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+'\"
+'\" Copyright (c) 2008-2012 Donal K. Fellows
+'\"
+'\" See the file "license.terms" for information on usage and redistribution
+'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
+'\"
+.TH zlib n 8.6 Tcl "Tcl Built-In Commands"
+.so man.macros
+.BS
+'\" Note:  do not modify the .SH NAME line immediately below!
+.SH NAME
+zlib \- compression and decompression operations
+.SH SYNOPSIS
+.nf
+\fBzlib \fIsubcommand arg ...\fR
+.fi
+.BE
+.SH DESCRIPTION
+.PP
+The \fBzlib\fR command provides access to the compression and check-summing
+facilities of the Zlib library by Jean-loup Gailly and Mark Adler. It has the
+following subcommands.
+.SS "COMPRESSION SUBCOMMANDS"
+.TP
+\fBzlib compress\fI string\fR ?\fIlevel\fR?
+.
+Returns the zlib-format compressed binary data of the binary string in
+\fIstring\fR. If present, \fIlevel\fR gives the compression level to use (from
+0, which is uncompressed, to 9, maximally compressed).
+.TP
+\fBzlib decompress\fI string\fR ?\fIbufferSize\fR?
+.
+Returns the uncompressed version of the raw compressed binary data in
+\fIstring\fR. If present, \fIbufferSize\fR is a hint as to what size of buffer
+is to be used to receive the data.
+.TP
+\fBzlib deflate\fI string\fR ?\fIlevel\fR?
+.
+Returns the raw compressed binary data of the binary string in \fIstring\fR.
+If present, \fIlevel\fR gives the compression level to use (from 0, which is
+uncompressed, to 9, maximally compressed).
+.TP
+\fBzlib gunzip\fI string\fR ?\fB\-headerVar \fIvarName\fR?
+.
+Return the uncompressed contents of binary string \fIstring\fR, which must
+have been in gzip format. If \fB\-headerVar\fR is given, store a dictionary
+describing the contents of the gzip header in the variable called
+\fIvarName\fR. The keys of the dictionary that may be present are:
+.RS
+.TP
+\fBcomment\fR
+.
+The comment field from the header, if present.
+.TP
+\fBcrc\fR
+.
+A boolean value describing whether a CRC of the header is computed.
+.TP
+\fBfilename\fR
+.
+The filename field from the header, if present.
+.TP
+\fBos\fR
+.
+The operating system type code field from the header (if not the
+QW unknown
+value). See RFC 1952 for the meaning of these codes.
+.TP
+\fBsize\fR
+.
+The size of the uncompressed data.
+.TP
+\fBtime\fR
+.
+The time field from the header if non-zero, expected to be time that the file
+named by the \fBfilename\fR field was modified. Suitable for use with
+\fBclock format\fR.
+.TP
+\fBtype\fR
+.
+The type of the uncompressed data (\fBbinary\fR or \fBtext\fR) if known.
+.RE
+.TP
+\fBzlib gzip\fI string\fR ?\fB\-level \fIlevel\fR? ?\fB\-header \fIdict\fR?
+.
+Return the compressed contents of binary string \fIstring\fR in gzip format.
+If \fB\-level\fR is given, \fIlevel\fR gives the compression level to use
+(from 0, which is uncompressed, to 9, maximally compressed). If \fB\-header\fR
+is given, \fIdict\fR is a dictionary containing values used for the gzip
+header. The following keys may be defined:
+.RS
+.TP
+\fBcomment\fR
+.
+Add the given comment to the header of the gzip-format data.
+.TP
+\fBcrc\fR
+.
+A boolean saying whether to compute a CRC of the header. Note that if the data
+is to be interchanged with the \fBgzip\fR program, a header CRC should
+\fInot\fR be computed.
+.TP
+\fBfilename\fR
+.
+The name of the file that the data to be compressed came from.
+.TP
+\fBos\fR
+.
+The operating system type code, which should be one of the values described in
+RFC 1952.
+.TP
+\fBtime\fR
+.
+The time that the file named in the \fBfilename\fR key was last modified. This
+will be in the same as is returned by \fBclock seconds\fR or \fBfile mtime\fR.
+.TP
+\fBtype\fR
+.
+The type of the data being compressed, being \fBbinary\fR or \fBtext\fR.
+.RE
+.TP
+\fBzlib inflate\fI string\fR ?\fIbufferSize\fR?
+.
+Returns the uncompressed version of the raw compressed binary data in
+\fIstring\fR. If present, \fIbufferSize\fR is a hint as to what size of buffer
+is to be used to receive the data.
+.SS "CHANNEL SUBCOMMAND"
+.TP
+\fBzlib push\fI mode channel\fR ?\fIoptions ...\fR?
+.
+Pushes a compressing or decompressing transformation onto the channel
+\fIchannel\fR.
+The transformation can be removed again with \fBchan pop\fR.
+The \fImode\fR argument determines what type of transformation
+is pushed; the following are supported:
+.RS
+.TP
+\fBcompress\fR
+.
+The transformation will be a compressing transformation that produces
+zlib-format data on \fIchannel\fR, which must be writable.
+.TP
+\fBdecompress\fR
+.
+The transformation will be a decompressing transformation that reads
+zlib-format data from \fIchannel\fR, which must be readable.
+.TP
+\fBdeflate\fR
+.
+The transformation will be a compressing transformation that produces raw
+compressed data on \fIchannel\fR, which must be writable.
+.TP
+\fBgunzip\fR
+.
+The transformation will be a decompressing transformation that reads
+gzip-format data from \fIchannel\fR, which must be readable.
+.TP
+\fBgzip\fR
+.
+The transformation will be a compressing transformation that produces
+gzip-format data on \fIchannel\fR, which must be writable.
+.TP
+\fBinflate\fR
+.
+The transformation will be a decompressing transformation that reads raw
+compressed data from \fIchannel\fR, which must be readable.
+.PP
+The following options may be set when creating a transformation via
+the
+.QW "\fIoptions ...\fR"
+to the \fBzlib push\fR command:
+.TP
+\fB\-dictionary\fI binData\fR
+.VS "TIP 400"
+Sets the compression dictionary to use when working with compressing or
+decompressing the data to be \fIbinData\fR. Not valid for transformations that
+work with gzip-format data.  The dictionary should consist of strings (byte
+sequences) that are likely to be encountered later in the data to be compressed,
+with the most commonly used strings preferably put towards the end of the
+dictionary. Tcl provides no mechanism for choosing a good such dictionary for
+a particular data sequence.
+.VE
+.TP
+\fB\-header\fI dictionary\fR
+.
+Passes a description of the gzip header to create, in the same format that
+\fBzlib gzip\fR understands.
+.TP
+\fB\-level\fI compressionLevel\fR
+.
+How hard to compress the data. Must be an integer from 0 (uncompressed) to 9
+(maximally compressed).
+.TP
+\fB\-limit\fI readaheadLimit\fR
+.
+The maximum number of bytes ahead to read when decompressing. This defaults to
+1, which ensures that data is always decompressed correctly, but may be
+increased to improve performance. This is more useful when the channel is
+non-blocking.
+.PP
+Both compressing and decompressing channel transformations add extra
+configuration options that may be accessed through \fBchan configure\fR. The
+options are:
+.TP
+\fB\-checksum\fI checksum\fR
+.
+This read-only option gets the current checksum for the uncompressed data that
+the compression engine has seen so far. It is valid for both compressing and
+decompressing transforms, but not for the raw inflate and deflate formats. The
+compression algorithm depends on what format is being produced or consumed.
+.TP
+\fB\-dictionary\fI binData\fR
+.VS "TIP 400"
+This read-write options gets or sets the initial compression dictionary to use
+when working with compressing or decompressing the data to be \fIbinData\fR.
+It is not valid for transformations that work with gzip-format data, and should
+not normally be set on compressing transformations other than at the point where
+the transformation is stacked. Note that this cannot be used to get the
+current active compression dictionary mid-stream, as that information is not
+exposed by the underlying library.
+.VE
+.TP
+\fB\-flush\fI type\fR
+.
+This write-only operation flushes the current state of the compressor to the
+underlying channel. It is only valid for compressing transformations. The
+\fItype\fR must be either \fBsync\fR or \fBfull\fR for a normal flush or an
+expensive flush respectively. Flushing degrades the compression ratio, but
+makes it easier for a decompressor to recover more of the file in the case of
+data corruption.
+.TP
+\fB\-header\fI dictionary\fR
+.
+This read-only option, only valid for decompressing transforms that are
+processing gzip-format data, returns the dictionary describing the header read
+off the data stream.
+.TP
+\fB\-limit\fI readaheadLimit\fR
+.
+This read-write option is used by decompressing channels to control the
+maximum number of bytes ahead to read from the underlying data source. This
+defaults to 1, which ensures that data is always decompressed correctly, but
+may be increased to improve performance. This is more useful when the channel
+is non-blocking.
+.RE
+.SS "STREAMING SUBCOMMAND"
+.TP
+\fBzlib stream\fI mode\fR ?\fIoptions\fR?
+.
+Creates a streaming compression or decompression command based on the
+\fImode\fR, and return the name of the command. For a description of how that
+command works, see \fBSTREAMING INSTANCE COMMAND\fR below. The following modes
+and \fIoptions\fR are supported:
+.RS
+.TP
+\fBzlib stream compress\fR ?\fB\-dictionary \fIbindata\fR? ?\fB\-level \fIlevel\fR?
+.
+The stream will be a compressing stream that produces zlib-format output,
+using compression level \fIlevel\fR (if specified) which will be an integer
+from 0 to 9,
+.VS "TIP 400"
+and the compression dictionary \fIbindata\fR (if specified).
+.VE
+.TP
+\fBzlib stream decompress\fR ?\fB\-dictionary \fIbindata\fR?
+.
+The stream will be a decompressing stream that takes zlib-format input and
+produces uncompressed output.
+.VS "TIP 400"
+If \fIbindata\fR is supplied, it is a compression dictionary to use if
+required.
+.VE
+.TP
+\fBzlib stream deflate\fR ?\fB\-dictionary \fIbindata\fR? ?\fB\-level \fIlevel\fR?
+.
+The stream will be a compressing stream that produces raw output, using
+compression level \fIlevel\fR (if specified) which will be an integer from 0
+to 9,
+.VS "TIP 400"
+and the compression dictionary \fIbindata\fR (if specified). Note that
+the raw compressed data includes no metadata about what compression
+dictionary was used, if any; that is a feature of the zlib-format data.
+.VE
+.TP
+\fBzlib stream gunzip\fR
+.
+The stream will be a decompressing stream that takes gzip-format input and
+produces uncompressed output.
+.TP
+\fBzlib stream gzip\fR ?\fB\-header \fIheader\fR? ?\fB\-level \fIlevel\fR?
+.
+The stream will be a compressing stream that produces gzip-format output,
+using compression level \fIlevel\fR (if specified) which will be an integer
+from 0 to 9, and the header descriptor dictionary \fIheader\fR (if specified;
+for keys see \fBzlib gzip\fR).
+.TP
+\fBzlib stream inflate\fR ?\fB\-dictionary \fIbindata\fR?
+.
+The stream will be a decompressing stream that takes raw compressed input and
+produces uncompressed output.
+.VS "TIP 400"
+If \fIbindata\fR is supplied, it is a compression dictionary to use. Note that
+there are no checks in place to determine whether the compression dictionary
+is correct.
+.VE
+.RE
+.SS "CHECKSUMMING SUBCOMMANDS"
+.TP
+\fBzlib adler32\fI string\fR ?\fIinitValue\fR?
+.
+Compute a checksum of binary string \fIstring\fR using the Adler-32 algorithm.
+If given, \fIinitValue\fR is used to initialize the checksum engine.
+.TP
+\fBzlib crc32\fI string\fR ?\fIinitValue\fR?
+.
+Compute a checksum of binary string \fIstring\fR using the CRC-32 algorithm.
+If given, \fIinitValue\fR is used to initialize the checksum engine.
+.SH "STREAMING INSTANCE COMMAND"
+.PP
+Streaming compression instance commands are produced by the \fBzlib stream\fR
+command. They are used by calling their \fBput\fR subcommand one or more times
+to load data in, and their \fBget\fR subcommand one or more times to extract
+the transformed data.
+.PP
+The full set of subcommands supported by a streaming instance command,
+\fIstream\fR, is as follows:
+.TP
+\fIstream \fBadd\fR ?\fIoption...\fR? \fIdata\fR
+.
+A short-cut for
+.QW "\fIstream \fBput \fR?\fIoption...\fR? \fIdata\fR"
+followed by
+.QW "\fIstream \fBget\fR" .
+.TP
+\fIstream \fBchecksum\fR
+.
+Returns the checksum of the uncompressed data seen so far by this stream.
+.TP
+\fIstream \fBclose\fR
+.
+Deletes this stream and frees up all resources associated with it.
+.TP
+\fIstream \fBeof\fR
+.
+Returns a boolean indicating whether the end of the stream (as determined by
+the compressed data itself) has been reached. Not all formats support
+detection of the end of the stream.
+.TP
+\fIstream \fBfinalize\fR
+.
+A short-cut for
+.QW "\fIstream \fBput \-finalize {}\fR" .
+.TP
+\fIstream \fBflush\fR
+.
+A short-cut for
+.QW "\fIstream \fBput \-flush {}\fR" .
+.TP
+\fIstream \fBfullflush\fR
+.
+A short-cut for
+.QW "\fIstream \fBput \-fullflush {}\fR" .
+.TP
+\fIstream \fBget \fR?\fIcount\fR?
+.
+Return up to \fIcount\fR bytes from \fIstream\fR's internal buffers with the
+transformation applied. If \fIcount\fR is omitted, the entire contents of the
+buffers are returned.
+.
+\fIstream \fBheader\fR
+.
+Return the gzip header description dictionary extracted from the stream. Only
+supported for streams created with their \fImode\fR parameter set to
+\fBgunzip\fR.
+.TP
+\fIstream \fBput\fR ?\fIoption...\fR? \fIdata\fR
+.
+Append the contents of the binary string \fIdata\fR to \fIstream\fR's internal
+buffers while applying the transformation. The following \fIoption\fRs are
+supported (or an unambiguous prefix of them), which are used to modify the
+way in which the transformation is applied:
+.RS
+.TP
+\fB\-dictionary\fI binData\fR
+.VS "TIP 400"
+Sets the compression dictionary to use when working with compressing or
+decompressing the data to be \fIbinData\fR.
+.VE
+.TP
+\fB\-finalize\fR
+.
+Mark the stream as finished, ensuring that all bytes have been wholly
+compressed or decompressed. For gzip streams, this also ensures that the
+footer is written to the stream. The stream will need to be reset before
+having more data written to it after this, though data can still be read out
+of the stream with the \fBget\fR subcommand.
+.RS
+.PP
+This option is mutually exclusive with the \fB\-flush\fR and \fB\-fullflush\fR
+options.
+.RE
+.TP
+\fB\-flush\fR
+.
+Ensure that a decompressor consuming the bytes that the current (compressing)
+stream is producing will be able to produce all the bytes that have been
+compressed so far, at some performance penalty.
+.RS
+.PP
+This option is mutually exclusive with the \fB\-finalize\fR and
+\fB\-fullflush\fR options.
+.RE
+.TP
+\fB\-fullflush\fR
+.
+Ensure that not only can a decompressor handle all the bytes produced so far
+(as with \fB\-flush\fR above) but also that it can restart from this point if
+it detects that the stream is partially corrupt. This incurs a substantial
+performance penalty.
+.RS
+.PP
+This option is mutually exclusive with the \fB\-finalize\fR and \fB\-flush\fR
+options.
+.RE
+.RE
+.TP
+\fIstream \fBreset\fR
+.
+Puts any stream, including those that have been finalized or that have reached
+eof, back into a state where it can process more data. Throws away all
+internally buffered data.
+.SH EXAMPLES
+.PP
+To compress a Tcl string, it should be first converted to a particular charset
+encoding since the \fBzlib\fR command always operates on binary strings.
+.PP
+.CS
+set binData [encoding convertto utf-8 $string]
+set compData [\fBzlib compress\fR $binData]
+.CE
+.PP
+When converting back, it is also important to reverse the charset encoding:
+.PP
+.CS
+set binData [\fBzlib decompress\fR $compData]
+set string [encoding convertfrom utf-8 $binData]
+.CE
+.PP
+The compression operation from above can also be done with streams, which is
+especially helpful when you want to accumulate the data by stages:
+.PP
+.CS
+set strm [\fBzlib stream\fR compress]
+$\fIstrm \fBput\fR [encoding convertto utf-8 $string]
+# ...
+$\fIstrm \fBfinalize\fR
+set compData [$\fIstrm \fBget\fR]
+$\fIstrm \fBclose\fR
+.CE
+.SH "SEE ALSO"
+binary(n), chan(n), encoding(n), Tcl_ZlibDeflate(3), RFC1950 \- RFC1952
+.SH "KEYWORDS"
+compress, decompress, deflate, gzip, inflate, zlib
+'\" Local Variables:
+'\" mode: nroff
+'\" End: