1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
|
#!/usr/bin/perl
my ($chunk, $seek, $bytes) = @ARGV;
$bytes =~ s/../chr(hex($&))/ge;
binmode STDIN;
binmode STDOUT;
# A few helpers to read bytes, or read and copy them to the
# output.
sub get {
my $n = shift;
return unless $n;
read(STDIN, my $buf, $n)
or die "read error or eof: $!\n";
return $buf;
}
sub copy {
my $buf = get(@_);
print $buf;
return $buf;
}
# read until we find table-of-contents entry for chunk;
# note that we cheat a bit by assuming 4-byte alignment and
# that no ToC entry will accidentally look like a header.
#
# If we don't find the entry, copy() will hit EOF and exit
# (which should cause the caller to fail the test).
while (copy(4) ne $chunk) { }
my $offset = unpack("Q>", copy(8));
# In clear mode, our length will change. So figure out
# the length by comparing to the offset of the next chunk, and
# then adjust that offset (and all subsequent) ones.
my $len;
if ($seek eq "clear") {
my $id;
do {
$id = copy(4);
my $next = unpack("Q>", get(8));
if (!defined $len) {
$len = $next - $offset;
}
print pack("Q>", $next - $len + length($bytes));
} while (unpack("N", $id));
}
# and now copy up to our existing chunk data
copy($offset - tell(STDIN));
if ($seek eq "clear") {
# if clearing, skip past existing data
get($len);
} else {
# otherwise, copy up to the requested offset,
# and skip past the overwritten bytes
copy($seek);
get(length($bytes));
}
# now write out the requested bytes, along
# with any other remaining data
print $bytes;
while (read(STDIN, my $buf, 4096)) {
print $buf;
}
|
