ЭЛЕКТРОННАЯ БИБЛИОТЕКА КОАПП |
Сборники Художественной, Технической, Справочной, Английской, Нормативной, Исторической, и др. литературы. |
8.13. Updating a Random-Access FileProblemYou want to read in an old record from a binary file, change its values, and write back the record. SolutionAfter 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: $!"; DiscussionYou don't have to use anything fancier than The example program shown in Example 8.4, weekearly, takes one argument: the user whose record you want to backdate by a week. (Of course, in practice, you wouldn't really want to (nor be able to!) mess with the system accounting files.) This program requires write access to the file to be updated, since it opens the file in update mode. After fetching and altering the record, it packs it up again, skips backwards in the file one record, and writes it out. Example 8.4: weekearly#!/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: $!"; See AlsoThe |