DragonFly On-Line Manual Pages
ARCHIVE_WRITE_DISK(3) DragonFly Library Functions Manual
archive_write_disk_set_user_lookup - functions for creating objects on
Streaming Archive Library (libarchive, -larchive)
struct archive *
archive_write_disk_set_options(struct archive *, int flags);
archive_write_disk_set_skip_file(struct archive *, dev_t, ino_t);
archive_write_disk_set_group_lookup(struct archive *, void *,
gid_t (*)(void *, const char *gname, gid_t gid),
void (*cleanup)(void *));
archive_write_disk_set_standard_lookup(struct archive *);
archive_write_disk_set_user_lookup(struct archive *, void *,
uid_t (*)(void *, const char *uname, uid_t uid),
void (*cleanup)(void *));
These functions provide a complete API for creating objects on disk from
struct archive_entry descriptions. They are most naturally used when
extracting objects from an archive using the archive_read() interface.
The general process is to read struct archive_entry objects from an
archive, then write those objects to a struct archive object created
using the archive_write_disk() family functions. This interface is
deliberately very similar to the archive_write() interface used to write
objects to a streaming archive.
Allocates and initializes a struct archive object suitable for
writing objects to disk.
Records the device and inode numbers of a file that should not be
overwritten. This is typically used to ensure that an extraction
process does not overwrite the archive from which objects are
being read. This capability is technically unnecessary but can
be a significant performance optimization in practice.
The options field consists of a bitwise OR of one or more of the
Attempt to restore Access Control Lists. By default,
extended ACLs are ignored.
Before removing a file system object prior to replacing
it, clear platform-specific file flags which might
prevent its removal.
Attempt to restore file attributes (file flags). By
default, file attributes are ignored. See chattr(1)
(Linux) or chflags(1) (FreeBSD, Mac OS X) for more
information on file attributes.
Mac OS X specific. Restore metadata using copyfile(3).
By default, copyfile(3) metadata is ignored.
Existing files on disk will not be overwritten. By
default, existing regular files are truncated and
overwritten; existing directories will have their
permissions updated; other pre-existing objects are
unlinked and recreated from scratch.
The user and group IDs should be set on the restored
file. By default, the user and group IDs are not
Full permissions (including SGID, SUID, and sticky bits)
should be restored exactly as specified, without obeying
the current umask. Note that SUID and SGID bits can only
be restored if the user and group ID of the object on
disk are correct. If ARCHIVE_EXTRACT_OWNER is not
specified, then SUID and SGID bits will only be restored
if the default user and group IDs of newly-created
objects on disk happen to match those specified in the
archive entry. By default, only basic permissions are
restored, and umask is obeyed.
Extract files atomically, by first creating a unique
temporary file and then renaming it to its required
destination name. This avoids a race where an
application might see a partial file (or no file) during
Refuse to extract an absolute path. The default is to
not refuse such paths.
Refuse to extract a path that contains a .. element
anywhere within it. The default is to not refuse such
paths. Note that paths ending in .. always cause an
error, regardless of this flag.
Refuse to extract any object whose final location would
be altered by a symlink on disk. This is intended to
help guard against a variety of mischief caused by
archives that (deliberately or otherwise) extract files
outside of the current directory. The default is not to
perform this check. If ARCHIVE_EXTRACT_UNLINK is
specified together with this option, the library will
remove any intermediate symlinks it finds and return an
error only if such symlink could not be removed.
Scan data for blocks of NUL bytes and try to recreate
them with holes. This results in sparse files,
independent of whether the archive format supports or
The timestamps (mtime, ctime, and atime) should be
restored. By default, they are ignored. Note that
restoring of atime is not currently supported.
Existing files on disk will be unlinked before any
attempt to create them. In some cases, this can prove to
be a significant performance improvement. By default,
existing files are truncated and rewritten, but the file
is not recreated. In particular, the default behavior
does not break existing hard links.
Attempt to restore extended file attributes. By default,
they are ignored. See xattr(7) (Linux), xattr(2) (Mac OS
X), or getextattr(8) (FreeBSD) for more information on
extended file attributes.
The struct archive_entry objects contain both names and ids that
can be used to identify users and groups. These names and ids
describe the ownership of the file itself and also appear in ACL
lists. By default, the library uses the ids and ignores the
names, but this can be overridden by registering user and group
lookup functions. To register, you must provide a lookup
function which accepts both a name and id and returns a suitable
id. You may also provide a void * pointer to a private data
structure and a cleanup function for that data. The cleanup
function will be invoked when the struct archive object is
This convenience function installs a standard set of user and
group lookup functions. These functions use getpwnam(3) and
getgrnam(3) to convert names to ids, defaulting to the ids if the
names cannot be looked up. These functions also implement a
simple memory cache to reduce the number of calls to getpwnam(3)
More information about the struct archive object and the overall design
of the library can be found in the libarchive(3) overview. Many of these
functions are also documented under archive_write(3).
Most functions return ARCHIVE_OK (zero) on success, or one of several
non-zero error codes for errors. Specific error codes include:
ARCHIVE_RETRY for operations that might succeed if retried, ARCHIVE_WARN
for unusual conditions that do not prevent further operations, and
ARCHIVE_FATAL for serious errors that make remaining operations
archive_write_disk_new() returns a pointer to a newly-allocated struct
archive_write_data() returns a count of the number of bytes actually
written, or -1 on error.
Detailed error codes and textual descriptions are available from the
archive_errno() and archive_error_string() functions.
tar(1), archive_read(3), archive_write(3), libarchive(3)
The libarchive library first appeared in FreeBSD 5.3. The
archive_write_disk interface was added to libarchive 2.0 and first
appeared in FreeBSD 6.3.
The libarchive library was written by Tim Kientzle <email@example.com>.
Directories are actually extracted in two distinct phases. Directories
are created during archive_write_header(), but final permissions are not
set until archive_write_close(). This separation is necessary to
correctly handle borderline cases such as a non-writable directory
containing files, but can cause unexpected results. In particular,
directory permissions are not fully restored until the archive is closed.
If you use chdir(2) to change the current directory between calls to
archive_read_extract() or before calling archive_read_close(), you may
confuse the permission-setting logic with the result that directory
permissions are restored incorrectly.
The library attempts to create objects with filenames longer than
PATH_MAX by creating prefixes of the full path and changing the current
directory. Currently, this logic is limited in scope; the fixup pass
does not work correctly for such objects and the symlink security check
option disables the support for very long pathnames.
Restoring the path aa/../bb does create each intermediate directory. In
particular, the directory aa is created as well as the final object bb.
In theory, this can be exploited to create an entire directory hierarchy
with a single request. Of course, this does not work if the
ARCHIVE_EXTRACT_NODOTDOT option is specified.
Implicit directories are always created obeying the current umask.
Explicit objects are created obeying the current umask unless
ARCHIVE_EXTRACT_PERM is specified, in which case they current umask is
SGID and SUID bits are restored only if the correct user and group could
be set. If ARCHIVE_EXTRACT_OWNER is not specified, then no attempt is
made to set the ownership. In this case, SGID and SUID bits are restored
only if the user and group of the final object happen to match those
specified in the entry.
The "standard" user-id and group-id lookup functions are not the defaults
because getgrnam(3) and getpwnam(3) are sometimes too large for
particular applications. The current design allows the application
author to use a more compact implementation when appropriate.
There should be a corresponding archive_read_disk interface that walks a
directory hierarchy and returns archive entry objects.
DragonFly 6.3-DEVELOPMENT January 19, 2020 DragonFly 6.3-DEVELOPMENT