perl 更新一个随机存取文件

[perl]代码库

#-----------------------------
use Fcntl;                          # for SEEK_SET and SEEK_CUR

$ADDRESS = $RECSIZE * $RECNO;
seek(FH, $ADDRESS, SEEK_SET)        or die "Seeking: $!";
read(FH, $BUFFER, $RECSIZE) == $RECSIZE
                                    or die "Reading: $!";
@FIELDS = unpack($FORMAT, $BUFFER);
# update fields, then
$BUFFER = pack($FORMAT, @FIELDS);
seek(FH, -$RECSIZE, SEEK_CUR)       or die "Seeking: $!";
print FH $BUFFER;
close FH                            or die "Closing: $!";
#-----------------------------
# download the following standalone program
#!/usr/bin/perl
# weekearly -- set someone's login date back a week
use User::pwent;
use IO::Seekable;

$typedef = 'L A12 A16';         # linux fmt; sunos is "L A8 A16"
$sizeof  = length(pack($typedef, ()));
$user    = shift(@ARGV) || $ENV{USER} || $ENV{LOGNAME};

$address = getpwnam($user)->uid * $sizeof;

open (LASTLOG, "+</var/log/lastlog")
    or die "can't update /usr/adm/lastlog: $!";
seek(LASTLOG, $address, SEEK_SET)
    or die "seek failed: $!";
read(LASTLOG, $buffer, $sizeof) == $sizeof
    or die "read failed: $!";

($time, $line, $host) = unpack($typedef, $buffer);
$time  -= 24 * 7 * 60 * 60;         # back-date a week
$buffer = pack($typedef, $time, $line, $time);

seek(LASTLOG, -$sizeof, SEEK_CUR)   # backup one record
    or die "seek failed: $!";
print LASTLOG $record;
    
close(LASTLOG)
    or die "close failed: $!";

#-----------------------------