]> wimlib.net Git - wimlib/blobdiff - src/write.c
Implement advisory locking for WIM overwrites
[wimlib] / src / write.c
index 7c4d985c7885903faaca9c0d0b9c73707ae5f4b4..773de0b7b6553e61ead5ef8b726e00f218b54f6a 100644 (file)
 #include "dentry.h"
 #include "lookup_table.h"
 #include "xml.h"
+#include "lzx.h"
+#include "xpress.h"
 #include <unistd.h>
 
+#ifdef ENABLE_MULTITHREADED_COMPRESSION
+#include <semaphore.h>
+#include <pthread.h>
+#endif
 
-/* Reopens the FILE* for a WIM read-write. */
-static int reopen_rw(WIMStruct *w)
-{
-       FILE *fp;
+#include <errno.h>
 
-       if (fclose(w->fp) != 0)
-               ERROR_WITH_ERRNO("Failed to close the file `%s'", w->filename);
-       w->fp = NULL;
-       fp = fopen(w->filename, "r+b");
-       if (!fp) {
-               ERROR_WITH_ERRNO("Failed to open `%s' for reading and writing",
-                                w->filename);
-               return WIMLIB_ERR_OPEN;
+#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
+
+#if defined(HAVE_SYS_FILE_H) && defined(HAVE_FLOCK)
+#include <sys/file.h>
+#endif
+
+
+static int do_fflush(FILE *fp)
+{
+       int ret = fflush(fp);
+       if (ret != 0) {
+               ERROR_WITH_ERRNO("Failed to flush data to output WIM file");
+               return WIMLIB_ERR_WRITE;
        }
-       w->fp = fp;
        return 0;
 }
 
+static int fflush_and_ftruncate(FILE *fp, off_t size)
+{
+       int ret;
 
+       ret = do_fflush(fp);
+       if (ret != 0)
+               return ret;
+       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;
+       }
+       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];
+};
 
 /*
- * Writes a WIM file to the original file that it was read from, overwriting it.
+ * Allocates and initializes a chunk table, and reserves space for it in the
+ * output file.
  */
-WIMLIBAPI int wimlib_overwrite(WIMStruct *w, int write_flags)
+static int
+begin_wim_resource_chunk_tab(const struct lookup_table_entry *lte,
+                            FILE *out_fp,
+                            off_t file_offset,
+                            struct chunk_table **chunk_tab_ret)
 {
-       const char *wimfile_name;
-       size_t wim_name_len;
+       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 (!w)
-               return WIMLIB_ERR_INVALID_PARAM;
-
-       write_flags &= ~WIMLIB_WRITE_FLAG_NO_LOOKUP_TABLE;
+       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;
+       }
 
-       wimfile_name = w->filename;
+       ret = 0;
+out:
+       *chunk_tab_ret = chunk_tab;
+       return ret;
+}
 
-       DEBUG("Replacing WIM file `%s'.", wimfile_name);
+/*
+ * Pointer to function to compresses a chunk of a WIM resource.
+ *
+ * @chunk:             Uncompressed data of the chunk.
+ * @chunk_size:                Size of the uncompressed chunk in bytes.
+ * @compressed_chunk:  Pointer to output buffer of size at least
+ *                             (@chunk_size - 1) bytes.
+ * @compressed_chunk_len_ret:  Pointer to an unsigned int into which the size
+ *                                     of the compressed chunk will be
+ *                                     returned.
+ *
+ * Returns zero if compressed succeeded, and nonzero if the chunk could not be
+ * compressed to any smaller than @chunk_size.  This function cannot fail for
+ * any other reasons.
+ */
+typedef int (*compress_func_t)(const void *, unsigned, void *, unsigned *);
 
-       if (!wimfile_name)
-               return WIMLIB_ERR_NO_FILENAME;
+compress_func_t get_compress_func(int out_ctype)
+{
+       if (out_ctype == WIMLIB_COMPRESSION_TYPE_LZX)
+               return lzx_compress;
+       else
+               return xpress_compress;
+}
 
-       /* 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';
+/*
+ * Writes a chunk of a WIM resource to an output file.
+ *
+ * @chunk:       Uncompressed data of the chunk.
+ * @chunk_size:          Size of the chunk (<= WIM_CHUNK_SIZE)
+ * @out_fp:      FILE * to write tho chunk to.
+ * @out_ctype:   Compression type to use when writing the chunk (ignored if no
+ *                     chunk table provided)
+ * @chunk_tab:   Pointer to chunk table being created.  It is updated with the
+ *                     offset of the chunk we write.
+ *
+ * Returns 0 on success; nonzero on failure.
+ */
+static int write_wim_resource_chunk(const u8 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 (chunk_tab) {
+               u8 *compressed_chunk = alloca(chunk_size);
+               int ret;
+
+               ret = compress(chunk, chunk_size, compressed_chunk,
+                              &out_chunk_size);
+               if (ret == 0) {
+                       out_chunk = compressed_chunk;
+               } else {
+                       out_chunk = chunk;
+                       out_chunk_size = chunk_size;
+               }
+               *chunk_tab->cur_offset_p++ = chunk_tab->cur_offset;
+               chunk_tab->cur_offset += out_chunk_size;
+       } else {
+               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;
+}
 
-       ret = wimlib_write(w, tmpfile, WIM_ALL_IMAGES, write_flags);
-       if (ret != 0) {
-               ERROR("Failed to write the WIM file `%s'", tmpfile);
-               if (unlink(tmpfile) != 0)
-                       WARNING("Failed to remove `%s'", tmpfile);
-               return ret;
+/*
+ * Finishes a WIM chunk tale and writes it to the output file at the correct
+ * offset.
+ *
+ * The final size of the full compressed resource is returned in the
+ * @compressed_size_p.
+ */
+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;
        }
 
-       DEBUG("Closing original WIM file.");
-       /* 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'", wimfile_name);
-               }
-               w->fp = NULL;
+       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;
+}
 
-       DEBUG("Renaming `%s' to `%s'", tmpfile, wimfile_name);
+/* Prepare for multiple reads to a resource by caching a FILE * or NTFS
+ * attribute pointer in the lookup table entry. */
+static int prepare_resource_for_read(struct lookup_table_entry *lte
 
-       /* Rename the new file to the old file .*/
-       if (rename(tmpfile, wimfile_name) != 0) {
-               ERROR_WITH_ERRNO("Failed to rename `%s' to `%s'",
-                                tmpfile, wimfile_name);
-               /* Remove temporary file. */
-               if (unlink(tmpfile) != 0)
-                       ERROR_WITH_ERRNO("Failed to remove `%s'", tmpfile);
-               return WIMLIB_ERR_RENAME;
+                                       #ifdef WITH_NTFS_3G
+                                       , ntfs_inode **ni_ret
+                                       #endif
+               )
+{
+       if (lte->resource_location == RESOURCE_IN_FILE_ON_DISK
+            && !lte->file_on_disk_fp)
+       {
+               wimlib_assert(lte->file_on_disk);
+               lte->file_on_disk_fp = fopen(lte->file_on_disk, "rb");
+               if (!lte->file_on_disk_fp) {
+                       ERROR_WITH_ERRNO("Failed to open the file `%s' for "
+                                        "reading", lte->file_on_disk);
+                       return WIMLIB_ERR_OPEN;
+               }
        }
-
+#ifdef WITH_NTFS_3G
+       else if (lte->resource_location == RESOURCE_IN_NTFS_VOLUME
+                 && !lte->attr)
+       {
+               struct ntfs_location *loc = lte->ntfs_loc;
+               ntfs_inode *ni;
+               wimlib_assert(loc);
+               ni = ntfs_pathname_to_inode(*loc->ntfs_vol_p, NULL, loc->path_utf8);
+               if (!ni) {
+                       ERROR_WITH_ERRNO("Failed to open inode `%s' in NTFS "
+                                        "volume", loc->path_utf8);
+                       return WIMLIB_ERR_NTFS_3G;
+               }
+               lte->attr = ntfs_attr_open(ni,
+                                          loc->is_reparse_point ? AT_REPARSE_POINT : AT_DATA,
+                                          (ntfschar*)loc->stream_name_utf16,
+                                          loc->stream_name_utf16_num_chars);
+               if (!lte->attr) {
+                       ERROR_WITH_ERRNO("Failed to open attribute of `%s' in "
+                                        "NTFS volume", loc->path_utf8);
+                       ntfs_inode_close(ni);
+                       return WIMLIB_ERR_NTFS_3G;
+               }
+               *ni_ret = ni;
+       }
+#endif
        return 0;
 }
 
-static int check_resource_offset(struct lookup_table_entry *lte, void *arg)
+/* Undo prepare_resource_for_read() by closing the cached FILE * or NTFS
+ * attribute. */
+static void end_wim_resource_read(struct lookup_table_entry *lte
+                               #ifdef WITH_NTFS_3G
+                                       , ntfs_inode *ni
+                               #endif
+                                       )
 {
-       u64 xml_data_offset = *(u64*)arg;
-       if (lte->resource_entry.offset > xml_data_offset) {
-               ERROR("The following resource is *after* the XML data:");
-               print_lookup_table_entry(lte);
-               return WIMLIB_ERR_RESOURCE_ORDER;
+       if (lte->resource_location == RESOURCE_IN_FILE_ON_DISK
+           && lte->file_on_disk_fp) {
+               fclose(lte->file_on_disk_fp);
+               lte->file_on_disk_fp = NULL;
        }
-       return 0;
+#ifdef WITH_NTFS_3G
+       else if (lte->resource_location == RESOURCE_IN_NTFS_VOLUME) {
+               if (lte->attr) {
+                       ntfs_attr_close(lte->attr);
+                       lte->attr = NULL;
+               }
+               if (ni)
+                       ntfs_inode_close(ni);
+       }
+#endif
 }
 
-WIMLIBAPI int wimlib_overwrite_xml_and_header(WIMStruct *w, int write_flags)
+/*
+ * Writes a WIM resource to a FILE * opened for writing.  The resource may be
+ * written uncompressed or compressed depending on the @out_ctype parameter.
+ *
+ * If by chance the resource compresses to more than the original size (this may
+ * happen with random data or files than are pre-compressed), the resource is
+ * instead written uncompressed (and this is reflected in the @out_res_entry by
+ * removing the WIM_RESHDR_FLAG_COMPRESSED flag).
+ *
+ * @lte:       The lookup table entry for the WIM resource.
+ * @out_fp:    The FILE * to write the resource to.
+ * @out_ctype:  The compression type of the resource to write.  Note: if this is
+ *                     the same as the compression type of the WIM resource we
+ *                     need to read, we simply copy the data (i.e. we do not
+ *                     uncompress it, then compress it again).
+ * @out_res_entry:  If non-NULL, a resource entry that is filled in with the
+ *                 offset, original size, compressed size, and compression flag
+ *                 of the output resource.
+ *
+ * Returns 0 on success; nonzero on failure.
+ */
+int write_wim_resource(struct lookup_table_entry *lte,
+                      FILE *out_fp, int out_ctype,
+                      struct resource_entry *out_res_entry,
+                      int flags)
 {
+       u64 bytes_remaining;
+       u64 original_size;
+       u64 old_compressed_size;
+       u64 new_compressed_size;
+       u64 offset;
        int ret;
-       FILE *fp;
-       u8 *integrity_table = NULL;
-       off_t xml_end;
-       off_t xml_size;
-       size_t bytes_written;
+       struct chunk_table *chunk_tab = NULL;
+       bool raw;
+       off_t file_offset;
+       compress_func_t compress = NULL;
+#ifdef WITH_NTFS_3G
+       ntfs_inode *ni = NULL;
+#endif
+
+       wimlib_assert(lte);
+
+       /* Original size of the resource */
+       original_size = wim_resource_size(lte);
+
+       /* Compressed size of the resource (as it exists now) */
+       old_compressed_size = wim_resource_compressed_size(lte);
+
+       /* Current offset in output file */
+       file_offset = ftello(out_fp);
+       if (file_offset == -1) {
+               ERROR_WITH_ERRNO("Failed to get offset in output "
+                                "stream");
+               return WIMLIB_ERR_WRITE;
+       }
 
-       DEBUG("Overwriting XML and header of `%s', write_flags = %#x",
-             w->filename, write_flags);
+       /* Are the compression types the same?  If so, do a raw copy (copy
+        * without decompressing and recompressing the data). */
+       raw = (wim_resource_compression_type(lte) == out_ctype
+              && out_ctype != WIMLIB_COMPRESSION_TYPE_NONE
+              && !(flags & WIMLIB_RESOURCE_FLAG_RECOMPRESS));
 
-       if (!w->filename)
-               return WIMLIB_ERR_NO_FILENAME;
+       if (raw) {
+               flags |= WIMLIB_RESOURCE_FLAG_RAW;
+               bytes_remaining = old_compressed_size;
+       } else {
+               flags &= ~WIMLIB_RESOURCE_FLAG_RAW;
+               bytes_remaining = original_size;
+       }
 
-       write_flags &= ~WIMLIB_WRITE_FLAG_NO_LOOKUP_TABLE;
+       /* Empty resource; nothing needs to be done, so just return success. */
+       if (bytes_remaining == 0)
+               return 0;
 
-       /* 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
-        * destroyed by this function. */
-       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;
-       }
+       /* Buffer for reading chunks for the resource */
+       u8 buf[min(WIM_CHUNK_SIZE, bytes_remaining)];
 
-       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 we are writing a compressed resource and not doing a raw copy, we
+        * need to initialize the chunk table */
+       if (out_ctype != WIMLIB_COMPRESSION_TYPE_NONE && !raw) {
+               ret = begin_wim_resource_chunk_tab(lte, out_fp, file_offset,
+                                                  &chunk_tab);
+               if (ret != 0)
+                       goto out;
        }
 
-       ret = for_lookup_table_entry(w->lookup_table, check_resource_offset,
-                                    &w->hdr.xml_res_entry.offset);
+       /* If the WIM resource is in an external file, open a FILE * to it so we
+        * don't have to open a temporary one in read_wim_resource() for each
+        * chunk. */
+#ifdef WITH_NTFS_3G
+       ret = prepare_resource_for_read(lte, &ni);
+#else
+       ret = prepare_resource_for_read(lte);
+#endif
        if (ret != 0)
-               return ret;
+               goto out;
 
-       ret = reopen_rw(w);
-       if (ret != 0)
-               return ret;
+       /* If we aren't doing a raw copy, we will compute the SHA1 message
+        * digest of the resource as we read it, and verify it's the same as the
+        * hash given in the lookup table entry once we've finished reading the
+        * resource. */
+       SHA_CTX ctx;
+       if (!raw) {
+               sha1_init(&ctx);
+               compress = get_compress_func(out_ctype);
+       }
+       offset = 0;
+
+       /* While there are still bytes remaining in the WIM resource, read a
+        * chunk of the resource, update SHA1, then write that chunk using the
+        * desired compression type. */
+       do {
+               u64 to_read = min(bytes_remaining, WIM_CHUNK_SIZE);
+               ret = read_wim_resource(lte, buf, to_read, offset, flags);
+               if (ret != 0)
+                       goto out_fclose;
+               if (!raw)
+                       sha1_update(&ctx, buf, to_read);
+               ret = write_wim_resource_chunk(buf, to_read, out_fp,
+                                              compress, chunk_tab);
+               if (ret != 0)
+                       goto out_fclose;
+               bytes_remaining -= to_read;
+               offset += to_read;
+       } while (bytes_remaining);
+
+       /* Raw copy:  The new compressed size is the same as the old compressed
+        * size
+        *
+        * Using WIMLIB_COMPRESSION_TYPE_NONE:  The new compressed size is the
+        * original size
+        *
+        * Using a different compression type:  Call
+        * finish_wim_resource_chunk_tab() and it will provide the new
+        * compressed size.
+        */
+       if (raw) {
+               new_compressed_size = old_compressed_size;
+       } else {
+               if (out_ctype == WIMLIB_COMPRESSION_TYPE_NONE)
+                       new_compressed_size = original_size;
+               else {
+                       ret = finish_wim_resource_chunk_tab(chunk_tab, out_fp,
+                                                           &new_compressed_size);
+                       if (ret != 0)
+                               goto out_fclose;
+               }
+       }
 
-       fp = w->fp;
+       /* Verify SHA1 message digest of the resource, unless we are doing a raw
+        * write (in which case we never even saw the uncompressed data).  Or,
+        * if the hash we had before is all 0's, just re-set it to be the new
+        * hash. */
+       if (!raw) {
+               u8 md[SHA1_HASH_SIZE];
+               sha1_final(md, &ctx);
+               if (is_zero_hash(lte->hash)) {
+                       copy_hash(lte->hash, md);
+               } else if (!hashes_equal(md, lte->hash)) {
+                       ERROR("WIM resource has incorrect hash!");
+                       if (lte->resource_location == RESOURCE_IN_FILE_ON_DISK) {
+                               ERROR("We were reading it from `%s'; maybe it changed "
+                                     "while we were reading it.",
+                                     lte->file_on_disk);
+                       }
+                       ret = WIMLIB_ERR_INVALID_RESOURCE_HASH;
+                       goto out_fclose;
+               }
+       }
 
-       /* 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 ((write_flags & WIMLIB_WRITE_FLAG_CHECK_INTEGRITY)
-            && w->hdr.integrity.offset != 0)
+       if (!raw && new_compressed_size >= original_size &&
+           out_ctype != WIMLIB_COMPRESSION_TYPE_NONE)
        {
-               DEBUG("Reading existing integrity table.");
-               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);
+               /* Oops!  We compressed the resource to larger than the original
+                * size.  Write the resource uncompressed instead. */
+               if (fseeko(out_fp, file_offset, SEEK_SET) != 0) {
+                       ERROR_WITH_ERRNO("Failed to seek to byte %"PRIu64" "
+                                        "of output WIM file", file_offset);
+                       ret = WIMLIB_ERR_WRITE;
+                       goto out_fclose;
+               }
+               ret = write_wim_resource(lte, out_fp, WIMLIB_COMPRESSION_TYPE_NONE,
+                                        out_res_entry, flags);
+               if (ret != 0)
+                       goto out_fclose;
+
+               ret = fflush_and_ftruncate(out_fp, file_offset + out_res_entry->size);
                if (ret != 0)
-                       goto err;
-               DEBUG("Done reading existing integrity table.");
+                       goto out_fclose;
+       } else {
+               if (out_res_entry) {
+                       out_res_entry->size          = new_compressed_size;
+                       out_res_entry->original_size = original_size;
+                       out_res_entry->offset        = file_offset;
+                       out_res_entry->flags         = lte->resource_entry.flags
+                                                       & ~WIM_RESHDR_FLAG_COMPRESSED;
+                       if (out_ctype != WIMLIB_COMPRESSION_TYPE_NONE)
+                               out_res_entry->flags |= WIM_RESHDR_FLAG_COMPRESSED;
+               }
        }
+       ret = 0;
+out_fclose:
+#ifdef WITH_NTFS_3G
+       end_wim_resource_read(lte, ni);
+#else
+       end_wim_resource_read(lte);
+#endif
+out:
+       FREE(chunk_tab);
+       return ret;
+}
 
-       DEBUG("Overwriting XML data.");
-       /* Overwrite the XML data. */
-       if (fseeko(fp, w->hdr.xml_res_entry.offset, SEEK_SET) != 0) {
-               ERROR_WITH_ERRNO("Failed to seek to byte %"PRIu64" "
-                                "for XML data", w->hdr.xml_res_entry.offset);
-               ret = WIMLIB_ERR_WRITE;
-               goto err;
+#ifdef ENABLE_MULTITHREADED_COMPRESSION
+struct shared_queue {
+       sem_t filled_slots;
+       sem_t empty_slots;
+       pthread_mutex_t lock;
+       unsigned front;
+       unsigned back;
+       void **array;
+       unsigned size;
+};
+
+static int shared_queue_init(struct shared_queue *q, unsigned size)
+{
+       q->array = CALLOC(sizeof(q->array[0]), size);
+       if (!q->array)
+               return WIMLIB_ERR_NOMEM;
+
+       sem_init(&q->filled_slots, 0, 0);
+       sem_init(&q->empty_slots, 0, size);
+       pthread_mutex_init(&q->lock, NULL);
+       q->front = 0;
+       q->back = size - 1;
+       q->size = size;
+       return 0;
+}
+
+static void shared_queue_destroy(struct shared_queue *q)
+{
+       sem_destroy(&q->filled_slots);
+       sem_destroy(&q->empty_slots);
+       pthread_mutex_destroy(&q->lock);
+       FREE(q->array);
+}
+
+static void shared_queue_put(struct shared_queue *q, void *obj)
+{
+       sem_wait(&q->empty_slots);
+       pthread_mutex_lock(&q->lock);
+
+       q->back = (q->back + 1) % q->size;
+       q->array[q->back] = obj;
+
+       sem_post(&q->filled_slots);
+       pthread_mutex_unlock(&q->lock);
+}
+
+static void *shared_queue_get(struct shared_queue *q)
+{
+       sem_wait(&q->filled_slots);
+       pthread_mutex_lock(&q->lock);
+
+       void *obj = q->array[q->front];
+       q->array[q->front] = NULL;
+       q->front = (q->front + 1) % q->size;
+
+       sem_post(&q->empty_slots);
+       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 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);
+               int ret = compress(msg->uncompressed_chunks[i],
+                                  msg->uncompressed_chunk_sizes[i],
+                                  msg->compressed_chunks[i],
+                                  &msg->compressed_chunk_sizes[i]);
+               if (ret == 0) {
+                       msg->out_compressed_chunks[i] = msg->compressed_chunks[i];
+               } else {
+                       msg->out_compressed_chunks[i] = msg->uncompressed_chunks[i];
+                       msg->compressed_chunk_sizes[i] = msg->uncompressed_chunk_sizes[i];
+               }
+       }
+}
+
+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
+
+static int do_write_stream_list(struct list_head *my_resources,
+                               FILE *out_fp,
+                               int out_ctype,
+                               wimlib_progress_func_t progress_func,
+                               union wimlib_progress_info *progress,
+                               int write_resource_flags)
+{
+       int ret;
+       struct lookup_table_entry *lte, *tmp;
+
+       list_for_each_entry_safe(lte, tmp, my_resources, staging_list) {
+               ret = write_wim_resource(lte,
+                                        out_fp,
+                                        out_ctype,
+                                        &lte->output_resource_entry,
+                                        write_resource_flags);
+               if (ret != 0)
+                       return ret;
+               list_del(&lte->staging_list);
+               progress->write_streams.completed_bytes +=
+                       wim_resource_size(lte);
+               progress->write_streams.completed_streams++;
+               if (progress_func) {
+                       progress_func(WIMLIB_PROGRESS_MSG_WRITE_STREAMS,
+                                     progress);
+               }
+       }
+       return 0;
+}
+
+static int write_stream_list_serial(struct list_head *stream_list,
+                                   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, 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;
        }
-       ret = write_xml_data(w->wim_info, WIM_ALL_IMAGES, fp, 0);
+       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 queue_size,
+                                  int write_flags,
+                                  wimlib_progress_func_t progress_func,
+                                  union wimlib_progress_info *progress)
+{
+       int ret;
+
+       struct message msgs[queue_size];
+       ZERO_ARRAY(msgs);
+
+       // Initially, all the messages are available to use.
+       LIST_HEAD(available_msgs);
+       for (size_t i = 0; i < ARRAY_LEN(msgs); 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;
+       struct lookup_table_entry *next_lte = container_of(next_resource,
+                                                          struct lookup_table_entry,
+                                                          staging_list);
+       next_resource = next_resource->next;
+       u64 next_chunk = 0;
+       u64 next_num_chunks = wim_resource_chunks(next_lte);
+       INIT_LIST_HEAD(&next_lte->msg_list);
+       list_add_tail(&next_lte->staging_list, &outstanding_resources);
+
+       // As in write_wim_resource(), each resource we read is checksummed.
+       SHA_CTX next_sha_ctx;
+       sha1_init(&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 lookup_table_entry *cur_lte = next_lte;
+       struct chunk_table *cur_chunk_tab = NULL;
+       struct message *msg;
+
+#ifdef WITH_NTFS_3G
+       ntfs_inode *ni = NULL;
+#endif
+
+#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 err;
+               goto out;
 
-       DEBUG("Updating XML resource entry.");
-       /* Update the XML resource entry in the WIM header. */
-       xml_end = ftello(fp);
-       if (xml_end == -1) {
-               ret = WIMLIB_ERR_WRITE;
-               goto err;
+       DEBUG("Initializing buffers for uncompressed "
+             "and compressed data (%zu bytes needed)",
+             queue_size * 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 < ARRAY_LEN(msgs); i++) {
+               for (size_t j = 0; j < MAX_CHUNKS_PER_MSG; j++) {
+                       msgs[i].compressed_chunks[j] = MALLOC(WIM_CHUNK_SIZE);
+                       msgs[i].uncompressed_chunks[j] = MALLOC(WIM_CHUNK_SIZE);
+                       if (msgs[i].compressed_chunks[j] == NULL ||
+                           msgs[i].uncompressed_chunks[j] == NULL)
+                       {
+                               ERROR("Could not allocate enough memory for "
+                                     "multi-threaded compression");
+                               ret = WIMLIB_ERR_NOMEM;
+                               goto out;
+                       }
+               }
        }
-       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;
-       /* XML data offset is unchanged. */
 
-       if (write_flags & WIMLIB_WRITE_FLAG_CHECK_INTEGRITY) {
-               DEBUG("Writing integrity table.");
-               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_WITH_ERRNO("Failed to write integrity "
-                                                "table");
-                               ret = WIMLIB_ERR_WRITE;
-                               goto err;
+       // 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) && next_lte != NULL) {
+
+                       // 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 &&
+                                    wim_resource_size(next_lte) % WIM_CHUNK_SIZE != 0)
+                               {
+                                       size = 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++;
                        }
-                       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,
-                                       write_flags & WIMLIB_WRITE_FLAG_SHOW_PROGRESS);
-                       if (ret != 0)
-                               return ret;
 
-                       off_t end_integrity = ftello(fp);
-                       if (end_integrity == -1)
-                               return WIMLIB_ERR_WRITE;
+                       // 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 (next_chunk != next_num_chunks)
+                               // More chunks to send for this resource
+                               continue;
+
+                       // Done sending compression requests for a resource!
+                       // Check the SHA1 message digest.
+                       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 `%s'; 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) {
+                                       next_lte = NULL;
+                                       break;
+                               }
+                       #ifdef WITH_NTFS_3G
+                               end_wim_resource_read(next_lte, ni);
+                               ni = NULL;
+                       #else
+                               end_wim_resource_read(next_lte);
+                       #endif
+
+                               next_lte = container_of(next_resource,
+                                                       struct lookup_table_entry,
+                                                       staging_list);
+                               next_resource = next_resource->next;
+                               if ((!(write_flags & WIMLIB_WRITE_FLAG_RECOMPRESS)
+                                     && next_lte->resource_location == RESOURCE_IN_WIM
+                                     && wimlib_get_compression_type(next_lte->wim) == 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;
+                                       DEBUG2("Updated next_lte");
+                                       break;
+                               }
+                       }
+               }
+
+               // If there are no outstanding resources, there are no more
+               // resources that need to be written.
+               if (list_empty(&outstanding_resources)) {
+                       DEBUG("No outstanding resources! Done");
+                       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.
+               DEBUG2("Waiting for message");
+               msg = shared_queue_get(compressed_res_queue);
+               msg->complete = true;
+
+               DEBUG2("Received msg (begin_chunk=%"PRIu64")", msg->begin_chunk);
 
-                       off_t integrity_size           = end_integrity - xml_end;
-                       w->hdr.integrity.size          = integrity_size;
-                       w->hdr.integrity.original_size = integrity_size;
-                       w->hdr.integrity.flags         = 0;
+               list_for_each_entry(msg, &cur_lte->msg_list, list) {
+                       DEBUG2("complete=%d", msg->complete);
                }
+
+               // 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 (!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;
+
+                               progress->write_streams.completed_bytes +=
+                                               wim_resource_size(cur_lte);
+                               progress->write_streams.completed_streams++;
+
+                               if (progress_func) {
+                                       progress_func(WIMLIB_PROGRESS_MSG_WRITE_STREAMS,
+                                                     progress);
+                               }
+
+                               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;
+
+                               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) {
+                                       DEBUG("No more outstanding resources");
+                                       ret = 0;
+                                       goto out;
+                               } else {
+                                       cur_lte = container_of(cur_lte->staging_list.next,
+                                                              struct 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:
+#ifdef WITH_NTFS_3G
+       end_wim_resource_read(cur_lte, ni);
+#else
+       end_wim_resource_read(cur_lte);
+#endif
+       if (ret == 0) {
+               ret = do_write_stream_list(&my_resources, out_fp,
+                                          out_ctype, progress_func,
+                                          progress, 0);
        } else {
-               DEBUG("Truncating file to end of XML data.");
-               /* 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_WITH_ERRNO("Failed to flush stream for file `%s'",
-                                        w->filename);
-                       return WIMLIB_ERR_WRITE;
+               size_t num_available_msgs = 0;
+               struct list_head *cur;
+
+               list_for_each(cur, &available_msgs) {
+                       num_available_msgs++;
                }
-               if (ftruncate(fileno(fp), xml_end) != 0) {
-                       ERROR_WITH_ERRNO("Failed to truncate `%s' to %"PRIu64" "
-                                        "bytes", w->filename, xml_end);
-                       return WIMLIB_ERR_WRITE;
+
+               while (num_available_msgs < ARRAY_LEN(msgs)) {
+                       shared_queue_get(compressed_res_queue);
+                       num_available_msgs++;
                }
-               memset(&w->hdr.integrity, 0, sizeof(struct resource_entry));
        }
 
-       DEBUG("Overwriting header.");
-       /* Overwrite the header. */
-       if (fseeko(fp, 0, SEEK_SET) != 0) {
-               ERROR_WITH_ERRNO("Failed to seek to beginning of `%s'",
-                                w->filename);
-               return WIMLIB_ERR_WRITE;
+       for (size_t i = 0; i < ARRAY_LEN(msgs); i++) {
+               for (size_t j = 0; j < MAX_CHUNKS_PER_MSG; j++) {
+                       FREE(msgs[i].compressed_chunks[j]);
+                       FREE(msgs[i].uncompressed_chunks[j]);
+               }
        }
 
-       ret = write_header(&w->hdr, fp);
+       if (cur_chunk_tab != NULL)
+               FREE(cur_chunk_tab);
+       return ret;
+}
+
+
+static int write_stream_list_parallel(struct list_head *stream_list,
+                                     FILE *out_fp,
+                                     int out_ctype,
+                                     int write_flags,
+                                     unsigned num_threads,
+                                     wimlib_progress_func_t progress_func,
+                                     union wimlib_progress_info *progress)
+{
+       int ret;
+       struct shared_queue res_to_compress_queue;
+       struct shared_queue compressed_res_queue;
+       pthread_t *compressor_threads = NULL;
+
+       if (num_threads == 0) {
+               long nthreads = sysconf(_SC_NPROCESSORS_ONLN);
+               if (nthreads < 1) {
+                       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));
+
+       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("Failed to join compressor thread %u: %s",
+                               i, strerror(errno));
+               }
+       }
+       FREE(compressor_threads);
+       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,
+                                       out_fp,
+                                       out_ctype,
+                                       write_flags,
+                                       progress_func,
+                                       progress);
 
-       DEBUG("Closing `%s'.", w->filename);
-       if (fclose(fp) != 0) {
-               ERROR_WITH_ERRNO("Failed to close `%s'", w->filename);
-               return WIMLIB_ERR_WRITE;
+}
+#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, FILE *out_fp,
+                            int out_ctype, int write_flags,
+                            unsigned num_threads,
+                            wimlib_progress_func_t progress_func)
+{
+       struct lookup_table_entry *lte;
+       size_t num_streams = 0;
+       u64 total_bytes = 0;
+       bool compression_needed = false;
+       union wimlib_progress_info progress;
+       int ret;
+
+       list_for_each_entry(lte, stream_list, staging_list) {
+               num_streams++;
+               total_bytes += wim_resource_size(lte);
+               if (!compression_needed
+                   &&
+                   (out_ctype != WIMLIB_COMPRESSION_TYPE_NONE
+                      && (lte->resource_location != RESOURCE_IN_WIM
+                          || wimlib_get_compression_type(lte->wim) != out_ctype
+                          || (write_flags & WIMLIB_WRITE_FLAG_REBUILD)))
+                   && wim_resource_size(lte) != 0)
+                       compression_needed = true;
        }
-       w->fp = NULL;
-       DEBUG("Done.");
-       return 0;
-err:
-       FREE(integrity_table);
+       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;
+
+       if (num_streams == 0) {
+               ret = 0;
+               goto out;
+       }
+
+#ifdef ENABLE_MULTITHREADED_COMPRESSION
+       if (compression_needed && total_bytes >= 1000000 && num_threads != 1) {
+               ret = write_stream_list_parallel(stream_list,
+                                                out_fp,
+                                                out_ctype,
+                                                write_flags,
+                                                num_threads,
+                                                progress_func,
+                                                &progress);
+       }
+       else
+#endif
+       {
+               ret = write_stream_list_serial(stream_list,
+                                              out_fp,
+                                              out_ctype,
+                                              write_flags,
+                                              progress_func,
+                                              &progress);
+       }
+out:
        return ret;
 }
 
 
-/* Write the file resources for the current image. */
-static int write_file_resources(WIMStruct *w)
+static int dentry_find_streams_to_write(struct dentry *dentry,
+                                       void *wim)
 {
-       DEBUG("Writing file resources for image %u.", w->current_image);
-       return for_dentry_in_tree(wim_root_dentry(w), write_dentry_resources, w);
+       WIMStruct *w = wim;
+       struct list_head *stream_list = w->private;
+       struct lookup_table_entry *lte;
+       for (unsigned i = 0; i <= dentry->d_inode->num_ads; i++) {
+               lte = inode_stream_lte(dentry->d_inode, i, w->lookup_table);
+               if (lte && ++lte->out_refcnt == 1)
+                       list_add_tail(&lte->staging_list, stream_list);
+       }
+       return 0;
+}
+
+static int find_streams_to_write(WIMStruct *w)
+{
+       return for_dentry_in_tree(wim_root_dentry(w),
+                                 dentry_find_streams_to_write, w);
+}
+
+static int write_wim_streams(WIMStruct *w, int image, int write_flags,
+                            unsigned num_threads,
+                            wimlib_progress_func_t progress_func)
+{
+
+       for_lookup_table_entry(w->lookup_table, lte_zero_out_refcnt, NULL);
+       LIST_HEAD(stream_list);
+       w->private = &stream_list;
+       for_image(w, image, find_streams_to_write);
+       return write_stream_list(&stream_list, w->out_fp,
+                                wimlib_get_compression_type(w), write_flags,
+                                num_threads, progress_func);
 }
 
 /*
- * Write the lookup table, xml data, and integrity table, then overwrite the WIM
- * header.
+ * Finish writing a WIM file: write the lookup table, xml data, and integrity
+ * table (optional), then overwrite the WIM header.
+ *
+ * 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)
-{
-       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;
+int finish_write(WIMStruct *w, int image, int write_flags,
+                wimlib_progress_func_t progress_func)
+{
        int ret;
        struct wim_header hdr;
        FILE *out = w->out_fp;
 
-       if (!(write_flags & WIMLIB_WRITE_FLAG_NO_LOOKUP_TABLE)) {
-               /* Write the lookup table. */
-               lookup_table_offset = ftello(out);
-               if (lookup_table_offset == -1)
-                       return WIMLIB_ERR_WRITE;
-
-               DEBUG("Writing lookup table (offset %"PRIu64")",
-                     lookup_table_offset);
-               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;
-
        /* @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_flags & WIMLIB_WRITE_FLAG_NO_LOOKUP_TABLE)) {
-               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;
+               ret = write_lookup_table(w->lookup_table, out, &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;
 
-       DEBUG("Writing XML data (offset %"PRIu64")", xml_data_offset);
        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);
+                             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) {
+                               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;
+                       }
 
-       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;
+                       if (fseeko(out, 0, SEEK_END) != 0) {
+                               ret = WIMLIB_ERR_WRITE;
+                               goto out;
+                       }
+               }
 
-       if (write_flags & WIMLIB_WRITE_FLAG_CHECK_INTEGRITY) {
-               ret = write_integrity_table(out, WIM_HEADER_DISK_SIZE,
-                                           xml_data_offset,
-                                           write_flags & WIMLIB_WRITE_FLAG_SHOW_PROGRESS);
+               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;
+
+               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.");
 
        /*
         * In the WIM header, there is room for the resource entry for a
@@ -392,7 +1446,7 @@ int finish_write(WIMStruct *w, int image, int write_flags)
         * marked as bootable.  This is not well documented...
         */
        if (hdr.boot_idx == 0 || !w->image_metadata
-                       || (image != WIM_ALL_IMAGES && image != hdr.boot_idx)) {
+                       || (image != WIMLIB_ALL_IMAGES && image != hdr.boot_idx)) {
                memset(&hdr.boot_metadata_res_entry, 0,
                       sizeof(struct resource_entry));
        } else {
@@ -403,7 +1457,7 @@ int finish_write(WIMStruct *w, int image, int write_flags)
        }
 
        /* 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;
@@ -411,97 +1465,444 @@ int finish_write(WIMStruct *w, int image, int write_flags)
                        hdr.boot_idx = 0;
        }
 
+       if (fseeko(out, 0, SEEK_SET) != 0) {
+               ret = WIMLIB_ERR_WRITE;
+               goto out;
+       }
 
-       if (fseeko(out, 0, SEEK_SET) != 0)
-               return WIMLIB_ERR_WRITE;
+       ret = write_header(&hdr, out);
+       if (ret != 0)
+               goto out;
 
-       return write_header(&hdr, 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 write_flags)
+static int open_wim_writable(WIMStruct *w, const char *path,
+                            bool trunc, bool readable)
 {
        const char *mode;
-       DEBUG("Opening `%s' for new WIM", path);
-
-       /* 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 is
-        * written-- this would be faster, but a bit more complicated) */
-       if (write_flags & WIMLIB_WRITE_FLAG_CHECK_INTEGRITY)
-               mode = "w+b";
+       int ret = 0;
+       if (trunc)
+               if (readable)
+                       mode = "w+b";
+               else
+                       mode = "wb";
        else
-               mode = "wb";
+               mode = "r+b";
 
+       DEBUG("Opening `%s' read-write", path);
+       wimlib_assert(w->out_fp == NULL);
+       wimlib_assert(path != NULL);
        w->out_fp = fopen(path, mode);
        if (!w->out_fp) {
-               ERROR_WITH_ERRNO("Failed to open the file `%s' for writing",
-                                path);
+               ERROR_WITH_ERRNO("Failed to open `%s' for writing", path);
                return WIMLIB_ERR_OPEN;
        }
+#if defined(HAVE_SYS_FILE_H) && defined(HAVE_FLOCK)
+       if (!trunc) {
+               ret = flock(fileno(w->out_fp), LOCK_EX | LOCK_NB);
+               if (ret != 0) {
+                       if (errno == EWOULDBLOCK) {
+                               ERROR("`%s' is already being modified "
+                                     "by another process", path);
+                               ret = WIMLIB_ERR_ALREADY_LOCKED;
+                               fclose(w->out_fp);
+                               w->out_fp = NULL;
+                       } else {
+                               WARNING("Failed to lock `%s': %s",
+                                       path, strerror(errno));
+                               ret = 0;
+                       }
+               }
+       }
+#endif
+       return ret;
+}
+
+
+static void close_wim_writable(WIMStruct *w)
+{
+       if (w->out_fp) {
+               if (fclose(w->out_fp) != 0) {
+                       WARNING("Failed to close output WIM: %s",
+                               strerror(errno));
+               }
+               w->out_fp = NULL;
+       }
+}
+
+/* Open file stream and write dummy header for WIM. */
+int begin_write(WIMStruct *w, const char *path, int write_flags)
+{
+       int ret;
+       bool need_readable = false;
+       bool trunc = true;
+       if (write_flags & WIMLIB_WRITE_FLAG_CHECK_INTEGRITY)
+               need_readable = true;
 
+       ret = open_wim_writable(w, path, trunc, need_readable);
+       if (ret != 0)
+               return ret;
        /* Write dummy header. It will be overwritten later. */
        return write_header(&w->hdr, w->out_fp);
 }
 
 /* Writes a stand-alone WIM to a file.  */
 WIMLIBAPI int wimlib_write(WIMStruct *w, const char *path,
-                          int image, int write_flags)
+                          int image, int write_flags, unsigned num_threads,
+                          wimlib_progress_func_t progress_func)
 {
        int ret;
 
        if (!w || !path)
                return WIMLIB_ERR_INVALID_PARAM;
 
-       write_flags &= ~WIMLIB_WRITE_FLAG_NO_LOOKUP_TABLE;
+       write_flags &= WIMLIB_WRITE_MASK_PUBLIC;
 
-       if (image != WIM_ALL_IMAGES &&
+       if (image != WIMLIB_ALL_IMAGES &&
             (image < 1 || image > w->hdr.image_count))
                return WIMLIB_ERR_INVALID_IMAGE;
 
-
        if (w->hdr.total_parts != 1) {
                ERROR("Cannot call wimlib_write() on part of a split WIM");
                return WIMLIB_ERR_SPLIT_UNSUPPORTED;
        }
 
-       if (image == WIM_ALL_IMAGES)
-               DEBUG("Writing all images to `%s'.", path);
-       else
-               DEBUG("Writing image %d to `%s'.", image, path);
-
        ret = begin_write(w, path, write_flags);
        if (ret != 0)
                goto out;
 
-       for_lookup_table_entry(w->lookup_table, lte_zero_out_refcnt, NULL);
+       ret = write_wim_streams(w, image, write_flags, num_threads,
+                               progress_func);
+       if (ret != 0)
+               goto out;
 
-       w->write_flags = write_flags;
+       if (progress_func)
+               progress_func(WIMLIB_PROGRESS_MSG_WRITE_METADATA_BEGIN, NULL);
 
-       ret = for_image(w, image, write_file_resources);
-       if (ret != 0) {
-               ERROR("Failed to write WIM file resources to `%s'", path);
+       ret = for_image(w, image, write_metadata_resource);
+       if (ret != 0)
                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);
+       return ret;
+}
+
+static int lte_overwrite_prepare(struct lookup_table_entry *lte,
+                                void *ignore)
+{
+       memcpy(&lte->output_resource_entry, &lte->resource_entry,
+              sizeof(struct resource_entry));
+       lte->out_refcnt = 0;
+       return 0;
+}
+
+static int check_resource_offset(struct lookup_table_entry *lte, void *arg)
+{
+       off_t end_offset = *(u64*)arg;
+
+       wimlib_assert(lte->out_refcnt <= lte->refcnt);
+       if (lte->out_refcnt < lte->refcnt) {
+               if (lte->resource_entry.offset + lte->resource_entry.size > end_offset) {
+                       ERROR("The following resource is after the XML data:");
+                       print_lookup_table_entry(lte);
+                       return WIMLIB_ERR_RESOURCE_ORDER;
+               }
        }
+       return 0;
+}
 
-       ret = for_image(w, image, write_metadata_resource);
+static int find_new_streams(struct lookup_table_entry *lte, void *arg)
+{
+       if (lte->out_refcnt == lte->refcnt)
+               list_add(&lte->staging_list, (struct list_head*)arg);
+       else
+               lte->out_refcnt = lte->refcnt;
+       return 0;
+}
+
+/*
+ * 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 modified_image_idx)
+{
+       int ret;
+       struct list_head stream_list;
+       off_t old_wim_end;
+
+       DEBUG("Overwriting `%s' 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;
+       }
+
+       DEBUG("Identifying newly added streams");
+       for_lookup_table_entry(w->lookup_table, lte_overwrite_prepare, NULL);
+       INIT_LIST_HEAD(&stream_list);
+       for (int i = modified_image_idx; i < w->hdr.image_count; i++) {
+               DEBUG("Identifiying streams in image %d", i + 1);
+               wimlib_assert(w->image_metadata[i].modified);
+               wimlib_assert(!w->image_metadata[i].has_been_mounted_rw);
+               wimlib_assert(w->image_metadata[i].root_dentry != NULL);
+               wimlib_assert(w->image_metadata[i].metadata_lte != NULL);
+               w->private = &stream_list;
+               for_dentry_in_tree(w->image_metadata[i].root_dentry,
+                                  dentry_find_streams_to_write, w);
+       }
+
+       if (w->hdr.integrity.offset)
+               old_wim_end = w->hdr.integrity.offset + w->hdr.integrity.size;
+       else
+               old_wim_end = w->hdr.xml_res_entry.offset + w->hdr.xml_res_entry.size;
+
+       ret = for_lookup_table_entry(w->lookup_table, check_resource_offset,
+                                    &old_wim_end);
+       if (ret != 0)
+               return ret;
+
+       if (modified_image_idx == w->hdr.image_count && !w->deletion_occurred) {
+               /* If no images have been modified and no images have been
+                * deleted, a new lookup table does not need to be written. */
+               wimlib_assert(list_empty(&stream_list));
+               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);
+       for_lookup_table_entry(w->lookup_table, find_new_streams,
+                              &stream_list);
+
+       ret = open_wim_writable(w, w->filename, false,
+                               (write_flags & WIMLIB_WRITE_FLAG_CHECK_INTEGRITY) != 0);
+       if (ret != 0)
+               return ret;
+
+       if (fseeko(w->out_fp, old_wim_end, SEEK_SET) != 0) {
+               ERROR_WITH_ERRNO("Can't seek to end of WIM");
+               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->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 = modified_image_idx; i < w->hdr.image_count; i++) {
+               select_wim_image(w, i + 1);
+               ret = write_metadata_resource(w);
+               if (ret != 0)
+                       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) {
-               /*ERROR("Failed to write WIM image metadata to `%s'", path);*/
-               goto out;
+               WARNING("Truncating `%s' to its original size (%"PRIu64" bytes)",
+                       w->filename, old_wim_end);
+               truncate(w->filename, old_wim_end);
        }
+       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;
 
-       ret = finish_write(w, image, write_flags);
+       DEBUG("Overwriting `%s' via a temporary file", w->filename);
 
-out:
-       DEBUG("Closing output file.");
-       if (w->out_fp != NULL) {
-               if (fclose(w->out_fp) != 0) {
-                       ERROR_WITH_ERRNO("Failed to close the file `%s'", path);
-                       ret = WIMLIB_ERR_WRITE;
-               }
-               w->out_fp = NULL;
+       /* Write the WIM to a temporary file in the same directory as the
+        * original WIM. */
+       wim_name_len = strlen(w->filename);
+       char tmpfile[wim_name_len + 10];
+       memcpy(tmpfile, w->filename, wim_name_len);
+       randomize_char_array_with_alnum(tmpfile + wim_name_len, 9);
+       tmpfile[wim_name_len + 9] = '\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 the WIM file `%s'", tmpfile);
+               goto err;
+       }
+
+       /* Close the original WIM file that was opened for reading. */
+       if (w->fp != NULL) {
+               fclose(w->fp);
+               w->fp = NULL;
+       }
+
+       DEBUG("Renaming `%s' to `%s'", tmpfile, w->filename);
+
+       /* Rename the new file to the old file .*/
+       if (rename(tmpfile, w->filename) != 0) {
+               ERROR_WITH_ERRNO("Failed to rename `%s' to `%s'",
+                                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);
+       }
+
+       /* Re-open the WIM read-only. */
+       w->fp = fopen(w->filename, "rb");
+       if (w->fp == NULL) {
+               ret = WIMLIB_ERR_REOPEN;
+               WARNING("Failed to re-open `%s' read-only: %s",
+                       w->filename, strerror(errno));
        }
        return ret;
+err:
+       /* Remove temporary file. */
+       if (unlink(tmpfile) != 0)
+               WARNING("Failed to remove `%s': %s", tmpfile, strerror(errno));
+       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)
+{
+       if (!w)
+               return WIMLIB_ERR_INVALID_PARAM;
+
+       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 i, modified_image_idx;
+               for (i = 0; i < w->hdr.image_count && !w->image_metadata[i].modified; i++)
+                       ;
+               modified_image_idx = i;
+               for (; i < w->hdr.image_count && w->image_metadata[i].modified &&
+                       !w->image_metadata[i].has_been_mounted_rw; i++)
+                       ;
+               if (i == w->hdr.image_count) {
+                       return overwrite_wim_inplace(w, write_flags, num_threads,
+                                                    progress_func,
+                                                    modified_image_idx);
+               }
+       }
+       return overwrite_wim_via_tmpfile(w, write_flags, num_threads,
+                                        progress_func);
 }