archive_write_data_block3

Usage

Library

Streaming Archive Library (libarchive, -larchive)

Overview

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.