]> wimlib.net Git - wimlib/blobdiff - src/write.c
Fixes, comments
[wimlib] / src / write.c
index 3bd2f7804137d8b63a0c33062aeb893967de90ac..ccad7f07caa5c3f4b7af309995ab5974d2742aa0 100644 (file)
  */
 
 /*
- * Copyright (C) 2010 Carl Thijssen
- * 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 is free software; you can redistribute it and/or modify it under the
- * terms of the GNU Lesser General Public License as published by the Free
- * Software Foundation; either version 2.1 of the License, or (at your option)
+ * terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 3 of the License, or (at your option)
  * any later version.
  *
  * wimlib is distributed in the hope that it will be useful, but WITHOUT ANY
  * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
- * A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more
+ * A PARTICULAR PURPOSE. See the GNU General Public License for more
  * details.
  *
- * You should have received a copy of the GNU Lesser General Public License
+ * You should have received a copy of the GNU General Public License
  * along with wimlib; if not, see http://www.gnu.org/licenses/.
  */
 
+#include "config.h"
+
+#if defined(HAVE_SYS_FILE_H) && defined(HAVE_FLOCK)
+/* On BSD, this should be included before "list.h" so that "list.h" can
+ * overwrite the LIST_HEAD macro. */
+#  include <sys/file.h>
+#endif
+
+#ifdef __WIN32__
+#  include "win32.h"
+#endif
+
+#include "list.h"
 #include "wimlib_internal.h"
-#include "io.h"
+#include "buffer_io.h"
 #include "dentry.h"
 #include "lookup_table.h"
 #include "xml.h"
+
+#ifdef ENABLE_MULTITHREADED_COMPRESSION
+#  include <pthread.h>
+#endif
+
 #include <unistd.h>
+#include <errno.h>
 
-/* Reopens the FILE* for a WIM read-write. */
-static int reopen_rw(WIMStruct *w)
+#ifdef WITH_NTFS_3G
+#  include <time.h>
+#  include <ntfs-3g/attrib.h>
+#  include <ntfs-3g/inode.h>
+#  include <ntfs-3g/dir.h>
+#endif
+
+#ifdef HAVE_ALLOCA_H
+#  include <alloca.h>
+#else
+#  include <stdlib.h>
+#endif
+
+#include <limits.h>
+
+#if defined(__WIN32__) && !defined(INVALID_HANDLE_VALUE)
+#  define INVALID_HANDLE_VALUE ((HANDLE)(-1))
+#endif
+
+static int
+fflush_and_ftruncate(FILE *fp, off_t size)
 {
-       FILE *fp;
+       int ret;
 
-       if (fclose(w->fp) != 0)
-               ERROR("Failed to close the file `%s': %m\n", w->filename);
-       fp = fopen(w->filename, "r+b");
-       if (!fp) {
-               ERROR("Failed to open `%s' for reading and writing: "
-                               "%m\n", w->filename);
-               return WIMLIB_ERR_OPEN;
+       ret = fflush(fp);
+       if (ret != 0) {
+               ERROR_WITH_ERRNO("Failed to flush data to output WIM file");
+               return WIMLIB_ERR_WRITE;
+       }
+       ret = ftruncate(fileno(fp), size);
+       if (ret != 0) {
+               ERROR_WITH_ERRNO("Failed to truncate output WIM file to "
+                                "%"PRIu64" bytes", size);
+               return WIMLIB_ERR_WRITE;
        }
-       w->fp = fp;
        return 0;
 }
 
+/* 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.
+ */
+static int
+begin_wim_resource_chunk_tab(const struct wim_lookup_table_entry *lte,
+                            FILE *out_fp,
+                            off_t file_offset,
+                            struct chunk_table **chunk_tab_ret)
+{
+       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);
+       int ret;
+
+       if (!chunk_tab) {
+               ERROR("Failed to allocate chunk table for %"PRIu64" byte "
+                     "resource", size);
+               ret = WIMLIB_ERR_NOMEM;
+               goto out;
+       }
+       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;
+       }
 
+       ret = 0;
+out:
+       *chunk_tab_ret = chunk_tab;
+       return ret;
+}
 
-/* 
- * Writes a WIM file to the original file that it was read from, overwriting it.
+/*
+ * compress_func_t- Pointer to a function to compresses a chunk
+ *                  of a WIM resource.  This may be either
+ *                  wimlib_xpress_compress() (xpress-compress.c) or
+ *                  wimlib_lzx_compress() (lzx-compress.c).
+ *
+ * @chunk:       Uncompressed data of the chunk.
+ * @chunk_size:          Size of the uncompressed chunk, in bytes.
+ * @out:         Pointer to output buffer of size at least (@chunk_size - 1) bytes.
+ *
+ * Returns the size of the compressed data written to @out in bytes, or 0 if the
+ * data could not be compressed to (@chunk_size - 1) bytes or fewer.
+ *
+ * As a special requirement, the compression code is optimized for the WIM
+ * format and therefore requires (@chunk_size <= 32768).
+ *
+ * As another special requirement, the compression code will read up to 8 bytes
+ * off the end of the @chunk array for performance reasons.  The values of these
+ * bytes will not affect the output of the compression, but the calling code
+ * must make sure that the buffer holding the uncompressed chunk is actually at
+ * least (@chunk_size + 8) bytes, or at least that these extra bytes are in
+ * mapped memory that will not cause a memory access violation if accessed.
  */
-WIMLIBAPI int wimlib_overwrite(WIMStruct *w, int flags)
+typedef unsigned (*compress_func_t)(const void *chunk, unsigned chunk_size,
+                                   void *out);
+
+compress_func_t
+get_compress_func(int out_ctype)
+{
+       if (out_ctype == WIMLIB_COMPRESSION_TYPE_LZX)
+               return wimlib_lzx_compress;
+       else
+               return wimlib_xpress_compress;
+}
+
+/*
+ * 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 the chunk to.
+ * @compress:     Compression function to use (NULL if writing uncompressed
+ *                     data).
+ * @chunk_tab:   Pointer to chunk table being created.  It is updated with the
+ *                     offset of the chunk we write.
+ *
+ * Returns 0 on success; nonzero on failure.
+ */
+static int
+write_wim_resource_chunk(const void *chunk, unsigned chunk_size,
+                        FILE *out_fp, compress_func_t compress,
+                        struct chunk_table *chunk_tab)
+{
+       const u8 *out_chunk;
+       unsigned out_chunk_size;
+       if (compress) {
+               u8 *compressed_chunk = alloca(chunk_size);
+
+               out_chunk_size = compress(chunk, chunk_size, compressed_chunk);
+               if (out_chunk_size) {
+                       /* Write compressed */
+                       out_chunk = compressed_chunk;
+               } else {
+                       /* Write uncompressed */
+                       out_chunk = chunk;
+                       out_chunk_size = chunk_size;
+               }
+               *chunk_tab->cur_offset_p++ = chunk_tab->cur_offset;
+               chunk_tab->cur_offset += out_chunk_size;
+       } else {
+               /* Write uncompressed */
+               out_chunk = chunk;
+               out_chunk_size = chunk_size;
+       }
+       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;
+       }
+       return 0;
+}
+
+/*
+ * Finishes a WIM chunk table 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.
+ */
+static int
+finish_wim_resource_chunk_tab(struct chunk_table *chunk_tab,
+                             FILE *out_fp, u64 *compressed_size_p)
+{
+       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;
+       }
+
+       if (chunk_tab->bytes_per_chunk_entry == 8) {
+               array_cpu_to_le64(chunk_tab->offsets, chunk_tab->num_chunks);
+       } 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;
+       }
+       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;
+}
+
+static int
+write_uncompressed_resource_and_truncate(struct wim_lookup_table_entry *lte,
+                                        FILE *out_fp,
+                                        off_t file_offset,
+                                        struct resource_entry *out_res_entry)
+{
+       int ret;
+       if (fseeko(out_fp, file_offset, SEEK_SET) != 0) {
+               ERROR_WITH_ERRNO("Failed to seek to byte %"PRIu64" of "
+                                "output WIM file", file_offset);
+               return WIMLIB_ERR_WRITE;
+       }
+       ret = write_wim_resource(lte, out_fp,
+                                WIMLIB_COMPRESSION_TYPE_NONE,
+                                out_res_entry,
+                                0);
+       if (ret)
+               return ret;
+
+       return fflush_and_ftruncate(out_fp,
+                                   file_offset + wim_resource_size(lte));
+}
+
+struct write_resource_ctx {
+       compress_func_t compress;
+       struct chunk_table *chunk_tab;
+       FILE *out_fp;
+       SHA_CTX sha_ctx;
+       bool doing_sha;
+};
+
+static int
+write_resource_cb(const void *chunk, size_t chunk_size, void *_ctx)
+{
+       struct write_resource_ctx *ctx = _ctx;
+
+       if (ctx->doing_sha)
+               sha1_update(&ctx->sha_ctx, chunk, chunk_size);
+
+       if (ctx->compress) {
+               return write_wim_resource_chunk(chunk, chunk_size,
+                                               ctx->out_fp, ctx->compress,
+                                               ctx->chunk_tab);
+       } else {
+               if (fwrite(chunk, 1, chunk_size, ctx->out_fp) != chunk_size) {
+                       ERROR_WITH_ERRNO("Error writing to output WIM");
+                       return WIMLIB_ERR_WRITE;
+               } else {
+                       return 0;
+               }
+       }
+}
+
+int
+write_wim_resource(struct wim_lookup_table_entry *lte,
+                  FILE *out_fp, int out_ctype,
+                  struct resource_entry *out_res_entry,
+                  int flags)
+{
+       struct write_resource_ctx write_ctx;
+       u64 new_size;
+       off_t offset;
+       int ret;
+
+       if (wim_resource_size(lte) == 0) {
+               /* Empty resource; nothing needs to be done, so just return
+                * success. */
+               return 0;
+       }
+
+       offset = ftello(out_fp);
+       if (offset == -1) {
+               ERROR_WITH_ERRNO("Can't get position in output WIM");
+               return WIMLIB_ERR_WRITE;
+       }
+
+       /* Can we simply copy the compressed data without recompressing it? */
+
+       if (!(flags & WIMLIB_RESOURCE_FLAG_RECOMPRESS) &&
+           lte->resource_location == RESOURCE_IN_WIM &&
+           wimlib_get_compression_type(lte->wim) == out_ctype)
+       {
+               flags |= WIMLIB_RESOURCE_FLAG_RAW;
+               write_ctx.doing_sha = false;
+       } else {
+               write_ctx.doing_sha = true;
+               sha1_init(&write_ctx.sha_ctx);
+       }
+
+       /* Initialize the chunk table and set the compression function if
+        * compressing the resource. */
+       if (out_ctype == WIMLIB_COMPRESSION_TYPE_NONE ||
+           (flags & WIMLIB_RESOURCE_FLAG_RAW)) {
+               write_ctx.compress = NULL;
+               write_ctx.chunk_tab = NULL;
+       } else {
+               write_ctx.compress = get_compress_func(out_ctype);
+               ret = begin_wim_resource_chunk_tab(lte, out_fp,
+                                                  offset,
+                                                  &write_ctx.chunk_tab);
+               if (ret)
+                       return ret;
+       }
+
+       /* Write the data */
+       write_ctx.out_fp = out_fp;
+       ret = read_resource_prefix(lte, wim_resource_size(lte),
+                                  write_resource_cb, &write_ctx, 0);
+
+       /* Verify SHA1 message digest of the resource, or set the hash for the
+        * first time. */
+       if (write_ctx.doing_sha) {
+               u8 md[SHA1_HASH_SIZE];
+               sha1_final(md, &write_ctx.sha_ctx);
+               if (lte->unhashed) {
+                       copy_hash(lte->hash, md);
+               } else if (!hashes_equal(md, lte->hash)) {
+                       ERROR("WIM resource has incorrect hash!");
+                       if (lte_filename_valid(lte)) {
+                               ERROR("We were reading it from \"%"TS"\"; maybe "
+                                     "it changed while we were reading it.",
+                                     lte->file_on_disk);
+                       }
+                       ret = WIMLIB_ERR_INVALID_RESOURCE_HASH;
+                       goto out_free_chunk_tab;
+               }
+       }
+
+       out_res_entry->flags = lte->resource_entry.flags;
+       out_res_entry->original_size = wim_resource_size(lte);
+       out_res_entry->offset = offset;
+       if (flags & WIMLIB_RESOURCE_FLAG_RAW) {
+               /* Doing a raw write:  The new compressed size is the same as
+                * the compressed size in the other WIM. */
+               new_size = lte->resource_entry.size;
+       } else if (out_ctype == WIMLIB_COMPRESSION_TYPE_NONE) {
+               /* Using WIMLIB_COMPRESSION_TYPE_NONE:  The new compressed size
+                * is the original size. */
+               new_size = lte->resource_entry.original_size;
+               out_res_entry->flags &= ~WIM_RESHDR_FLAG_COMPRESSED;
+       } else {
+               /* Using a different compression type:  Call
+                * finish_wim_resource_chunk_tab() and it will provide the new
+                * compressed size. */
+               ret = finish_wim_resource_chunk_tab(write_ctx.chunk_tab, out_fp,
+                                                   &new_size);
+               if (ret)
+                       goto out_free_chunk_tab;
+               if (new_size >= wim_resource_size(lte)) {
+                       /* Oops!  We compressed the resource to larger than the original
+                        * size.  Write the resource uncompressed instead. */
+                       ret = write_uncompressed_resource_and_truncate(lte,
+                                                                      out_fp,
+                                                                      offset,
+                                                                      out_res_entry);
+                       goto out_free_chunk_tab;
+               }
+               out_res_entry->flags |= WIM_RESHDR_FLAG_COMPRESSED;
+       }
+       out_res_entry->size = new_size;
+       ret = 0;
+out_free_chunk_tab:
+       FREE(write_ctx.chunk_tab);
+       return ret;
+}
+
+#ifdef ENABLE_MULTITHREADED_COMPRESSION
+
+/* Blocking shared queue (solves the producer-consumer problem) */
+struct shared_queue {
+       unsigned size;
+       unsigned front;
+       unsigned back;
+       unsigned filled_slots;
+       void **array;
+       pthread_mutex_t lock;
+       pthread_cond_t msg_avail_cond;
+       pthread_cond_t space_avail_cond;
+};
+
+static int
+shared_queue_init(struct shared_queue *q, unsigned size)
+{
+       wimlib_assert(size != 0);
+       q->array = CALLOC(sizeof(q->array[0]), size);
+       if (!q->array)
+               return WIMLIB_ERR_NOMEM;
+       q->filled_slots = 0;
+       q->front = 0;
+       q->back = size - 1;
+       q->size = size;
+       pthread_mutex_init(&q->lock, NULL);
+       pthread_cond_init(&q->msg_avail_cond, NULL);
+       pthread_cond_init(&q->space_avail_cond, NULL);
+       return 0;
+}
+
+static void
+shared_queue_destroy(struct shared_queue *q)
+{
+       FREE(q->array);
+       pthread_mutex_destroy(&q->lock);
+       pthread_cond_destroy(&q->msg_avail_cond);
+       pthread_cond_destroy(&q->space_avail_cond);
+}
+
+static void
+shared_queue_put(struct shared_queue *q, void *obj)
+{
+       pthread_mutex_lock(&q->lock);
+       while (q->filled_slots == q->size)
+               pthread_cond_wait(&q->space_avail_cond, &q->lock);
+
+       q->back = (q->back + 1) % q->size;
+       q->array[q->back] = obj;
+       q->filled_slots++;
+
+       pthread_cond_broadcast(&q->msg_avail_cond);
+       pthread_mutex_unlock(&q->lock);
+}
+
+static void *
+shared_queue_get(struct shared_queue *q)
+{
+       void *obj;
+
+       pthread_mutex_lock(&q->lock);
+       while (q->filled_slots == 0)
+               pthread_cond_wait(&q->msg_avail_cond, &q->lock);
+
+       obj = q->array[q->front];
+       q->array[q->front] = NULL;
+       q->front = (q->front + 1) % q->size;
+       q->filled_slots--;
+
+       pthread_cond_broadcast(&q->space_avail_cond);
+       pthread_mutex_unlock(&q->lock);
+       return obj;
+}
+
+struct compressor_thread_params {
+       struct shared_queue *res_to_compress_queue;
+       struct shared_queue *compressed_res_queue;
+       compress_func_t compress;
+};
+
+#define MAX_CHUNKS_PER_MSG 2
+
+struct message {
+       struct wim_lookup_table_entry *lte;
+       u8 *uncompressed_chunks[MAX_CHUNKS_PER_MSG];
+       u8 *out_compressed_chunks[MAX_CHUNKS_PER_MSG];
+       u8 *compressed_chunks[MAX_CHUNKS_PER_MSG];
+       unsigned uncompressed_chunk_sizes[MAX_CHUNKS_PER_MSG];
+       unsigned compressed_chunk_sizes[MAX_CHUNKS_PER_MSG];
+       unsigned num_chunks;
+       struct list_head list;
+       bool complete;
+       u64 begin_chunk;
+};
+
+static void
+compress_chunks(struct message *msg, compress_func_t compress)
+{
+       for (unsigned i = 0; i < msg->num_chunks; i++) {
+               DEBUG2("compress chunk %u of %u", i, msg->num_chunks);
+               unsigned len = compress(msg->uncompressed_chunks[i],
+                                       msg->uncompressed_chunk_sizes[i],
+                                       msg->compressed_chunks[i]);
+               if (len) {
+                       /* To be written compressed */
+                       msg->out_compressed_chunks[i] = msg->compressed_chunks[i];
+                       msg->compressed_chunk_sizes[i] = len;
+               } else {
+                       /* To be written uncompressed */
+                       msg->out_compressed_chunks[i] = msg->uncompressed_chunks[i];
+                       msg->compressed_chunk_sizes[i] = msg->uncompressed_chunk_sizes[i];
+
+               }
+       }
+}
+
+/* Compressor thread routine.  This is a lot simpler than the main thread
+ * routine: just repeatedly get a group of chunks from the
+ * res_to_compress_queue, compress them, and put them in the
+ * compressed_res_queue.  A NULL pointer indicates that the thread should stop.
+ * */
+static void *
+compressor_thread_proc(void *arg)
+{
+       struct compressor_thread_params *params = arg;
+       struct shared_queue *res_to_compress_queue = params->res_to_compress_queue;
+       struct shared_queue *compressed_res_queue = params->compressed_res_queue;
+       compress_func_t compress = params->compress;
+       struct message *msg;
+
+       DEBUG("Compressor thread ready");
+       while ((msg = shared_queue_get(res_to_compress_queue)) != NULL) {
+               compress_chunks(msg, compress);
+               shared_queue_put(compressed_res_queue, msg);
+       }
+       DEBUG("Compressor thread terminating");
+       return NULL;
+}
+#endif /* ENABLE_MULTITHREADED_COMPRESSION */
+
+static void
+do_write_streams_progress(union wimlib_progress_info *progress,
+                         wimlib_progress_func_t progress_func,
+                         uint64_t size_added)
+{
+       progress->write_streams.completed_bytes += size_added;
+       progress->write_streams.completed_streams++;
+       if (progress_func &&
+           progress->write_streams.completed_bytes >= progress->write_streams._private)
+       {
+               progress_func(WIMLIB_PROGRESS_MSG_WRITE_STREAMS,
+                             progress);
+               if (progress->write_streams._private == progress->write_streams.total_bytes) {
+                       progress->write_streams._private = ~0;
+               } else {
+                       progress->write_streams._private =
+                               min(progress->write_streams.total_bytes,
+                                   progress->write_streams.completed_bytes +
+                                       progress->write_streams.total_bytes / 100);
+               }
+       }
+}
+
+static int
+sha1_chunk(const void *buf, size_t len, void *ctx)
+{
+       sha1_update(ctx, buf, len);
+       return 0;
+}
+
+static int
+sha1_resource(struct wim_lookup_table_entry *lte)
+{
+       int ret;
+       SHA_CTX sha_ctx;
+
+       sha1_init(&sha_ctx);
+       ret = read_resource_prefix(lte, wim_resource_size(lte),
+                                  sha1_chunk, &sha_ctx, 0);
+       if (ret == 0)
+               sha1_final(lte->hash, &sha_ctx);
+       return ret;
+}
+
+enum {
+       STREAMS_MERGED = 0,
+       STREAMS_NOT_MERGED = 1,
+};
+
+static int
+do_write_stream_list(struct list_head *my_resources,
+                    struct wim_lookup_table *lookup_table,
+                    FILE *out_fp,
+                    int out_ctype,
+                    wimlib_progress_func_t progress_func,
+                    union wimlib_progress_info *progress,
+                    int write_resource_flags)
+{
+       int ret;
+       struct wim_lookup_table_entry *lte;
+
+       while (!list_empty(my_resources)) {
+               lte = container_of(my_resources->next,
+                                  struct wim_lookup_table_entry,
+                                  write_streams_list);
+               list_del(&lte->write_streams_list);
+               if (lte->unhashed && !lte->unique_size) {
+                       struct wim_lookup_table_entry *duplicate_lte;
+                       struct wim_lookup_table_entry **my_ptr;
+
+                       my_ptr = lte->my_ptr;
+                       ret = sha1_resource(lte);
+                       if (ret)
+                               return ret;
+                       duplicate_lte = __lookup_resource(lookup_table, lte->hash);
+                       if (duplicate_lte) {
+                               bool new_stream = (duplicate_lte->out_refcnt == 0);
+                               duplicate_lte->refcnt += lte->refcnt;
+                               duplicate_lte->out_refcnt += lte->refcnt;
+                               *my_ptr = duplicate_lte;
+                               free_lookup_table_entry(lte);
+
+                               if (new_stream) {
+                                       lte = duplicate_lte;
+                                       DEBUG("Stream of length %"PRIu64" is duplicate "
+                                             "with one already in WIM",
+                                             wim_resource_size(duplicate_lte));
+                               } else {
+                                       DEBUG("Discarding duplicate stream of length %"PRIu64,
+                                             wim_resource_size(duplicate_lte));
+                                       goto skip_to_progress;
+                               }
+
+                       } else {
+                               lookup_table_insert(lookup_table, lte);
+                               lte->out_refcnt = lte->refcnt;
+                               lte->unhashed = 0;
+                       }
+               }
+
+               wimlib_assert(lte->out_refcnt != 0);
+
+               ret = write_wim_resource(lte,
+                                        out_fp,
+                                        out_ctype,
+                                        &lte->output_resource_entry,
+                                        write_resource_flags);
+               if (ret)
+                       return ret;
+               if (lte->unhashed) {
+                       lookup_table_insert(lookup_table, lte);
+                       lte->unhashed = 0;
+               }
+       skip_to_progress:
+               do_write_streams_progress(progress,
+                                         progress_func,
+                                         wim_resource_size(lte));
+       }
+       return 0;
+}
+
+static int
+write_stream_list_serial(struct list_head *stream_list,
+                        struct wim_lookup_table *lookup_table,
+                        FILE *out_fp,
+                        int out_ctype,
+                        int write_flags,
+                        wimlib_progress_func_t progress_func,
+                        union wimlib_progress_info *progress)
+{
+       int write_resource_flags;
+
+       if (write_flags & WIMLIB_WRITE_FLAG_RECOMPRESS)
+               write_resource_flags = WIMLIB_RESOURCE_FLAG_RECOMPRESS;
+       else
+               write_resource_flags = 0;
+       progress->write_streams.num_threads = 1;
+       if (progress_func)
+               progress_func(WIMLIB_PROGRESS_MSG_WRITE_STREAMS, progress);
+       return do_write_stream_list(stream_list,
+                                   lookup_table,
+                                   out_fp,
+                                   out_ctype, progress_func,
+                                   progress, write_resource_flags);
+}
+
+#ifdef ENABLE_MULTITHREADED_COMPRESSION
+static int
+write_wim_chunks(struct message *msg, FILE *out_fp,
+                struct chunk_table *chunk_tab)
+{
+       for (unsigned i = 0; i < msg->num_chunks; i++) {
+               unsigned chunk_csize = msg->compressed_chunk_sizes[i];
+
+               DEBUG2("Write wim chunk %u of %u (csize = %u)",
+                     i, msg->num_chunks, chunk_csize);
+
+               if (fwrite(msg->out_compressed_chunks[i], 1, chunk_csize, out_fp)
+                   != chunk_csize)
+               {
+                       ERROR_WITH_ERRNO("Failed to write WIM chunk");
+                       return WIMLIB_ERR_WRITE;
+               }
+
+               *chunk_tab->cur_offset_p++ = chunk_tab->cur_offset;
+               chunk_tab->cur_offset += chunk_csize;
+       }
+       return 0;
+}
+
+/*
+ * This function is executed by the main thread when the resources are being
+ * compressed in parallel.  The main thread is in change of all reading of the
+ * uncompressed data and writing of the compressed data.  The compressor threads
+ * *only* do compression from/to in-memory buffers.
+ *
+ * Each unit of work given to a compressor thread is up to MAX_CHUNKS_PER_MSG
+ * chunks of compressed data to compress, represented in a `struct message'.
+ * Each message is passed from the main thread to a worker thread through the
+ * res_to_compress_queue, and it is passed back through the
+ * compressed_res_queue.
+ */
+static int
+main_writer_thread_proc(struct list_head *stream_list,
+                       FILE *out_fp,
+                       int out_ctype,
+                       struct shared_queue *res_to_compress_queue,
+                       struct shared_queue *compressed_res_queue,
+                       size_t num_messages,
+                       int write_flags,
+                       wimlib_progress_func_t progress_func,
+                       union wimlib_progress_info *progress)
+{
+       int ret;
+       struct chunk_table *cur_chunk_tab = NULL;
+       struct message *msgs = CALLOC(num_messages, sizeof(struct message));
+       struct wim_lookup_table_entry *next_lte = NULL;
+
+       // Initially, all the messages are available to use.
+       LIST_HEAD(available_msgs);
+
+       if (!msgs) {
+               ret = WIMLIB_ERR_NOMEM;
+               goto out;
+       }
+
+       for (size_t i = 0; i < num_messages; i++)
+               list_add(&msgs[i].list, &available_msgs);
+
+       // outstanding_resources is the list of resources that currently have
+       // had chunks sent off for compression.
+       //
+       // The first stream in outstanding_resources is the stream that is
+       // currently being written (cur_lte).
+       //
+       // The last stream in outstanding_resources is the stream that is
+       // currently being read and chunks fed to the compressor threads
+       // (next_lte).
+       //
+       // Depending on the number of threads and the sizes of the resource,
+       // the outstanding streams list may contain streams between cur_lte and
+       // next_lte that have all their chunks compressed or being compressed,
+       // but haven't been written yet.
+       //
+       LIST_HEAD(outstanding_resources);
+       struct list_head *next_resource = stream_list->next;
+       u64 next_chunk = 0;
+       u64 next_num_chunks = 0;
+
+       // As in write_wim_resource(), each resource we read is checksummed.
+       SHA_CTX next_sha_ctx;
+       u8 next_hash[SHA1_HASH_SIZE];
+
+       // Resources that don't need any chunks compressed are added to this
+       // list and written directly by the main thread.
+       LIST_HEAD(my_resources);
+
+       struct wim_lookup_table_entry *cur_lte = NULL;
+       struct message *msg;
+
+#ifdef WITH_NTFS_3G
+       ntfs_inode *ni = NULL;
+#endif
+
+       DEBUG("Initializing buffers for uncompressed "
+             "and compressed data (%zu bytes needed)",
+             num_messages * MAX_CHUNKS_PER_MSG * WIM_CHUNK_SIZE * 2);
+
+       // Pre-allocate all the buffers that will be needed to do the chunk
+       // compression.
+       for (size_t i = 0; i < num_messages; i++) {
+               for (size_t j = 0; j < MAX_CHUNKS_PER_MSG; j++) {
+                       msgs[i].compressed_chunks[j] = MALLOC(WIM_CHUNK_SIZE);
+
+                       // The extra 8 bytes is because longest_match() in
+                       // lz77.c may read a little bit off the end of the
+                       // uncompressed data.  It doesn't need to be
+                       // initialized--- we really just need to avoid accessing
+                       // an unmapped page.
+                       msgs[i].uncompressed_chunks[j] = MALLOC(WIM_CHUNK_SIZE + 8);
+                       if (msgs[i].compressed_chunks[j] == NULL ||
+                           msgs[i].uncompressed_chunks[j] == NULL)
+                       {
+                               ret = WIMLIB_ERR_NOMEM;
+                               goto out;
+                       }
+               }
+       }
+
+       // This loop is executed until all resources have been written, except
+       // possibly a few that have been added to the @my_resources list for
+       // writing later.
+       while (1) {
+               // Send chunks to the compressor threads until either (a) there
+               // are no more messages available since they were all sent off,
+               // or (b) there are no more resources that need to be
+               // compressed.
+               while (!list_empty(&available_msgs)) {
+                       if (next_chunk == next_num_chunks) {
+                               // If next_chunk == next_num_chunks, there are
+                               // no more chunks to write in the current
+                               // stream.  So, check the SHA1 message digest of
+                               // the stream that was just finished (unless
+                               // next_lte == NULL, which is the case the very
+                               // first time this loop is entered, and also
+                               // near the very end of the compression when
+                               // there are no more streams.)  Then, advance to
+                               // the next stream (if there is one).
+                               if (next_lte != NULL) {
+                               #ifdef WITH_NTFS_3G
+                                       end_wim_resource_read(next_lte, ni);
+                                       ni = NULL;
+                               #else
+                                       end_wim_resource_read(next_lte);
+                               #endif
+                                       DEBUG2("Finalize SHA1 md (next_num_chunks=%zu)",
+                                              next_num_chunks);
+                                       sha1_final(next_hash, &next_sha_ctx);
+                                       if (!hashes_equal(next_lte->hash, next_hash)) {
+                                               ERROR("WIM resource has incorrect hash!");
+                                               if (next_lte->resource_location ==
+                                                   RESOURCE_IN_FILE_ON_DISK)
+                                               {
+                                                       ERROR("We were reading it from `%"TS"'; "
+                                                             "maybe it changed while we were "
+                                                             "reading it.",
+                                                             next_lte->file_on_disk);
+                                               }
+                                               ret = WIMLIB_ERR_INVALID_RESOURCE_HASH;
+                                               goto out;
+                                       }
+                               }
+
+                               // Advance to the next resource.
+                               //
+                               // If the next resource needs no compression, just write
+                               // it with this thread (not now though--- we could be in
+                               // the middle of writing another resource.)  Keep doing
+                               // this until we either get to the end of the resources
+                               // list, or we get to a resource that needs compression.
+                               while (1) {
+                                       if (next_resource == stream_list) {
+                                               // No more resources to send for
+                                               // compression
+                                               next_lte = NULL;
+                                               break;
+                                       }
+                                       next_lte = container_of(next_resource,
+                                                               struct wim_lookup_table_entry,
+                                                               staging_list);
+                                       next_resource = next_resource->next;
+                                       if ((!(write_flags & WIMLIB_WRITE_FLAG_RECOMPRESS)
+                                              && wim_resource_compression_type(next_lte) == out_ctype)
+                                           || wim_resource_size(next_lte) == 0)
+                                       {
+                                               list_add_tail(&next_lte->staging_list,
+                                                             &my_resources);
+                                       } else {
+                                               list_add_tail(&next_lte->staging_list,
+                                                             &outstanding_resources);
+                                               next_chunk = 0;
+                                               next_num_chunks = wim_resource_chunks(next_lte);
+                                               sha1_init(&next_sha_ctx);
+                                               INIT_LIST_HEAD(&next_lte->msg_list);
+                                       #ifdef WITH_NTFS_3G
+                                               ret = prepare_resource_for_read(next_lte, &ni);
+                                       #else
+                                               ret = prepare_resource_for_read(next_lte);
+                                       #endif
+
+                                               if (ret != 0)
+                                                       goto out;
+                                               if (cur_lte == NULL) {
+                                                       // Set cur_lte for the
+                                                       // first time
+                                                       cur_lte = next_lte;
+                                               }
+                                               break;
+                                       }
+                               }
+                       }
+
+                       if (next_lte == NULL) {
+                               // No more resources to send for compression
+                               break;
+                       }
+
+                       // Get a message from the available messages
+                       // list
+                       msg = container_of(available_msgs.next,
+                                          struct message,
+                                          list);
+
+                       // ... and delete it from the available messages
+                       // list
+                       list_del(&msg->list);
+
+                       // Initialize the message with the chunks to
+                       // compress.
+                       msg->num_chunks = min(next_num_chunks - next_chunk,
+                                             MAX_CHUNKS_PER_MSG);
+                       msg->lte = next_lte;
+                       msg->complete = false;
+                       msg->begin_chunk = next_chunk;
+
+                       unsigned size = WIM_CHUNK_SIZE;
+                       for (unsigned i = 0; i < msg->num_chunks; i++) {
+
+                               // Read chunk @next_chunk of the stream into the
+                               // message so that a compressor thread can
+                               // compress it.
+
+                               if (next_chunk == next_num_chunks - 1) {
+                                       size = MODULO_NONZERO(wim_resource_size(next_lte),
+                                                             WIM_CHUNK_SIZE);
+                               }
+
+                               DEBUG2("Read resource (size=%u, offset=%zu)",
+                                     size, next_chunk * WIM_CHUNK_SIZE);
+
+                               msg->uncompressed_chunk_sizes[i] = size;
+
+                               ret = read_wim_resource(next_lte,
+                                                       msg->uncompressed_chunks[i],
+                                                       size,
+                                                       next_chunk * WIM_CHUNK_SIZE,
+                                                       0);
+                               if (ret != 0)
+                                       goto out;
+                               sha1_update(&next_sha_ctx,
+                                           msg->uncompressed_chunks[i], size);
+                               next_chunk++;
+                       }
+
+                       // Send the compression request
+                       list_add_tail(&msg->list, &next_lte->msg_list);
+                       shared_queue_put(res_to_compress_queue, msg);
+                       DEBUG2("Compression request sent");
+               }
+
+               // If there are no outstanding resources, there are no more
+               // resources that need to be written.
+               if (list_empty(&outstanding_resources)) {
+                       ret = 0;
+                       goto out;
+               }
+
+               // Get the next message from the queue and process it.
+               // The message will contain 1 or more data chunks that have been
+               // compressed.
+               msg = shared_queue_get(compressed_res_queue);
+               msg->complete = true;
+
+               // Is this the next chunk in the current resource?  If it's not
+               // (i.e., an earlier chunk in a same or different resource
+               // hasn't been compressed yet), do nothing, and keep this
+               // message around until all earlier chunks are received.
+               //
+               // Otherwise, write all the chunks we can.
+               while (cur_lte != NULL &&
+                      !list_empty(&cur_lte->msg_list) &&
+                      (msg = container_of(cur_lte->msg_list.next,
+                                          struct message,
+                                          list))->complete)
+               {
+                       DEBUG2("Complete msg (begin_chunk=%"PRIu64")", msg->begin_chunk);
+                       if (msg->begin_chunk == 0) {
+                               DEBUG2("Begin chunk tab");
+
+                               // This is the first set of chunks.  Leave space
+                               // for the chunk table in the output file.
+                               off_t cur_offset = ftello(out_fp);
+                               if (cur_offset == -1) {
+                                       ret = WIMLIB_ERR_WRITE;
+                                       goto out;
+                               }
+                               ret = begin_wim_resource_chunk_tab(cur_lte,
+                                                                  out_fp,
+                                                                  cur_offset,
+                                                                  &cur_chunk_tab);
+                               if (ret != 0)
+                                       goto out;
+                       }
+
+                       // Write the compressed chunks from the message.
+                       ret = write_wim_chunks(msg, out_fp, cur_chunk_tab);
+                       if (ret != 0)
+                               goto out;
+
+                       list_del(&msg->list);
+
+                       // This message is available to use for different chunks
+                       // now.
+                       list_add(&msg->list, &available_msgs);
+
+                       // Was this the last chunk of the stream?  If so, finish
+                       // it.
+                       if (list_empty(&cur_lte->msg_list) &&
+                           msg->begin_chunk + msg->num_chunks == cur_chunk_tab->num_chunks)
+                       {
+                               DEBUG2("Finish wim chunk tab");
+                               u64 res_csize;
+                               ret = finish_wim_resource_chunk_tab(cur_chunk_tab,
+                                                                   out_fp,
+                                                                   &res_csize);
+                               if (ret != 0)
+                                       goto out;
+
+                               if (res_csize >= wim_resource_size(cur_lte)) {
+                                       /* Oops!  We compressed the resource to
+                                        * larger than the original size.  Write
+                                        * the resource uncompressed instead. */
+                                       ret = write_uncompressed_resource_and_truncate(
+                                                        cur_lte,
+                                                        out_fp,
+                                                        cur_chunk_tab->file_offset,
+                                                        &cur_lte->output_resource_entry);
+                                       if (ret != 0)
+                                               goto out;
+                               } else {
+                                       cur_lte->output_resource_entry.size =
+                                               res_csize;
+
+                                       cur_lte->output_resource_entry.original_size =
+                                               cur_lte->resource_entry.original_size;
+
+                                       cur_lte->output_resource_entry.offset =
+                                               cur_chunk_tab->file_offset;
+
+                                       cur_lte->output_resource_entry.flags =
+                                               cur_lte->resource_entry.flags |
+                                                       WIM_RESHDR_FLAG_COMPRESSED;
+                               }
+
+                               do_write_streams_progress(progress, progress_func,
+                                                         wim_resource_size(cur_lte));
+
+                               FREE(cur_chunk_tab);
+                               cur_chunk_tab = NULL;
+
+                               struct list_head *next = cur_lte->staging_list.next;
+                               list_del(&cur_lte->staging_list);
+
+                               if (next == &outstanding_resources)
+                                       cur_lte = NULL;
+                               else
+                                       cur_lte = container_of(cur_lte->staging_list.next,
+                                                              struct wim_lookup_table_entry,
+                                                              staging_list);
+
+                               // Since we just finished writing a stream,
+                               // write any streams that have been added to the
+                               // my_resources list for direct writing by the
+                               // main thread (e.g. resources that don't need
+                               // to be compressed because the desired
+                               // compression type is the same as the previous
+                               // compression type).
+                               ret = do_write_stream_list(&my_resources,
+                                                          out_fp,
+                                                          out_ctype,
+                                                          progress_func,
+                                                          progress,
+                                                          0);
+                               if (ret != 0)
+                                       goto out;
+                       }
+               }
+       }
+
+out:
+       if (ret == WIMLIB_ERR_NOMEM) {
+               ERROR("Could not allocate enough memory for "
+                     "multi-threaded compression");
+       }
+
+       if (next_lte) {
+#ifdef WITH_NTFS_3G
+               end_wim_resource_read(next_lte, ni);
+#else
+               end_wim_resource_read(next_lte);
+#endif
+       }
+
+       if (ret == 0) {
+               ret = do_write_stream_list(&my_resources, out_fp,
+                                          out_ctype, progress_func,
+                                          progress, 0);
+       } else {
+               if (msgs) {
+                       size_t num_available_msgs = 0;
+                       struct list_head *cur;
+
+                       list_for_each(cur, &available_msgs) {
+                               num_available_msgs++;
+                       }
+
+                       while (num_available_msgs < num_messages) {
+                               shared_queue_get(compressed_res_queue);
+                               num_available_msgs++;
+                       }
+               }
+       }
+
+       if (msgs) {
+               for (size_t i = 0; i < num_messages; i++) {
+                       for (size_t j = 0; j < MAX_CHUNKS_PER_MSG; j++) {
+                               FREE(msgs[i].compressed_chunks[j]);
+                               FREE(msgs[i].uncompressed_chunks[j]);
+                       }
+               }
+               FREE(msgs);
+       }
+
+       FREE(cur_chunk_tab);
+       return ret;
+}
+
+static long
+get_default_num_threads()
+{
+#ifdef __WIN32__
+       return win32_get_number_of_processors();
+#else
+       return sysconf(_SC_NPROCESSORS_ONLN);
+#endif
+}
+
+static int
+write_stream_list_parallel(struct list_head *stream_list,
+                          struct wim_lookup_table *lookup_table,
+                          FILE *out_fp,
+                          int out_ctype,
+                          int write_flags,
+                          unsigned num_threads,
+                          wimlib_progress_func_t progress_func,
+                          union wimlib_progress_info *progress)
 {
-       const char *wimfile_name;
-       size_t wim_name_len;
        int ret;
-       
-       wimfile_name = w->filename;
+       struct shared_queue res_to_compress_queue;
+       struct shared_queue compressed_res_queue;
+       pthread_t *compressor_threads = NULL;
+
+       if (num_threads == 0) {
+               long nthreads = get_default_num_threads();
+               if (nthreads < 1 || nthreads > UINT_MAX) {
+                       WARNING("Could not determine number of processors! Assuming 1");
+                       goto out_serial;
+               } else {
+                       num_threads = nthreads;
+               }
+       }
+
+       progress->write_streams.num_threads = num_threads;
+       wimlib_assert(stream_list->next != stream_list);
+
+       static const double MESSAGES_PER_THREAD = 2.0;
+       size_t queue_size = (size_t)(num_threads * MESSAGES_PER_THREAD);
+
+       DEBUG("Initializing shared queues (queue_size=%zu)", queue_size);
+
+       ret = shared_queue_init(&res_to_compress_queue, queue_size);
+       if (ret != 0)
+               goto out_serial;
+
+       ret = shared_queue_init(&compressed_res_queue, queue_size);
+       if (ret != 0)
+               goto out_destroy_res_to_compress_queue;
+
+       struct compressor_thread_params params;
+       params.res_to_compress_queue = &res_to_compress_queue;
+       params.compressed_res_queue = &compressed_res_queue;
+       params.compress = get_compress_func(out_ctype);
+
+       compressor_threads = MALLOC(num_threads * sizeof(pthread_t));
+       if (!compressor_threads) {
+               ret = WIMLIB_ERR_NOMEM;
+               goto out_destroy_compressed_res_queue;
+       }
+
+       for (unsigned i = 0; i < num_threads; i++) {
+               DEBUG("pthread_create thread %u", i);
+               ret = pthread_create(&compressor_threads[i], NULL,
+                                    compressor_thread_proc, &params);
+               if (ret != 0) {
+                       ret = -1;
+                       ERROR_WITH_ERRNO("Failed to create compressor "
+                                        "thread %u", i);
+                       num_threads = i;
+                       goto out_join;
+               }
+       }
+
+       if (progress_func)
+               progress_func(WIMLIB_PROGRESS_MSG_WRITE_STREAMS, progress);
+
+       ret = main_writer_thread_proc(stream_list,
+                                     out_fp,
+                                     out_ctype,
+                                     &res_to_compress_queue,
+                                     &compressed_res_queue,
+                                     queue_size,
+                                     write_flags,
+                                     progress_func,
+                                     progress);
+out_join:
+       for (unsigned i = 0; i < num_threads; i++)
+               shared_queue_put(&res_to_compress_queue, NULL);
+
+       for (unsigned i = 0; i < num_threads; i++) {
+               if (pthread_join(compressor_threads[i], NULL)) {
+                       WARNING_WITH_ERRNO("Failed to join compressor "
+                                          "thread %u", i);
+               }
+       }
+       FREE(compressor_threads);
+out_destroy_compressed_res_queue:
+       shared_queue_destroy(&compressed_res_queue);
+out_destroy_res_to_compress_queue:
+       shared_queue_destroy(&res_to_compress_queue);
+       if (ret >= 0 && ret != WIMLIB_ERR_NOMEM)
+               return ret;
+out_serial:
+       WARNING("Falling back to single-threaded compression");
+       return write_stream_list_serial(stream_list,
+                                       lookup_table,
+                                       out_fp,
+                                       out_ctype,
+                                       write_flags,
+                                       progress_func,
+                                       progress);
+
+}
+#endif
+
+/*
+ * Write a list of streams to a WIM (@out_fp) using the compression type
+ * @out_ctype and up to @num_threads compressor threads.
+ */
+static int
+write_stream_list(struct list_head *stream_list,
+                 struct wim_lookup_table *lookup_table,
+                 FILE *out_fp, int out_ctype, int write_flags,
+                 unsigned num_threads, wimlib_progress_func_t progress_func)
+{
+       struct wim_lookup_table_entry *lte;
+       size_t num_streams = 0;
+       u64 total_bytes = 0;
+       u64 total_compression_bytes = 0;
+       union wimlib_progress_info progress;
+       int ret;
+
+       list_for_each_entry(lte, stream_list, write_streams_list) {
+               num_streams++;
+               total_bytes += wim_resource_size(lte);
+               if (out_ctype != WIMLIB_COMPRESSION_TYPE_NONE
+                      && (wim_resource_compression_type(lte) != out_ctype ||
+                          (write_flags & WIMLIB_WRITE_FLAG_RECOMPRESS)))
+               {
+                       total_compression_bytes += wim_resource_size(lte);
+               }
+       }
+       progress.write_streams.total_bytes       = total_bytes;
+       progress.write_streams.total_streams     = num_streams;
+       progress.write_streams.completed_bytes   = 0;
+       progress.write_streams.completed_streams = 0;
+       progress.write_streams.num_threads       = num_threads;
+       progress.write_streams.compression_type  = out_ctype;
+       progress.write_streams._private          = 0;
 
-       DEBUG("Replacing WIM file `%s'\n", wimfile_name);
+#ifdef ENABLE_MULTITHREADED_COMPRESSION
+       if (total_compression_bytes >= 1000000 && num_threads != 1)
+               ret = write_stream_list_parallel(stream_list,
+                                                lookup_table,
+                                                out_fp,
+                                                out_ctype,
+                                                write_flags,
+                                                num_threads,
+                                                progress_func,
+                                                &progress);
+       else
+#endif
+               ret = write_stream_list_serial(stream_list,
+                                              lookup_table,
+                                              out_fp,
+                                              out_ctype,
+                                              write_flags,
+                                              progress_func,
+                                              &progress);
+       return ret;
+}
 
-       if (!wimfile_name)
-               return WIMLIB_ERR_NO_FILENAME;
+struct lte_overwrite_prepare_args {
+       WIMStruct *wim;
+       off_t end_offset;
+       struct list_head *stream_list;
+};
 
-       /* Write the WIM to a temporary file. */
-       /* XXX should the temporary file be somewhere else? */
-       wim_name_len = strlen(wimfile_name);
-       char tmpfile[wim_name_len + 10];
-       memcpy(tmpfile, wimfile_name, wim_name_len);
-       randomize_char_array_with_alnum(tmpfile + wim_name_len, 9);
-       tmpfile[wim_name_len + 9] = '\0';
+static int
+lte_overwrite_prepare(struct wim_lookup_table_entry *lte, void *arg)
+{
+       struct lte_overwrite_prepare_args *args = arg;
 
-       ret = wimlib_write(w, tmpfile, WIM_ALL_IMAGES, flags);
-       if (ret != 0) {
-               ERROR("Failed to write the WIM file `%s'!\n", tmpfile);
-               return ret;
+       if (lte->resource_location == RESOURCE_IN_WIM &&
+           lte->wim == args->wim &&
+           lte->resource_entry.offset + lte->resource_entry.size > args->end_offset)
+       {
+       #ifdef ENABLE_ERROR_MESSAGES
+               ERROR("The following resource is after the XML data:");
+               print_lookup_table_entry(lte, stderr);
+       #endif
+               return WIMLIB_ERR_RESOURCE_ORDER;
        }
 
-       DEBUG("Closing original WIM file.\n");
-       /* Close the original WIM file that was opened for reading. */
-       if (w->fp) {
-               if (fclose(w->fp) != 0) {
-                       WARNING("Failed to close the file `%s'\n",
-                                       wimfile_name);
-               }
-               w->fp = NULL;
-       }
+       lte->out_refcnt = lte->refcnt;
+       memcpy(&lte->output_resource_entry, &lte->resource_entry,
+              sizeof(struct resource_entry));
+       if (!(lte->resource_entry.flags & WIM_RESHDR_FLAG_METADATA))
+               if (lte->resource_location != RESOURCE_IN_WIM || lte->wim != args->wim)
+                       list_add(&lte->staging_list, args->stream_list);
+       return 0;
+}
 
-       DEBUG("Renaming `%s' to `%s'\n", tmpfile, wimfile_name);
+static int
+wim_prepare_streams(WIMStruct *wim, off_t end_offset,
+                   struct list_head *stream_list)
+{
+       struct lte_overwrite_prepare_args args = {
+               .wim         = wim,
+               .end_offset  = end_offset,
+               .stream_list = stream_list,
+       };
+       int ret;
 
-       /* Rename the new file to the old file .*/
-       if (rename(tmpfile, wimfile_name) != 0) {
-               ERROR("Failed to rename `%s' to `%s': %m\n", tmpfile, 
-                                                               wimfile_name);
-               /* Remove temporary file. */
-               if (unlink(tmpfile) != 0)
-                       ERROR("Failed to remove `%s': %m\n", tmpfile);
-               return WIMLIB_ERR_RENAME;
+       for (int i = 0; i < wim->hdr.image_count; i++) {
+               ret = lte_overwrite_prepare(wim->image_metadata[i]->metadata_lte,
+                                           &args);
+               if (ret)
+                       return ret;
        }
+       return for_lookup_table_entry(wim->lookup_table,
+                                     lte_overwrite_prepare, &args);
+}
 
+struct stream_size_table {
+       struct hlist_head *array;
+       size_t num_entries;
+       size_t capacity;
+};
+
+static int
+init_stream_size_table(struct stream_size_table *tab, size_t capacity)
+{
+       tab->array = CALLOC(capacity, sizeof(tab->array[0]));
+       if (!tab->array)
+               return WIMLIB_ERR_NOMEM;
+       tab->num_entries = 0;
+       tab->capacity = capacity;
        return 0;
 }
 
+static void
+destroy_stream_size_table(struct stream_size_table *tab)
+{
+       FREE(tab->array);
+}
 
-WIMLIBAPI int wimlib_overwrite_xml_and_header(WIMStruct *w, int flags)
+static int
+stream_size_table_insert(struct wim_lookup_table_entry *lte, void *_tab)
 {
-       int ret;
-       FILE *fp;
-       u8 *integrity_table = NULL;
-       off_t xml_end;
-       off_t xml_size;
-       size_t bytes_written;
+       struct stream_size_table *tab = _tab;
+       size_t pos;
+       struct wim_lookup_table_entry *hashed_lte;
+       struct hlist_node *tmp;
 
-       DEBUG("Overwriting XML and header of `%s', flags = %d\n", 
-                               w->filename, flags);
-       if (!w->filename)
-               return WIMLIB_ERR_NO_FILENAME;
+       pos = hash_u64(wim_resource_size(lte)) % tab->capacity;
+       lte->unique_size = 1;
+       hlist_for_each_entry(hashed_lte, tmp, &tab->array[pos], hash_list_2) {
+               if (wim_resource_size(hashed_lte) == wim_resource_size(lte)) {
+                       lte->unique_size = 0;
+                       hashed_lte->unique_size = 0;
+                       break;
+               }
+       }
 
-       ret = reopen_rw(w);
-       if (ret != 0)
-               return ret;
+       hlist_add_head(&lte->hash_list_2, &tab->array[pos]);
+       tab->num_entries++;
+       return 0;
+}
 
-       fp = w->fp;
-
-       /* The old integrity table is still OK, as the SHA1 message digests in
-        * the integrity table include neither the header nor the XML data.
-        * Save it for later if it exists and an integrity table was required.
-        * */
-       if (flags & WIMLIB_WRITE_FLAG_CHECK_INTEGRITY && 
-                       w->hdr.integrity.offset != 0) {
-               DEBUG("Reading existing integrity table.\n");
-               integrity_table = MALLOC(w->hdr.integrity.size);
-               if (!integrity_table)
-                       return WIMLIB_ERR_NOMEM;
-
-               ret = read_uncompressed_resource(fp, w->hdr.integrity.offset,
-                                                w->hdr.integrity.original_size,
-                                                integrity_table);
-               if (ret != 0)
-                       goto err;
-               DEBUG("Done reading existing integrity table.\n");
-       }
 
-       DEBUG("Overwriting XML data.\n");
-       /* Overwrite the XML data. */
-       if (fseeko(fp, w->hdr.xml_res_entry.offset, SEEK_SET) != 0) {
-               ERROR("Failed to seek to byte %"PRIu64" for XML data: "
-                               "%m\n", w->hdr.xml_res_entry.offset);
-               ret = WIMLIB_ERR_WRITE;
-               goto err;
-       }
-       ret = write_xml_data(w->wim_info, WIM_ALL_IMAGES, fp, 0);
-       if (ret != 0)
-               goto err;
+struct find_streams_ctx {
+       struct list_head stream_list;
+       struct stream_size_table stream_size_tab;
+};
 
-       DEBUG("Updating XML resource entry.\n");
-       /* Update the XML resource entry in the WIM header. */
-       xml_end = ftello(fp);
-       if (xml_end == -1) {
-               ret = WIMLIB_ERR_WRITE;
-               goto err;
-       }
-       xml_size = xml_end - w->hdr.xml_res_entry.offset;
-       w->hdr.xml_res_entry.size = xml_size;
-       w->hdr.xml_res_entry.original_size = xml_size;
-
-       if (flags & WIMLIB_WRITE_FLAG_CHECK_INTEGRITY) {
-               DEBUG("Writing integrity table.\n");
-               w->hdr.integrity.offset        = xml_end;
-               if (integrity_table) {
-                       /* The existing integrity table was saved. */
-                       bytes_written = fwrite(integrity_table, 1, 
-                                              w->hdr.integrity.size, fp);
-                       if (bytes_written != w->hdr.integrity.size) {
-                               ERROR("Failed to write integrity table: %m\n");
-                               ret = WIMLIB_ERR_WRITE;
-                               goto err;
+static int
+inode_find_streams_to_write(struct wim_inode *inode,
+                           struct wim_lookup_table *table,
+                           struct list_head *stream_list,
+                           struct stream_size_table *tab)
+{
+       struct wim_lookup_table_entry *lte;
+       for (unsigned i = 0; i <= inode->i_num_ads; i++) {
+               lte = inode_stream_lte(inode, i, table);
+               if (lte) {
+                       if (lte->out_refcnt == 0) {
+                               if (lte->unhashed)
+                                       stream_size_table_insert(lte, tab);
+                               list_add_tail(&lte->write_streams_list, stream_list);
                        }
-                       FREE(integrity_table);
-               } else {
-                       /* There was no existing integrity table, so a new one
-                        * must be calculated. */
-                       ret = write_integrity_table(fp, WIM_HEADER_DISK_SIZE,
-                                       w->hdr.lookup_table_res_entry.offset + 
-                                       w->hdr.lookup_table_res_entry.size,
-                                       flags & WIMLIB_WRITE_FLAG_SHOW_PROGRESS);
-                       if (ret != 0)
-                               goto err;
-
-                       off_t integrity_size           = ftello(fp) - xml_end;
-                       w->hdr.integrity.size          = integrity_size;
-                       w->hdr.integrity.original_size = integrity_size;
-                       w->hdr.integrity.flags         = 0;
-               }
-       } else {
-               DEBUG("Truncating file to end of XML data.\n");
-               /* No integrity table to write.  The file should be truncated
-                * because it's possible that the old file was longer (due to it
-                * including an integrity table, or due to its XML data being
-                * longer) */
-               if (fflush(fp) != 0) {
-                       ERROR("Failed to flush stream for file `%s': %m\n",
-                                       w->filename);
-                       return WIMLIB_ERR_WRITE;
-               }
-               if (ftruncate(fileno(fp), xml_end) != 0) {
-                       ERROR("Failed to truncate `%s' to %"PRIu64" "
-                                       "bytes: %m\n", 
-                                       w->filename, xml_end);
-                       return WIMLIB_ERR_WRITE;
+                       lte->out_refcnt += inode->i_nlink;
                }
-               memset(&w->hdr.integrity, 0, sizeof(struct resource_entry));
        }
+       return 0;
+}
 
-       DEBUG("Overwriting header.\n");
-       /* Overwrite the header. */
-       if (fseeko(fp, 0, SEEK_SET) != 0) {
-               ERROR("Failed to seek to beginning of `%s': %m\n",
-                               w->filename);
-               return WIMLIB_ERR_WRITE;
-       }
+static int
+image_find_streams_to_write(WIMStruct *w)
+{
+       struct wim_image_metadata *imd;
+       struct find_streams_ctx *ctx;
+       struct wim_inode *inode;
+       struct wim_lookup_table_entry *lte;
 
-       ret = write_header(&w->hdr, fp);
-       if (ret != 0)
-               return ret;;
+       ctx = w->private;
+       imd = wim_get_current_image_metadata(w);
 
-       DEBUG("Closing file.\n");
-       if (fclose(fp) != 0) {
-               ERROR("Failed to close `%s': %m\n", w->filename);
-               return WIMLIB_ERR_WRITE;
+       image_for_each_unhashed_stream(lte, imd) {
+               lte->out_refcnt = 0;
+               wimlib_assert(lte->unhashed);
+               wimlib_assert(lte->my_ptr != NULL);
+       }
+
+       /* Go through this image's inodes to find any streams that have not been
+        * found yet. */
+       image_for_each_inode(inode, imd) {
+               inode_find_streams_to_write(inode, w->lookup_table,
+                                           &ctx->stream_list,
+                                           &ctx->stream_size_tab);
        }
-       w->fp = NULL;
-       DEBUG("Done.\n");
        return 0;
-err:
-       FREE(integrity_table);
-       return ret;
 }
 
+/* Given a WIM that from which one or all of the images is being written, build
+ * the list of unique streams ('struct wim_lookup_table_entry's) that must be
+ * written, plus any unhashed streams that need to be written but may be
+ * identical to other hashed or unhashed streams being written.  These unhashed
+ * streams are checksummed while the streams are being written.  To aid this
+ * process, the member @unique_size is set to 1 on streams that have a unique
+ * size and therefore must be written.
+ *
+ * The out_refcnt member of each 'struct wim_lookup_table_entry' is set to
+ * indicate the number of times the stream is referenced in only the streams
+ * that are being written; this may still be adjusted later when unhashed
+ * streams are being resolved.
+ */
+static int
+prepare_stream_list(WIMStruct *wim, int image, struct list_head *stream_list)
+{
+       int ret;
+       struct find_streams_ctx ctx;
+
+       for_lookup_table_entry(wim->lookup_table, lte_zero_out_refcnt, NULL);
+       ret = init_stream_size_table(&ctx.stream_size_tab, 9001);
+       if (ret)
+               return ret;
+       for_lookup_table_entry(wim->lookup_table, stream_size_table_insert,
+                              &ctx.stream_size_tab);
+       INIT_LIST_HEAD(&ctx.stream_list);
+       wim->private = &ctx;
+       for_image(wim, image, image_find_streams_to_write);
+       destroy_stream_size_table(&ctx.stream_size_tab);
+
+       INIT_LIST_HEAD(stream_list);
+       list_splice(&ctx.stream_list, stream_list);
+       return 0;
+}
 
-/* Write the file resources for the current image. */
-static int write_file_resources(WIMStruct *w)
+/* Writes the streams for the specified @image in @wim to @wim->out_fp.
+ */
+static int
+write_wim_streams(WIMStruct *wim, int image, int write_flags,
+                 unsigned num_threads,
+                 wimlib_progress_func_t progress_func)
 {
+       int ret;
+       struct list_head stream_list;
 
-       DEBUG("Writing file resources for image %u\n", w->current_image);
-       return for_dentry_in_tree(wim_root_dentry(w), write_file_resource, w);
+       ret = prepare_stream_list(wim, image, &stream_list);
+       if (ret)
+               return ret;
+       return write_stream_list(&stream_list,
+                                wim->lookup_table,
+                                wim->out_fp,
+                                wimlib_get_compression_type(wim),
+                                write_flags,
+                                num_threads,
+                                progress_func);
 }
 
-/* Write lookup table, xml data, lookup table, and rewrite header 
+/*
+ * Finish writing a WIM file: write the lookup table, xml data, and integrity
+ * table (optional), then overwrite the WIM header.
  *
- * write_lt is zero iff the lookup table is not to be written; i.e. it is
- * handled elsewhere. */
-int finish_write(WIMStruct *w, int image, int flags, int write_lt)
-{
-       off_t lookup_table_offset;
-       off_t xml_data_offset;
-       off_t lookup_table_size;
-       off_t integrity_offset;
-       off_t xml_data_size;
-       off_t end_offset;
-       off_t integrity_size;
+ * write_flags is a bitwise OR of the following:
+ *
+ *     (public)  WIMLIB_WRITE_FLAG_CHECK_INTEGRITY:
+ *             Include an integrity table.
+ *
+ *     (public)  WIMLIB_WRITE_FLAG_SHOW_PROGRESS:
+ *             Show progress information when (if) writing the integrity table.
+ *
+ *     (private) WIMLIB_WRITE_FLAG_NO_LOOKUP_TABLE:
+ *             Don't write the lookup table.
+ *
+ *     (private) WIMLIB_WRITE_FLAG_REUSE_INTEGRITY_TABLE:
+ *             When (if) writing the integrity table, re-use entries from the
+ *             existing integrity table, if possible.
+ *
+ *     (private) WIMLIB_WRITE_FLAG_CHECKPOINT_AFTER_XML:
+ *             After writing the XML data but before writing the integrity
+ *             table, write a temporary WIM header and flush the stream so that
+ *             the WIM is less likely to become corrupted upon abrupt program
+ *             termination.
+ *
+ *     (private) WIMLIB_WRITE_FLAG_FSYNC:
+ *             fsync() the output file before closing it.
+ *
+ */
+int
+finish_write(WIMStruct *w, int image, int write_flags,
+            wimlib_progress_func_t progress_func)
+{
        int ret;
-       int i;
        struct wim_header hdr;
        FILE *out = w->out_fp;
 
-       if (write_lt) {
-               lookup_table_offset = ftello(out);
-               if (lookup_table_offset == -1)
-                       return WIMLIB_ERR_WRITE;
-
-               DEBUG("Writing lookup table (offset %"PRIu64")\n", lookup_table_offset);
-               /* Write the lookup table. */
-               ret = write_lookup_table(w->lookup_table, out);
-               if (ret != 0)
-                       return ret;
-       }
-
-
-       xml_data_offset = ftello(out);
-       if (xml_data_offset == -1)
-               return WIMLIB_ERR_WRITE;
-       DEBUG("Writing XML data (offset %"PRIu64")\n", xml_data_offset);
-
        /* @hdr will be the header for the new WIM.  First copy all the data
         * from the header in the WIMStruct; then set all the fields that may
         * have changed, including the resource entries, boot index, and image
         * count.  */
        memcpy(&hdr, &w->hdr, sizeof(struct wim_header));
-       if (write_lt) {
-               lookup_table_size = xml_data_offset - lookup_table_offset;
-               hdr.lookup_table_res_entry.offset        = lookup_table_offset;
-               hdr.lookup_table_res_entry.size          = lookup_table_size;
+
+       if (!(write_flags & WIMLIB_WRITE_FLAG_NO_LOOKUP_TABLE)) {
+               ret = write_lookup_table(w, image, &hdr.lookup_table_res_entry);
+               if (ret != 0)
+                       goto out;
        }
-       hdr.lookup_table_res_entry.original_size = hdr.lookup_table_res_entry.size;
-       hdr.lookup_table_res_entry.flags         = WIM_RESHDR_FLAG_METADATA;
 
-       ret = write_xml_data(w->wim_info, image, out, 
-                            write_lt ? 0 : wim_info_get_total_bytes(w->wim_info));
+       ret = write_xml_data(w->wim_info, image, out,
+                            (write_flags & WIMLIB_WRITE_FLAG_NO_LOOKUP_TABLE) ?
+                             wim_info_get_total_bytes(w->wim_info) : 0,
+                            &hdr.xml_res_entry);
        if (ret != 0)
-               return ret;
+               goto out;
 
-       integrity_offset = ftello(out);
-       if (integrity_offset == -1)
-               return WIMLIB_ERR_WRITE;
-       xml_data_size = integrity_offset - xml_data_offset;
+       if (write_flags & WIMLIB_WRITE_FLAG_CHECK_INTEGRITY) {
+               if (write_flags & WIMLIB_WRITE_FLAG_CHECKPOINT_AFTER_XML) {
+                       struct wim_header checkpoint_hdr;
+                       memcpy(&checkpoint_hdr, &hdr, sizeof(struct wim_header));
+                       memset(&checkpoint_hdr.integrity, 0, sizeof(struct resource_entry));
+                       if (fseeko(out, 0, SEEK_SET) != 0) {
+                               ERROR_WITH_ERRNO("Failed to seek to beginning "
+                                                "of WIM being written");
+                               ret = WIMLIB_ERR_WRITE;
+                               goto out;
+                       }
+                       ret = write_header(&checkpoint_hdr, out);
+                       if (ret != 0)
+                               goto out;
+
+                       if (fflush(out) != 0) {
+                               ERROR_WITH_ERRNO("Can't write data to WIM");
+                               ret = WIMLIB_ERR_WRITE;
+                               goto out;
+                       }
+
+                       if (fseeko(out, 0, SEEK_END) != 0) {
+                               ERROR_WITH_ERRNO("Failed to seek to end "
+                                                "of WIM being written");
+                               ret = WIMLIB_ERR_WRITE;
+                               goto out;
+                       }
+               }
 
-       hdr.xml_res_entry.offset                 = xml_data_offset;
-       hdr.xml_res_entry.size                   = xml_data_size;
-       hdr.xml_res_entry.original_size          = xml_data_size;
-       hdr.xml_res_entry.flags                  = 0;
+               off_t old_lookup_table_end;
+               off_t new_lookup_table_end;
+               if (write_flags & WIMLIB_WRITE_FLAG_REUSE_INTEGRITY_TABLE) {
+                       old_lookup_table_end = w->hdr.lookup_table_res_entry.offset +
+                                              w->hdr.lookup_table_res_entry.size;
+               } else {
+                       old_lookup_table_end = 0;
+               }
+               new_lookup_table_end = hdr.lookup_table_res_entry.offset +
+                                      hdr.lookup_table_res_entry.size;
 
-       if (flags & WIMLIB_WRITE_FLAG_CHECK_INTEGRITY) {
-               ret = write_integrity_table(out, WIM_HEADER_DISK_SIZE, 
-                                           xml_data_offset, 
-                                           flags & WIMLIB_WRITE_FLAG_SHOW_PROGRESS);
+               ret = write_integrity_table(out,
+                                           &hdr.integrity,
+                                           new_lookup_table_end,
+                                           old_lookup_table_end,
+                                           progress_func);
                if (ret != 0)
-                       return ret;
-               end_offset = ftello(out);
-               if (end_offset == -1)
-                       return WIMLIB_ERR_WRITE;
-               integrity_size = end_offset - integrity_offset;
-               hdr.integrity.offset = integrity_offset;
-               hdr.integrity.size   = integrity_size;
-               hdr.integrity.original_size = integrity_size;
+                       goto out;
        } else {
-               hdr.integrity.offset        = 0;
-               hdr.integrity.size          = 0;
-               hdr.integrity.original_size = 0;
+               memset(&hdr.integrity, 0, sizeof(struct resource_entry));
        }
-       hdr.integrity.flags = 0;
-
-       DEBUG("Updating WIM header.\n");
 
-       /* 
+       /*
         * In the WIM header, there is room for the resource entry for a
         * metadata resource labeled as the "boot metadata".  This entry should
         * be zeroed out if there is no bootable image (boot_idx 0).  Otherwise,
         * it should be a copy of the resource entry for the image that is
         * marked as bootable.  This is not well documented...
         */
-       if (hdr.boot_idx == 0 || !w->image_metadata
-                       || (image != WIM_ALL_IMAGES && image != hdr.boot_idx)) {
-               memset(&hdr.boot_metadata_res_entry, 0, 
-                      sizeof(struct resource_entry));
-       } else {
-               memcpy(&hdr.boot_metadata_res_entry, 
-                      &w->image_metadata[hdr.boot_idx - 1].lookup_table_entry->
-                                       output_resource_entry,
-                                       sizeof(struct resource_entry));
-       }
 
        /* Set image count and boot index correctly for single image writes */
-       if (image != WIM_ALL_IMAGES) {
+       if (image != WIMLIB_ALL_IMAGES) {
                hdr.image_count = 1;
                if (hdr.boot_idx == image)
                        hdr.boot_idx = 1;
@@ -364,81 +1679,443 @@ int finish_write(WIMStruct *w, int image, int flags, int write_lt)
                        hdr.boot_idx = 0;
        }
 
+       if (hdr.boot_idx == 0) {
+               memset(&hdr.boot_metadata_res_entry, 0,
+                      sizeof(struct resource_entry));
+       } else {
+               memcpy(&hdr.boot_metadata_res_entry,
+                      &w->image_metadata[
+                         hdr.boot_idx - 1]->metadata_lte->output_resource_entry,
+                      sizeof(struct resource_entry));
+       }
 
-       if (fseeko(out, 0, SEEK_SET) != 0)
-               return WIMLIB_ERR_WRITE;
+       if (fseeko(out, 0, SEEK_SET) != 0) {
+               ERROR_WITH_ERRNO("Failed to seek to beginning of WIM "
+                                "being written");
+               ret = WIMLIB_ERR_WRITE;
+               goto out;
+       }
 
-       return write_header(&hdr, out);
+       ret = write_header(&hdr, out);
+       if (ret)
+               goto out;
+
+       if (write_flags & WIMLIB_WRITE_FLAG_FSYNC) {
+               if (fflush(out) != 0
+                   || fsync(fileno(out)) != 0)
+               {
+                       ERROR_WITH_ERRNO("Error flushing data to WIM file");
+                       ret = WIMLIB_ERR_WRITE;
+               }
+       }
+out:
+       if (fclose(out) != 0) {
+               ERROR_WITH_ERRNO("Failed to close the WIM file");
+               if (ret == 0)
+                       ret = WIMLIB_ERR_WRITE;
+       }
+       w->out_fp = NULL;
+       return ret;
 }
 
-/* Open file stream and write dummy header for WIM. */
-int begin_write(WIMStruct *w, const char *path, int flags)
+#if defined(HAVE_SYS_FILE_H) && defined(HAVE_FLOCK)
+int
+lock_wim(WIMStruct *w, FILE *fp)
 {
-       const char *mode;
-       DEBUG("Opening `%s' for new WIM\n", path);
+       int ret = 0;
+       if (fp && !w->wim_locked) {
+               ret = flock(fileno(fp), LOCK_EX | LOCK_NB);
+               if (ret != 0) {
+                       if (errno == EWOULDBLOCK) {
+                               ERROR("`%"TS"' is already being modified or has been "
+                                     "mounted read-write\n"
+                                     "        by another process!", w->filename);
+                               ret = WIMLIB_ERR_ALREADY_LOCKED;
+                       } else {
+                               WARNING_WITH_ERRNO("Failed to lock `%"TS"'",
+                                                  w->filename);
+                               ret = 0;
+                       }
+               } else {
+                       w->wim_locked = 1;
+               }
+       }
+       return ret;
+}
+#endif
 
-       /* checking the integrity requires going back over the file to read it.
-        * XXX 
-        * (It also would be possible to keep a running sha1sum as the file
-        * as written-- this would be faster, but a bit more complicated) */
-       if (flags & WIMLIB_WRITE_FLAG_CHECK_INTEGRITY) 
-               mode = "w+b";
+static int
+open_wim_writable(WIMStruct *w, const tchar *path,
+                 bool trunc, bool also_readable)
+{
+       const tchar *mode;
+       if (trunc)
+               if (also_readable)
+                       mode = T("w+b");
+               else
+                       mode = T("wb");
        else
-               mode = "wb";
+               mode = T("r+b");
 
-       w->out_fp = fopen(path, mode);
-       if (!w->out_fp) {
-               ERROR("Failed to open the file `%s' for writing!\n", path);
+       wimlib_assert(w->out_fp == NULL);
+       w->out_fp = tfopen(path, mode);
+       if (w->out_fp) {
+               return 0;
+       } else {
+               ERROR_WITH_ERRNO("Failed to open `%"TS"' for writing", path);
                return WIMLIB_ERR_OPEN;
        }
+}
+
+
+void
+close_wim_writable(WIMStruct *w)
+{
+       if (w->out_fp) {
+               if (fclose(w->out_fp) != 0) {
+                       WARNING_WITH_ERRNO("Failed to close output WIM");
+               }
+               w->out_fp = NULL;
+       }
+}
 
+/* Open file stream and write dummy header for WIM. */
+int
+begin_write(WIMStruct *w, const tchar *path, int write_flags)
+{
+       int ret;
+       ret = open_wim_writable(w, path, true,
+                               (write_flags & WIMLIB_WRITE_FLAG_CHECK_INTEGRITY) != 0);
+       if (ret != 0)
+               return ret;
        /* Write dummy header. It will be overwritten later. */
        return write_header(&w->hdr, w->out_fp);
 }
 
-/* Writes the WIM to a file.  */
-WIMLIBAPI int wimlib_write(WIMStruct *w, const char *path, int image, int flags)
+/* Writes a stand-alone WIM to a file.  */
+WIMLIBAPI int
+wimlib_write(WIMStruct *w, const tchar *path,
+            int image, int write_flags, unsigned num_threads,
+            wimlib_progress_func_t progress_func)
 {
        int ret;
 
-       if (image != WIM_ALL_IMAGES && 
-                       (image < 1 || image > w->hdr.image_count))
+       if (!path)
+               return WIMLIB_ERR_INVALID_PARAM;
+
+       write_flags &= WIMLIB_WRITE_MASK_PUBLIC;
+
+       if (image != WIMLIB_ALL_IMAGES &&
+            (image < 1 || image > w->hdr.image_count))
                return WIMLIB_ERR_INVALID_IMAGE;
 
-       if (image == WIM_ALL_IMAGES)
-               DEBUG("Writing all images to `%s'\n", path);
+       if (w->hdr.total_parts != 1) {
+               ERROR("Cannot call wimlib_write() on part of a split WIM");
+               return WIMLIB_ERR_SPLIT_UNSUPPORTED;
+       }
+
+       ret = begin_write(w, path, write_flags);
+       if (ret)
+               goto out;
+
+       ret = write_wim_streams(w, image, write_flags, num_threads,
+                               progress_func);
+       if (ret)
+               goto out;
+
+       if (progress_func)
+               progress_func(WIMLIB_PROGRESS_MSG_WRITE_METADATA_BEGIN, NULL);
+
+       ret = for_image(w, image, write_metadata_resource);
+       if (ret)
+               goto out;
+
+       if (progress_func)
+               progress_func(WIMLIB_PROGRESS_MSG_WRITE_METADATA_END, NULL);
+
+       ret = finish_write(w, image, write_flags, progress_func);
+out:
+       close_wim_writable(w);
+       DEBUG("wimlib_write(path=%"TS") = %d", path, ret);
+       return ret;
+}
+
+static bool
+any_images_modified(WIMStruct *w)
+{
+       for (int i = 0; i < w->hdr.image_count; i++)
+               if (w->image_metadata[i]->modified)
+                       return true;
+       return false;
+}
+
+/*
+ * Overwrite a WIM, possibly appending streams to it.
+ *
+ * A WIM looks like (or is supposed to look like) the following:
+ *
+ *                   Header (212 bytes)
+ *                   Streams and metadata resources (variable size)
+ *                   Lookup table (variable size)
+ *                   XML data (variable size)
+ *                   Integrity table (optional) (variable size)
+ *
+ * If we are not adding any streams or metadata resources, the lookup table is
+ * unchanged--- so we only need to overwrite the XML data, integrity table, and
+ * header.  This operation is potentially unsafe if the program is abruptly
+ * terminated while the XML data or integrity table are being overwritten, but
+ * before the new header has been written.  To partially alleviate this problem,
+ * a special flag (WIMLIB_WRITE_FLAG_CHECKPOINT_AFTER_XML) is passed to
+ * finish_write() to cause a temporary WIM header to be written after the XML
+ * data has been written.  This may prevent the WIM from becoming corrupted if
+ * the program is terminated while the integrity table is being calculated (but
+ * no guarantees, due to write re-ordering...).
+ *
+ * If we are adding new streams or images (metadata resources), the lookup table
+ * needs to be changed, and those streams need to be written.  In this case, we
+ * try to perform a safe update of the WIM file by writing the streams *after*
+ * the end of the previous WIM, then writing the new lookup table, XML data, and
+ * (optionally) integrity table following the new streams.  This will produce a
+ * layout like the following:
+ *
+ *                   Header (212 bytes)
+ *                   (OLD) Streams and metadata resources (variable size)
+ *                   (OLD) Lookup table (variable size)
+ *                   (OLD) XML data (variable size)
+ *                   (OLD) Integrity table (optional) (variable size)
+ *                   (NEW) Streams and metadata resources (variable size)
+ *                   (NEW) Lookup table (variable size)
+ *                   (NEW) XML data (variable size)
+ *                   (NEW) Integrity table (optional) (variable size)
+ *
+ * At all points, the WIM is valid as nothing points to the new data yet.  Then,
+ * the header is overwritten to point to the new lookup table, XML data, and
+ * integrity table, to produce the following layout:
+ *
+ *                   Header (212 bytes)
+ *                   Streams and metadata resources (variable size)
+ *                   Nothing (variable size)
+ *                   More Streams and metadata resources (variable size)
+ *                   Lookup table (variable size)
+ *                   XML data (variable size)
+ *                   Integrity table (optional) (variable size)
+ *
+ * This method allows an image to be appended to a large WIM very quickly, and
+ * is is crash-safe except in the case of write re-ordering, but the
+ * disadvantage is that a small hole is left in the WIM where the old lookup
+ * table, xml data, and integrity table were.  (These usually only take up a
+ * small amount of space compared to the streams, however.)
+ */
+static int
+overwrite_wim_inplace(WIMStruct *w, int write_flags,
+                     unsigned num_threads,
+                     wimlib_progress_func_t progress_func)
+{
+       int ret;
+       struct list_head stream_list;
+       off_t old_wim_end;
+
+       DEBUG("Overwriting `%"TS"' in-place", w->filename);
+
+       /* Make sure that the integrity table (if present) is after the XML
+        * data, and that there are no stream resources, metadata resources, or
+        * lookup tables after the XML data.  Otherwise, these data would be
+        * overwritten. */
+       if (w->hdr.integrity.offset != 0 &&
+           w->hdr.integrity.offset < w->hdr.xml_res_entry.offset) {
+               ERROR("Didn't expect the integrity table to be before the XML data");
+               return WIMLIB_ERR_RESOURCE_ORDER;
+       }
+
+       if (w->hdr.lookup_table_res_entry.offset > w->hdr.xml_res_entry.offset) {
+               ERROR("Didn't expect the lookup table to be after the XML data");
+               return WIMLIB_ERR_RESOURCE_ORDER;
+       }
+
+
+       if (w->hdr.integrity.offset)
+               old_wim_end = w->hdr.integrity.offset + w->hdr.integrity.size;
        else
-               DEBUG("Writing image %d to `%s'\n", image, path);
+               old_wim_end = w->hdr.xml_res_entry.offset + w->hdr.xml_res_entry.size;
 
-       ret = begin_write(w, path, flags);
+       if (!w->deletion_occurred && !any_images_modified(w)) {
+               /* If no images have been modified and no images have been
+                * deleted, a new lookup table does not need to be written. */
+               old_wim_end = w->hdr.lookup_table_res_entry.offset +
+                             w->hdr.lookup_table_res_entry.size;
+               write_flags |= WIMLIB_WRITE_FLAG_NO_LOOKUP_TABLE |
+                              WIMLIB_WRITE_FLAG_CHECKPOINT_AFTER_XML;
+       }
+       INIT_LIST_HEAD(&stream_list);
+       ret = wim_prepare_streams(w, old_wim_end, &stream_list);
        if (ret != 0)
-               goto done;
+               return ret;
 
-       for_lookup_table_entry(w->lookup_table, zero_out_refcnts, NULL);
+       ret = open_wim_writable(w, w->filename, false,
+                               (write_flags & WIMLIB_WRITE_FLAG_CHECK_INTEGRITY) != 0);
+       if (ret != 0)
+               return ret;
 
-       ret = for_image(w, image, write_file_resources);
+       ret = lock_wim(w, w->out_fp);
        if (ret != 0) {
-               ERROR("Failed to write file resources!\n");
-               goto done;
+               fclose(w->out_fp);
+               w->out_fp = NULL;
+               return ret;
        }
 
-       ret = for_image(w, image, write_metadata_resource);
+       if (fseeko(w->out_fp, old_wim_end, SEEK_SET) != 0) {
+               ERROR_WITH_ERRNO("Can't seek to end of WIM");
+               fclose(w->out_fp);
+               w->out_fp = NULL;
+               w->wim_locked = 0;
+               return WIMLIB_ERR_WRITE;
+       }
+
+       if (!list_empty(&stream_list)) {
+               DEBUG("Writing newly added streams (offset = %"PRIu64")",
+                     old_wim_end);
+               ret = write_stream_list(&stream_list,
+                                       w->lookup_table,
+                                       w->out_fp,
+                                       wimlib_get_compression_type(w),
+                                       write_flags, num_threads,
+                                       progress_func);
+               if (ret != 0)
+                       goto out_ftruncate;
+       } else {
+               DEBUG("No new streams were added");
+       }
 
+       for (int i = 0; i < w->hdr.image_count; i++) {
+               if (w->image_metadata[i]->modified) {
+                       select_wim_image(w, i + 1);
+                       ret = write_metadata_resource(w);
+                       if (ret)
+                               goto out_ftruncate;
+               }
+       }
+       write_flags |= WIMLIB_WRITE_FLAG_REUSE_INTEGRITY_TABLE;
+       ret = finish_write(w, WIMLIB_ALL_IMAGES, write_flags,
+                          progress_func);
+out_ftruncate:
+       close_wim_writable(w);
+       if (ret != 0 && !(write_flags & WIMLIB_WRITE_FLAG_NO_LOOKUP_TABLE)) {
+               WARNING("Truncating `%"TS"' to its original size (%"PRIu64" bytes)",
+                       w->filename, old_wim_end);
+               /* Return value of truncate() is ignored because this is already
+                * an error path. */
+               (void)ttruncate(w->filename, old_wim_end);
+       }
+       w->wim_locked = 0;
+       return ret;
+}
+
+static int
+overwrite_wim_via_tmpfile(WIMStruct *w, int write_flags,
+                         unsigned num_threads,
+                         wimlib_progress_func_t progress_func)
+{
+       size_t wim_name_len;
+       int ret;
+
+       DEBUG("Overwriting `%"TS"' via a temporary file", w->filename);
+
+       /* Write the WIM to a temporary file in the same directory as the
+        * original WIM. */
+       wim_name_len = tstrlen(w->filename);
+       tchar tmpfile[wim_name_len + 10];
+       tmemcpy(tmpfile, w->filename, wim_name_len);
+       randomize_char_array_with_alnum(tmpfile + wim_name_len, 9);
+       tmpfile[wim_name_len + 9] = T('\0');
+
+       ret = wimlib_write(w, tmpfile, WIMLIB_ALL_IMAGES,
+                          write_flags | WIMLIB_WRITE_FLAG_FSYNC,
+                          num_threads, progress_func);
        if (ret != 0) {
-               ERROR("Failed to write image metadata!\n");
-               goto done;
+               ERROR("Failed to write the WIM file `%"TS"'", tmpfile);
+               goto err;
        }
 
-       ret = finish_write(w, image, flags, 1);
+       DEBUG("Renaming `%"TS"' to `%"TS"'", tmpfile, w->filename);
 
-done:
-       DEBUG("Closing output file.\n");
-       if (w->out_fp != NULL) {
-               if (fclose(w->out_fp) != 0) {
-                       ERROR("Failed to close the file `%s': %m\n", path);
-                       ret = WIMLIB_ERR_WRITE;
-               }
-               w->out_fp = NULL;
+#ifdef __WIN32__
+       /* Windows won't let you delete open files unless FILE_SHARE_DELETE was
+        * specified to CreateFile().  The WIM was opened with fopen(), which
+        * didn't provided this flag to CreateFile, so the handle must be closed
+        * before executing the rename(). */
+       if (w->fp != NULL) {
+               fclose(w->fp);
+               w->fp = NULL;
+       }
+#endif
+
+       /* Rename the new file to the old file .*/
+       if (trename(tmpfile, w->filename) != 0) {
+               ERROR_WITH_ERRNO("Failed to rename `%"TS"' to `%"TS"'",
+                                tmpfile, w->filename);
+               ret = WIMLIB_ERR_RENAME;
+               goto err;
+       }
+
+       if (progress_func) {
+               union wimlib_progress_info progress;
+               progress.rename.from = tmpfile;
+               progress.rename.to = w->filename;
+               progress_func(WIMLIB_PROGRESS_MSG_RENAME, &progress);
+       }
+
+       /* Close the original WIM file that was opened for reading. */
+       if (w->fp != NULL) {
+               fclose(w->fp);
+               w->fp = NULL;
+       }
+
+       /* Re-open the WIM read-only. */
+       w->fp = tfopen(w->filename, T("rb"));
+       if (w->fp == NULL) {
+               ret = WIMLIB_ERR_REOPEN;
+               WARNING_WITH_ERRNO("Failed to re-open `%"TS"' read-only",
+                                  w->filename);
+               FREE(w->filename);
+               w->filename = NULL;
        }
        return ret;
+err:
+       /* Remove temporary file. */
+       if (tunlink(tmpfile) != 0)
+               WARNING_WITH_ERRNO("Failed to remove `%"TS"'", tmpfile);
+       return ret;
+}
+
+/*
+ * Writes a WIM file to the original file that it was read from, overwriting it.
+ */
+WIMLIBAPI int
+wimlib_overwrite(WIMStruct *w, int write_flags,
+                unsigned num_threads,
+                wimlib_progress_func_t progress_func)
+{
+       write_flags &= WIMLIB_WRITE_MASK_PUBLIC;
+
+       if (!w->filename)
+               return WIMLIB_ERR_NO_FILENAME;
+
+       if (w->hdr.total_parts != 1) {
+               ERROR("Cannot modify a split WIM");
+               return WIMLIB_ERR_SPLIT_UNSUPPORTED;
+       }
+
+       if ((!w->deletion_occurred || (write_flags & WIMLIB_WRITE_FLAG_SOFT_DELETE))
+           && !(write_flags & WIMLIB_WRITE_FLAG_REBUILD))
+       {
+               int ret;
+               ret = overwrite_wim_inplace(w, write_flags, num_threads,
+                                           progress_func);
+               if (ret == WIMLIB_ERR_RESOURCE_ORDER)
+                       WARNING("Falling back to re-building entire WIM");
+               else
+                       return ret;
+       }
+       return overwrite_wim_via_tmpfile(w, write_flags, num_threads,
+                                        progress_func);
 }