لطفا منتظر باشید ...
 |  |
8.13. Updating a Random-Access File
8.13.1. Problem
You want to read an old
record from a binary file, change its values, and write back the
record.
8.13.2. Solution
After read ing the old record,
pack up the updated values,
seek to the previous address, and write it
back.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: $!";
8.13.3. Discussion
You don't have to use anything fancier than print
in Perl to output a record. Remember that the opposite of
read is not write but
print, although oddly enough, the opposite of
sysread is
syswrite.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 backward in the
file one record, and writes it out.
Example 8-4. weekearly
#!/usr/bin/perl -w
# 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, "+<:raw", "/var/log/lastlog")
or die "can't update /var/log/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: $!";
8.13.4. See Also
The PerlIO(3) manpage; the
open, seek,
read, pack, and
unpack functions in the
perlfunc(1) and in Chapter 29 of
Programming Perl; Recipe 8.12; Recipe 8.14
|  | |
| 8.12. Using Random-Access I/O |  | 8.14. Reading a String from a Binary File |
Copyright © 2003 O'Reilly & Associates. All rights reserved.
