4 * A dentry (directory entry) contains the metadata for a file. In the WIM file
5 * format, the dentries are stored in the "metadata resource" section right
6 * after the security data. Each image in the WIM file has its own metadata
7 * resource with its own security data and dentry tree. Dentries in different
8 * images may share file resources by referring to the same lookup table
14 * Copyright (C) 2010 Carl Thijssen
15 * Copyright (C) 2012 Eric Biggers
17 * This file is part of wimlib, a library for working with WIM files.
19 * wimlib is free software; you can redistribute it and/or modify it under the
20 * terms of the GNU Lesser General Public License as published by the Free
21 * Software Foundation; either version 2.1 of the License, or (at your option)
24 * wimlib is distributed in the hope that it will be useful, but WITHOUT ANY
25 * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
26 * A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more
29 * You should have received a copy of the GNU Lesser General Public License
30 * along with wimlib; if not, see http://www.gnu.org/licenses/.
33 #include "wimlib_internal.h"
36 #include "timestamp.h"
37 #include "lookup_table.h"
43 /* Transfers file attributes from a `stat' buffer to a struct dentry. */
44 void stbuf_to_dentry(const struct stat *stbuf, struct dentry *dentry)
46 if (S_ISDIR(stbuf->st_mode))
47 dentry->attributes = WIM_FILE_ATTRIBUTE_DIRECTORY;
49 dentry->attributes = WIM_FILE_ATTRIBUTE_NORMAL;
52 /* Transfers file attributes from a struct dentry to a `stat' buffer. */
53 void dentry_to_stbuf(const struct dentry *dentry, struct stat *stbuf,
54 const struct lookup_table *table)
56 struct lookup_table_entry *lte;
58 if (dentry_is_directory(dentry))
59 stbuf->st_mode = S_IFDIR | 0755;
61 stbuf->st_mode = S_IFREG | 0644;
64 lte = lookup_resource(table, dentry->hash);
69 stbuf->st_nlink = lte->refcnt;
70 stbuf->st_size = lte->resource_entry.original_size;
75 stbuf->st_uid = getuid();
76 stbuf->st_gid = getgid();
77 stbuf->st_atime = ms_timestamp_to_unix(dentry->last_access_time);
78 stbuf->st_mtime = ms_timestamp_to_unix(dentry->last_write_time);
79 stbuf->st_ctime = ms_timestamp_to_unix(dentry->creation_time);
80 stbuf->st_blocks = (stbuf->st_size + 511) / 512;
83 /* Makes all timestamp fields for the dentry be the current time. */
84 void dentry_update_all_timestamps(struct dentry *dentry)
86 u64 now = get_timestamp();
87 dentry->creation_time = now;
88 dentry->last_access_time = now;
89 dentry->last_write_time = now;
93 * Calls a function on all directory entries in a directory tree. It is called
94 * on a parent before its children.
96 int for_dentry_in_tree(struct dentry *root,
97 int (*visitor)(struct dentry*, void*), void *arg)
100 struct dentry *child;
102 ret = visitor(root, arg);
107 child = root->children;
113 ret = for_dentry_in_tree(child, visitor, arg);
117 } while (child != root->children);
122 * Like for_dentry_in_tree(), but the visitor function is always called on a
123 * dentry's children before on itself.
125 int for_dentry_in_tree_depth(struct dentry *root,
126 int (*visitor)(struct dentry*, void*), void *arg)
129 struct dentry *child;
132 child = root->children;
136 ret = for_dentry_in_tree_depth(child, visitor, arg);
140 } while (child != root->children);
142 return visitor(root, arg);
146 * Calculate the full path of @dentry, based on its parent's full path and on
147 * its UTF-8 file name.
149 int calculate_dentry_full_path(struct dentry *dentry, void *ignore)
153 char *parent_full_path;
156 FREE(dentry->full_path_utf8);
158 if (dentry_is_root(dentry)) {
159 dentry->full_path_utf8 = MALLOC(2);
160 if (!dentry->full_path_utf8) {
161 ERROR("Out of memory!\n");
162 return WIMLIB_ERR_NOMEM;
165 dentry->full_path_utf8[0] = '/';
166 dentry->full_path_utf8[1] = '\0';
167 dentry->full_path_utf8_len = 1;
171 if (dentry_is_root(dentry->parent)) {
173 parent_full_path = "";
175 parent_len = dentry->parent->full_path_utf8_len;
176 parent_full_path = dentry->parent->full_path_utf8;
179 len = parent_len + 1 + dentry->file_name_utf8_len;
180 full_path = MALLOC(len + 1);
182 ERROR("Out of memory!\n");
183 return WIMLIB_ERR_NOMEM;
186 memcpy(full_path, parent_full_path, parent_len);
187 full_path[parent_len] = '/';
188 memcpy(full_path + parent_len + 1, dentry->file_name_utf8,
189 dentry->file_name_utf8_len);
190 full_path[len] = '\0';
191 dentry->full_path_utf8 = full_path;
192 dentry->full_path_utf8_len = len;
197 * Recursively calculates the subdir offsets for a directory tree.
199 * @dentry: The root of the directory tree.
200 * @subdir_offset_p: The current subdirectory offset; i.e., the subdirectory
201 * offset for @dentry.
203 void calculate_subdir_offsets(struct dentry *dentry, u64 *subdir_offset_p)
205 struct dentry *child;
207 child = dentry->children;
208 dentry->subdir_offset = *subdir_offset_p;
211 /* Advance the subdir offset by the amount of space the children
212 * of this dentry take up. */
214 *subdir_offset_p += child->length;
216 } while (child != dentry->children);
218 /* End-of-directory dentry on disk. */
219 *subdir_offset_p += 8;
221 /* Recursively call calculate_subdir_offsets() on all the
224 calculate_subdir_offsets(child, subdir_offset_p);
226 } while (child != dentry->children);
228 /* On disk, childless directories have a valid subdir_offset
229 * that points to an 8-byte end-of-directory dentry. Regular
230 * files have a subdir_offset of 0. */
231 if (dentry_is_directory(dentry))
232 *subdir_offset_p += 8;
234 dentry->subdir_offset = 0;
239 /* Returns the child of @dentry that has the file name @name.
240 * Returns NULL if no child has the name. */
241 struct dentry *get_dentry_child_with_name(const struct dentry *dentry,
244 struct dentry *child;
247 child = dentry->children;
249 name_len = strlen(name);
251 if (dentry_has_name(child, name, name_len))
254 } while (child != dentry->children);
259 /* Retrieves the dentry that has the UTF-8 @path relative to the dentry
260 * @cur_dir. Returns NULL if no dentry having the path is found. */
261 static struct dentry *get_dentry_relative_path(struct dentry *cur_dir, const char *path)
263 struct dentry *child;
265 const char *new_path;
270 child = cur_dir->children;
272 new_path = path_next_part(path, &base_len);
274 if (dentry_has_name(child, path, base_len))
275 return get_dentry_relative_path(child, new_path);
277 } while (child != cur_dir->children);
282 /* Returns the dentry corresponding to the UTF-8 @path, or NULL if there is no
284 struct dentry *get_dentry(WIMStruct *w, const char *path)
286 struct dentry *root = wim_root_dentry(w);
289 return get_dentry_relative_path(root, path);
292 /* Returns the parent directory for the @path. */
293 struct dentry *get_parent_dentry(WIMStruct *w, const char *path)
295 size_t path_len = strlen(path);
296 char buf[path_len + 1];
298 memcpy(buf, path, path_len + 1);
300 to_parent_name(buf, path_len);
302 return get_dentry(w, buf);
305 /* Prints the full path of a dentry. */
306 int print_dentry_full_path(struct dentry *dentry, void *ignore)
308 if (dentry->full_path_utf8)
309 puts(dentry->full_path_utf8);
313 /* Prints a directory entry. @lookup_table is a pointer to the lookup table, or
314 * NULL if the resource entry for the dentry is not to be printed. */
315 int print_dentry(struct dentry *dentry, void *lookup_table)
317 struct lookup_table_entry *lte;
318 printf("[DENTRY]\n");
319 printf("Length = %"PRIu64"\n", dentry->length);
320 printf("Attributes = 0x%x\n", dentry->attributes);
321 #ifdef ENABLE_SECURITY_DATA
322 printf("Security ID = %d\n", dentry->security_id);
324 printf("Subdir offset = %"PRIu64"\n", dentry->subdir_offset);
325 /*printf("Unused1 = %"PRIu64"\n", dentry->unused1);*/
326 /*printf("Unused2 = %"PRIu64"\n", dentry->unused2);*/
327 printf("Creation Time = %"PRIu64"\n", dentry->creation_time);
328 printf("Last Access Time = %"PRIu64"\n", dentry->last_access_time);
329 printf("Last Write Time = %"PRIu64"\n", dentry->last_write_time);
330 printf("Creation Time = 0x%"PRIx64"\n", dentry->creation_time);
332 print_hash(dentry->hash);
334 /*printf("Reparse Tag = %u\n", dentry->reparse_tag);*/
335 printf("Hard Link Group = %"PRIu64"\n", dentry->hard_link);
336 /*printf("Number of Streams = %hu\n", dentry->streams);*/
337 printf("Filename = \"");
338 print_string(dentry->file_name, dentry->file_name_len);
340 printf("Filename Length = %hu\n", dentry->file_name_len);
341 printf("Filename (UTF-8) = \"%s\"\n", dentry->file_name_utf8);
342 printf("Filename (UTF-8) Length = %hu\n", dentry->file_name_utf8_len);
343 printf("Short Name = \"");
344 print_string(dentry->short_name, dentry->short_name_len);
346 printf("Short Name Length = %hu\n", dentry->short_name_len);
347 printf("Full Path (UTF-8) = \"%s\"\n", dentry->full_path_utf8);
349 lte = lookup_resource(lookup_table, dentry->hash);
351 print_lookup_table_entry(lte, NULL);
360 static inline void dentry_common_init(struct dentry *dentry)
362 memset(dentry, 0, sizeof(struct dentry));
364 #ifdef ENABLE_SECURITY_DATA
365 dentry->security_id = -1;
370 * Creates an unlinked directory entry.
372 * @name: The base name of the new dentry.
373 * @return: A pointer to the new dentry, or NULL if out of memory.
375 struct dentry *new_dentry(const char *name)
377 struct dentry *dentry;
379 dentry = MALLOC(sizeof(struct dentry));
383 dentry_common_init(dentry);
384 if (change_dentry_name(dentry, name) != 0) {
389 dentry_update_all_timestamps(dentry);
390 dentry->next = dentry;
391 dentry->prev = dentry;
392 dentry->parent = dentry;
397 void free_dentry(struct dentry *dentry)
399 FREE(dentry->file_name);
400 FREE(dentry->file_name_utf8);
401 FREE(dentry->short_name);
402 FREE(dentry->full_path_utf8);
406 /* Arguments for do_free_dentry(). */
407 struct free_dentry_args {
408 struct lookup_table *lookup_table;
409 bool decrement_refcnt;
413 * This function is passed as an argument to for_dentry_in_tree_depth() in order
414 * to free a directory tree. __args is a pointer to a `struct free_dentry_args'.
416 static int do_free_dentry(struct dentry *dentry, void *__args)
418 struct free_dentry_args *args = (struct free_dentry_args*)__args;
420 if (args->decrement_refcnt && !dentry_is_directory(dentry)) {
421 lookup_table_decrement_refcnt(args->lookup_table,
425 wimlib_assert(dentry->refcnt >= 1);
426 if (--dentry->refcnt == 0)
432 * Unlinks and frees a dentry tree.
434 * @root: The root of the tree.
435 * @lookup_table: The lookup table for dentries.
436 * @decrement_refcnt: True if the dentries in the tree are to have their
437 * reference counts in the lookup table decremented.
439 void free_dentry_tree(struct dentry *root, struct lookup_table *lookup_table,
440 bool decrement_refcnt)
442 if (!root || !root->parent)
445 struct free_dentry_args args;
446 args.lookup_table = lookup_table;
447 args.decrement_refcnt = decrement_refcnt;
448 for_dentry_in_tree_depth(root, do_free_dentry, &args);
451 int increment_dentry_refcnt(struct dentry *dentry, void *ignore)
458 * Links a dentry into the directory tree.
460 * @dentry: The dentry to link.
461 * @parent: The dentry that will be the parent of @dentry.
463 void link_dentry(struct dentry *dentry, struct dentry *parent)
465 dentry->parent = parent;
466 if (parent->children) {
467 /* Not an only child; link to siblings. */
468 dentry->next = parent->children;
469 dentry->prev = parent->children->prev;
470 dentry->next->prev = dentry;
471 dentry->prev->next = dentry;
473 /* Only child; link to parent. */
474 parent->children = dentry;
475 dentry->next = dentry;
476 dentry->prev = dentry;
480 /* Unlink a dentry from the directory tree. */
481 void unlink_dentry(struct dentry *dentry)
483 if (dentry_is_root(dentry))
485 if (dentry_is_only_child(dentry)) {
486 dentry->parent->children = NULL;
488 if (dentry_is_first_sibling(dentry))
489 dentry->parent->children = dentry->next;
490 dentry->next->prev = dentry->prev;
491 dentry->prev->next = dentry->next;
496 /* Recalculates the length of @dentry based on its file name length and short
498 static inline void recalculate_dentry_size(struct dentry *dentry)
500 dentry->length = WIM_DENTRY_DISK_SIZE + dentry->file_name_len +
501 2 + dentry->short_name_len;
502 /* Must be multiple of 8. */
503 dentry->length += (8 - dentry->length % 8) % 8;
506 /* Changes the name of a dentry to @new_name. Only changes the file_name and
507 * file_name_utf8 fields; does not change the short_name, short_name_utf8, or
508 * full_path_utf8 fields. Also recalculates its length. */
509 int change_dentry_name(struct dentry *dentry, const char *new_name)
514 FREE(dentry->file_name);
516 utf8_len = strlen(new_name);
518 dentry->file_name = utf8_to_utf16(new_name, utf8_len, &utf16_len);
520 if (!dentry->file_name)
521 return WIMLIB_ERR_NOMEM;
523 FREE(dentry->file_name_utf8);
524 dentry->file_name_utf8 = MALLOC(utf8_len + 1);
525 if (!dentry->file_name_utf8) {
526 FREE(dentry->file_name);
527 dentry->file_name = NULL;
528 return WIMLIB_ERR_NOMEM;
531 dentry->file_name_len = utf16_len;
532 dentry->file_name_utf8_len = utf8_len;
533 memcpy(dentry->file_name_utf8, new_name, utf8_len + 1);
534 recalculate_dentry_size(dentry);
538 /* Parameters for calculate_dentry_statistics(). */
539 struct image_statistics {
540 struct lookup_table *lookup_table;
544 u64 *hard_link_bytes;
547 static int calculate_dentry_statistics(struct dentry *dentry, void *arg)
549 struct image_statistics *stats;
550 struct lookup_table_entry *lte;
553 lte = lookup_resource(stats->lookup_table, dentry->hash);
555 if (dentry_is_directory(dentry) && !dentry_is_root(dentry))
558 ++*stats->file_count;
561 u64 size = lte->resource_entry.original_size;
562 *stats->total_bytes += size;
563 if (++lte->out_refcnt == 1)
564 *stats->hard_link_bytes += size;
569 void calculate_dir_tree_statistics(struct dentry *root, struct lookup_table *table,
570 u64 *dir_count_ret, u64 *file_count_ret,
571 u64 *total_bytes_ret,
572 u64 *hard_link_bytes_ret)
574 struct image_statistics stats;
577 *total_bytes_ret = 0;
578 *hard_link_bytes_ret = 0;
579 stats.lookup_table = table;
580 stats.dir_count = dir_count_ret;
581 stats.file_count = file_count_ret;
582 stats.total_bytes = total_bytes_ret;
583 stats.hard_link_bytes = hard_link_bytes_ret;
584 for_lookup_table_entry(table, zero_out_refcnts, NULL);
585 for_dentry_in_tree(root, calculate_dentry_statistics, &stats);
589 * Reads a directory entry from the metadata resource.
591 int read_dentry(const u8 metadata_resource[], u64 metadata_resource_len,
592 u64 offset, struct dentry *dentry)
597 char *file_name_utf8;
601 size_t file_name_utf8_len;
603 dentry_common_init(dentry);
605 /*Make sure the dentry really fits into the metadata resource.*/
606 if (offset + 8 > metadata_resource_len) {
607 ERROR("Directory entry starting at %"PRIu64" ends past the "
608 "end of the metadata resource (size %"PRIu64")!\n",
609 offset, metadata_resource_len);
610 return WIMLIB_ERR_INVALID_DENTRY;
613 /* Before reading the whole entry, we need to read just the length.
614 * This is because an entry of length 8 (that is, just the length field)
615 * terminates the list of sibling directory entries. */
617 p = get_u64(&metadata_resource[offset], &dentry->length);
619 /* A zero length field (really a length of 8, since that's how big the
620 * directory entry is...) indicates that this is the end of directory
621 * dentry. We do not read it into memory as an actual dentry, so just
622 * return true in that case. */
623 if (dentry->length == 0)
626 if (offset + dentry->length >= metadata_resource_len) {
627 ERROR("Directory entry at offset %"PRIu64" and with size "
628 "%"PRIu64" ends past the end of the metadata resource "
629 "(size %"PRIu64")!\n", offset, dentry->length,
630 metadata_resource_len);
631 return WIMLIB_ERR_INVALID_DENTRY;
634 /* If it is a recognized length, read the rest of the directory entry.
635 * Note: The root directory entry has no name, and its length does not
636 * include the short name length field. */
637 if (dentry->length < WIM_DENTRY_DISK_SIZE) {
638 ERROR("Directory entry has invalid length of "
639 "%"PRIu64" bytes\n", dentry->length);
640 return WIMLIB_ERR_INVALID_DENTRY;
643 p = get_u32(p, &dentry->attributes);
644 #ifdef ENABLE_SECURITY_DATA
645 p = get_u32(p, (u32*)&dentry->security_id);
649 p = get_u64(p, &dentry->subdir_offset);
651 /* 2 unused fields */
652 p += 2 * sizeof(u64);
654 p = get_u64(p, &dentry->creation_time);
655 p = get_u64(p, &dentry->last_access_time);
656 p = get_u64(p, &dentry->last_write_time);
658 p = get_bytes(p, WIM_HASH_SIZE, dentry->hash);
660 /* Currently ignoring reparse_tag. */
663 /* The reparse_reserved field does not actually exist. */
665 p = get_u64(p, &dentry->hard_link);
667 /* Currently ignoring streams. */
670 p = get_u16(p, &short_name_len);
671 p = get_u16(p, &file_name_len);
673 calculated_size = WIM_DENTRY_DISK_SIZE + file_name_len + 2 +
676 if (dentry->length < calculated_size) {
677 ERROR("Unexpected end of directory entry! (Expected "
678 "%"PRIu64" bytes, got %"PRIu64" bytes. "
679 "short_name_len = %hu, file_name_len = %hu)\n",
680 calculated_size, dentry->length,
681 short_name_len, file_name_len);
682 return WIMLIB_ERR_INVALID_DENTRY;
685 /* Read the filename. */
686 file_name = MALLOC(file_name_len);
688 ERROR("Failed to allocate %hu bytes for dentry file name!\n",
690 return WIMLIB_ERR_NOMEM;
692 p = get_bytes(p, file_name_len, file_name);
694 /* Convert filename to UTF-8. */
695 file_name_utf8 = utf16_to_utf8(file_name, file_name_len,
696 &file_name_utf8_len);
698 if (!file_name_utf8) {
699 ERROR("Failed to allocate memory to convert UTF16 "
700 "filename (%hu bytes) to UTF8\n",
705 /* Undocumented padding between file name and short name. This probably
706 * is supposed to be a terminating NULL character. */
709 /* Read the short filename. */
710 short_name = MALLOC(short_name_len);
712 ERROR("Failed to allocate %hu bytes for short filename\n",
717 get_bytes(p, short_name_len, short_name);
719 dentry->file_name = file_name;
720 dentry->file_name_utf8 = file_name_utf8;
721 dentry->short_name = short_name;
722 dentry->file_name_len = file_name_len;
723 dentry->file_name_utf8_len = file_name_utf8_len;
724 dentry->short_name_len = short_name_len;
727 FREE(dentry->file_name_utf8);
729 FREE(dentry->file_name);
730 return WIMLIB_ERR_NOMEM;
734 * Writes a dentry to an output buffer.
736 * @dentry: The dentry structure.
737 * @p: The memory location to write the data to.
738 * @return: True on success, false on failure.
740 static u8 *write_dentry(const struct dentry *dentry, u8 *p)
743 memset(p, 0, dentry->length);
744 p = put_u64(p, dentry->length);
745 p = put_u32(p, dentry->attributes);
746 #ifdef ENABLE_SECURITY_DATA
747 p = put_u32(p, dentry->security_id);
749 p = put_u32(p, (u32)(-1));
751 p = put_u64(p, dentry->subdir_offset);
752 p = put_u64(p, 0); /* unused1 */
753 p = put_u64(p, 0); /* unused2 */
754 p = put_u64(p, dentry->creation_time);
755 p = put_u64(p, dentry->last_access_time);
756 p = put_u64(p, dentry->last_write_time);
757 if (!is_empty_file_hash(dentry->hash))
758 memcpy(p, dentry->hash, WIM_HASH_SIZE);
760 DEBUG("zero hash for %s\n", dentry->file_name_utf8);
762 p = put_u32(p, 0); /* reparse_tag */
763 p = put_u64(p, dentry->hard_link);
764 p = put_u16(p, 0); /*streams */
765 p = put_u16(p, dentry->short_name_len);
766 p = put_u16(p, dentry->file_name_len);
767 p = put_bytes(p, dentry->file_name_len, (u8*)dentry->file_name);
768 p = put_u16(p, 0); /* filename padding, 2 bytes. */
769 p = put_bytes(p, dentry->short_name_len, (u8*)dentry->short_name);
770 return orig_p + dentry->length;
773 /* Recursive function that writes a dentry tree rooted at @tree, not including
774 * @tree itself, which has already been written, except in the case of the root
775 * dentry, which is written right away, along with an end-of-directory entry. */
776 u8 *write_dentry_tree(const struct dentry *tree, u8 *p)
778 const struct dentry *child;
780 if (dentry_is_root(tree)) {
781 p = write_dentry(tree, p);
783 /* write end of directory entry */
786 /* Nothing to do for a regular file. */
787 if (dentry_is_regular_file(tree))
791 /* Write child dentries and end-of-directory entry. */
792 child = tree->children;
795 p = write_dentry(child, p);
797 } while (child != tree->children);
800 /* write end of directory entry */
803 /* Recurse on children. */
806 p = write_dentry_tree(child, p);
808 } while (child != tree->children);
813 /* Reads the children of a dentry, and all their children, ..., etc. from the
814 * metadata resource and into the dentry tree.
816 * @metadata_resource: An array that contains the uncompressed metadata
817 * resource for the WIM file.
818 * @metadata_resource_len: The length of @metadata_resource.
819 * @dentry: A pointer to a struct dentry that is the root of the directory tree
820 * and has already been read from the metadata resource. It does not
821 * need to be the real root, because this procedure is called
823 * @return: True on success, false on failure.
825 int read_dentry_tree(const u8 metadata_resource[], u64 metadata_resource_len,
826 struct dentry *dentry)
828 u64 cur_offset = dentry->subdir_offset;
829 struct dentry *prev_child = NULL;
830 struct dentry *first_child = NULL;
831 struct dentry *child;
832 struct dentry cur_child;
835 /* If @dentry is a regular file, nothing more needs to be done for this
840 /* Find and read all the children of @dentry. */
843 /* Read next child of @dentry into @cur_child. */
844 ret = read_dentry(metadata_resource, metadata_resource_len,
845 cur_offset, &cur_child);
849 /* Check for end of directory. */
850 if (cur_child.length == 0) {
855 /* Not end of directory. Allocate this child permanently and
856 * link it to the parent and previous child. */
857 child = MALLOC(sizeof(struct dentry));
859 ERROR("Failed to allocate %zu bytes for new dentry!\n",
860 sizeof(struct dentry));
861 ret = WIMLIB_ERR_NOMEM;
864 memcpy(child, &cur_child, sizeof(struct dentry));
867 prev_child->next = child;
868 child->prev = prev_child;
873 child->parent = dentry;
876 /* If there are children of this child, call this procedure
878 if (child->subdir_offset != 0) {
879 ret = read_dentry_tree(metadata_resource,
880 metadata_resource_len, child);
885 /* Advance to the offset of the next child. */
886 cur_offset += child->length;
889 /* Link last child to first one, and set parent's
890 * children pointer to the first child. */
892 prev_child->next = first_child;
893 first_child->prev = prev_child;
895 dentry->children = first_child;