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Perl Cookbook
book

Perl Cookbook

by Tom Christiansen, Nathan Torkington
August 1998
Intermediate to advanced content levelIntermediate to advanced
800 pages
39h 20m
English
O'Reilly Media, Inc.
Content preview from Perl Cookbook

Reading a String from a Binary File

Problem

You want to read a NUL-terminated string from a file, starting at a particular address.

Solution

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 restored

Discussion

The 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";
    }
}
                  
                  

See Also

The seek, getc, and ord functions in perlfunc(1) and in Chapter 3 of Programming ...

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Publisher Resources

ISBN: 1565922433Supplemental ContentCatalog PageErrata