PERL Handling Binary Scalar

1 [Question]:

I adopt select()...sysread()...syswrite() mechanism to handle socket messages, where messages are sysread() into $buffer (binary) before they are syswritten.

Now I want to change two bytes of the message, which denote the length of the whole message. At first, I use following code:

my $msglen=substr($buffer,0,2); # Get the first two bytes
my $declen=hex($msglen);
+= 3;
($buffer,0,2,$declen); # change the length

However, it doesn't work in this way. If the final value of $declen is 85, then the modified $buffer will be "0x35 0x35 0x00 0x02...". I insert digital number to $buffer but finally got ASCII!

I also tried this way:

my $msglen=substr($buffer,0,2); # Get the first two bytes,binary
+= 0b11; # Or $msglen += 3;
my $msgbody=substr($buffer,2); # Get the rest part of message, binary
=join("", $msglen, $msgbody);

Sadly, this method also failed. The result is such as"0x33 0x 0x00 0x02..." I just wonder why two binary scalar can't be joined into a binary scalar?

Can you help me? Thank you!

[Answer 1]: 


my $msglen=substr($buffer,0,2); # Get the first two bytes
my $number = unpack("S",$msglen);
+= 3;
my $number_bin = pack("S",$number);
($buffer,0,2,$number_bin); # change the length


[Answer 2]:



Perl code
my $buffer = "/x00/x01"; print unpack("H*", $buffer), "/n"; vec($buffer, 0, 16) += 3; print unpack("H*", $buffer), "/n";


I am writing a Perl program that sends and receives messages from two sockets and acta as a switch. I have to modify the received messages received from one socket, prepend 3 bytes to the data, and finally send the modified messages to another socket. I adopt select()...sysread()...syswrite() mechanism to poll for messages between sockets. The received messages are stored in $buffer during modification.

Now I can use following way to get the received messages:

my $hexmsg = unpack("H*", $buffer);
my @msg = ( $hexmsg =~ m/../g );

then I can insert 3 bytes to @msg. However, I don't know how to pack the message in @msg into a scalar(such as $buffer) and send it to another socket by syswrite(). Can anybody help me? Thank you in advance!

BTW, are messages in $buffer binary?



Yes, messages in $buffer are binary (if I'm guessing what you mean by that correctly). If your only reason for unpacking it into @msg is to insert the bytes, don't. Use substr instead, and just write out the changed $buffer. For instance:

substr( $buffer, 0, 0, "/x01/x02/x03" ); # insert 3 bytes at beginning.

If you are doing other things with @msg, you could continue to use that as well as doing the substr insert before writing it out, or you could use substr or pack or split or vec or a regex to parse out the pieces you need. You'd need to describe what you are doing to get more specific help.