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.TH AR 6
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.SH NAME
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ar \- archive (library) file format
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.SH SYNOPSIS
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.B #include <ar.h>
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.SH DESCRIPTION
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The archive command
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.IR ar (1)
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is used to combine several files into
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one.
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Archives are used mainly as libraries to be searched
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by the loaders
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.IR 8l (1)
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.I et al.
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.PP
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A file produced by
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.I ar
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has a magic string at the start,
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followed by the constituent files, each preceded by a file header.
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The magic number and header layout as described in the
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include file are:
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.IP
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.EX
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.ta \w'#define 'u +\w'SAR_HDR 'u
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.ec %
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#define	ARMAG	"!<arch>\n"
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#define	SARMAG	8
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#define	ARFMAG	"`\n"
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struct ar_hdr {
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	char	name[16];
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	char	date[12];
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	char	uid[6];
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	char	gid[6];
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	char	mode[8];
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	char	size[10];
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	char	fmag[2];
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};
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#define	SAR_HDR	60
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.ec \
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.EE
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.LP
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The
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.B name
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is a blank-padded string.
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The
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.L fmag
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field contains
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.L ARFMAG
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to help verify the presence of a header.
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The other fields are left-adjusted, blank-padded numbers.
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They are decimal except for
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.LR mode ,
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which is octal.
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The date is the modification date of the file (see
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.IR stat (2))
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at the time of its insertion into the archive.
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The mode is the low 9 bits of the file permission mode.
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The length of the header is
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.LR SAR_HDR .
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Because the
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.L ar_hdr
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structure is padded in an architecture-dependent manner,
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the structure should never be read or written as a unit;
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instead, each field should be read or written independently.
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.PP
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Each file begins on an even (0 mod 2) boundary;
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a newline is inserted between files if necessary.
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Nevertheless
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.B size
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reflects the
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actual size of the file exclusive of padding.
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.PP
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When all members of an archive are object files of
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the same architecture,
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.B ar
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automatically adds an extra file, named
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.BR __.SYMDEF ,
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as the first member of the archive.  This file
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contains an index used by the loaders to locate all
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externally defined text and data symbols in the archive.
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.PP
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There is no provision for empty areas in an archive
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file.
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.SH "SEE ALSO"
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.IR ar (1),
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.IR 8l (1),
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.IR nm (1),
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.IR stat (2)
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.SH BUGS
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The
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.B uid
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and
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.B gid
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fields are unused in Plan 9.
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They provide compatibility with Unix
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.I ar
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format.