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From: lwall@jpl-devvax.jpl.nasa.gov (Larry Wall)
Newsgroups: comp.lang.perl
Subject: Re: Binary fixed-length records
Message-ID: <1991Apr24.002156.7082@jpl-devvax.jpl.nasa.gov>
Date: 24 Apr 91 00:21:56 GMT
References: <1991Apr23.202307.2454@kfw.COM>
Reply-To: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Organization: Jet Propulsion Laboratory, Pasadena, CA
Lines: 49

In article <1991Apr23.202307.2454@kfw.COM> dan@kfw.com (Dan Mick) writes:
: Often, I want to dump a file full of fixed-length records.  The one way I've
: been using is
: 
: #!/usr/local/bin/perl
: 
: $stbuf_def="ccCCCCCCSSII"; 		# unpack codes for struct
: $streclen=20;				# length of above in bytes
: $/ = 0777;				# slurp in the whole file
: $_ = <>;				# into $_
: 
: $inputlen = length($_);
: $offset = 0;
: 
: while ($inputlen > 0) {
: 	$rec = substr($_, $offset, $streclen);
: 
: 	<unpack and print $rec>
: 
: 	$inputlen -= $streclen;
: 	$offset += $streclen;
: }
: 
: 
: ...but it strikes me that 1) the "slurp in whole file" will fail someday,
: and 2) the copy in the substr() can't be efficient.
: 
: Anyone willing to take a crack?  (I'm aware that the surrounding goo
: might not be as efficient as possible; what I'm mostly worried about are
: the two points above).

Recalling that read actually uses stdio for efficiency, and desiring to
emulate the <> semantics, which may be overkill, we get:

    #!/usr/local/bin/perl

    $stbuf_def = "ccCCCCCCSSII"; 		# unpack codes for struct
    $streclen = length(pack($stbuf_def, 0));	# length of above in bytes

    @ARGV = '-' unless @ARGV;

    foreach $file (@ARGV) {
	open(IN, $file) || do { warn "Can't open $file: $!\n"; next; };
	while (read(IN, $_, $streclen)) {
		<unpack and print $rec>
	}
    }

Larry