August 1998
Intermediate to advanced
800 pages
39h 20m
English
You want to read a NUL-terminated string from a file, starting at a particular address.
Set $/ to an ASCII NUL, and read the string with
<>:
$old_rs = $/; # save old $/ $/ = "\0"; # NULL seek(FH, $addr, SEEK_SET) or die "Seek error: $!\n"; $string = <FH>; # read string chomp $string; # remove NULL $/ = $old_rs; # restore old $/
You can use local to save and restore
$/ if you want:
{
local $/ = "\0";
# ...
} # $/ is automatically restoredThe example program shown in Example 8.5, bgets , accepts a filename and one or more byte addresses as arguments. Decimal, octal, or hexadecimal addresses may be specified. For each address, the program reads and prints the NULL- or EOF-terminated string at that position:
Example 8-5. bgets
#!/usr/bin/perl
# bgets - get a string from an address in a binary file
use IO::Seekable;
($file, @addrs) = @ARGV or die "usage: $0 file addr ...";
open(FH, $file) or die "cannot open $file: $!";
$/ = "\000";
foreach $addr (@addrs) {
$addr = oct $addr if $addr =~ /^0/;
seek(FH, $addr, SEEK_SET)
or die "can't seek to $addr in $file: $!";
printf qq{%#x %#o %d "%s"\n}, $addr, $addr, $addr, scalar <>;
}Here’s a simple implementation of the Unix strings program:
Example 8-6. strings
#!/usr/bin/perl
# strings - pull strings out of a binary file
$/ = "\0";
while (<>) {
while (/([\040-\176\s]{4,})/g) {
print $1, "\n";
}
}
The seek, getc, and
ord functions in perlfunc(1)
and in Chapter 3 of Programming ...