libarchive/archive_write_disk.3

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.Dd January 19, 2020
.Dt ARCHIVE_WRITE_DISK 3
.Os
.Sh NAME
.Nm archive_write_disk_new ,
.Nm archive_write_disk_set_options ,
.Nm archive_write_disk_set_skip_file ,
.Nm archive_write_disk_set_group_lookup ,
.Nm archive_write_disk_set_standard_lookup ,
.Nm archive_write_disk_set_user_lookup
.Nd functions for creating objects on disk
.Sh LIBRARY
Streaming Archive Library (libarchive, -larchive)
.Sh SYNOPSIS
.In archive.h
.Ft struct archive *
.Fn archive_write_disk_new "void"
.Ft int
.Fn archive_write_disk_set_options "struct archive *" "int flags"
.Ft int
.Fn archive_write_disk_set_skip_file "struct archive *" "dev_t" "ino_t"
.Ft int
.Fo archive_write_disk_set_group_lookup
.Fa "struct archive *"
.Fa "void *"
.Fa "gid_t (*)(void *, const char *gname, gid_t gid)"
.Fa "void (*cleanup)(void *)"
.Fc
.Ft int
.Fn archive_write_disk_set_standard_lookup "struct archive *"
.Ft int
.Fo archive_write_disk_set_user_lookup
.Fa "struct archive *"
.Fa "void *"
.Fa "uid_t (*)(void *, const char *uname, uid_t uid)"
.Fa "void (*cleanup)(void *)"
.Fc
.Sh DESCRIPTION
These functions provide a complete API for creating objects on
disk from
.Tn struct archive_entry
descriptions.
They are most naturally used when extracting objects from an archive
using the
.Fn archive_read
interface.
The general process is to read
.Tn struct archive_entry
objects from an archive, then write those objects to a
.Tn struct archive
object created using the
.Fn archive_write_disk
family functions.
This interface is deliberately very similar to the
.Fn archive_write
interface used to write objects to a streaming archive.
.Bl -tag -width indent
.It Fn archive_write_disk_new
Allocates and initializes a
.Tn struct archive
object suitable for writing objects to disk.
.It Fn 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.
.It Fn archive_write_disk_set_options
The options field consists of a bitwise OR of one or more of the
following values:
.Bl -tag -compact -width "indent"
.It Cm ARCHIVE_EXTRACT_ACL
Attempt to restore Access Control Lists.
By default, extended ACLs are ignored.
.It Cm 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.
.It Cm ARCHIVE_EXTRACT_FFLAGS
Attempt to restore file attributes (file flags).
By default, file attributes are ignored.
See
.Xr chattr 1
.Pq Linux
or
.Xr chflags 1
.Pq FreeBSD, Mac OS X
for more information on file attributes.
.It Cm ARCHIVE_EXTRACT_MAC_METADATA
Mac OS X specific.
Restore metadata using
.Xr copyfile 3 .
By default,
.Xr copyfile 3
metadata is ignored.
.It Cm 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.
.It Cm 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.
.It Cm 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
.Cm 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.
.It Cm 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.
.It Cm ARCHIVE_EXTRACT_SECURE_NOABSOLUTEPATHS
Refuse to extract an absolute path.
The default is to not refuse such paths.
.It Cm ARCHIVE_EXTRACT_SECURE_NODOTDOT
Refuse to extract a path that contains a
.Pa ..
element anywhere within it.
The default is to not refuse such paths.
Note that paths ending in
.Pa ..
always cause an error, regardless of this flag.
.It Cm 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
.Cm 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.
.It Cm 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.
.It Cm 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.
.It Cm 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.
.It Cm ARCHIVE_EXTRACT_XATTR
Attempt to restore extended file attributes.
By default, they are ignored.
See
.Xr xattr 7
.Pq Linux ,
.Xr xattr 2
.Pq Mac OS X ,
or
.Xr getextattr 8
.Pq FreeBSD
for more information on extended file attributes.
.El
.It Xo
.Fn archive_write_disk_set_group_lookup ,
.Fn archive_write_disk_set_user_lookup
.Xc
The
.Tn 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
.Tn void *
pointer to a private data structure and a cleanup function for
that data.
The cleanup function will be invoked when the
.Tn struct archive
object is destroyed.
.It Fn archive_write_disk_set_standard_lookup
This convenience function installs a standard set of user
and group lookup functions.
These functions use
.Xr getpwnam 3
and
.Xr 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
.Xr getpwnam 3
and
.Xr getgrnam 3 .
.El
More information about the
.Va struct archive
object and the overall design of the library can be found in the
.Xr libarchive 3
overview.
Many of these functions are also documented under
.Xr archive_write 3 .
.Sh RETURN VALUES
Most functions return
.Cm ARCHIVE_OK
(zero) on success, or one of several non-zero
error codes for errors.
Specific error codes include:
.Cm ARCHIVE_RETRY
for operations that might succeed if retried,
.Cm ARCHIVE_WARN
for unusual conditions that do not prevent further operations, and
.Cm ARCHIVE_FATAL
for serious errors that make remaining operations impossible.
.Pp
.Fn archive_write_disk_new
returns a pointer to a newly-allocated
.Tn struct archive
object.
.Pp
.Fn archive_write_data
returns a count of the number of bytes actually written,
or
.Li -1
on error.
.\"
.Sh ERRORS
Detailed error codes and textual descriptions are available from the
.Fn archive_errno
and
.Fn archive_error_string
functions.
.\"
.Sh SEE ALSO
.Xr tar 1 ,
.Xr archive_read 3 ,
.Xr archive_write 3 ,
.Xr libarchive 3
.Sh HISTORY
The
.Nm libarchive
library first appeared in
.Fx 5.3 .
The
.Nm archive_write_disk
interface was added to
.Nm libarchive 2.0
and first appeared in
.Fx 6.3 .
.Sh AUTHORS
.An -nosplit
The
.Nm libarchive
library was written by
.An Tim Kientzle Aq kientzle@acm.org .
.Sh BUGS
Directories are actually extracted in two distinct phases.
Directories are created during
.Fn archive_write_header ,
but final permissions are not set until
.Fn 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
.Xr chdir 2
to change the current directory between calls to
.Fn archive_read_extract
or before calling
.Fn archive_read_close ,
you may confuse the permission-setting logic with
the result that directory permissions are restored
incorrectly.
.Pp
The library attempts to create objects with filenames longer than
.Cm 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.
.Pp
Restoring the path
.Pa aa/../bb
does create each intermediate directory.
In particular, the directory
.Pa aa
is created as well as the final object
.Pa 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
.Cm ARCHIVE_EXTRACT_NODOTDOT
option is specified.
.Pp
Implicit directories are always created obeying the current umask.
Explicit objects are created obeying the current umask unless
.Cm ARCHIVE_EXTRACT_PERM
is specified, in which case they current umask is ignored.
.Pp
SGID and SUID bits are restored only if the correct user and
group could be set.
If
.Cm 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.
.Pp
The
.Dq standard
user-id and group-id lookup functions are not the defaults because
.Xr getgrnam 3
and
.Xr getpwnam 3
are sometimes too large for particular applications.
The current design allows the application author to use a more
compact implementation when appropriate.
.Pp
There should be a corresponding
.Nm archive_read_disk
interface that walks a directory hierarchy and returns archive
entry objects.