mirror of https://github.com/mkerrisk/man-pages
217 lines
6.9 KiB
Groff
217 lines
6.9 KiB
Groff
.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
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.TH "COMPRESS" 1P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
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.\" compress
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.SH NAME
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compress \- compress data
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.SH SYNOPSIS
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.LP
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\fBcompress\fP \fB[\fP\fB-fv\fP\fB][\fP\fB-b\fP \fIbits\fP\fB][\fP\fIfile\fP
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\fB\&...\fP\fB]\fP\fB
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.br
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.sp
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compress\fP \fB[\fP\fB-cfv\fP\fB][\fP\fB-b\fP \fIbits\fP\fB][\fP\fIfile\fP\fB]\fP\fB\fP
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\fB
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.br
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\fP
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.SH DESCRIPTION
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.LP
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The \fIcompress\fP utility shall attempt to reduce the size of the
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named files by using adaptive Lempel-Ziv coding algorithm.
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.TP 7
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\fBNote:\fP
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Lempel-Ziv is US Patent 4464650, issued to William Eastman, Abraham
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Lempel, Jacob Ziv, Martin Cohn on August 7th, 1984, and
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assigned to Sperry Corporation.
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.LP
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Lempel-Ziv-Welch compression is covered by US Patent 4558302, issued
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to Terry A. Welch on December 10th, 1985, and assigned to
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Sperry Corporation.
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.sp
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On systems not supporting adaptive Lempel-Ziv coding algorithm, the
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input files shall not be changed and an
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error value greater than two shall be returned. Except when the output
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is to the standard output, each file shall be replaced by
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one with the extension \fB.Z\fP. If the invoking process has appropriate
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privileges, the ownership, modes, access time, and
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modification time of the original file are preserved. If appending
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the \fB.Z\fP to the filename would make the name exceed
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{NAME_MAX} bytes, the command shall fail. If no files are specified,
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the standard input shall be compressed to the standard
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output.
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.SH OPTIONS
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.LP
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The \fIcompress\fP utility shall conform to the Base Definitions volume
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of IEEE\ Std\ 1003.1-2001, Section 12.2, Utility Syntax Guidelines.
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.LP
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The following options shall be supported:
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.TP 7
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\fB-b\ \fP \fIbits\fP
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Specify the maximum number of bits to use in a code. For a conforming
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application, the \fIbits\fP argument shall be:
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.sp
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.RS
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.nf
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\fB9 <=\fP \fIbits\fP \fB<= 14
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\fP
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.fi
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.RE
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.LP
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The implementation may allow \fIbits\fP values of greater than 14.
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The default is 14, 15, or 16.
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.TP 7
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\fB-c\fP
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Cause \fIcompress\fP to write to the standard output; the input file
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is not changed, and no \fB.Z\fP files are created.
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.TP 7
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\fB-f\fP
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Force compression of \fIfile\fP, even if it does not actually reduce
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the size of the file, or if the corresponding \fIfile\fP
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\fB\&.Z\fP file already exists. If the \fB-f\fP option is not given,
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and the process is not running in the background, the user is
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prompted as to whether an existing \fIfile\fP \fB.Z\fP file should
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be overwritten.
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.TP 7
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\fB-v\fP
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Write the percentage reduction of each file to standard error.
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.sp
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.SH OPERANDS
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.LP
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The following operand shall be supported:
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.TP 7
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\fIfile\fP
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A pathname of a file to be compressed.
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.sp
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.SH STDIN
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.LP
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The standard input shall be used only if no \fIfile\fP operands are
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specified, or if a \fIfile\fP operand is \fB'-'\fP
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\&.
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.SH INPUT FILES
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.LP
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If \fIfile\fP operands are specified, the input files contain the
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data to be compressed.
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.SH ENVIRONMENT VARIABLES
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.LP
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The following environment variables shall affect the execution of
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\fIcompress\fP:
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.TP 7
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\fILANG\fP
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Provide a default value for the internationalization variables that
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are unset or null. (See the Base Definitions volume of
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IEEE\ Std\ 1003.1-2001, Section 8.2, Internationalization Variables
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for
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the precedence of internationalization variables used to determine
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the values of locale categories.)
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.TP 7
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\fILC_ALL\fP
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If set to a non-empty string value, override the values of all the
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other internationalization variables.
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.TP 7
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\fILC_CTYPE\fP
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Determine the locale for the interpretation of sequences of bytes
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of text data as characters (for example, single-byte as
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opposed to multi-byte characters in arguments).
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.TP 7
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\fILC_MESSAGES\fP
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Determine the locale that should be used to affect the format and
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contents of diagnostic messages written to standard
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error.
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.TP 7
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\fINLSPATH\fP
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Determine the location of message catalogs for the processing of \fILC_MESSAGES
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\&.\fP
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.sp
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.SH ASYNCHRONOUS EVENTS
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.LP
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Default.
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.SH STDOUT
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.LP
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If no \fIfile\fP operands are specified, or if a \fIfile\fP operand
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is \fB'-'\fP , or if the \fB-c\fP option is specified,
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the standard output contains the compressed output.
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.SH STDERR
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.LP
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The standard error shall be used only for diagnostic and prompt messages
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and the output from \fB-v\fP.
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.SH OUTPUT FILES
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.LP
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The output files shall contain the compressed output. The format of
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compressed files is unspecified and interchange of such
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files between implementations (including access via unspecified file
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sharing mechanisms) is not required by
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IEEE\ Std\ 1003.1-2001.
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.SH EXTENDED DESCRIPTION
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.LP
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None.
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.SH EXIT STATUS
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.LP
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The following exit values shall be returned:
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.TP 7
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\ 0
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Successful completion.
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.TP 7
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\ 1
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An error occurred.
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.TP 7
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\ 2
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One or more files were not compressed because they would have increased
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in size (and the \fB-f\fP option was not
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specified).
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.TP 7
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>2
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An error occurred.
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.sp
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.SH CONSEQUENCES OF ERRORS
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.LP
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The input file shall remain unmodified.
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.LP
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\fIThe following sections are informative.\fP
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.SH APPLICATION USAGE
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.LP
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The amount of compression obtained depends on the size of the input,
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the number of \fIbits\fP per code, and the distribution of
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common substrings. Typically, text such as source code or English
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is reduced by 50-60%. Compression is generally much better than
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that achieved by Huffman coding or adaptive Huffman coding ( \fIcompact\fP),
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and takes less time to compute.
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.LP
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Although \fIcompress\fP strictly follows the default actions upon
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receipt of a signal or when an error occurs, some unexpected
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results may occur. In some implementations it is likely that a partially
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compressed file is left in place, alongside its
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uncompressed input file. Since the general operation of \fIcompress\fP
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is to delete the uncompressed file only after the \fB.Z\fP
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file has been successfully filled, an application should always carefully
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check the exit status of \fIcompress\fP before
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arbitrarily deleting files that have like-named neighbors with \fB.Z\fP
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suffixes.
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.LP
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The limit of 14 on the \fIbits\fP option-argument is to achieve portability
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to all systems (within the restrictions imposed by
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the lack of an explicit published file format). Some implementations
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based on 16-bit architectures cannot support 15 or 16-bit
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uncompression.
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.SH EXAMPLES
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.LP
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None.
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.SH RATIONALE
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.LP
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None.
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.SH FUTURE DIRECTIONS
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.LP
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None.
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.SH SEE ALSO
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.LP
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\fIuncompress\fP , \fIzcat\fP
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.SH COPYRIGHT
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Portions of this text are reprinted and reproduced in electronic form
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from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
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-- Portable Operating System Interface (POSIX), The Open Group Base
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Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
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Electrical and Electronics Engineers, Inc and The Open Group. In the
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event of any discrepancy between this version and the original IEEE and
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The Open Group Standard, the original IEEE and The Open Group Standard
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is the referee document. The original Standard can be obtained online at
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http://www.opengroup.org/unix/online.html .
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