TclZlib(3) Tcl Library Procedures TclZlib(3) ______________________________________________________________________________
NAME
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
SYNOPSIS
#include <tcl.h>
int
Tcl_ZlibDeflate(interp, format, dataObj, level, dictObj)
int
Tcl_ZlibInflate(interp, format, dataObj, bufferSize, dictObj)
unsigned int
Tcl_ZlibCRC32(initValue, bytes, length)
unsigned int
Tcl_ZlibAdler32(initValue, bytes, length)
int
Tcl_ZlibStreamInit(interp, mode, format, level, dictObj, zshandlePtr)
Tcl_Obj *
Tcl_ZlibStreamGetCommandName(zshandle)
int
Tcl_ZlibStreamEof(zshandle)
int
Tcl_ZlibStreamClose(zshandle)
int
Tcl_ZlibStreamReset(zshandle)
int
Tcl_ZlibStreamChecksum(zshandle)
int
Tcl_ZlibStreamPut(zshandle, dataObj, flush)
int
Tcl_ZlibStreamGet(zshandle, dataObj, count)
Tcl_ZlibStreamSetCompressionDictionary(zshandle, compDict)
ARGUMENTS
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
Tcl_ZlibStreamGet.
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 dictionary.
unsigned int initValue (in) The initial value for the
checksum algorithm.
int bufferSize (in) A hint as to what size of buffer
is to be used to receive the
data. Use 0 to use a geric guess
based on the input data.
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
decompression.
______________________________________________________________________________
DESCRIPTION
These functions form the interface from the Tcl library to the Zlib
library by Jean-loup Gailly and Mark Adler.
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.
ZLIB STREAMS
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 reached.
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).
GZIP OPTIONS DICTIONARY
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.
comment
This holds the comment field of the header, if present. If
absent, no comment was supplied (on decompression) or will be
created (on compression).
crc 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.
filename
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.
os The operating system type code field from the header (if not the
"unknown" value). See RFC 1952 for the meaning of these codes.
On compression, if this is absent then the field will be set to
the "unknown" value.
size 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 compression engine.
time 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.
type The type of the uncompressed data (either binary or text) if
known.
PORTABILITY NOTES
These functions will fail gracefully if Tcl is not linked with the zlib
library.
SEE ALSO
Tcl_NewByteArrayObj(3), zlib(n)
KEYWORDS
compress, decompress, deflate, gzip, inflate
Tcl 8.6 TclZlib(3)
tcl 8.6.15 - Generated Tue Dec 3 16:44:22 CST 2024