Tcl_ZlibAdler32, Tcl_ZlibCRC32, Tcl_ZlibDeflate, Tcl_ZlibInflate, Tcl_ZlibStreamChecksum, Tcl_ZlibStreamClose, Tcl_ZlibStreamEof, Tcl_ZlibStreamGet, Tcl_ZlibStreamGetCommandName, Tcl_ZlibStreamInit, Tcl_ZlibStreamPut — compression and decompression functions
- Tcl_ZlibAdler32, Tcl_ZlibCRC32, Tcl_ZlibDeflate, Tcl_ZlibInflate, Tcl_ZlibStreamChecksum, Tcl_ZlibStreamClose, Tcl_ZlibStreamEof, Tcl_ZlibStreamGet, Tcl_ZlibStreamGetCommandName, Tcl_ZlibStreamInit, Tcl_ZlibStreamPut — compression and decompression functions
- #include <tcl.h>
- Tcl_ZlibDeflate(interp, format, dataObj, level, dictObj)
- Tcl_ZlibInflate(interp, format, dataObj, dictObj)
- unsigned int
- Tcl_ZlibCRC32(initValue, bytes, length)
- unsigned int
- Tcl_ZlibAdler32(initValue, bytes, length)
- Tcl_ZlibStreamInit(interp, mode, format, level, dictObj, zshandlePtr)
- Tcl_Obj *
- Tcl_ZlibStreamPut(zshandle, dataObj, flush)
- Tcl_ZlibStreamGet(zshandle, dataObj, count)
- Tcl_ZlibStreamSetCompressionDictionary(zshandle, compDict)
- ZLIB STREAMS
- GZIP OPTIONS DICTIONARY
- REFERENCE COUNT MANAGEMENT
- PORTABILITY NOTES
- SEE ALSO
Tcl_ZlibDeflate(interp, format, dataObj, level, dictObj)
Tcl_ZlibInflate(interp, format, dataObj, dictObj)
Tcl_ZlibCRC32(initValue, bytes, length)
Tcl_ZlibAdler32(initValue, bytes, length)
Tcl_ZlibStreamInit(interp, mode, format, level, dictObj, zshandlePtr)
Tcl_ZlibStreamPut(zshandle, dataObj, flush)
Tcl_ZlibStreamGet(zshandle, dataObj, count)
These functions form the interface from the Tcl library to the Zlib
library by Jean-loup Gailly and Mark Adler.
- Tcl_Interp *interp (in)
The interpreter to store resulting compressed or uncompressed data in. Also
where any error messages are written. For Tcl_ZlibStreamInit, this can
be NULL to create a stream that is not bound to a command.
- int format (in)
What format of compressed data to work with. Must be one of
TCL_ZLIB_FORMAT_ZLIB for zlib-format data, TCL_ZLIB_FORMAT_GZIP
for gzip-format data, or TCL_ZLIB_FORMAT_RAW for raw compressed data. In
addition, for decompression only, TCL_ZLIB_FORMAT_AUTO may also be
chosen which can automatically detect whether the compressed data was in zlib
or gzip format.
- Tcl_Obj *dataObj (in/out)
A byte-array value containing the data to be compressed or decompressed, or
to which the data extracted from the stream is appended when passed to
- int level (in)
What level of compression to use. Should be a number from 0 to 9 or one of the
following: TCL_ZLIB_COMPRESS_NONE for no compression,
TCL_ZLIB_COMPRESS_FAST for fast but inefficient compression,
TCL_ZLIB_COMPRESS_BEST for slow but maximal compression, or
TCL_ZLIB_COMPRESS_DEFAULT for the level recommended by the zlib library.
- Tcl_Obj *dictObj (in/out)
A dictionary that contains, or which will be updated to contain, a description
of the gzip header associated with the compressed data. Only useful when the
format is TCL_ZLIB_FORMAT_GZIP or TCL_ZLIB_FORMAT_AUTO. If
a NULL is passed, a default header will be used on compression and the header
will be ignored (apart from integrity checks) on decompression. See the
section GZIP OPTIONS DICTIONARY for details about the contents of this
- unsigned int initValue (in)
The initial value for the checksum algorithm.
- unsigned char *bytes (in)
An array of bytes to run the checksum algorithm over, or NULL to get the
recommended initial value for the checksum algorithm.
- int length (in)
The number of bytes in the array.
- int mode (in)
What mode to operate the stream in. Should be either
TCL_ZLIB_STREAM_DEFLATE for a compressing stream or
TCL_ZLIB_STREAM_INFLATE for a decompressing stream.
- Tcl_ZlibStream *zshandlePtr (out)
A pointer to a variable in which to write the abstract token for the stream
upon successful creation.
- Tcl_ZlibStream zshandle (in)
The abstract token for the stream to operate on.
- int flush (in)
Whether and how to flush the stream after writing the data to it. Must be one
of: TCL_ZLIB_NO_FLUSH if no flushing is to be done, TCL_ZLIB_FLUSH
if the currently compressed data must be made available for access using
Tcl_ZlibStreamGet, TCL_ZLIB_FULLFLUSH if the stream must be put
into a state where the decompressor can recover from on corruption, or
TCL_ZLIB_FINALIZE to ensure that the stream is finished and that any
trailer demanded by the format is written.
- int count (in)
The maximum number of bytes to get from the stream, or -1 to get all remaining
bytes from the stream's buffers.
- Tcl_Obj *compDict (in)
A byte array value that is the compression dictionary to use with the stream.
Note that this is not a Tcl dictionary, and it is recommended that this
only ever be used with streams that were created with their format set
to TCL_ZLIB_FORMAT_ZLIB because the other formats have no mechanism to
indicate whether a compression dictionary was present other than to fail on
Tcl_ZlibDeflate and Tcl_ZlibInflate respectively compress and
decompress the data contained in the dataObj argument, according to the
format and, for compression, level arguments. The dictionary in
the dictObj parameter is used to convey additional header information
about the compressed data when the compression format supports it; currently,
the dictionary is only used when the format parameter is
TCL_ZLIB_FORMAT_GZIP or TCL_ZLIB_FORMAT_AUTO. For details of the
contents of the dictionary, see the GZIP OPTIONS DICTIONARY section
below. Upon success, both functions leave the resulting compressed or
decompressed data in a byte-array value that is the Tcl interpreter's result;
the returned value is a standard Tcl result code.
Tcl_ZlibAdler32 and Tcl_ZlibCRC32 compute checksums on arrays of
bytes, returning the computed checksum. Checksums are computed incrementally,
allowing data to be processed one block at a time, but this requires the
caller to maintain the current checksum and pass it in as the initValue
parameter; the initial value to use for this can be obtained by using NULL for
the bytes parameter instead of a pointer to the array of bytes to
compute the checksum over. Thus, typical usage in the single data block case
is like this:
checksum = Tcl_ZlibCRC32(Tcl_ZlibCRC32(0,NULL,0), data, length);
Note that the Adler-32 algorithm is not a real checksum, but instead is a
related type of hash that works best on longer data.
Tcl_ZlibStreamInit creates a compressing or decompressing stream that is
linked to a Tcl command, according to its arguments, and provides an abstract
token for the stream and returns a normal Tcl result code;
Tcl_ZlibStreamGetCommandName returns the name of that command given the
stream token, or NULL if the stream has no command. Streams are not designed
to be thread-safe; each stream should only ever be used from the thread that
created it. When working with gzip streams, a dictionary (fields as given in
the GZIP OPTIONS DICTIONARY section below) can be given via the
dictObj parameter that on compression allows control over the generated
headers, and on decompression allows discovery of the existing headers. Note
that the dictionary will be written to on decompression once sufficient data
has been read to have a complete header. This means that the dictionary must
be an unshared value in that case; a blank value created with
Tcl_NewObj is suggested.
Once a stream has been constructed, Tcl_ZlibStreamPut is used to add
data to the stream and Tcl_ZlibStreamGet is used to retrieve data from
the stream after processing. Both return normal Tcl result codes and leave an
error message in the result of the interpreter that the stream is registered
with in the error case (if such a registration has been performed). With
Tcl_ZlibStreamPut, the data buffer value passed to it should not be
modified afterwards. With Tcl_ZlibStreamGet, the data buffer value
passed to it will have the data bytes appended to it. Internally to the
stream, data is kept compressed so as to minimize the cost of buffer space.
Tcl_ZlibStreamChecksum returns the checksum computed over the
uncompressed data according to the format, and Tcl_ZlibStreamEof returns
a boolean value indicating whether the end of the uncompressed data has been
Tcl_ZlibStreamSetCompressionDictionary is used to control the
compression dictionary used with the stream, a compression dictionary being an
array of bytes (such as might be created with Tcl_NewByteArrayObj) that
is used to initialize the compression engine rather than leaving it to create
it on the fly from the data being compressed. Setting a compression dictionary
allows for more efficient compression in the case where the start of the data
is highly regular, but it does require both the compressor and the
decompressor to agreee on the value to use. Compression dictionaries are only
fully supported for zlib-format data; on compression, they must be set before
any data is sent in with Tcl_ZlibStreamPut, and on decompression they
should be set when Tcl_ZlibStreamGet produces an error with its
-errorcode set to
“ZLIB NEED_DICT code”;
the code will be the Adler-32 checksum (see Tcl_ZlibAdler32) of
the compression dictionary sought. (Note that this is only true for
zlib-format streams; gzip streams ignore compression dictionaries as the
format specification doesn't permit them, and raw streams just produce a data
error if the compression dictionary is missing or incorrect.)
If you wish to clear a stream and reuse it for a new compression or
decompression action, Tcl_ZlibStreamReset will do this and return a
normal Tcl result code to indicate whether it was successful; if the stream is
registered with an interpreter, an error message will be left in the
interpreter result when this function returns TCL_ERROR.
Finally, Tcl_ZlibStreamClose will clean up the stream and delete the
associated command: using Tcl_DeleteCommand on the stream's command is
equivalent (when such a command exists).
The dictObj parameter to Tcl_ZlibDeflate, Tcl_ZlibInflate
and Tcl_ZlibStreamInit is used to pass a dictionary of options about
that is used to describe the gzip header in the compressed data. When creating
compressed data, the dictionary is read and when unpacking compressed data the
dictionary is written (in which case the dictObj parameter must refer to
an unshared dictionary value).
The following fields in the dictionary value are understood. All other fields
are ignored. No field is required when creating a gzip-format stream.
Tcl_ZlibDeflate and Tcl_ZlibInflate take a value with arbitrary
reference count for their dataObj and dictObj arguments (the
latter often being NULL instead), and set the interpreter result with their
output value (or an error). The existing interpreter result should not be
passed as any argument value unless an additional reference is held.
This holds the comment field of the header, if present. If absent, no comment
was supplied (on decompression) or will be created (on compression).
A boolean value describing whether a CRC of the header is computed. Note that
the gzip program does not use or allow a CRC on the header.
The name of the file that held the uncompressed data. This should not contain
any directory separators, and should be sanitized before use on decompression
with file tail.
The operating system type code field from the header (if not the
value). See RFC 1952 for the meaning of these codes. On compression, if this
is absent then the field will be set to the
The size of the uncompressed data. This is ignored on compression; the size
of the data compressed depends on how much data is supplied to the
The time field from the header if non-zero, expected to be the time that the
file named by the filename field was modified. Suitable for use with
clock format. On creation, the right value to use is that from
clock seconds or file mtime.
The type of the uncompressed data (either binary or text) if
Tcl_ZlibStreamInit takes a value with arbitrary reference count for its
dictObj argument; it only reads from it. The existing interpreter result
should not be passed unless an additional reference is held.
Tcl_ZlibStreamGetCommandName returns a zero reference count value, much
The dataObj argument to Tcl_ZlibStreamPut is a value with
arbitrary reference count; it is only ever read from.
The dataObj argument to Tcl_ZlibStreamGet is an unshared value
(see Tcl_IsShared) that will be updated by the function.
The compDict argument to Tcl_ZlibStreamSetCompressionDictionary,
if non-NULL, may be duplicated or may have its reference count incremented.
Using a zero reference count value is not recommended.
These functions will fail gracefully if Tcl is not linked with the zlib
compress, decompress, deflate, gzip, inflate
Copyright © 2008 Donal K. Fellows