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8.14. Reading a String from a Binary File

8.14.1. Problem

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

8.14.2. Solution

Ensure you're working with a binary file, set $/
to an ASCII NUL, and read the string with
<>:

binmode(FH);    # binary mode
$old_rs = $/;          # save old $/
$/ = "\0";           # ASCII 0: NUL
seek(FH, $addr, SEEK_SET)   or die "Seek error: $!\n";
$string = <FH>;       # read string
chomp $string;    # remove NUL
$/ = $old_rs;     # restore old $/

You can use local to save and restore
$/:

{
local $/ = "\0";
# ...
}  # $/ is automatically restored

8.14.3. 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 -w
# bgets - get a string from an address in a binary file
use IO::Seekable;
use open IO => ":raw"; # binary mode on all opened handles
($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 <>;
}

Example 8-6 is a simple implementation of
the Unix strings program.

Example 8-6. strings

  #!/usr/bin/perl -w
# strings - pull strings out of a binary file
$/ = "\0";
use open IO => ":raw";
while (<>) {
while (/([\040-\176\s]{4,})/g) {
print $1, "\n";
}
}

8.14.4. See Also

The PerlIO(3) manpage; the
seek, getc, and
ord functions in perlfunc(1)
and in Chapter 29 of Programming Perl; the
discussion of qq// in the "Quote and Quote-Like
Operators" section of the perlop(1) manpage, and
in the "Pick Your Own Quotes" section of Chapter 2 of
Programming Perl