
+               /* Read the chunk table into memory.  */
+
+               /* Calculate the number of chunk entries are actually needed to
+                * read the requested part of the resource.  Include an entry
+                * for the first chunk even though that doesn't exist in the
+                * on-disk table, but take into account that if the last chunk
+                * required for the read is not the last chunk of the resource,
+                * an extra chunk entry is needed so that the compressed size of
+                * the last chunk of the read can be determined.  */
+               const u64 num_alloc_chunk_entries = end_chunk - start_chunk +
+                                                   1 + (end_chunk != num_chunks - 1);
+
+               /* Allocate a buffer to hold a subset of the chunk table.  It
+                * will only contain offsets for the chunks that are actually
+                * needed for this read.  For speed, allocate the buffer on the
+                * stack unless it's too large.  */
+               if ((size_t)(num_alloc_chunk_entries * sizeof(u64)) !=
+                           (num_alloc_chunk_entries * sizeof(u64)))
+                       goto oom;
+
+               if (num_alloc_chunk_entries <= STACK_MAX / sizeof(u64)) {
+                       chunk_offsets = alloca(num_alloc_chunk_entries * sizeof(u64));
                } else {
-                       /* The last compressed chunk consists of the remaining
-                        * bytes in the file resource, and the last uncompressed
-                        * chunk has size equal to however many bytes are left-
-                        * that is, the remainder of the uncompressed size when
-                        * divided by WIM_CHUNK_SIZE.
-                        *
-                        * Note that the resource_compressed_size includes the
-                        * chunk table, so the size of it must be subtracted. */
-                       compressed_chunk_size = resource_compressed_size -
-                                               chunk_table_size -
-                                               chunk_offsets[i - start_chunk];
-
-                       uncompressed_chunk_size = resource_uncompressed_size %
-                                                               WIM_CHUNK_SIZE;
-
-                       /* If the remainder is 0, the last chunk actually
-                        * uncompresses to a full WIM_CHUNK_SIZE bytes. */
-                       if (uncompressed_chunk_size == 0)
-                               uncompressed_chunk_size = WIM_CHUNK_SIZE;
+                       chunk_offsets = MALLOC(num_alloc_chunk_entries * sizeof(u64));
+                       if (chunk_offsets == NULL)
+                               goto oom;
+                       chunk_offsets_malloced = true;
                }
 
-               DEBUG2("compressed_chunk_size = %u, "
-                      "uncompressed_chunk_size = %u",
-                      compressed_chunk_size, uncompressed_chunk_size);
+               /* Set the implicit offset of the first chunk if it's included
+                * in the needed chunks.  */
+               if (start_chunk == 0)
+                       chunk_offsets[0] = 0;
+
+               /* Calculate the index of the first needed entry in the chunk
+                * table.  */
+               const u64 start_table_idx = (start_chunk == 0) ? 0 : start_chunk - 1;
+
+               /* Calculate the number of entries that need to be read from the
+                * chunk table.  */
+               const u64 num_needed_chunk_entries = (start_chunk == 0) ?
+                                       num_alloc_chunk_entries - 1 : num_alloc_chunk_entries;
+
+               /* Calculate the number of bytes of data that need to be read
+                * from the chunk table.  */
+               const size_t chunk_table_needed_size =
+                                       num_needed_chunk_entries * chunk_entry_size;
+
+               /* Calculate the byte offset, in the WIM file, of the first
+                * chunk table entry to read.  Take into account that if the WIM
+                * file is in the special "pipable" format, then the chunk table
+                * is at the end of the resource, not the beginning.  */
+               const u64 file_offset_of_needed_chunk_entries =
+                       lte->resource_entry.offset
+                       + (start_table_idx * chunk_entry_size)
+                       + (lte->is_pipable ? (lte->resource_entry.size - chunk_table_size) : 0);
+
+               /* Read the needed chunk table entries into the end of the
+                * chunk_offsets buffer.  */
+               void * const chunk_tab_data = (u8*)&chunk_offsets[num_alloc_chunk_entries] -
+                                             chunk_table_needed_size;
+               ret = full_pread(in_fd, chunk_tab_data, chunk_table_needed_size,
+                                file_offset_of_needed_chunk_entries);
+               if (ret)
+                       goto read_error;
+
+               /* Now fill in chunk_offsets from the entries we have read in
+                * chunk_tab_data.  Careful: chunk_offsets aliases
+                * chunk_tab_data, which breaks C's aliasing rules when we read
+                * 32-bit integers and store 64-bit integers.  But since the
+                * operations are safe as long as the compiler doesn't mess with
+                * their order, we use the gcc may_alias extension to tell the
+                * compiler that loads from the 32-bit integers may alias stores
+                * to the 64-bit integers.  */
+               {
+                       typedef le64 __attribute__((may_alias)) aliased_le64_t;
+                       typedef le32 __attribute__((may_alias)) aliased_le32_t;
+                       u64 * const chunk_offsets_p = chunk_offsets + (start_chunk == 0);
+                       u64 i;
+
+                       if (chunk_entry_size == 4) {
+                               aliased_le32_t *raw_entries = (aliased_le32_t*)chunk_tab_data;
+                               for (i = 0; i < num_needed_chunk_entries; i++)
+                                       chunk_offsets_p[i] = le32_to_cpu(raw_entries[i]);
+                       } else {
+                               aliased_le64_t *raw_entries = (aliased_le64_t*)chunk_tab_data;
+                               for (i = 0; i < num_needed_chunk_entries; i++)
+                                       chunk_offsets_p[i] = le64_to_cpu(raw_entries[i]);
+                       }
+               }
 
+               /* Set offset to beginning of first chunk to read.  */
+               cur_read_offset += chunk_table_size + chunk_offsets[0];
+       }
 
-               /* Figure out how much of this chunk we actually need to read */
-               u64 start_offset;
-               if (i == start_chunk)
-                       start_offset = start_chunk_offset;
-               else
-                       start_offset = 0;
-               u64 end_offset;
-               if (i == end_chunk)
-                       end_offset = end_chunk_offset;
-               else
-                       end_offset = WIM_CHUNK_SIZE - 1;
-
-               u64 partial_chunk_size = end_offset + 1 - start_offset;
-               bool is_partial_chunk = (partial_chunk_size !=
-                                               uncompressed_chunk_size);
-
-               DEBUG2("start_offset = %u, end_offset = %u", start_offset,
-                                       end_offset);
-               DEBUG2("partial_chunk_size = %u", partial_chunk_size);
-
-               /* This is undocumented, but chunks can be uncompressed.  This
-                * appears to always be the case when the compressed chunk size
-                * is equal to the uncompressed chunk size. */
-               if (compressed_chunk_size == uncompressed_chunk_size) {
-                       /* Probably an uncompressed chunk */
-
-                       if (start_offset != 0) {
-                               if (fseeko(fp, start_offset, SEEK_CUR) != 0) {
-                                       ERROR_WITH_ERRNO("Uncompressed partial "
-                                                        "chunk fseek() error");
-                                       return WIMLIB_ERR_READ;
-                               }
-                       }
-                       if (fread(out_p, 1, partial_chunk_size, fp) !=
-                                       partial_chunk_size)
-                               goto err;
+       /* If using a callback function, allocate a temporary buffer that will
+        * be used to pass data to it.  If writing directly to a buffer instead,
+        * arrange to write data directly into it.  */
+       size_t out_buf_size;
+       u8 *out_buf_end, *out_p;
+       if (cb) {
+               out_buf_size = max(cb_chunk_size, orig_chunk_size);
+               if (out_buf_size <= STACK_MAX) {
+                       out_buf = alloca(out_buf_size);
+               } else {
+                       out_buf = MALLOC(out_buf_size);
+                       if (out_buf == NULL)
+                               goto oom;
+                       out_buf_malloced = true;
+               }
+       } else {
+               out_buf_size = size;
+               out_buf = ctx_or_buf;
+       }
+       out_buf_end = out_buf + out_buf_size;
+       out_p = out_buf;
+
+       /* Unless the raw compressed data was requested, allocate a temporary
+        * buffer for reading compressed chunks, each of which can be at most
+        * @orig_chunk_size - 1 bytes.  This excludes compressed chunks that are
+        * a full @orig_chunk_size bytes, which are actually stored
+        * uncompressed.  */
+       if (!(flags & WIMLIB_READ_RESOURCE_FLAG_RAW_CHUNKS)) {
+               if (orig_chunk_size - 1 <= STACK_MAX) {
+                       compressed_buf = alloca(orig_chunk_size - 1);
                } else {
-                       /* Compressed chunk */
-                       int ret;
+                       compressed_buf = MALLOC(orig_chunk_size - 1);
+                       if (compressed_buf == NULL)
+                               goto oom;
+                       compressed_buf_malloced = true;
+               }
+       }
 
-                       /* Read the compressed data into compressed_buf. */
-                       if (fread(compressed_buf, 1, compressed_chunk_size,
-                                               fp) != compressed_chunk_size)
-                               goto err;
+       /* Allocate yet another temporary buffer, this one for decompressing
+        * chunks for which only part of the data is needed.  */
+       if (start_offset_in_chunk != 0 ||
+           (end_offset_in_chunk != orig_chunk_size - 1 &&
+            offset + size != wim_resource_size(lte)))
+       {
+               if (orig_chunk_size <= STACK_MAX) {
+                       tmp_buf = alloca(orig_chunk_size);
+               } else {
+                       tmp_buf = MALLOC(orig_chunk_size);
+                       if (tmp_buf == NULL)
+                               goto oom;
+                       tmp_buf_malloced = true;
+               }
+       }
 
-                       /* For partial chunks we must buffer the uncompressed
-                        * data because we don't need all of it. */
-                       if (is_partial_chunk) {
-                               u8 uncompressed_buf[uncompressed_chunk_size];
+       /* Read, and possibly decompress, each needed chunk, either writing the
+        * data directly into the @ctx_or_buf buffer or passing it to the @cb
+        * callback function.  */
+       for (u64 i = actual_start_chunk; i <= end_chunk; i++) {
 
-                               ret = decompress(compressed_buf,
-                                               compressed_chunk_size,
-                                               uncompressed_buf,
-                                               uncompressed_chunk_size);
-                               if (ret != 0)
-                                       return WIMLIB_ERR_DECOMPRESSION;
-                               memcpy(out_p, uncompressed_buf + start_offset,
-                                               partial_chunk_size);
+               /* Calculate uncompressed size of next chunk.  */
+               u32 chunk_usize;
+               if ((i == num_chunks - 1) && (wim_resource_size(lte) & (orig_chunk_size - 1)))
+                       chunk_usize = (wim_resource_size(lte) & (orig_chunk_size - 1));
+               else
+                       chunk_usize = orig_chunk_size;
+
+               /* Calculate compressed size of next chunk.  */
+               u32 chunk_csize;
+               if (is_pipe_read) {
+                       struct pwm_chunk_hdr chunk_hdr;
+
+                       ret = full_pread(in_fd, &chunk_hdr,
+                                        sizeof(chunk_hdr), cur_read_offset);
+                       if (ret)
+                               goto read_error;
+                       chunk_csize = le32_to_cpu(chunk_hdr.compressed_size);
+               } else {
+                       if (i == num_chunks - 1) {
+                               chunk_csize = lte->resource_entry.size -
+                                             chunk_table_size -
+                                             chunk_offsets[i - start_chunk];
+                               if (lte->is_pipable)
+                                       chunk_csize -= num_chunks * sizeof(struct pwm_chunk_hdr);
                        } else {
-                               ret = decompress(compressed_buf,
-                                               compressed_chunk_size,
-                                               out_p,
-                                               uncompressed_chunk_size);
-                               if (ret != 0)
-                                       return WIMLIB_ERR_DECOMPRESSION;
+                               chunk_csize = chunk_offsets[i + 1 - start_chunk] -
+                                             chunk_offsets[i - start_chunk];
                        }
                }
+               if (chunk_csize == 0 || chunk_csize > orig_chunk_size) {
+                       ERROR("Invalid chunk size in compressed resource!");
+                       errno = EINVAL;
+                       ret = WIMLIB_ERR_INVALID_CHUNK_SIZE;
+                       goto out_free_memory;
+               }
+               if (lte->is_pipable)
+                       cur_read_offset += sizeof(struct pwm_chunk_hdr);
 
-               /* Advance the pointer into the uncompressed output data by the
-                * number of uncompressed bytes that were written.  */
-               out_p += partial_chunk_size;
-       }
+               if (i >= start_chunk) {
+                       /* Calculate how much of this chunk needs to be read.  */
+                       u32 chunk_needed_size;
+                       u32 start_offset = 0;
+                       u32 end_offset = orig_chunk_size - 1;
 
-       return 0;
+                       if (flags & WIMLIB_READ_RESOURCE_FLAG_RAW_CHUNKS) {
+                               chunk_needed_size = chunk_csize;
+                       } else {
+                               if (i == start_chunk)
+                                       start_offset = start_offset_in_chunk;
 
-err:
-       if (feof(fp))
-               ERROR("Unexpected EOF in compressed file resource");
-       else
-               ERROR_WITH_ERRNO("Error reading compressed file resource");
-       return WIMLIB_ERR_READ;
-}
+                               if (i == end_chunk)
+                                       end_offset = end_offset_in_chunk;
 
-/*
- * Reads uncompressed data from an open file stream.
- */
-int read_uncompressed_resource(FILE *fp, u64 offset, u64 len,
-                              u8 contents_ret[])
-{
-       if (fseeko(fp, offset, SEEK_SET) != 0) {
-               ERROR("Failed to seek to byte %"PRIu64" of input file "
-                     "to read uncompressed resource (len = %"PRIu64")",
-                     offset, len);
-               return WIMLIB_ERR_READ;
-       }
-       if (fread(contents_ret, 1, len, fp) != len) {
-               if (feof(fp)) {
-                       ERROR("Unexpected EOF in uncompressed file resource");
-               } else {
-                       ERROR("Failed to read %"PRIu64" bytes from "
-                             "uncompressed resource at offset %"PRIu64,
-                             len, offset);
-               }
-               return WIMLIB_ERR_READ;
-       }
-       return 0;
-}
+                               chunk_needed_size = end_offset + 1 - start_offset;
+                       }
 
+                       if (chunk_csize == chunk_usize ||
+                           (flags & WIMLIB_READ_RESOURCE_FLAG_RAW_CHUNKS))
+                       {
+                               /* Read the raw chunk data.  */
+
+                               ret = full_pread(in_fd,
+                                                out_p,
+                                                chunk_needed_size,
+                                                cur_read_offset + start_offset);
+                               if (ret)
+                                       goto read_error;
+                       } else {
+                               /* Read and decompress the chunk.  */
 
+                               u8 *target;
 
+                               ret = full_pread(in_fd,
+                                                compressed_buf,
+                                                chunk_csize,
+                                                cur_read_offset);
+                               if (ret)
+                                       goto read_error;
 
-/* Reads the contents of a struct resource_entry, as represented in the on-disk
- * format, from the memory pointed to by @p, and fills in the fields of @entry.
- * A pointer to the byte after the memory read at @p is returned. */
-const u8 *get_resource_entry(const u8 *p, struct resource_entry *entry)
-{
-       u64 size;
-       u8 flags;
+                               if (chunk_needed_size == chunk_usize)
+                                       target = out_p;
+                               else
+                                       target = tmp_buf;
 
-       p = get_u56(p, &size);
-       p = get_u8(p, &flags);
-       entry->size = size;
-       entry->flags = flags;
+                               ret = decompress(compressed_buf,
+                                                chunk_csize,
+                                                target,
+                                                chunk_usize,
+                                                wim_resource_compression_type(lte),
+                                                orig_chunk_size);
+                               if (ret) {
+                                       ERROR("Failed to decompress data!");
+                                       ret = WIMLIB_ERR_DECOMPRESSION;
+                                       errno = EINVAL;
+                                       goto out_free_memory;
+                               }
+                               if (chunk_needed_size != chunk_usize)
+                                       memcpy(out_p, tmp_buf + start_offset,
+                                              chunk_needed_size);
+                       }
 
-       /* offset and original_size are truncated to 62 bits to avoid possible
-        * overflows, when converting to a signed 64-bit integer (off_t) or when
-        * adding size or original_size.  This is okay since no one would ever
-        * actually have a WIM bigger than 4611686018427387903 bytes... */
-       p = get_u64(p, &entry->offset);
-       if (entry->offset & 0xc000000000000000ULL) {
-               WARNING("Truncating offset in resource entry");
-               entry->offset &= 0x3fffffffffffffffULL;
+                       out_p += chunk_needed_size;
+
+                       if (cb) {
+                               /* Feed the data to the callback function.  */
+
+                               if (flags & WIMLIB_READ_RESOURCE_FLAG_RAW_CHUNKS) {
+                                       ret = cb(out_buf, out_p - out_buf, ctx_or_buf);
+                                       if (ret)
+                                               goto out_free_memory;
+                                       out_p = out_buf;
+                               } else if (i == end_chunk || out_p == out_buf_end) {
+                                       size_t bytes_sent;
+                                       const u8 *p;
+
+                                       for (p = out_buf; p != out_p; p += bytes_sent) {
+                                               bytes_sent = min(cb_chunk_size, out_p - p);
+                                               ret = cb(p, bytes_sent, ctx_or_buf);
+                                               if (ret)
+                                                       goto out_free_memory;
+                                       }
+                                       out_p = out_buf;
+                               }
+                       }
+                       cur_read_offset += chunk_csize;
+               } else {
+                       u8 dummy;
+
+                       /* Skip data only.  */
+                       cur_read_offset += chunk_csize;
+                       ret = full_pread(in_fd, &dummy, 1, cur_read_offset - 1);
+                       if (ret)
+                               goto read_error;
+               }
        }
-       p = get_u64(p, &entry->original_size);
-       if (entry->original_size & 0xc000000000000000ULL) {
-               WARNING("Truncating original_size in resource entry");
-               entry->original_size &= 0x3fffffffffffffffULL;
+
+       if (is_pipe_read
+           && size == lte->resource_entry.original_size
+           && chunk_table_size)
+       {
+               u8 dummy;
+               /* Skip chunk table at end of pipable resource.  */
+
+               cur_read_offset += chunk_table_size;
+               ret = full_pread(in_fd, &dummy, 1, cur_read_offset - 1);
+               if (ret)
+                       goto read_error;
        }
-       return p;
-}
+       ret = 0;
+out_free_memory:
+       errno_save = errno;
+       if (chunk_offsets_malloced)
+               FREE(chunk_offsets);
+       if (out_buf_malloced)
+               FREE(out_buf);
+       if (compressed_buf_malloced)
+               FREE(compressed_buf);
+       if (tmp_buf_malloced)
+               FREE(tmp_buf);
+       errno = errno_save;
+       return ret;
 
-/* Copies the struct resource_entry @entry to the memory pointed to by @p in the
- * on-disk format.  A pointer to the byte after the memory written at @p is
- * returned. */
-u8 *put_resource_entry(u8 *p, const struct resource_entry *entry)
-{
-       p = put_u56(p, entry->size);
-       p = put_u8(p, entry->flags);
-       p = put_u64(p, entry->offset);
-       p = put_u64(p, entry->original_size);
-       return p;
+oom:
+       ERROR("Not enough memory available to read size=%"PRIu64" bytes "
+             "from compressed resource!", size);
+       errno = ENOMEM;
+       ret = WIMLIB_ERR_NOMEM;
+       goto out_free_memory;
+
+read_error:
+       ERROR_WITH_ERRNO("Error reading compressed file resource!");
+       goto out_free_memory;
 }
 
-/*
- * Reads some data from the resource corresponding to a WIM lookup table entry.
- *
- * @lte:       The WIM lookup table entry for the resource.
- * @buf:       Buffer into which to write the data.
- * @size:      Number of bytes to read.
- * @offset:    Offset at which to start reading the resource.
- * @raw:       If %true, compressed data is read literally rather than being
- *                     decompressed first.
- *
- * Returns zero on success, nonzero on failure.
- */
-int read_wim_resource(const struct lookup_table_entry *lte, u8 buf[],
-                     size_t size, u64 offset, bool raw)
+/* Read raw data from a file descriptor at the specified offset.  */
+static int
+read_raw_file_data(struct filedes *in_fd,
+                  u64 size,
+                  consume_data_callback_t cb,
+                  u32 cb_chunk_size,
+                  void *ctx_or_buf,
+                  u64 offset)
 {
-       int ctype;
-       int ret = 0;
-       FILE *fp;
-
-       /* We shouldn't be allowing read over-runs in any part of the library.
-        * */
-       if (raw)
-               wimlib_assert(offset + size <= lte->resource_entry.size);
-       else
-               wimlib_assert(offset + size <= lte->resource_entry.original_size);
-
-       switch (lte->resource_location) {
-       case RESOURCE_IN_WIM:
-               /* The resource is in a WIM file, and its WIMStruct is given by
-                * the lte->wim member.  The resource may be either compressed
-                * or uncompressed. */
-               wimlib_assert(lte->wim != NULL);
-               wimlib_assert(lte->wim->fp != NULL);
-               ctype = wim_resource_compression_type(lte);
-
-               wimlib_assert(ctype != WIM_COMPRESSION_TYPE_NONE ||
-                             (lte->resource_entry.original_size ==
-                              lte->resource_entry.size));
-
-               if (raw || ctype == WIM_COMPRESSION_TYPE_NONE)
-                       ret = read_uncompressed_resource(lte->wim->fp,
-                                                        lte->resource_entry.offset + offset,
-                                                        size, buf);
-               else
-                       ret = read_compressed_resource(lte->wim->fp,
-                                                      lte->resource_entry.size,
-                                                      lte->resource_entry.original_size,
-                                                      lte->resource_entry.offset,
-                                                      ctype, size, offset, buf);
-               break;
-       case RESOURCE_IN_STAGING_FILE:
-       case RESOURCE_IN_FILE_ON_DISK:
-               /* The resource is in some file on the external filesystem and
-                * needs to be read uncompressed */
-               wimlib_assert(lte->file_on_disk);
-               wimlib_assert(&lte->file_on_disk == &lte->staging_file_name);
-               /* Use existing file pointer if available; otherwise open one
-                * temporarily */
-               if (lte->file_on_disk_fp) {
-                       fp = lte->file_on_disk_fp;
+       int ret;
+       u8 *tmp_buf;
+       bool tmp_buf_malloced = false;
+
+       if (cb) {
+               /* Send data to callback function in chunks.  */
+               if (cb_chunk_size <= STACK_MAX) {
+                       tmp_buf = alloca(cb_chunk_size);
                } else {
-                       fp = fopen(lte->file_on_disk, "rb");
-                       if (!fp) {
-                               ERROR_WITH_ERRNO("Failed to open the file "
-                                                "`%s'", lte->file_on_disk);
-                               ret = WIMLIB_ERR_OPEN;
-                               break;
+                       tmp_buf = MALLOC(cb_chunk_size);
+                       if (tmp_buf == NULL) {
+                               ret = WIMLIB_ERR_NOMEM;
+                               goto out;
                        }
+                       tmp_buf_malloced = true;
                }
-               ret = read_uncompressed_resource(fp, offset, size, buf);
-               if (fp != lte->file_on_disk_fp)
-                       fclose(fp);
-               break;
-       case RESOURCE_IN_ATTACHED_BUFFER:
-               /* The resource is directly attached uncompressed in an
-                * in-memory buffer. */
-               wimlib_assert(lte->attached_buffer != NULL);
-               memcpy(buf, lte->attached_buffer + offset, size);
-               break;
-#ifdef WITH_NTFS_3G
-       case RESOURCE_IN_NTFS_VOLUME:
-               wimlib_assert(lte->ntfs_loc != NULL);
-               wimlib_assert(lte->attr != NULL);
-               {
-                       u64 adjusted_offset;
-                       if (lte->ntfs_loc->is_reparse_point)
-                               adjusted_offset = offset + 8;
-                       else
-                               adjusted_offset = offset;
-                       if (ntfs_attr_pread(lte->attr, offset, size, buf) != size) {
-                               ERROR_WITH_ERRNO("Error reading NTFS attribute "
-                                                "at `%s'",
-                                                lte->ntfs_loc->path_utf8);
-                               ret = WIMLIB_ERR_NTFS_3G;
-                       }
-                       break;
+
+               while (size) {
+                       size_t bytes_to_read = min(cb_chunk_size, size);
+                       ret = full_pread(in_fd, tmp_buf, bytes_to_read,
+                                        offset);
+                       if (ret)
+                               goto read_error;
+                       ret = cb(tmp_buf, bytes_to_read, ctx_or_buf);
+                       if (ret)
+                               goto out;
+                       size -= bytes_to_read;
+                       offset += bytes_to_read;
                }
-#endif
-       default:
-               wimlib_assert(0);
-               ret = -1;
-               break;
+       } else {
+               /* Read data directly into buffer.  */
+               ret = full_pread(in_fd, ctx_or_buf, size, offset);
+               if (ret)
+                       goto read_error;
        }
+       ret = 0;
+       goto out;
+
+read_error:
+       ERROR_WITH_ERRNO("Read error");
+out:
+       if (tmp_buf_malloced)
+               FREE(tmp_buf);
        return ret;
 }
 
 /*
- * Reads all the data from the resource corresponding to a WIM lookup table
- * entry.
+ * read_partial_wim_resource()-
  *
- * @lte:       The WIM lookup table entry for the resource.
- * @buf:       Buffer into which to write the data.  It must be at least
- *             wim_resource_size(lte) bytes long.
+ * Read a range of data from a uncompressed or compressed resource in a WIM
+ * file.  Data is written into a buffer or fed into a callback function, as
+ * documented in read_resource_prefix().
  *
- * Returns 0 on success; nonzero on failure.
- */
-int read_full_wim_resource(const struct lookup_table_entry *lte, u8 buf[])
-{
-       return read_wim_resource(lte, buf, wim_resource_size(lte), 0, false);
-}
-
-/* Chunk table that's located at the beginning of each compressed resource in
- * the WIM.  (This is not the on-disk format; the on-disk format just has an
- * array of offsets.) */
-struct chunk_table {
-       off_t file_offset;
-       u64 num_chunks;
-       u64 original_resource_size;
-       u64 bytes_per_chunk_entry;
-       u64 table_disk_size;
-       u64 cur_offset;
-       u64 *cur_offset_p;
-       u64 offsets[0];
-};
-
-/*
- * Allocates and initializes a chunk table, and reserves space for it in the
- * output file.
+ * By default, this function provides the uncompressed data of the resource, and
+ * @size and @offset and interpreted relative to the uncompressed contents of
+ * the resource.  The behavior can be modified by any of the following flags:
+ *
+ * WIMLIB_READ_RESOURCE_FLAG_RAW_FULL:
+ *     Read @size bytes at @offset of the raw contents of the compressed
+ *     resource.  In the case of pipable resources, this excludes the stream
+ *     header.  Exclusive with WIMLIB_READ_RESOURCE_FLAG_RAW_CHUNKS.
+ *
+ * WIMLIB_READ_RESOURCE_FLAG_RAW_CHUNKS:
+ *     Read the raw compressed chunks of the compressed resource.  @size must
+ *     be the full uncompressed size, @offset must be 0, and @cb_chunk_size
+ *     must be the resource chunk size.
+ *
+ * Return values:
+ *     WIMLIB_ERR_SUCCESS (0)
+ *     WIMLIB_ERR_READ                   (errno set)
+ *     WIMLIB_ERR_UNEXPECTED_END_OF_FILE (errno set to 0)
+ *     WIMLIB_ERR_NOMEM                  (errno set to ENOMEM)
+ *     WIMLIB_ERR_DECOMPRESSION          (errno set to EINVAL)
+ *     WIMLIB_ERR_INVALID_CHUNK_SIZE    (errno set to EINVAL)
+ *
+ *     or other error code returned by the @cb function.
  */
-static int
-begin_wim_resource_chunk_tab(const struct lookup_table_entry *lte,
-                            FILE *out_fp,
-                            off_t file_offset,
-                            struct chunk_table **chunk_tab_ret)
+int
+read_partial_wim_resource(const struct wim_lookup_table_entry *lte,
+                         u64 size, consume_data_callback_t cb,
+                         u32 cb_chunk_size,
+                         void *ctx_or_buf, int flags, u64 offset)
 {
-       u64 size = wim_resource_size(lte);
-       u64 num_chunks = (size + WIM_CHUNK_SIZE - 1) / WIM_CHUNK_SIZE;
-       size_t alloc_size = sizeof(struct chunk_table) + num_chunks * sizeof(u64);
-       struct chunk_table *chunk_tab = CALLOC(1, alloc_size);
+       struct filedes *in_fd;
        int ret;
 
-       if (!chunk_tab) {
-               ERROR("Failed to allocate chunk table for %"PRIu64" byte "
-                     "resource", size);
-               ret = WIMLIB_ERR_NOMEM;
-               goto out;
+       /* Verify parameters.  */
+       wimlib_assert(lte->resource_location == RESOURCE_IN_WIM);
+       in_fd = &lte->wim->in_fd;
+       if (cb)
+               wimlib_assert(is_power_of_2(cb_chunk_size));
+       if (flags & WIMLIB_READ_RESOURCE_FLAG_RAW_CHUNKS) {
+               /* Raw chunks mode is subject to the restrictions noted.  */
+               wimlib_assert(!(flags & WIMLIB_READ_RESOURCE_FLAG_RAW_FULL));
+               wimlib_assert(cb_chunk_size == wim_resource_chunk_size(lte));
+               wimlib_assert(size == lte->resource_entry.original_size);
+               wimlib_assert(offset == 0);
+       } else if (flags & WIMLIB_READ_RESOURCE_FLAG_RAW_FULL) {
+               /* Raw full mode:  read must not overrun end of store size.  */
+               wimlib_assert(offset + size >= size &&
+                             offset + size <= lte->resource_entry.size);
+       } else {
+               /* Normal mode:  read must not overrun end of original size.  */
+               wimlib_assert(offset + size >= size &&
+                             offset + size <= lte->resource_entry.original_size);
        }
-       chunk_tab->file_offset = file_offset;
-       chunk_tab->num_chunks = num_chunks;
-       chunk_tab->original_resource_size = size;
-       chunk_tab->bytes_per_chunk_entry = (size >= (1ULL << 32)) ? 8 : 4;
-       chunk_tab->table_disk_size = chunk_tab->bytes_per_chunk_entry *
-                                    (num_chunks - 1);
-       chunk_tab->cur_offset = 0;
-       chunk_tab->cur_offset_p = chunk_tab->offsets;
-
-       if (fwrite(chunk_tab, 1, chunk_tab->table_disk_size, out_fp) !=
-                  chunk_tab->table_disk_size) {
-               ERROR_WITH_ERRNO("Failed to write chunk table in compressed "
-                                "file resource");
-               ret = WIMLIB_ERR_WRITE;
-               goto out;
+
+       DEBUG("Reading WIM resource: %"PRIu64" @ +%"PRIu64" "
+             "from %"PRIu64"(%"PRIu64") @ +%"PRIu64" "
+             "(readflags 0x%08x, resflags 0x%02x%s)",
+             size, offset,
+             lte->resource_entry.size,
+             lte->resource_entry.original_size,
+             lte->resource_entry.offset,
+             flags, lte->resource_entry.flags,
+             (lte->is_pipable ? ", pipable" : ""));
+
+       if ((flags & WIMLIB_READ_RESOURCE_FLAG_RAW_FULL) ||
+           !resource_is_compressed(&lte->resource_entry)) {
+               return read_raw_file_data(in_fd,
+                                         size,
+                                         cb,
+                                         cb_chunk_size,
+                                         ctx_or_buf,
+                                         offset + lte->resource_entry.offset);
+       } else {
+               return read_compressed_wim_resource(lte, size, cb,
+                                                   cb_chunk_size,
+                                                   ctx_or_buf, flags, offset);
        }
+}
 
-       ret = 0;
-out:
-       *chunk_tab_ret = chunk_tab;
-       return ret;
+int
+read_partial_wim_resource_into_buf(const struct wim_lookup_table_entry *lte,
+                                  size_t size, u64 offset, void *buf)
+{
+       return read_partial_wim_resource(lte, size, NULL, 0, buf, 0, offset);
 }
 
-/*
- * Compresses a chunk of a WIM resource.
- *
- * @chunk:             Uncompressed data of the chunk.
- * @chunk_size:                Size of the uncompressed chunk in bytes.
- * @compressed_chunk:  Pointer to output buffer of size at least
- *                             (@chunk_size - 1) bytes.
- * @compressed_chunk_len_ret:  Pointer to an unsigned int into which the size
- *                                     of the compressed chunk will be
- *                                     returned.
- * @ctype:     Type of compression to use.  Must be WIM_COMPRESSION_TYPE_LZX
- *             or WIM_COMPRESSION_TYPE_XPRESS.
- *
- * Returns zero if compressed succeeded, and nonzero if the chunk could not be
- * compressed to any smaller than @chunk_size.  This function cannot fail for
- * any other reasons.
- */
-static int compress_chunk(const u8 chunk[], unsigned chunk_size,
-                         u8 compressed_chunk[],
-                         unsigned *compressed_chunk_len_ret,
-                         int ctype)
+static int
+read_wim_resource_prefix(const struct wim_lookup_table_entry *lte,
+                        u64 size,
+                        consume_data_callback_t cb,
+                        u32 cb_chunk_size,
+                        void *ctx_or_buf,
+                        int flags)
 {
-       int (*compress)(const void *, unsigned, void *, unsigned *);
-       switch (ctype) {
-       case WIM_COMPRESSION_TYPE_LZX:
-               compress = lzx_compress;
-               break;
-       case WIM_COMPRESSION_TYPE_XPRESS:
-               compress = xpress_compress;
-               break;
-       default:
-               wimlib_assert(0);
-               break;
-       }
-       return (*compress)(chunk, chunk_size, compressed_chunk,
-                          compressed_chunk_len_ret);
+       return read_partial_wim_resource(lte, size, cb, cb_chunk_size,
+                                        ctx_or_buf, flags, 0);
 }
 
-/*
- * Writes a chunk of a WIM resource to an output file.
- *
- * @chunk:       Uncompressed data of the chunk.
- * @chunk_size:          Size of the chunk (<= WIM_CHUNK_SIZE)
- * @out_fp:      FILE * to write tho chunk to.
- * @out_ctype:   Compression type to use when writing the chunk (ignored if no
- *                     chunk table provided)
- * @chunk_tab:   Pointer to chunk table being created.  It is updated with the
- *                     offset of the chunk we write.
+#ifndef __WIN32__
+/* This function handles reading resource data that is located in an external
+ * file,  such as a file that has been added to the WIM image through execution
+ * of a wimlib_add_command.
  *
- * Returns 0 on success; nonzero on failure.
+ * This assumes the file can be accessed using the standard POSIX open(),
+ * read(), and close().  On Windows this will not necessarily be the case (since
+ * the file may need FILE_FLAG_BACKUP_SEMANTICS to be opened, or the file may be
+ * encrypted), so Windows uses its own code for its equivalent case.
  */
-static int write_wim_resource_chunk(const u8 chunk[], unsigned chunk_size,
-                                   FILE *out_fp, int out_ctype,
-                                   struct chunk_table *chunk_tab)
+static int
+read_file_on_disk_prefix(const struct wim_lookup_table_entry *lte,
+                        u64 size,
+                        consume_data_callback_t cb,
+                        u32 cb_chunk_size,
+                        void *ctx_or_buf,
+                        int _ignored_flags)
 {
-       const u8 *out_chunk;
-       unsigned out_chunk_size;
-
-       wimlib_assert(chunk_size <= WIM_CHUNK_SIZE);
-
-       if (!chunk_tab) {
-               out_chunk = chunk;
-               out_chunk_size = chunk_size;
-       } else {
-               u8 *compressed_chunk = alloca(chunk_size);
-               int ret;
+       int ret;
+       int raw_fd;
+       struct filedes fd;
 
-               ret = compress_chunk(chunk, chunk_size, compressed_chunk,
-                                    &out_chunk_size, out_ctype);
-               if (ret == 0) {
-                       out_chunk = compressed_chunk;
-               } else {
-                       out_chunk = chunk;
-                       out_chunk_size = chunk_size;
-               }
-               *chunk_tab->cur_offset_p++ = chunk_tab->cur_offset;
-               chunk_tab->cur_offset += out_chunk_size;
-       }
+       wimlib_assert(size <= wim_resource_size(lte));
+       DEBUG("Reading %"PRIu64" bytes from \"%"TS"\"", size, lte->file_on_disk);
 
-       if (fwrite(out_chunk, 1, out_chunk_size, out_fp) != out_chunk_size) {
-               ERROR_WITH_ERRNO("Failed to write WIM resource chunk");
-               return WIMLIB_ERR_WRITE;
+       raw_fd = open(lte->file_on_disk, O_BINARY | O_RDONLY);
+       if (raw_fd < 0) {
+               ERROR_WITH_ERRNO("Can't open \"%"TS"\"", lte->file_on_disk);
+               return WIMLIB_ERR_OPEN;
        }
-       return 0;
+       filedes_init(&fd, raw_fd);
+       ret = read_raw_file_data(&fd, size, cb, cb_chunk_size, ctx_or_buf, 0);
+       filedes_close(&fd);
+       return ret;
 }
+#endif /* !__WIN32__ */
 
-/*
- * Finishes a WIM chunk tale and writes it to the output file at the correct
- * offset.
- *
- * The final size of the full compressed resource is returned in the
- * @compressed_size_p.
- */
+/* This function handles the trivial case of reading resource data that is, in
+ * fact, already located in an in-memory buffer.  */
 static int
-finish_wim_resource_chunk_tab(struct chunk_table *chunk_tab,
-                             FILE *out_fp, u64 *compressed_size_p)
+read_buffer_prefix(const struct wim_lookup_table_entry *lte,
+                  u64 size, consume_data_callback_t cb,
+                  u32 cb_chunk_size,
+                  void *ctx_or_buf, int _ignored_flags)
 {
-       size_t bytes_written;
-       if (fseeko(out_fp, chunk_tab->file_offset, SEEK_SET) != 0) {
-               ERROR_WITH_ERRNO("Failed to seek to byte %"PRIu64" of output "
-                                "WIM file", chunk_tab->file_offset);
-               return WIMLIB_ERR_WRITE;
-       }
+       wimlib_assert(size <= wim_resource_size(lte));
 
-       if (chunk_tab->bytes_per_chunk_entry == 8) {
-               array_cpu_to_le64(chunk_tab->offsets, chunk_tab->num_chunks);
+       if (cb) {
+               /* Feed the data into the callback function in
+                * appropriately-sized chunks.  */
+               int ret;
+               u32 chunk_size;
+
+               for (u64 offset = 0; offset < size; offset += chunk_size) {
+                       chunk_size = min(cb_chunk_size, size - offset);
+                       ret = cb((const u8*)lte->attached_buffer + offset,
+                                chunk_size, ctx_or_buf);
+                       if (ret)
+                               return ret;
+               }
        } else {
-               for (u64 i = 0; i < chunk_tab->num_chunks; i++)
-                       ((u32*)chunk_tab->offsets)[i] =
-                               cpu_to_le32(chunk_tab->offsets[i]);
-       }
-       bytes_written = fwrite((u8*)chunk_tab->offsets +
-                                       chunk_tab->bytes_per_chunk_entry,
-                              1, chunk_tab->table_disk_size, out_fp);
-       if (bytes_written != chunk_tab->table_disk_size) {
-               ERROR_WITH_ERRNO("Failed to write chunk table in compressed "
-                                "file resource");
-               return WIMLIB_ERR_WRITE;
+               /* Copy the data directly into the specified buffer.  */
+               memcpy(ctx_or_buf, lte->attached_buffer, size);
        }
-       if (fseeko(out_fp, 0, SEEK_END) != 0) {
-               ERROR_WITH_ERRNO("Failed to seek to end of output WIM file");
-               return WIMLIB_ERR_WRITE;
-       }
-       *compressed_size_p = chunk_tab->cur_offset + chunk_tab->table_disk_size;
        return 0;
 }
 
+typedef int (*read_resource_prefix_handler_t)(const struct wim_lookup_table_entry *lte,
+                                             u64 size,
+                                             consume_data_callback_t cb,
+                                             u32 cb_chunk_size,
+                                             void *ctx_or_buf,
+                                             int flags);
+
 /*
- * Writes a WIM resource to a FILE * opened for writing.  The resource may be
- * written uncompressed or compressed depending on the @out_ctype parameter.
+ * read_resource_prefix()-
+ *
+ * Reads the first @size bytes from a generic "resource", which may be located
+ * in any one of several locations, such as in a WIM file (compressed or
+ * uncompressed), in an external file, or directly in an in-memory buffer.
  *
- * If by chance the resource compresses to more than the original size (this may
- * happen with random data or files than are pre-compressed), the resource is
- * instead written uncompressed (and this is reflected in the @out_res_entry by
- * removing the WIM_RESHDR_FLAG_COMPRESSED flag).
+ * This function feeds the data either to a callback function (@cb != NULL,
+ * passing it @ctx_or_buf), or write it directly into a buffer (@cb == NULL,
+ * @ctx_or_buf specifies the buffer, which must have room for at least @size
+ * bytes).
  *
- * @lte:       The lookup table entry for the WIM resource.
- * @out_fp:    The FILE * to write the resource to.
- * @out_ctype:  The compression type of the resource to write.  Note: if this is
- *                     the same as the compression type of the WIM resource we
- *                     need to read, we simply copy the data (i.e. we do not
- *                     uncompress it, then compress it again).
- * @out_res_entry:  If non-NULL, a resource entry that is filled in with the
- *                 offset, original size, compressed size, and compression flag
- *                 of the output resource.
+ * When (@cb != NULL), @cb_chunk_size specifies the maximum size of data chunks
+ * to feed the callback function.  @cb_chunk_size must be positive, and if the
+ * resource is in a WIM file, must be a power of 2.  All chunks, except possibly
+ * the last one, will be this size.  If (@cb == NULL), @cb_chunk_size is
+ * ignored.
  *
- * Returns 0 on success; nonzero on failure.
+ * If the resource is located in a WIM file, @flags can be set as documented in
+ * read_partial_wim_resource().  Otherwise @flags are ignored.
+ *
+ * Returns 0 on success; nonzero on error.  A nonzero value will be returned if
+ * the resource data cannot be successfully read (for a number of different
+ * reasons, depending on the resource location), or if a callback function was
+ * specified and it returned nonzero.
  */
-static int write_wim_resource(struct lookup_table_entry *lte,
-                             FILE *out_fp, int out_ctype,
-                             struct resource_entry *out_res_entry)
+int
+read_resource_prefix(const struct wim_lookup_table_entry *lte,
+                    u64 size, consume_data_callback_t cb, u32 cb_chunk_size,
+                    void *ctx_or_buf, int flags)
 {
-       u64 bytes_remaining;
-       u64 original_size;
-       u64 old_compressed_size;
-       u64 new_compressed_size;
-       u64 offset;
-       int ret;
-       struct chunk_table *chunk_tab = NULL;
-       bool raw;
-       off_t file_offset;
-#ifdef WITH_NTFS_3G
-       ntfs_inode *ni = NULL;
-#endif
-
-       wimlib_assert(lte);
+       /* This function merely verifies several preconditions, then passes
+        * control to an appropriate function for understanding each possible
+        * resource location.  */
+       static const read_resource_prefix_handler_t handlers[] = {
+               [RESOURCE_IN_WIM]             = read_wim_resource_prefix,
+       #ifdef __WIN32__
+               [RESOURCE_IN_FILE_ON_DISK]    = read_win32_file_prefix,
+       #else
+               [RESOURCE_IN_FILE_ON_DISK]    = read_file_on_disk_prefix,
+       #endif
+               [RESOURCE_IN_ATTACHED_BUFFER] = read_buffer_prefix,
+       #ifdef WITH_FUSE
+               [RESOURCE_IN_STAGING_FILE]    = read_file_on_disk_prefix,
+       #endif
+       #ifdef WITH_NTFS_3G
+               [RESOURCE_IN_NTFS_VOLUME]     = read_ntfs_file_prefix,
+       #endif
+       #ifdef __WIN32__
+               [RESOURCE_WIN32_ENCRYPTED]    = read_win32_encrypted_file_prefix,
+       #endif
+       };
+       wimlib_assert(lte->resource_location < ARRAY_LEN(handlers)
+                     && handlers[lte->resource_location] != NULL);
+       wimlib_assert(cb == NULL || cb_chunk_size > 0);
+       return handlers[lte->resource_location](lte, size, cb, cb_chunk_size,
+                                               ctx_or_buf, flags);
+}
 
-       /* Original size of the resource */
-       original_size = wim_resource_size(lte);
+/* Read the full uncompressed data of the specified resource into the specified
+ * buffer, which must have space for at least lte->resource_entry.original_size
+ * bytes.  */
+int
+read_full_resource_into_buf(const struct wim_lookup_table_entry *lte,
+                           void *buf)
+{
+       return read_resource_prefix(lte, wim_resource_size(lte),
+                                   NULL, 0, buf, 0);
+}
 
-       /* Compressed size of the resource (as it exists now) */
-       old_compressed_size = wim_resource_compressed_size(lte);
+/* Read the full uncompressed data of the specified resource.  A buffer
+ * sufficient to hold the data is allocated and returned in @buf_ret.  */
+int
+read_full_resource_into_alloc_buf(const struct wim_lookup_table_entry *lte,
+                                 void **buf_ret)
+{
+       int ret;
+       void *buf;
 
-       /* Current offset in output file */
-       file_offset = ftello(out_fp);
-       if (file_offset == -1) {
-               ERROR_WITH_ERRNO("Failed to get offset in output "
-                                "stream");
-               return WIMLIB_ERR_WRITE;
+       if ((size_t)lte->resource_entry.original_size !=
+           lte->resource_entry.original_size)
+       {
+               ERROR("Can't read %"PRIu64" byte resource into "
+                     "memory", lte->resource_entry.original_size);
+               return WIMLIB_ERR_NOMEM;
        }
 
-       /* Are the compression types the same?  If so, do a raw copy (copy
-        * without decompressing and recompressing the data). */
-       raw = (wim_resource_compression_type(lte) == out_ctype
-              && out_ctype != WIM_COMPRESSION_TYPE_NONE);
-       if (raw)
-               bytes_remaining = old_compressed_size;
-       else
-               bytes_remaining = original_size;
-
-       /* Empty resource; nothing needs to be done, so just return success. */
-       if (bytes_remaining == 0)
-               return 0;
-
-       /* Buffer for reading chunks for the resource */
-       u8 buf[min(WIM_CHUNK_SIZE, bytes_remaining)];
+       buf = MALLOC(lte->resource_entry.original_size);
+       if (buf == NULL)
+               return WIMLIB_ERR_NOMEM;
 
-       /* If we are writing a compressed resource and not doing a raw copy, we
-        * need to initialize the chunk table */
-       if (out_ctype != WIM_COMPRESSION_TYPE_NONE && !raw) {
-               ret = begin_wim_resource_chunk_tab(lte, out_fp, file_offset,
-                                                  &chunk_tab);
-               if (ret != 0)
-                       goto out;
+       ret = read_full_resource_into_buf(lte, buf);
+       if (ret) {
+               FREE(buf);
+               return ret;
        }
 
-       /* If the WIM resource is in an external file, open a FILE * to it so we
-        * don't have to open a temporary one in read_wim_resource() for each
-        * chunk. */
-       if (lte->resource_location == RESOURCE_IN_FILE_ON_DISK
-            && !lte->file_on_disk_fp)
-       {
-               wimlib_assert(lte->file_on_disk);
-               lte->file_on_disk_fp = fopen(lte->file_on_disk, "rb");
-               if (!lte->file_on_disk_fp) {
-                       ERROR_WITH_ERRNO("Failed to open the file `%s' for "
-                                        "reading", lte->file_on_disk);
-                       ret = WIMLIB_ERR_OPEN;
-                       goto out;
-               }
-       }
-#ifdef WITH_NTFS_3G
-       else if (lte->resource_location == RESOURCE_IN_NTFS_VOLUME
-                 && !lte->attr)
-       {
-               struct ntfs_location *loc = lte->ntfs_loc;
-               wimlib_assert(loc);
-               ni = ntfs_pathname_to_inode(*loc->ntfs_vol_p, NULL, loc->path_utf8);
-               if (!ni) {
-                       ERROR_WITH_ERRNO("Failed to open inode `%s' in NTFS "
-                                        "volume", loc->path_utf8);
-                       ret = WIMLIB_ERR_NTFS_3G;
-                       goto out;
-               }
-               lte->attr = ntfs_attr_open(ni,
-                                          loc->is_reparse_point ? AT_REPARSE_POINT : AT_DATA,
-                                          (ntfschar*)loc->stream_name_utf16,
-                                          loc->stream_name_utf16_num_chars);
-               if (!lte->attr) {
-                       ERROR_WITH_ERRNO("Failed to open attribute of `%s' in "
-                                        "NTFS volume", loc->path_utf8);
-                       ret = WIMLIB_ERR_NTFS_3G;
-                       goto out_fclose;
-               }
-       }
-#endif
+       *buf_ret = buf;
+       return 0;
+}
 
-       /* If we aren't doing a raw copy, we will compute the SHA1 message
-        * digest of the resource as we read it, and verify it's the same as the
-        * hash given in the lookup table entry once we've finished reading the
-        * resource. */
-       SHA_CTX ctx;
-       if (!raw)
-               sha1_init(&ctx);
-
-       /* While there are still bytes remaining in the WIM resource, read a
-        * chunk of the resource, update SHA1, then write that chunk using the
-        * desired compression type. */
-       offset = 0;
-       do {
-               u64 to_read = min(bytes_remaining, WIM_CHUNK_SIZE);
-               ret = read_wim_resource(lte, buf, to_read, offset, raw);
-               if (ret != 0)
-                       goto out_fclose;
-               if (!raw)
-                       sha1_update(&ctx, buf, to_read);
-               ret = write_wim_resource_chunk(buf, to_read, out_fp,
-                                              out_ctype, chunk_tab);
-               if (ret != 0)
-                       goto out_fclose;
-               bytes_remaining -= to_read;
-               offset += to_read;
-       } while (bytes_remaining);
-
-       /* Raw copy:  The new compressed size is the same as the old compressed
-        * size
-        *
-        * Using WIM_COMPRESSION_TYPE_NONE:  The new compressed size is the
-        * original size
-        *
-        * Using a different compression type:  Call
-        * finish_wim_resource_chunk_tab() and it will provide the new
-        * compressed size.
-        */
-       if (raw) {
-               new_compressed_size = old_compressed_size;
-       } else {
-               if (out_ctype == WIM_COMPRESSION_TYPE_NONE)
-                       new_compressed_size = original_size;
-               else {
-                       ret = finish_wim_resource_chunk_tab(chunk_tab, out_fp,
-                                                           &new_compressed_size);
-                       if (ret != 0)
-                               goto out_fclose;
-               }
-       }
+/* Retrieve the full uncompressed data of the specified WIM resource, provided
+ * as a raw `struct resource_entry'.  */
+int
+res_entry_to_data(const struct resource_entry *res_entry,
+                 WIMStruct *wim, void **buf_ret)
+{
+       int ret;
+       struct wim_lookup_table_entry *lte;
 
-       /* Verify SHA1 message digest of the resource, unless we are doing a raw
-        * write (in which case we never even saw the uncompressed data).  Or,
-        * if the hash we had before is all 0's, just re-set it to be the new
-        * hash. */
-       if (!raw) {
-               u8 md[SHA1_HASH_SIZE];
-               sha1_final(md, &ctx);
-               if (is_zero_hash(lte->hash)) {
-                       copy_hash(lte->hash, md);
-               } else if (!hashes_equal(md, lte->hash)) {
-                       ERROR("WIM resource has incorrect hash!");
-                       if (lte->resource_location == RESOURCE_IN_FILE_ON_DISK) {
-                               ERROR("We were reading it from `%s'; maybe it changed "
-                                     "while we were reading it.",
-                                     lte->file_on_disk);
-                       }
-                       ret = WIMLIB_ERR_INVALID_RESOURCE_HASH;
-                       goto out_fclose;
-               }
-       }
+       lte = new_lookup_table_entry();
+       if (lte == NULL)
+               return WIMLIB_ERR_NOMEM;
 
-       if (!raw && new_compressed_size >= original_size &&
-           out_ctype != WIM_COMPRESSION_TYPE_NONE)
-       {
-               /* Oops!  We compressed the resource to larger than the original
-                * size.  Write the resource uncompressed instead. */
-               if (fseeko(out_fp, file_offset, SEEK_SET) != 0) {
-                       ERROR_WITH_ERRNO("Failed to seek to byte %"PRIu64" "
-                                        "of output WIM file", file_offset);
-                       ret = WIMLIB_ERR_WRITE;
-                       goto out_fclose;
-               }
-               ret = write_wim_resource(lte, out_fp, WIM_COMPRESSION_TYPE_NONE,
-                                        out_res_entry);
-               if (ret != 0)
-                       goto out_fclose;
-               if (fflush(out_fp) != 0) {
-                       ERROR_WITH_ERRNO("Failed to flush output WIM file");
-                       ret = WIMLIB_ERR_WRITE;
-                       goto out_fclose;
-               }
-               if (ftruncate(fileno(out_fp), file_offset + out_res_entry->size) != 0) {
-                       ERROR_WITH_ERRNO("Failed to truncate output WIM file");
-                       ret = WIMLIB_ERR_WRITE;
-                       goto out_fclose;
-               }
-       } else {
-               if (out_res_entry) {
-                       out_res_entry->size          = new_compressed_size;
-                       out_res_entry->original_size = original_size;
-                       out_res_entry->offset        = file_offset;
-                       out_res_entry->flags         = lte->resource_entry.flags
-                                                       & ~WIM_RESHDR_FLAG_COMPRESSED;
-                       if (out_ctype != WIM_COMPRESSION_TYPE_NONE)
-                               out_res_entry->flags |= WIM_RESHDR_FLAG_COMPRESSED;
-               }
-       }
-       ret = 0;
-out_fclose:
-       if (lte->resource_location == RESOURCE_IN_FILE_ON_DISK
-           && lte->file_on_disk_fp) {
-               fclose(lte->file_on_disk_fp);
-               lte->file_on_disk_fp = NULL;
-       }
-#ifdef WITH_NTFS_3G
-       else if (lte->resource_location == RESOURCE_IN_NTFS_VOLUME) {
-               if (lte->attr) {
-                       ntfs_attr_close(lte->attr);
-                       lte->attr = NULL;
-               }
-               if (ni)
-                       ntfs_inode_close(ni);
-       }
-#endif
-out:
-       FREE(chunk_tab);
+       copy_resource_entry(&lte->resource_entry, res_entry);
+       lte->unhashed = 1;
+       lte->part_number = wim->hdr.part_number;
+       lte_init_wim(lte, wim);
+
+       ret = read_full_resource_into_alloc_buf(lte, buf_ret);
+       free_lookup_table_entry(lte);
        return ret;
 }
 
-/* Like write_wim_resource(), but the resource is specified by a buffer of
- * uncompressed data rather a lookup table entry; also writes the SHA1 hash of
- * the buffer to @hash.  */
-static int write_wim_resource_from_buffer(const u8 *buf, u64 buf_size,
-                                         FILE *out_fp, int out_ctype,
-                                         struct resource_entry *out_res_entry,
-                                         u8 hash[SHA1_HASH_SIZE])
+struct extract_ctx {
+       SHA_CTX sha_ctx;
+       consume_data_callback_t extract_chunk;
+       void *extract_chunk_arg;
+};
+
+static int
+extract_chunk_sha1_wrapper(const void *chunk, size_t chunk_size,
+                          void *_ctx)
 {
-       /* Set up a temporary lookup table entry to provide to
-        * write_wim_resource(). */
-       struct lookup_table_entry lte;
-       int ret;
-       lte.resource_entry.flags         = 0;
-       lte.resource_entry.original_size = buf_size;
-       lte.resource_entry.size          = buf_size;
-       lte.resource_entry.offset        = 0;
-       lte.resource_location            = RESOURCE_IN_ATTACHED_BUFFER;
-       lte.attached_buffer              = (u8*)buf;
-
-       zero_out_hash(lte.hash);
-       ret = write_wim_resource(&lte, out_fp, out_ctype, out_res_entry);
-       if (ret != 0)
-               return ret;
-       copy_hash(hash, lte.hash);
-       return 0;
+       struct extract_ctx *ctx = _ctx;
+
+       sha1_update(&ctx->sha_ctx, chunk, chunk_size);
+       return ctx->extract_chunk(chunk, chunk_size, ctx->extract_chunk_arg);
 }
 
-/*
- * Extracts the first @size bytes of the WIM resource specified by @lte to the
- * open file descriptor @fd.
+/* Extracts the first @size bytes of a resource to somewhere.  In the process,
+ * the SHA1 message digest of the uncompressed resource is checked if the full
+ * resource is being extracted.
  *
- * Returns 0 on success; nonzero on failure.
- */
-int extract_wim_resource_to_fd(const struct lookup_table_entry *lte, int fd,
-                              u64 size)
+ * @extract_chunk is a function that will be called to extract each chunk of the
+ * resource.  */
+int
+extract_wim_resource(const struct wim_lookup_table_entry *lte,
+                    u64 size,
+                    consume_data_callback_t extract_chunk,
+                    void *extract_chunk_arg)
 {
-       u64 bytes_remaining = size;
-       u8 buf[min(WIM_CHUNK_SIZE, bytes_remaining)];
-       u64 offset = 0;
-       int ret = 0;
-       u8 hash[SHA1_HASH_SIZE];
-
-       SHA_CTX ctx;
-       sha1_init(&ctx);
-
-       while (bytes_remaining) {
-               u64 to_read = min(bytes_remaining, WIM_CHUNK_SIZE);
-               ret = read_wim_resource(lte, buf, to_read, offset, false);
-               if (ret != 0)
-                       break;
-               sha1_update(&ctx, buf, to_read);
-               if (full_write(fd, buf, to_read) < to_read) {
-                       ERROR_WITH_ERRNO("Error extracting WIM resource");
-                       return WIMLIB_ERR_WRITE;
+       int ret;
+       if (size == wim_resource_size(lte)) {
+               /* Do SHA1 */
+               struct extract_ctx ctx;
+               ctx.extract_chunk = extract_chunk;
+               ctx.extract_chunk_arg = extract_chunk_arg;
+               sha1_init(&ctx.sha_ctx);
+               ret = read_resource_prefix(lte, size,
+                                          extract_chunk_sha1_wrapper,
+                                          wim_resource_chunk_size(lte),
+                                          &ctx, 0);
+               if (ret == 0) {
+                       u8 hash[SHA1_HASH_SIZE];
+                       sha1_final(hash, &ctx.sha_ctx);
+                       if (!hashes_equal(hash, lte->hash)) {
+                               if (wimlib_print_errors) {
+                                       ERROR("Invalid SHA1 message digest "
+                                             "on the following WIM resource:");
+                                       print_lookup_table_entry(lte, stderr);
+                                       if (lte->resource_location == RESOURCE_IN_WIM)
+                                               ERROR("The WIM file appears to be corrupt!");
+                               }
+                               ret = WIMLIB_ERR_INVALID_RESOURCE_HASH;
+                       }
                }
-               bytes_remaining -= to_read;
-               offset += to_read;
-       }
-       sha1_final(hash, &ctx);
-       if (!hashes_equal(hash, lte->hash)) {
-               ERROR("Invalid checksum on a WIM resource "
-                     "(detected when extracting to external file)");
-               ERROR("The following WIM resource is invalid:");
-               print_lookup_table_entry(lte);
-               return WIMLIB_ERR_INVALID_RESOURCE_HASH;
+       } else {
+               /* Don't do SHA1 */
+               ret = read_resource_prefix(lte, size, extract_chunk,
+                                          wim_resource_chunk_size(lte),
+                                          extract_chunk_arg, 0);
        }
-       return 0;
+       return ret;
 }
 
-/*
- * Extracts the WIM resource specified by @lte to the open file descriptor @fd.
- *
- * Returns 0 on success; nonzero on failure.
- */
-int extract_full_wim_resource_to_fd(const struct lookup_table_entry *lte, int fd)
+static int
+extract_wim_chunk_to_fd(const void *buf, size_t len, void *_fd_p)
 {
-       return extract_wim_resource_to_fd(lte, fd, wim_resource_size(lte));
+       struct filedes *fd = _fd_p;
+       int ret = full_write(fd, buf, len);
+       if (ret)
+               ERROR_WITH_ERRNO("Error writing to file descriptor");
+       return ret;
 }
 
-/*
- * Copies the file resource specified by the lookup table entry @lte from the
- * input WIM to the output WIM that has its FILE * given by
- * ((WIMStruct*)wim)->out_fp.
- *
- * The output_resource_entry, out_refcnt, and part_number fields of @lte are
- * updated.
- *
- * Metadata resources are not copied (they are handled elsewhere for joining and
- * splitting).
- */
-int copy_resource(struct lookup_table_entry *lte, void *wim)
+/* Extract the first @size bytes of the specified resource to the specified file
+ * descriptor.  If @size is the full size of the resource, its SHA1 message
+ * digest is also checked.  */
+int
+extract_wim_resource_to_fd(const struct wim_lookup_table_entry *lte,
+                          struct filedes *fd, u64 size)
 {
-       WIMStruct *w = wim;
-       int ret;
+       return extract_wim_resource(lte, size, extract_wim_chunk_to_fd, fd);
+}
 
-       if ((lte->resource_entry.flags & WIM_RESHDR_FLAG_METADATA) &&
-           !w->write_metadata)
-               return 0;
 
-       ret = write_wim_resource(lte, w->out_fp,
-                                wim_resource_compression_type(lte),
-                                &lte->output_resource_entry);
-       if (ret != 0)
-               return ret;
-       lte->out_refcnt = lte->refcnt;
-       lte->part_number = w->hdr.part_number;
+static int
+sha1_chunk(const void *buf, size_t len, void *ctx)
+{
+       sha1_update(ctx, buf, len);
        return 0;
 }
 
-/*
- * Writes a dentry's resources, including the main file resource as well as all
- * alternate data streams, to the output file.
- *
- * @dentry:  The dentry for the file.
- * @wim_p:   A pointer to the WIMStruct containing @dentry.
- *
- * @return zero on success, nonzero on failure.
- */
-int write_dentry_resources(struct dentry *dentry, void *wim_p)
+/* Calculate the SHA1 message digest of a resource, storing it in @lte->hash.  */
+int
+sha1_resource(struct wim_lookup_table_entry *lte)
 {
-       WIMStruct *w = wim_p;
-       int ret = 0;
-       struct lookup_table_entry *lte;
-       int ctype = wimlib_get_compression_type(w);
-
-       if (w->write_flags & WIMLIB_WRITE_FLAG_VERBOSE) {
-               wimlib_assert(dentry->full_path_utf8);
-               printf("Writing streams for `%s'\n", dentry->full_path_utf8);
-       }
+       int ret;
+       SHA_CTX sha_ctx;
+
+       sha1_init(&sha_ctx);
+       ret = read_resource_prefix(lte, wim_resource_size(lte),
+                                  sha1_chunk, wim_resource_chunk_size(lte),
+                                  &sha_ctx, 0);
+       if (ret == 0)
+               sha1_final(lte->hash, &sha_ctx);
 
-       for (unsigned i = 0; i <= dentry->d_inode->num_ads; i++) {
-               lte = inode_stream_lte(dentry->d_inode, i, w->lookup_table);
-               if (lte && ++lte->out_refcnt == 1) {
-                       ret = write_wim_resource(lte, w->out_fp, ctype,
-                                                &lte->output_resource_entry);
-                       if (ret != 0)
-                               break;
-               }
-       }
        return ret;
 }
 
-/*
- * Reads the metadata metadata resource from the WIM file.  The metadata
- * resource consists of the security data, followed by the directory entry for
- * the root directory, followed by all the other directory entries in the
- * filesystem.  The subdir_offset field of each directory entry gives the start
- * of its child entries from the beginning of the metadata resource.  An
- * end-of-directory is signaled by a directory entry of length '0', really of
- * length 8, because that's how long the 'length' field is.
- *
- * @fp:                The FILE* for the input WIM file.
- * @wim_ctype: The compression type of the WIM file.
- * @imd:       Pointer to the image metadata structure.  Its `metadata_lte'
- *             member specifies the lookup table entry for the metadata
- *             resource.  The rest of the image metadata entry will be filled
- *             in by this function.
- *
- * @return:    Zero on success, nonzero on failure.
- */
-int read_metadata_resource(WIMStruct *w, struct image_metadata *imd)
+/* Translates a WIM resource entry from the on-disk format into an in-memory
+ * format.  */
+void
+get_resource_entry(const struct resource_entry_disk *disk_entry,
+                  struct resource_entry *entry)
 {
-       u8 *buf;
-       u32 dentry_offset;
-       int ret;
-       struct dentry *dentry;
-       struct inode_table inode_tab;
-       const struct lookup_table_entry *metadata_lte;
-       u64 metadata_len;
-       u64 metadata_offset;
-       struct hlist_head inode_list;
-
-       metadata_lte = imd->metadata_lte;
-       metadata_len = wim_resource_size(metadata_lte);
-       metadata_offset = metadata_lte->resource_entry.offset;
-
-       DEBUG("Reading metadata resource: length = %"PRIu64", "
-             "offset = %"PRIu64"", metadata_len, metadata_offset);
-
-       /* There is no way the metadata resource could possibly be less than (8
-        * + WIM_DENTRY_DISK_SIZE) bytes, where the 8 is for security data (with
-        * no security descriptors) and WIM_DENTRY_DISK_SIZE is for the root
-        * dentry. */
-       if (metadata_len < 8 + WIM_DENTRY_DISK_SIZE) {
-               ERROR("Expected at least %u bytes for the metadata resource",
-                     8 + WIM_DENTRY_DISK_SIZE);
-               return WIMLIB_ERR_INVALID_RESOURCE_SIZE;
-       }
-
-       /* Allocate memory for the uncompressed metadata resource. */
-       buf = MALLOC(metadata_len);
+       /* Note: disk_entry may not be 8 byte aligned--- in that case, the
+        * offset and original_size members will be unaligned.  (This is okay
+        * since `struct resource_entry_disk' is declared as packed.)  */
+
+       /* Read the size and flags into a bitfield portably... */
+       entry->size = (((u64)disk_entry->size[0] <<  0) |
+                      ((u64)disk_entry->size[1] <<  8) |
+                      ((u64)disk_entry->size[2] << 16) |
+                      ((u64)disk_entry->size[3] << 24) |
+                      ((u64)disk_entry->size[4] << 32) |
+                      ((u64)disk_entry->size[5] << 40) |
+                      ((u64)disk_entry->size[6] << 48));
+       entry->flags = disk_entry->flags;
+       entry->offset = le64_to_cpu(disk_entry->offset);
+       entry->original_size = le64_to_cpu(disk_entry->original_size);
 
-       if (!buf) {
-               ERROR("Failed to allocate %"PRIu64" bytes for uncompressed "
-                     "metadata resource", metadata_len);
-               return WIMLIB_ERR_NOMEM;
+       /* offset and original_size are truncated to 62 bits to avoid possible
+        * overflows, when converting to a signed 64-bit integer (off_t) or when
+        * adding size or original_size.  This is okay since no one would ever
+        * actually have a WIM bigger than 4611686018427387903 bytes... */
+       if (entry->offset & 0xc000000000000000ULL) {
+               WARNING("Truncating offset in resource entry");
+               entry->offset &= 0x3fffffffffffffffULL;
        }
-
-       /* Read the metadata resource into memory.  (It may be compressed.) */
-       ret = read_full_wim_resource(metadata_lte, buf);
-       if (ret != 0)
-               goto out_free_buf;
-
-       DEBUG("Finished reading metadata resource into memory.");
-
-       /* The root directory entry starts after security data, aligned on an
-        * 8-byte boundary within the metadata resource.
-        *
-        * The security data starts with a 4-byte integer giving its total
-        * length, so if we round that up to an 8-byte boundary that gives us
-        * the offset of the root dentry.
-        *
-        * Here we read the security data into a wim_security_data structure,
-        * and if successful, go ahead and calculate the offset in the metadata
-        * resource of the root dentry. */
-
-       ret = read_security_data(buf, metadata_len, &imd->security_data);
-       if (ret != 0)
-               goto out_free_buf;
-
-       get_u32(buf, &dentry_offset);
-       if (dentry_offset == 0)
-               dentry_offset = 8;
-       dentry_offset = (dentry_offset + 7) & ~7;
-
-       /* Allocate memory for the root dentry and read it into memory */
-       dentry = MALLOC(sizeof(struct dentry));
-       if (!dentry) {
-               ERROR("Failed to allocate %zu bytes for root dentry",
-                     sizeof(struct dentry));
-               ret = WIMLIB_ERR_NOMEM;
-               goto out_free_security_data;
+       if (entry->original_size & 0xc000000000000000ULL) {
+               WARNING("Truncating original_size in resource entry");
+               entry->original_size &= 0x3fffffffffffffffULL;
        }
-
-       ret = read_dentry(buf, metadata_len, dentry_offset, dentry);
-
-       /* This is the root dentry, so set its pointers correctly. */
-       dentry->parent = dentry;
-       dentry->next   = dentry;
-       dentry->prev   = dentry;
-       if (ret != 0)
-               goto out_free_dentry_tree;
-       inode_add_dentry(dentry, dentry->d_inode);
-
-       /* Now read the entire directory entry tree into memory. */
-       DEBUG("Reading dentry tree");
-       ret = read_dentry_tree(buf, metadata_len, dentry);
-       if (ret != 0)
-               goto out_free_dentry_tree;
-
-       /* Calculate the full paths in the dentry tree. */
-       DEBUG("Calculating dentry full paths");
-       ret = for_dentry_in_tree(dentry, calculate_dentry_full_path, NULL);
-       if (ret != 0)
-               goto out_free_dentry_tree;
-
-       /* Build hash table that maps hard link group IDs to dentry sets */
-       DEBUG("Building link group table");
-       ret = init_inode_table(&inode_tab, 9001);
-       if (ret != 0)
-               goto out_free_dentry_tree;
-
-       for_dentry_in_tree(dentry, inode_table_insert, &inode_tab);
-
-       DEBUG("Fixing inconsistencies in the hard link groups");
-       ret = fix_inodes(&inode_tab, &inode_list);
-       destroy_inode_table(&inode_tab);
-       if (ret != 0)
-               goto out_free_dentry_tree;
-
-       DEBUG("Running miscellaneous verifications on the dentry tree");
-       for_lookup_table_entry(w->lookup_table, lte_zero_real_refcnt, NULL);
-       ret = for_dentry_in_tree(dentry, verify_dentry, w);
-       if (ret != 0)
-               goto out_free_dentry_tree;
-
-       DEBUG("Done reading image metadata");
-
-       imd->root_dentry = dentry;
-       imd->inode_list  = inode_list;
-       goto out_free_buf;
-out_free_dentry_tree:
-       free_dentry_tree(dentry, NULL);
-out_free_security_data:
-       free_security_data(imd->security_data);
-       imd->security_data = NULL;
-out_free_buf:
-       FREE(buf);
-       return ret;
 }
 
-/* Write the metadata resource for the current WIM image. */
-int write_metadata_resource(WIMStruct *w)
+/* Translates a WIM resource entry from an in-memory format into the on-disk
+ * format. */
+void
+put_resource_entry(const struct resource_entry *entry,
+                  struct resource_entry_disk *disk_entry)
 {
-       u8 *buf;
-       u8 *p;
-       int ret;
-       u64 subdir_offset;
-       struct dentry *root;
-       struct lookup_table_entry *lte;
-       u64 metadata_original_size;
-       const struct wim_security_data *sd;
-       const unsigned random_tail_len = 20;
-
-       DEBUG("Writing metadata resource for image %d", w->current_image);
-
-       root = wim_root_dentry(w);
-       sd = wim_security_data(w);
-
-       /* We do not allow the security data pointer to be NULL, although it may
-        * point to an empty security data with no entries. */
-       wimlib_assert(root != NULL);
-       wimlib_assert(sd != NULL);
-
-       /* Offset of first child of the root dentry.  It's equal to:
-        * - The total length of the security data, rounded to the next 8-byte
-        *   boundary,
-        * - plus the total length of the root dentry,
-        * - plus 8 bytes for an end-of-directory entry following the root
-        *   dentry (shouldn't really be needed, but just in case...)
-        */
-       subdir_offset = ((sd->total_length + 7) & ~7) +
-                       dentry_correct_total_length(root) + 8;
-
-       /* Calculate the subdirectory offsets for the entire dentry tree. */
-       calculate_subdir_offsets(root, &subdir_offset);
-
-       /* Total length of the metadata resource (uncompressed) */
-       metadata_original_size = subdir_offset + random_tail_len;
-
-       /* Allocate a buffer to contain the uncompressed metadata resource */
-       buf = MALLOC(metadata_original_size);
-       if (!buf) {
-               ERROR("Failed to allocate %"PRIu64" bytes for "
-                     "metadata resource", metadata_original_size);
-               return WIMLIB_ERR_NOMEM;
-       }
-
-       /* Write the security data into the resource buffer */
-       p = write_security_data(sd, buf);
-
-       /* Write the dentry tree into the resource buffer */
-       p = write_dentry_tree(root, p);
-
-       /*
-        * Append 20 random bytes to the metadata resource so that we don't have
-        * identical metadata resources if we happen to append exactly the same
-        * image twice without any changes in timestamps.  If this were to
-        * happen, it would cause confusion about the number and order of images
-        * in the WIM.
-        */
-       randomize_byte_array(p, random_tail_len);
-
-       /* We MUST have exactly filled the buffer; otherwise we calculated its
-        * size incorrectly or wrote the data incorrectly. */
-       wimlib_assert(p - buf + random_tail_len == metadata_original_size);
-
-       /* Get the lookup table entry for the metadata resource so we can update
-        * it. */
-       lte = wim_metadata_lookup_table_entry(w);
-
-       wimlib_assert(lte != NULL);
-
-       /* Write the metadata resource to the output WIM using the proper
-        * compression type.  The lookup table entry for the metadata resource
-        * is updated. */
-       ret = write_wim_resource_from_buffer(buf, metadata_original_size,
-                                            w->out_fp,
-                                            wimlib_get_compression_type(w),
-                                            &lte->output_resource_entry,
-                                            lte->hash);
-       if (ret != 0)
-               goto out;
-
-       /* It's very likely the SHA1 message digest of the metadata resource
-        * changed, so re-insert the lookup table entry into the lookup table.
-        * */
-       lookup_table_unlink(w->lookup_table, lte);
-       lookup_table_insert(w->lookup_table, lte);
-
-       /* We do not allow a metadata resource to be referenced multiple times,
-        * and the 20 random bytes appended to it should make it extremely
-        * likely for each metadata resource to be unique, even if the exact
-        * same image is captured. */
-       wimlib_assert(lte->out_refcnt == 0);
-       lte->out_refcnt = 1;
-
-       /* Make sure that the resource entry is written marked with the metadata
-        * flag. */
-       lte->output_resource_entry.flags |= WIM_RESHDR_FLAG_METADATA;
-out:
-       /* All the data has been written to the new WIM; no need for the buffer
-        * anymore */
-       FREE(buf);
-       return ret;
+       /* Note: disk_entry may not be 8 byte aligned--- in that case, the
+        * offset and original_size members will be unaligned.  (This is okay
+        * since `struct resource_entry_disk' is declared as packed.)  */
+       u64 size = entry->size;
+
+       disk_entry->size[0] = size >>  0;
+       disk_entry->size[1] = size >>  8;
+       disk_entry->size[2] = size >> 16;
+       disk_entry->size[3] = size >> 24;
+       disk_entry->size[4] = size >> 32;
+       disk_entry->size[5] = size >> 40;
+       disk_entry->size[6] = size >> 48;
+       disk_entry->flags = entry->flags;
+       disk_entry->offset = cpu_to_le64(entry->offset);
+       disk_entry->original_size = cpu_to_le64(entry->original_size);
 }