perlunpack

Unpack function requirement in Perl for varying data sizes

I have a function in Perl that reads a packet from a template in this way:

sub read_packet {
  my $contents = shift;
  my @decode = unpack('nnnN', $contents);

  my $version = $decode[0];
  my $identifier = $decode[1];
  my $type = $decode[2];
  my $size = $decode [3];

  @contents = ($version, $identifier, $type, $size);

  return (@contents);
}

A change was required to the template in the unpack function. Now $identifier must support 32 bits, so according to PerlMonks: Pack/Unpack Tutorial (aka How the System Stores Data) (N) -> n,N A 16/32 bit value in "network" (big-endian) order.

So the function was rewritten doing this small change. Now, the program that sends the "packet" could be sending 16/32 bit values. The customer required from me to support both 16/32 bit. What can I do to know when a packet contains an 16 or 32 bit identifier?

Solution

  • If those four fields are all that the packet contains then you can check the length of $contents

    However, if there is data following those four fields then you must try the most commonunpack template and check the result for sanity, using the alternative template if it fails