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, 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
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. What
format of compressed data to work with. Must be one of TCL_ZLIB_FOR-
MAT_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. 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. 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 recom-
mended by the zlib library. 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_FOR-
MAT_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 DIC-
TIONARY for details about the contents of this dictionary. The initial
value for the checksum algorithm. An array of bytes to run the check-
sum algorithm over, or NULL to get the recommended initial value for
the checksum algorithm. The number of bytes in the array. 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. A pointer to a variable in which to write the abstract token
for the stream upon successful creation. The abstract token for the
stream to operate on. Whether and how to flush the stream after writ-
ing 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 writ-
ten. The maximum number of bytes to get from the stream, or -1 to get
all remaining bytes from the stream's buffers. 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 decom-
press the data contained in the dataObj argument, according to the for-
mat and, for compression, level arguments. The dictionary in the dic-
tObj 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_FOR-
MAT_GZIP or TCL_ZLIB_FORMAT_AUTO. For details of the contents of the
dictionary, see the GZIP OPTIONS DICTIONARY section below. Upon suc-
cess, 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 init-
Value 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 dictio-
nary (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 uncom-
pressed 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 compres-
sion 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 compres-
sion 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. Compres-
sion dictionaries are only fully supported for zlib-format data; on
compression, they must be set before any data is sent in with Tcl_Zlib-
StreamPut, and on decompression they should be set when Tcl_Zlib-
StreamGet produces an error with its -errorcode set to the code will be
the Adler-32 checksum (see Tcl_ZlibAdler32) of the compression dictio-
nary 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_Zlib-
StreamInit is used to pass a dictionary of options about that is used
to describe the gzip header in the compressed data. When creating com-
pressed 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 com-
puted. 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 sani-
tized before use on decompression with file tail.
os The operating system type code field from the header (if not the
value). See RFC 1952 for the meaning of these codes. On compres-
sion, if this is absent then the field will be set to the value.
size The size of the uncompressed data. This is ignored on compres-
sion; 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 8.6.0 - Generated Sun Jan 13 10:36:27 CST 2013