From cb58f3291cd93a50489d646a198f5437fdffc3ef Mon Sep 17 00:00:00 2001 From: sean Date: Wed, 15 Jan 2020 11:54:20 +0000 Subject: compress: import Plan9 compress Add #define USED(x)... boilerplate compress: import Plan9 manpage. --- man/man1/compress.1 | 237 ++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 237 insertions(+) create mode 100644 man/man1/compress.1 (limited to 'man/man1/compress.1') diff --git a/man/man1/compress.1 b/man/man1/compress.1 new file mode 100644 index 00000000..7dda3e18 --- /dev/null +++ b/man/man1/compress.1 @@ -0,0 +1,237 @@ +.TH COMPRESS 1 +.SH NAME +compress, uncompress, zcat \- compress and expand data +.SH SYNOPSIS +.B compress +[ +.B \-f +] [ +.B \-v +] [ +.B \-c +] [ +.B \-V +] [ +.B \-b +.I bits +] [ +.I "name \&..." +] +.PP +.B uncompress +[ +.B \-f +] [ +.B \-v +] [ +.B \-c +] [ +.B \-V +] [ +.I "name \&..." +] +.PP +.B zcat +[ +.B \-V +] [ +.I "name \&..." +] +.SH DESCRIPTION +.I Compress +reduces the size of the named files using adaptive Lempel-Ziv coding. +Whenever possible, +each file is replaced by one with the extension +.B "\&.Z," +while keeping the same ownership modes, access and modification times. +If no files are specified, the standard input is compressed to the +standard output. +Compressed files can be restored to their original form using +.I uncompress +or +.I zcat. +.PP +The +.B \-f +option will force compression of +.I name. +This is useful for compressing an entire directory, +even if some of the files do not actually shrink. +If +.B \-f +is not given and +.I compress +is run in the foreground, +the user is prompted as to whether an existing file should be overwritten. +.PP +The +.B \-c +option makes +.I compress/uncompress +write to the standard output; no files are changed. +The nondestructive behavior of +.I zcat +is identical to that of +.I uncompress +.B \-c. +.PP +.I Compress +uses the modified Lempel-Ziv algorithm popularized in +"A Technique for High Performance Data Compression", +Terry A. Welch, +.I "IEEE Computer," +vol. 17, no. 6 (June 1984), pp. 8-19. +Common substrings in the file are first replaced by 9-bit codes 257 and up. +When code 512 is reached, the algorithm switches to 10-bit codes and +continues to use more bits until the +limit specified by the +.B \-b +flag is reached (default 16). +.I Bits +must be between 9 and 16. The default can be changed in the source to allow +.I compress +to be run on a smaller machine. +.PP +After the +.I bits +limit is attained, +.I compress +periodically checks the compression ratio. If it is increasing, +.I compress +continues to use the existing code dictionary. However, +if the compression ratio decreases, +.I compress +discards the table of substrings and rebuilds it from scratch. This allows +the algorithm to adapt to the next "block" of the file. +.PP +Note that the +.B \-b +flag is omitted for +.I uncompress, +since the +.I bits +parameter specified during compression +is encoded within the output, along with +a magic number to ensure that neither decompression of random data nor +recompression of compressed data is attempted. +.PP +.ne 8 +The amount of compression obtained depends on the size of the +input, the number of +.I bits +per code, and the distribution of common substrings. +Typically, text such as source code or English +is reduced by 50\-60%. +Compression is generally much better than that achieved by +Huffman coding (as used in +.IR pack ), +or adaptive Huffman coding +.RI ( compact ), +and takes less time to compute. +.PP +Under the +.B \-v +option, +a message is printed yielding the percentage of +reduction for each file compressed. +.PP +If the +.B \-V +option is specified, the current version and compile options are printed on +stderr. +.PP +Exit status is normally 0; +if the last file is larger after (attempted) compression, the status is 2; +if an error occurs, exit status is 1. +.SH "SEE ALSO" +pack(1), compact(1) +.SH "DIAGNOSTICS" +Usage: compress [\-dfvcV] [\-b maxbits] [file ...] +.in +8 +Invalid options were specified on the command line. +.in -8 +Missing maxbits +.in +8 +Maxbits must follow +.BR \-b \. +.in -8 +.IR file : +not in compressed format +.in +8 +The file specified to +.I uncompress +has not been compressed. +.in -8 +.IR file : +compressed with +.I xx +bits, can only handle +.I yy +bits +.in +8 +.I File +was compressed by a program that could deal with +more +.I bits +than the compress code on this machine. +Recompress the file with smaller +.IR bits \. +.in -8 +.IR file : +already has .Z suffix -- no change +.in +8 +The file is assumed to be already compressed. +Rename the file and try again. +.in -8 +.IR file : +filename too long to tack on .Z +.in +8 +The file cannot be compressed because its name is longer than +12 characters. +Rename and try again. +This message does not occur on BSD systems. +.in -8 +.I file +already exists; do you wish to overwrite (y or n)? +.in +8 +Respond "y" if you want the output file to be replaced; "n" if not. +.in -8 +uncompress: corrupt input +.in +8 +A SIGSEGV violation was detected which usually means that the input file has +been corrupted. +.in -8 +Compression: +.I "xx.xx%" +.in +8 +Percentage of the input saved by compression. +(Relevant only for +.BR \-v \.) +.in -8 +-- not a regular file: unchanged +.in +8 +When the input file is not a regular file, +(e.g. a directory), it is +left unaltered. +.in -8 +-- has +.I xx +other links: unchanged +.in +8 +The input file has links; it is left unchanged. See +.IR ln "(1)" +for more information. +.in -8 +-- file unchanged +.in +8 +No savings is achieved by +compression. The input remains virgin. +.in -8 +.SH SOURCE +.B \*9/src/cmd/compress/compress.c +.SH "BUGS" +Although compressed files are compatible between machines with large memory, +.BR \-b \12 +should be used for file transfer to architectures with +a small process data space (64KB or less, as exhibited by the DEC PDP +series, the Intel 80286, etc.) -- cgit v1.2.3