13.6. "tarfile" — Read and write tar archive files
**************************************************

**Source code:** Lib/tarfile.py

======================================================================

The "tarfile" module makes it possible to read and write tar archives,
including those using gzip, bz2 and lzma compression. Use the
"zipfile" module to read or write ".zip" files, or the higher-level
functions in shutil.

Some facts and figures:

* reads and writes "gzip", "bz2" and "lzma" compressed archives if
  the respective modules are available.

* read/write support for the POSIX.1-1988 (ustar) format.

* read/write support for the GNU tar format including *longname* and
  *longlink* extensions, read-only support for all variants of the
  *sparse* extension including restoration of sparse files.

* read/write support for the POSIX.1-2001 (pax) format.

* handles directories, regular files, hardlinks, symbolic links,
  fifos, character devices and block devices and is able to acquire
  and restore file information like timestamp, access permissions and
  owner.

Changed in version 3.3: Added support for "lzma" compression.

tarfile.open(name=None, mode='r', fileobj=None, bufsize=10240, **kwargs)

   Return a "TarFile" object for the pathname *name*. For detailed
   information on "TarFile" objects and the keyword arguments that are
   allowed, see TarFile Objects.

   *mode* has to be a string of the form "'filemode[:compression]'",
   it defaults to "'r'". Here is a full list of mode combinations:

   +--------------------+-----------------------------------------------+
   | mode               | action                                        |
   +====================+===============================================+
   | "'r' or 'r:*'"     | Open for reading with transparent compression |
   |                    | (recommended).                                |
   +--------------------+-----------------------------------------------+
   | "'r:'"             | Open for reading exclusively without          |
   |                    | compression.                                  |
   +--------------------+-----------------------------------------------+
   | "'r:gz'"           | Open for reading with gzip compression.       |
   +--------------------+-----------------------------------------------+
   | "'r:bz2'"          | Open for reading with bzip2 compression.      |
   +--------------------+-----------------------------------------------+
   | "'r:xz'"           | Open for reading with lzma compression.       |
   +--------------------+-----------------------------------------------+
   | "'x'" or "'x:'"    | Create a tarfile exclusively without          |
   |                    | compression. Raise an "FileExistsError"       |
   |                    | exception if it already exists.               |
   +--------------------+-----------------------------------------------+
   | "'x:gz'"           | Create a tarfile with gzip compression. Raise |
   |                    | an "FileExistsError" exception if it already  |
   |                    | exists.                                       |
   +--------------------+-----------------------------------------------+
   | "'x:bz2'"          | Create a tarfile with bzip2 compression.      |
   |                    | Raise an "FileExistsError" exception if it    |
   |                    | already exists.                               |
   +--------------------+-----------------------------------------------+
   | "'x:xz'"           | Create a tarfile with lzma compression. Raise |
   |                    | an "FileExistsError" exception if it already  |
   |                    | exists.                                       |
   +--------------------+-----------------------------------------------+
   | "'a' or 'a:'"      | Open for appending with no compression. The   |
   |                    | file is created if it does not exist.         |
   +--------------------+-----------------------------------------------+
   | "'w' or 'w:'"      | Open for uncompressed writing.                |
   +--------------------+-----------------------------------------------+
   | "'w:gz'"           | Open for gzip compressed writing.             |
   +--------------------+-----------------------------------------------+
   | "'w:bz2'"          | Open for bzip2 compressed writing.            |
   +--------------------+-----------------------------------------------+
   | "'w:xz'"           | Open for lzma compressed writing.             |
   +--------------------+-----------------------------------------------+

   Note that "'a:gz'", "'a:bz2'" or "'a:xz'" is not possible. If
   *mode* is not suitable to open a certain (compressed) file for
   reading, "ReadError" is raised. Use *mode* "'r'" to avoid this.  If
   a compression method is not supported, "CompressionError" is
   raised.

   If *fileobj* is specified, it is used as an alternative to a *file
   object* opened in binary mode for *name*. It is supposed to be at
   position 0.

   For modes "'w:gz'", "'r:gz'", "'w:bz2'", "'r:bz2'", "'x:gz'",
   "'x:bz2'", "tarfile.open()" accepts the keyword argument
   *compresslevel* (default "9") to specify the compression level of
   the file.

   For special purposes, there is a second format for *mode*:
   "'filemode|[compression]'".  "tarfile.open()" will return a
   "TarFile" object that processes its data as a stream of blocks.  No
   random seeking will be done on the file. If given, *fileobj* may be
   any object that has a "read()" or "write()" method (depending on
   the *mode*). *bufsize* specifies the blocksize and defaults to "20
   * 512" bytes. Use this variant in combination with e.g.
   "sys.stdin", a socket *file object* or a tape device. However, such
   a "TarFile" object is limited in that it does not allow random
   access, see Examples.  The currently possible modes:

   +---------------+----------------------------------------------+
   | Mode          | Action                                       |
   +===============+==============================================+
   | "'r|*'"       | Open a *stream* of tar blocks for reading    |
   |               | with transparent compression.                |
   +---------------+----------------------------------------------+
   | "'r|'"        | Open a *stream* of uncompressed tar blocks   |
   |               | for reading.                                 |
   +---------------+----------------------------------------------+
   | "'r|gz'"      | Open a gzip compressed *stream* for reading. |
   +---------------+----------------------------------------------+
   | "'r|bz2'"     | Open a bzip2 compressed *stream* for         |
   |               | reading.                                     |
   +---------------+----------------------------------------------+
   | "'r|xz'"      | Open an lzma compressed *stream* for         |
   |               | reading.                                     |
   +---------------+----------------------------------------------+
   | "'w|'"        | Open an uncompressed *stream* for writing.   |
   +---------------+----------------------------------------------+
   | "'w|gz'"      | Open a gzip compressed *stream* for writing. |
   +---------------+----------------------------------------------+
   | "'w|bz2'"     | Open a bzip2 compressed *stream* for         |
   |               | writing.                                     |
   +---------------+----------------------------------------------+
   | "'w|xz'"      | Open an lzma compressed *stream* for         |
   |               | writing.                                     |
   +---------------+----------------------------------------------+

   Changed in version 3.5: The "'x'" (exclusive creation) mode was
   added.

class tarfile.TarFile

   Class for reading and writing tar archives. Do not use this class
   directly: use "tarfile.open()" instead. See TarFile Objects.

tarfile.is_tarfile(name)

   Return "True" if *name* is a tar archive file, that the "tarfile"
   module can read.

The "tarfile" module defines the following exceptions:

exception tarfile.TarError

   Base class for all "tarfile" exceptions.

exception tarfile.ReadError

   Is raised when a tar archive is opened, that either cannot be
   handled by the "tarfile" module or is somehow invalid.

exception tarfile.CompressionError

   Is raised when a compression method is not supported or when the
   data cannot be decoded properly.

exception tarfile.StreamError

   Is raised for the limitations that are typical for stream-like
   "TarFile" objects.

exception tarfile.ExtractError

   Is raised for *non-fatal* errors when using "TarFile.extract()",
   but only if "TarFile.errorlevel""== 2".

exception tarfile.HeaderError

   Is raised by "TarInfo.frombuf()" if the buffer it gets is invalid.

The following constants are available at the module level:

tarfile.ENCODING

   The default character encoding: "'utf-8'" on Windows, the value
   returned by "sys.getfilesystemencoding()" otherwise.

Each of the following constants defines a tar archive format that the
"tarfile" module is able to create. See section Supported tar formats
for details.

tarfile.USTAR_FORMAT

   POSIX.1-1988 (ustar) format.

tarfile.GNU_FORMAT

   GNU tar format.

tarfile.PAX_FORMAT

   POSIX.1-2001 (pax) format.

tarfile.DEFAULT_FORMAT

   The default format for creating archives. This is currently
   "GNU_FORMAT".

See also:

  Module "zipfile"
     Documentation of the "zipfile" standard module.

  Archiving operations
     Documentation of the higher-level archiving facilities provided
     by the standard "shutil" module.

  GNU tar manual, Basic Tar Format
     Documentation for tar archive files, including GNU tar
     extensions.


13.6.1. TarFile Objects
=======================

The "TarFile" object provides an interface to a tar archive. A tar
archive is a sequence of blocks. An archive member (a stored file) is
made up of a header block followed by data blocks. It is possible to
store a file in a tar archive several times. Each archive member is
represented by a "TarInfo" object, see TarInfo Objects for details.

A "TarFile" object can be used as a context manager in a "with"
statement. It will automatically be closed when the block is
completed. Please note that in the event of an exception an archive
opened for writing will not be finalized; only the internally used
file object will be closed. See the Examples section for a use case.

New in version 3.2: Added support for the context management protocol.

class tarfile.TarFile(name=None, mode='r', fileobj=None, format=DEFAULT_FORMAT, tarinfo=TarInfo, dereference=False, ignore_zeros=False, encoding=ENCODING, errors='surrogateescape', pax_headers=None, debug=0, errorlevel=0)

   All following arguments are optional and can be accessed as
   instance attributes as well.

   *name* is the pathname of the archive. It can be omitted if
   *fileobj* is given. In this case, the file object’s "name"
   attribute is used if it exists.

   *mode* is either "'r'" to read from an existing archive, "'a'" to
   append data to an existing file, "'w'" to create a new file
   overwriting an existing one, or "'x'" to create a new file only if
   it does not already exist.

   If *fileobj* is given, it is used for reading or writing data. If
   it can be determined, *mode* is overridden by *fileobj*’s mode.
   *fileobj* will be used from position 0.

   Note: *fileobj* is not closed, when "TarFile" is closed.

   *format* controls the archive format. It must be one of the
   constants "USTAR_FORMAT", "GNU_FORMAT" or "PAX_FORMAT" that are
   defined at module level.

   The *tarinfo* argument can be used to replace the default "TarInfo"
   class with a different one.

   If *dereference* is "False", add symbolic and hard links to the
   archive. If it is "True", add the content of the target files to
   the archive. This has no effect on systems that do not support
   symbolic links.

   If *ignore_zeros* is "False", treat an empty block as the end of
   the archive. If it is "True", skip empty (and invalid) blocks and
   try to get as many members as possible. This is only useful for
   reading concatenated or damaged archives.

   *debug* can be set from "0" (no debug messages) up to "3" (all
   debug messages). The messages are written to "sys.stderr".

   If *errorlevel* is "0", all errors are ignored when using
   "TarFile.extract()". Nevertheless, they appear as error messages in
   the debug output, when debugging is enabled.  If "1", all *fatal*
   errors are raised as "OSError" exceptions. If "2", all *non-fatal*
   errors are raised as "TarError" exceptions as well.

   The *encoding* and *errors* arguments define the character encoding
   to be used for reading or writing the archive and how conversion
   errors are going to be handled. The default settings will work for
   most users. See section Unicode issues for in-depth information.

   The *pax_headers* argument is an optional dictionary of strings
   which will be added as a pax global header if *format* is
   "PAX_FORMAT".

   Changed in version 3.2: Use "'surrogateescape'" as the default for
   the *errors* argument.

   Changed in version 3.5: The "'x'" (exclusive creation) mode was
   added.

classmethod TarFile.open(...)

   Alternative constructor. The "tarfile.open()" function is actually
   a shortcut to this classmethod.

TarFile.getmember(name)

   Return a "TarInfo" object for member *name*. If *name* can not be
   found in the archive, "KeyError" is raised.

   Note: If a member occurs more than once in the archive, its last
     occurrence is assumed to be the most up-to-date version.

TarFile.getmembers()

   Return the members of the archive as a list of "TarInfo" objects.
   The list has the same order as the members in the archive.

TarFile.getnames()

   Return the members as a list of their names. It has the same order
   as the list returned by "getmembers()".

TarFile.list(verbose=True, *, members=None)

   Print a table of contents to "sys.stdout". If *verbose* is "False",
   only the names of the members are printed. If it is "True", output
   similar to that of **ls -l** is produced. If optional *members* is
   given, it must be a subset of the list returned by "getmembers()".

   Changed in version 3.5: Added the *members* parameter.

TarFile.next()

   Return the next member of the archive as a "TarInfo" object, when
   "TarFile" is opened for reading. Return "None" if there is no more
   available.

TarFile.extractall(path=".", members=None, *, numeric_owner=False)

   Extract all members from the archive to the current working
   directory or directory *path*. If optional *members* is given, it
   must be a subset of the list returned by "getmembers()". Directory
   information like owner, modification time and permissions are set
   after all members have been extracted. This is done to work around
   two problems: A directory’s modification time is reset each time a
   file is created in it. And, if a directory’s permissions do not
   allow writing, extracting files to it will fail.

   If *numeric_owner* is "True", the uid and gid numbers from the
   tarfile are used to set the owner/group for the extracted files.
   Otherwise, the named values from the tarfile are used.

   Warning: Never extract archives from untrusted sources without
     prior inspection. It is possible that files are created outside
     of *path*, e.g. members that have absolute filenames starting
     with ""/"" or filenames with two dots "".."".

   Changed in version 3.5: Added the *numeric_owner* parameter.

TarFile.extract(member, path="", set_attrs=True, *, numeric_owner=False)

   Extract a member from the archive to the current working directory,
   using its full name. Its file information is extracted as
   accurately as possible. *member* may be a filename or a "TarInfo"
   object. You can specify a different directory using *path*. File
   attributes (owner, mtime, mode) are set unless *set_attrs* is
   false.

   If *numeric_owner* is "True", the uid and gid numbers from the
   tarfile are used to set the owner/group for the extracted files.
   Otherwise, the named values from the tarfile are used.

   Note: The "extract()" method does not take care of several
     extraction issues. In most cases you should consider using the
     "extractall()" method.

   Warning: See the warning for "extractall()".

   Changed in version 3.2: Added the *set_attrs* parameter.

   Changed in version 3.5: Added the *numeric_owner* parameter.

TarFile.extractfile(member)

   Extract a member from the archive as a file object. *member* may be
   a filename or a "TarInfo" object. If *member* is a regular file or
   a link, an "io.BufferedReader" object is returned. Otherwise,
   "None" is returned.

   Changed in version 3.3: Return an "io.BufferedReader" object.

TarFile.add(name, arcname=None, recursive=True, exclude=None, *, filter=None)

   Add the file *name* to the archive. *name* may be any type of file
   (directory, fifo, symbolic link, etc.). If given, *arcname*
   specifies an alternative name for the file in the archive.
   Directories are added recursively by default. This can be avoided
   by setting *recursive* to "False". If *exclude* is given, it must
   be a function that takes one filename argument and returns a
   boolean value. Depending on this value the respective file is
   either excluded ("True") or added ("False"). If *filter* is
   specified it must be a keyword argument.  It should be a function
   that takes a "TarInfo" object argument and returns the changed
   "TarInfo" object. If it instead returns "None" the "TarInfo" object
   will be excluded from the archive. See Examples for an example.

   Changed in version 3.2: Added the *filter* parameter.

   Deprecated since version 3.2: The *exclude* parameter is
   deprecated, please use the *filter* parameter instead.

TarFile.addfile(tarinfo, fileobj=None)

   Add the "TarInfo" object *tarinfo* to the archive. If *fileobj* is
   given, it should be a *binary file*, and "tarinfo.size" bytes are
   read from it and added to the archive.  You can create "TarInfo"
   objects directly, or by using "gettarinfo()".

TarFile.gettarinfo(name=None, arcname=None, fileobj=None)

   Create a "TarInfo" object from the result of "os.stat()" or
   equivalent on an existing file.  The file is either named by
   *name*, or specified as a *file object* *fileobj* with a file
   descriptor.  If given, *arcname* specifies an alternative name for
   the file in the archive, otherwise, the name is taken from
   *fileobj*’s "name" attribute, or the *name* argument.  The name
   should be a text string.

   You can modify some of the "TarInfo"’s attributes before you add it
   using "addfile()". If the file object is not an ordinary file
   object positioned at the beginning of the file, attributes such as
   "size" may need modifying.  This is the case for objects such as
   "GzipFile". The "name" may also be modified, in which case
   *arcname* could be a dummy string.

TarFile.close()

   Close the "TarFile". In write mode, two finishing zero blocks are
   appended to the archive.

TarFile.pax_headers

   A dictionary containing key-value pairs of pax global headers.


13.6.2. TarInfo Objects
=======================

A "TarInfo" object represents one member in a "TarFile". Aside from
storing all required attributes of a file (like file type, size, time,
permissions, owner etc.), it provides some useful methods to determine
its type. It does *not* contain the file’s data itself.

"TarInfo" objects are returned by "TarFile"’s methods "getmember()",
"getmembers()" and "gettarinfo()".

class tarfile.TarInfo(name="")

   Create a "TarInfo" object.

classmethod TarInfo.frombuf(buf, encoding, errors)

   Create and return a "TarInfo" object from string buffer *buf*.

   Raises "HeaderError" if the buffer is invalid.

classmethod TarInfo.fromtarfile(tarfile)

   Read the next member from the "TarFile" object *tarfile* and return
   it as a "TarInfo" object.

TarInfo.tobuf(format=DEFAULT_FORMAT, encoding=ENCODING, errors='surrogateescape')

   Create a string buffer from a "TarInfo" object. For information on
   the arguments see the constructor of the "TarFile" class.

   Changed in version 3.2: Use "'surrogateescape'" as the default for
   the *errors* argument.

A "TarInfo" object has the following public data attributes:

TarInfo.name

   Name of the archive member.

TarInfo.size

   Size in bytes.

TarInfo.mtime

   Time of last modification.

TarInfo.mode

   Permission bits.

TarInfo.type

   File type.  *type* is usually one of these constants: "REGTYPE",
   "AREGTYPE", "LNKTYPE", "SYMTYPE", "DIRTYPE", "FIFOTYPE",
   "CONTTYPE", "CHRTYPE", "BLKTYPE", "GNUTYPE_SPARSE".  To determine
   the type of a "TarInfo" object more conveniently, use the "is*()"
   methods below.

TarInfo.linkname

   Name of the target file name, which is only present in "TarInfo"
   objects of type "LNKTYPE" and "SYMTYPE".

TarInfo.uid

   User ID of the user who originally stored this member.

TarInfo.gid

   Group ID of the user who originally stored this member.

TarInfo.uname

   User name.

TarInfo.gname

   Group name.

TarInfo.pax_headers

   A dictionary containing key-value pairs of an associated pax
   extended header.

A "TarInfo" object also provides some convenient query methods:

TarInfo.isfile()

   Return "True" if the "Tarinfo" object is a regular file.

TarInfo.isreg()

   Same as "isfile()".

TarInfo.isdir()

   Return "True" if it is a directory.

TarInfo.issym()

   Return "True" if it is a symbolic link.

TarInfo.islnk()

   Return "True" if it is a hard link.

TarInfo.ischr()

   Return "True" if it is a character device.

TarInfo.isblk()

   Return "True" if it is a block device.

TarInfo.isfifo()

   Return "True" if it is a FIFO.

TarInfo.isdev()

   Return "True" if it is one of character device, block device or
   FIFO.


13.6.3. Command-Line Interface
==============================

New in version 3.4.

The "tarfile" module provides a simple command-line interface to
interact with tar archives.

If you want to create a new tar archive, specify its name after the
"-c" option and then list the filename(s) that should be included:

   $ python -m tarfile -c monty.tar  spam.txt eggs.txt

Passing a directory is also acceptable:

   $ python -m tarfile -c monty.tar life-of-brian_1979/

If you want to extract a tar archive into the current directory, use
the "-e" option:

   $ python -m tarfile -e monty.tar

You can also extract a tar archive into a different directory by
passing the directory’s name:

   $ python -m tarfile -e monty.tar  other-dir/

For a list of the files in a tar archive, use the "-l" option:

   $ python -m tarfile -l monty.tar


13.6.3.1. Command-line options
------------------------------

-l <tarfile>
--list <tarfile>

   List files in a tarfile.

-c <tarfile> <source1> ... <sourceN>
--create <tarfile> <source1> ... <sourceN>

   Create tarfile from source files.

-e <tarfile> [<output_dir>]
--extract <tarfile> [<output_dir>]

   Extract tarfile into the current directory if *output_dir* is not
   specified.

-t <tarfile>
--test <tarfile>

   Test whether the tarfile is valid or not.

-v, --verbose

   Verbose output.


13.6.4. Examples
================

How to extract an entire tar archive to the current working directory:

   import tarfile
   tar = tarfile.open("sample.tar.gz")
   tar.extractall()
   tar.close()

How to extract a subset of a tar archive with "TarFile.extractall()"
using a generator function instead of a list:

   import os
   import tarfile

   def py_files(members):
       for tarinfo in members:
           if os.path.splitext(tarinfo.name)[1] == ".py":
               yield tarinfo

   tar = tarfile.open("sample.tar.gz")
   tar.extractall(members=py_files(tar))
   tar.close()

How to create an uncompressed tar archive from a list of filenames:

   import tarfile
   tar = tarfile.open("sample.tar", "w")
   for name in ["foo", "bar", "quux"]:
       tar.add(name)
   tar.close()

The same example using the "with" statement:

   import tarfile
   with tarfile.open("sample.tar", "w") as tar:
       for name in ["foo", "bar", "quux"]:
           tar.add(name)

How to read a gzip compressed tar archive and display some member
information:

   import tarfile
   tar = tarfile.open("sample.tar.gz", "r:gz")
   for tarinfo in tar:
       print(tarinfo.name, "is", tarinfo.size, "bytes in size and is", end="")
       if tarinfo.isreg():
           print("a regular file.")
       elif tarinfo.isdir():
           print("a directory.")
       else:
           print("something else.")
   tar.close()

How to create an archive and reset the user information using the
*filter* parameter in "TarFile.add()":

   import tarfile
   def reset(tarinfo):
       tarinfo.uid = tarinfo.gid = 0
       tarinfo.uname = tarinfo.gname = "root"
       return tarinfo
   tar = tarfile.open("sample.tar.gz", "w:gz")
   tar.add("foo", filter=reset)
   tar.close()


13.6.5. Supported tar formats
=============================

There are three tar formats that can be created with the "tarfile"
module:

* The POSIX.1-1988 ustar format ("USTAR_FORMAT"). It supports
  filenames up to a length of at best 256 characters and linknames up
  to 100 characters. The maximum file size is 8 GiB. This is an old
  and limited but widely supported format.

* The GNU tar format ("GNU_FORMAT"). It supports long filenames and
  linknames, files bigger than 8 GiB and sparse files. It is the de
  facto standard on GNU/Linux systems. "tarfile" fully supports the
  GNU tar extensions for long names, sparse file support is read-only.

* The POSIX.1-2001 pax format ("PAX_FORMAT"). It is the most
  flexible format with virtually no limits. It supports long filenames
  and linknames, large files and stores pathnames in a portable way.
  However, not all tar implementations today are able to handle pax
  archives properly.

  The *pax* format is an extension to the existing *ustar* format. It
  uses extra headers for information that cannot be stored otherwise.
  There are two flavours of pax headers: Extended headers only affect
  the subsequent file header, global headers are valid for the
  complete archive and affect all following files. All the data in a
  pax header is encoded in *UTF-8* for portability reasons.

There are some more variants of the tar format which can be read, but
not created:

* The ancient V7 format. This is the first tar format from Unix
  Seventh Edition, storing only regular files and directories. Names
  must not be longer than 100 characters, there is no user/group name
  information. Some archives have miscalculated header checksums in
  case of fields with non-ASCII characters.

* The SunOS tar extended format. This format is a variant of the
  POSIX.1-2001 pax format, but is not compatible.


13.6.6. Unicode issues
======================

The tar format was originally conceived to make backups on tape drives
with the main focus on preserving file system information. Nowadays
tar archives are commonly used for file distribution and exchanging
archives over networks. One problem of the original format (which is
the basis of all other formats) is that there is no concept of
supporting different character encodings. For example, an ordinary tar
archive created on a *UTF-8* system cannot be read correctly on a
*Latin-1* system if it contains non-*ASCII* characters. Textual
metadata (like filenames, linknames, user/group names) will appear
damaged. Unfortunately, there is no way to autodetect the encoding of
an archive. The pax format was designed to solve this problem. It
stores non-ASCII metadata using the universal character encoding
*UTF-8*.

The details of character conversion in "tarfile" are controlled by the
*encoding* and *errors* keyword arguments of the "TarFile" class.

*encoding* defines the character encoding to use for the metadata in
the archive. The default value is "sys.getfilesystemencoding()" or
"'ascii'" as a fallback. Depending on whether the archive is read or
written, the metadata must be either decoded or encoded. If *encoding*
is not set appropriately, this conversion may fail.

The *errors* argument defines how characters are treated that cannot
be converted. Possible values are listed in section Error Handlers.
The default scheme is "'surrogateescape'" which Python also uses for
its file system calls, see File Names, Command Line Arguments, and
Environment Variables.

In case of "PAX_FORMAT" archives, *encoding* is generally not needed
because all the metadata is stored using *UTF-8*. *encoding* is only
used in the rare cases when binary pax headers are decoded or when
strings with surrogate characters are stored.
