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_data_block
,
archive_write_finish_entry
,
archive_write_close
,
archive_write_finish
archive_write_free
—
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 *));
int
archive_write_header
(
struct
archive *,
struct archive_entry
*);
ssize_t
archive_write_data
(
struct
archive *,
const
void *,
size_t);
ssize_t
archive_write_data_block
(
struct
archive *,
const
void *,
size_t
size,
int64_t
offset);
int
archive_write_finish_entry
(
struct
archive *);
int
archive_write_close
(
struct
archive *);
int
archive_write_finish
(
struct
archive *);
int
archive_write_free
(
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:
- 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_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.
- 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 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.
- 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 ACLs. By default, extended ACLs are ignored.
- Attempt to restore extended file flags. By default, file flags are
ignored.
- Attempt to restore POSIX.1e extended attributes. By default, they are
ignored.
- 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.
- 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 an absolute path. The default is to not refuse such
paths.
- 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 creating 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_data_block
()
- Write data corresponding to the header just written. This is like
archive_write_data
() except that it
performs a seek on the file being written to the specified offset before
writing the data. This is useful when restoring sparse files from archive
formats that support sparse files. Returns number of bytes written or -1
on error. (Note: This is currently not supported for archive_write
handles, only for archive_write_disk handles.)
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.
However, some file attributes are written to disk only after the file is
closed, so this can be necessary if you need to work with the file on disk
right away.
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
()
- This is a deprecated synonym for
archive_write_free
().
archive_write_free
()
- 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.
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.
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 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.