Tribblix: manual page: archive_write

ARCHIVE_WRITE_DISK(3) Introduction to Library Functions

NAME

archive_write_disk_new, archive_write_disk_set_options,
archive_write_disk_set_skip_file, archive_write_disk_set_group_lookup,
archive_write_disk_set_standard_lookup,
archive_write_disk_set_user_lookup - functions for creating objects on
disk

LIBRARY

Streaming Archive Library (libarchive, -larchive)

SYNOPSIS

#include <archive.h>

struct archive *
archive_write_disk_new(void);

int
archive_write_disk_set_options(struct archive *, int flags);

int
archive_write_disk_set_skip_file(struct archive *, dev_t, ino_t);

int
archive_write_disk_set_group_lookup(struct archive *, void *,
gid_t (*)(void *, const char *gname, gid_t gid),
void (*cleanup)(void *));

int
archive_write_disk_set_standard_lookup(struct archive *);

int
archive_write_disk_set_user_lookup(struct archive *, void *,
uid_t (*)(void *, const char *uname, uid_t uid),
void (*cleanup)(void *));

DESCRIPTION

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.

archive_write_disk_new()
Allocates and initializes a struct archive object suitable for
writing objects to disk.

archive_write_disk_set_skip_file()
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.

archive_write_disk_set_options()
The options field consists of a bitwise OR of one or more of
the following values:
ARCHIVE_EXTRACT_ACL
Attempt to restore Access Control Lists. By default,
extended ACLs are ignored.
ARCHIVE_EXTRACT_CLEAR_NOCHANGE_FFLAGS
Before removing a file system object prior to replacing
it, clear platform-specific file flags which might
prevent its removal.
ARCHIVE_EXTRACT_FFLAGS
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.
ARCHIVE_EXTRACT_MAC_METADATA
Mac OS X specific. Restore metadata using copyfile(3).
By default, copyfile(3) metadata is ignored.
ARCHIVE_EXTRACT_NO_OVERWRITE
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.
ARCHIVE_EXTRACT_OWNER
The user and group IDs should be set on the restored
file. By default, the user and group IDs are not
restored.
ARCHIVE_EXTRACT_PERM
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.
ARCHIVE_EXTRACT_SAFE_WRITES
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 extraction.
ARCHIVE_EXTRACT_SECURE_NOABSOLUTEPATHS
Refuse to extract an absolute path. The default is to
not refuse such paths.
ARCHIVE_EXTRACT_SECURE_NODOTDOT
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.
ARCHIVE_EXTRACT_SECURE_SYMLINKS
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.
ARCHIVE_EXTRACT_SPARSE
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
uses them.
ARCHIVE_EXTRACT_TIME
The timestamps (mtime, ctime, and atime) should be
restored. By default, they are ignored. Note that
restoring of atime is not currently supported.
ARCHIVE_EXTRACT_UNLINK
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.
ARCHIVE_EXTRACT_XATTR
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.

archive_write_disk_set_group_lookup(),
archive_write_disk_set_user_lookup()
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 destroyed.

archive_write_disk_set_standard_lookup()
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) and getgrnam(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).

RETURN VALUES

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 impossible.

archive_write_disk_new() returns a pointer to a newly-allocated struct
archive object.

archive_write_data() returns a count of the number of bytes actually
written, or -1 on error.

ERRORS

Detailed error codes and textual descriptions are available from the
archive_errno() and archive_error_string() functions.

HISTORY

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.

AUTHORS

The libarchive library was written by Tim Kientzle <kientzle@acm.org>.

BUGS

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
ignored.

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.

illumos January 19, 2020 illumos