archive_write_disk(3) BSD Library Functions Manual archive_write_disk(3)
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, archive_write_header, archive_write_data,
archive_write_finish_entry, archive_write_close, archive_write_finish -- functions for creating objects on disk
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 *));
int
archive_write_header(struct archive *, struct archive_entry *);
ssize_t
archive_write_data(struct archive *, const void *, size_t);
int
archive_write_finish_entry(struct archive *);
int
archive_write_close(struct archive *);
int
archive_write_finish(struct archive *);
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_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_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_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_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 particu-
lar, the default behavior does not break existing hard links.
ARCHIVE_EXTRACT_ACL
Attempt to restore ACLs. By default, extended ACLs are ignored.
ARCHIVE_EXTRACT_FFLAGS
Attempt to restore extended file flags. By default, file flags are ignored.
ARCHIVE_EXTRACT_XATTR
Attempt to restore POSIX.1e extended attributes. By default, they are ignored.
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 direc-
tory. 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_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_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_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).
archive_write_header()
Build and write a header using the data in the provided struct archive_entry structure. See archive_entry(3) for information on cre-
ating and populating struct archive_entry objects.
archive_write_data()
Write data corresponding to the header just written. Returns number of bytes written or -1 on error.
archive_write_finish_entry()
Close out the entry just written. Ordinarily, clients never need to call this, as it is called automatically by
archive_write_next_header() and archive_write_close() as needed.
archive_write_close()
Set any attributes that could not be set during the initial restore. For example, directory timestamps are not restored initially
because restoring a subsequent file would alter that timestamp. Similarly, non-writable directories are initially created with write
permissions (so that their contents can be restored). The archive_write_disk_new library maintains a list of all such deferred
attributes and sets them when this function is invoked.
archive_write_finish()
Invokes archive_write_close() if it was not invoked manually, then releases all resources.
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. The archive_errno() and archive_error_string() functions can be
used to retrieve an appropriate error code and a textual error message.
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. On error, -1 is returned and the archive_errno() and
archive_error_string() functions will return appropriate values.
SEE ALSO
archive_read(3), archive_write(3), tar(1), libarchive(3)
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 con-
taining 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 heirarchy 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 heirarchy and returns archive entry objects.
BSD
August 5, 2008 BSD