|ARCHIVE_WRITE_DISK(3)||Library Functions Manual||ARCHIVE_WRITE_DISK(3)|
functions for creating objects on disk
Streaming Archive Library (libarchive, -larchive)
struct archive *
archive *, int
archive *, dev_t,
archive *, void *, gid_t
(*)(void *, const char *gname, gid_t gid), void
archive *, void *, uid_t
(*)(void *, const char *uname, uid_t uid), 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
interface. The general process is to read struct archive_entry objects from
an archive, then write those objects to a struct archive object created
family functions. This interface is deliberately very similar to the
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 following
- 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 restored.
- 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_OWNERis 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 extraction.
- 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
- 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_UNLINKis 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.
- 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 destroyed.
- 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).
Most functions return
ARCHIVE_OK (zero) on
success, or one of several non-zero error codes for errors. Specific error
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
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
Detailed error codes and textual descriptions are available from
libarchive library first appeared in
FreeBSD 5.3. The
archive_write_disk interface was added to
libarchive 2.0 and first appeared in
libarchive library was written by
Tim Kientzle ⟨firstname.lastname@example.org⟩.
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
archive_read_extract() or before calling
archive_read_close(), you may confuse the
permission-setting logic with the result that directory permissions are
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
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.
|January 19, 2020||Debian|