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

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.

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.

KEYWORDS

compress, decompress, deflate, gzip, inflate