Re: packing a C structure

On Oct 30, 4:19 am, rams <haai...@xxxxxxxxx> wrote:

Thanks a lot for your response.The pack command you provided gave me
much needed output.I got this almost done.

You're very welcome. I'm glad I could help.


The only problem right now
iam facing is to convert a variable into a 6 byte hexa value.
i have variable $i=123456789012 then i would like to convert this into
a hexa value 12 34 56 78 90 12
and i would like this hexa value also to pack along with my
structure..

Okay, before you do this, you must realize that $i (or
123456789012) should be a string (surrounded by quotes) and NOT a
number -- otherwise, any leading zeroes (or letters A-F) you had in
your number could be lost or misinterpreted.

So if you had a line like:

my $i = "123456789012"; # notice the "" marks

and you wanted a string with the bytes 12 34 56 78 90 12
(hexadecimal), you should use the "H*" pack-string, like this:

my $string = pack('H*', $i);

Now $string is equal to "\x{12}\x{34}\x{56}\x{78}\x{90}\x{12}".
You can do this to prove it to yourself (remember to use "eq" (and not
"==") when comparing strings) with the following code:

if ($string eq "\x{12}\x{34}\x{56}\x{78}\x{90}\x{12}")
{
print "The strings are equal!\n"
}

The "H*" pack template tells pack() to interpret the variable it
packs as the hexadecimal values of the bytes the final (packed) string
will have.

If that's a confusing explanation for you, consider this unpack()
line:

$i = unpack('H*', $string);

This line reads the $string and returns another string which contains
the $string's values in hexadecimal form. The pack() example,
naturally, does the exact opposite.

If you want (or need) a quick way to check the $string contents
(for debugging purposes), you can use this line of code:

printf "0x%x\n", ord($_) foreach split(//, $string);

This will print out each character's hexadecimal value, one character
at a time. In fact, if $string is indeed what you wanted (that is,
made up of bytes 12 34 56 78 90 12 (hexadecimal)), then this line of
code should give you the following output:

0x12
0x34
0x56
0x78
0x90
0x12

I hope this helps, Rams.

Take care and God bless.

-- Jean-Luc Romano

.

Relevant Pages

  • Re: Pack a linked DB file
    ... > lpReserved As String ... > Public Sub ShellWait ... > Dim proc As PROCESS_INFORMATION ... > pack all .dbf files in the folder. ...
    (microsoft.public.access.externaldata)
  • Re: packing a C structure
    ... giving the same result.how to send that value as a hex itself.. ... You said that when you tried sending 271 through the socket, ... packing what will happen to the Null Character of the string. ... if you're confused about how to use the pack() ...
    (comp.lang.perl.misc)
  • Re: unpack "B*", 15
    ... You seem to be misunderstanding what unpack and pack do, ... pack takes a list of perl values, and packs them into a 'string', which ... packing of structs from a perl program in a simiar way a C program would ...
    (comp.lang.perl.misc)
  • Re: packing a C structure
    ... char b; ... I have the impression that what you want is to pack values in Perl ... into a binary string of data that matches how a C program would do it. ... maximum of 15 characters), you probably want to replace "c16" in the ...
    (comp.lang.perl.misc)
  • Re: java ByteArrayOutputStream alternative in ruby.
    ... I want to pack bytes integers and all ... But My problem is I collect binary data as the program runs and then ... Well, String and Array can both be added too as needed, so I'm not sure how they fail to meet your needs. ... concatenate those two packed values, ...
    (comp.lang.ruby)