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.Dd April 3, 2017

.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_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

.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.

.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_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 archive_read 3 ,

.Xr archive_write 3 ,

.Xr tar 1 ,

.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.