archive_write_disk

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 ⟨[email protected]⟩.

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.

January 19, 2020

Debian