unpack

add a note User Contributed Notes

Justin dot SpahrSummers at gmail dot com
09-Oct-2005 03:10


I hadn't realized that if the number after the unpack type was 1 (i.e. "V1page"), that it would behave as if there was no number at all. I had been using a variable and didn't think to watch for this. For instance,



<?php



if ($something)

   $get = 2;

else

   $get = 1;



$arr = unpack("V" . $get . "page", $data);



?>



Now if $something was FALSE, then $arr will only have one entry named "page". If $something was TRUE, $arr would have "page1" and "page2".

Cristiano - finatofinato at yahoo dot com dot br
19-Jul-2005 09:31


STORE FILES IN DATABASE SQL_SERVER



After days of research, I obtained a solution to my "big-problem" working with php5/sqlserver/iis



//IN PHP.INI

//Valid range 0 - 2147483647.  Default = 4096.

mssql.textsize = 2147483647



=============<INSERT>==================

$dataString = file_get_contents($_FILES['myfile']['tmp_name']);

$arrData = unpack("H*hex", $dataString);

$ds = "0x".$arrData['hex'];



$sql = "SET TEXTSIZE 2147483647";

mssql_query($sql, $conn);    

$sql = " insert into table (myimage, name, extension) values (".$ds.", '".$name."', '".$extension."') ";

mssql_query($sql, $conn);    

=============</INSERT>==================



=============<RETRIEVE>==================

$sql = "SET TEXTSIZE 2147483647";

@mssql_query($sql, $conn);    

$sql = " select myimage, name, extension from table ";

$rs = @mssql_query($sql, $conn);    



header("Content-Disposition: attachment; filename=".$rs['name'].".".$rs['extension']." ");

echo $rs['myfile'];        



=============</RETRIEVE>==================



with apache does not work in some cases (work with txt files, but not work with pdf, .doc....), and you will try to do another solution (the solution is more simple than this of sqlserver)

info at dreystone dot com
05-May-2005 02:31


Here is my solution to reading a Big-Endian formatted double on an Little-Endian machine.



<?php



function ToDouble($data) {

    $t = unpack("C*", pack("S*", 256));

    if($t[1] == 1) {

        $a = unpack("d*", $data);

    } else {

        $a = unpack("d*", strrev($data));

    }

    return (double)$a[1];

}



?>

MagicalTux at FF dot st
28-Apr-2005 10:17


tomlove at gmail dot com gave a code to easily unpack a string. Here's a variant :



<?php

$packed = pack("s*", 123);

list($unpacked) = unpack("s*0", $packed);

?>



$unpacked now holds the value 123, not an array.



unpack() will use numeric keys if you give only numbers (probably same check as is_numeric(), but I didn't test). This can be useful in some cases (like for getting directly your data without using arrays).

tomlove at gmail dot com
02-Jan-2005 01:32


If you have one packed item, here's a quick way to get it out of its array when unpacking:



<?

$packed = pack("s*", 123);

list(,$unpacked) = unpack("s*", $packed);

?>



$unpacked now holds the value 123, not an array.

jjfoerch at earthlink dot net
21-Oct-2004 07:57


I had a situation where I had to unpack a file filled with little-endian order double-floats in a way that would work on either little-endian or big-endian machines.  PHP doesn't have a formatting code that will change the byte order of doubles, so I wrote this workaround.



<?php

/*The following code is a workaround for php's unpack function

which does not have the capability of unpacking double precision

floats that were packed in the opposite byte order of the current

machine.

*/

function big_endian_unpack ($format, $data) {

    $ar = unpack ($format, $data);

    $vals = array_values ($ar);

    $f = explode ('/', $format);

    $i = 0;

    foreach ($f as $f_k => $f_v) {

    $repeater = intval (substr ($f_v, 1));

    if ($repeater == 0) $repeater = 1;

    if ($f_v{1} == '*')

    {

        $repeater = count ($ar) - $i;

    }

    if ($f_v{0} != 'd') { $i += $repeater; continue; }

    $j = $i + $repeater;

    for ($a = $i; $a < $j; ++$a)

    {

        $p = pack ('d',$vals[$i]);

        $p = strrev ($p);

        list ($vals[$i]) = array_values (unpack ('d1d', $p));

        ++$i;

    }

    }

    $a = 0;

    foreach ($ar as $ar_k => $ar_v) {

    $ar[$ar_k] = $vals[$a];

    ++$a;

    }

    return $ar;

}



list ($endiantest) = array_values (unpack ('L1L', pack ('V',1)));

if ($endiantest != 1) define ('BIG_ENDIAN_MACHINE',1);

if (defined ('BIG_ENDIAN_MACHINE')) $unpack_workaround = 'big_endian_unpack';

else $unpack_workaround = 'unpack';

?>



This workaround is used like this:



<?php



function foo() {

        global $unpack_workaround;

    $bar = $unpack_workaround('N7N/V2V/d8d',$my_data);

//...

}



?>



On a little endian machine, $unpack_workaround will simply point to the function unpack.  On a big endian machine, it will call the workaround function.



Note, this solution only works for doubles.  In my project I had no need to check for single precision floats.

kennwhite dot nospam at hotmail dot com
29-Aug-2004 03:32


If having a zero-based index is useful/necessary, then instead of:



$int_list = unpack("s*", $some_binary_data);



 try:



$int_list = array_merge(unpack("s*", $some_binary_data));



This will return a 0-based array:



$int_list[0] = x

$int_list[1] = y

$int_list[2] = z

...



rather than the default 1-based array returned from unpack when no key is supplied:



$int_list[1] = x

$int_list[2] = y

$int_list[3] = z

...



It's not used often, but array_merge() with only one parameter will compress a sequentially-ordered numeric-index, starting with an index of [0].

Sergio Santana: ssantana at tlaloc dot imta dot mx
10-Jul-2004 01:54


This is about the last example of my previous post. For the sake of clarity, I'm including again here the example, which expands the one given in the formal documentation:



<?

  $binarydata = "AA\0A";

  $array = unpack("c2chars/nint", $binarydata);

  foreach ($array as $key => $value)

     echo "\$array[$key] = $value <br>\n";

?>



This outputs:



$array[chars1] = 65 

$array[chars2] = 65 

$array[int] = 65 



Here, we assume that the ascii code for character 'A' is decimal 65.



Remebering that the format string structure is:

<format-code> [<count>] [<array-key>] [/ ...],

in this example, the format string instructs the function to

  1. ("c2...") Read two chars from the second argument ("AA ...), 

  2. (...chars...) Use the array-keys "chars1", and "chars2" for 

      these two chars read,

  3. (.../n...) Read a short int from the second argument (...\0A"),

  4. (...int") Use the word "int" as the array key for the just read

      short.



I hope this is clearer now,



Sergio.

Sergio Santana: ssantana at tlaloc dot imta dot mx
09-Jul-2004 10:41


Suppose we need to get some kind of internal representation of an integer, say 65, as a four-byte long. Then we use, something like:



<?

  $i = 65;

  $s = pack("l", $i); // long 32 bit, machine byte order

  echo strlen($s) . "<br>\n";

  echo "***$s***<br>\n";

?>



The output is:



X-Powered-By: PHP/4.1.2

Content-type: text/html



4

***A*** 



(That is the string "A\0\0\0")



Now we want to go back from string "A\0\0\0" to number 65. In this case we can use:



<?

  $s = "A\0\0\0"; // This string is the bytes representation of number 65

  $arr = unpack("l", $s);

  foreach ($arr as $key => $value)

     echo "\$arr[$key] = $value<br>\n";

?>



And this outpus:

X-Powered-By: PHP/4.1.2

Content-type: text/html



$arr[] = 65



Let's give the array key a name, say "mykey". In this case, we can use: 



<?

  $s = "A\0\0\0"; // This string is the bytes representation of number  65

  $arr = unpack("lmykey", $s);

  foreach ($arr as $key => $value)

     echo "\$arr[$key] = $value\n";

?>



An this outpus:

X-Powered-By: PHP/4.1.2

Content-type: text/html



$arr[mykey] = 65



The "unpack" documentation is a little bit confusing. I think a more complete example could be:



<?

  $binarydata = "AA\0A";

  $array = unpack("c2chars/nint", $binarydata);

  foreach ($array as $key => $value)

    echo "\$array[$key] = $value <br>\n";

?>



whose output is:



X-Powered-By: PHP/4.1.2

Content-type: text/html



$array[chars1] = 65 <br>

$array[chars2] = 65 <br>

$array[int] = 65 <br>



Note that the format string is something like

<format-code> [<count>] [<array-key>] [/ ...]



I hope this clarifies something



Sergio

David Alsbury
25-Apr-2003 05:04


This is the best example I have found to use unpack.  I was finally able to make sense of the documentation afters seeing how they used the function.



http://fooassociates.com/phpfer/html/rn45re878.html



Quick Example:



<?

header("Content-type: text/plain");



/*

N unsigned long (always 32 bit, big endian byte order)

N = 4 bytes

"N2int_var" will read 8 bytes into 2 vars int_var1 and int_var2



binary string A324 = decimal 1,093,874,228

binary string &*12 = decimal 640,299,314



C unsigned char

"Cchar_var" will read 1 byte into char_var



binary string J = decimal 74

*/



$assoc_array = unpack("N2int_var/Cchar_var", "A324&*12J");

echo "int_var1 = ", $assoc_array['int_var1'], "\n";

echo "int_var2 = ", $assoc_array['int_var2'], "\n";

echo "char_var = ", $assoc_array['char_var'], "\n";

?>



Sample Output:

int_var1 = 1093874228

int_var2 = 640299314

char_var = 74

adam at adeptsoftware dot com
17-Jun-2002 12:01


If you just want to extract a dword/long int from a binary string, the following code works beautifully (intel endian):



$Number = ord($Buffer{0}) | (ord($Buffer{1})<<8) | (ord($Buffer{2})<<16) | (ord($Buffer{3})<<24);

DanRichter.at.programmer.dot.net
11-Apr-2001 02:26


If no key name is given [e.g., unpack('C*',$data)], the keys are simply integers starting at 1, and you have a standard array. (I know of no way to get the array to start at zero.)


<P>


If you use multiple types, you must give a key name for all of them (except optionally one), because the key counter is reset with each slash. For example, in unpack('n2/C*',$data), indices 1 and 2 of the returned array are filled by integers ('n'), then overwritten with characters ('C').

iredden at redden dot on dot ca
12-Mar-2000 08:34


function parse_pascalstr($bytes_parsed, $parse_str) { 


$parse_info = unpack("x$bytes_parsed/cstr_len", $parse_str);


$str_len = $parse_info["str_len"]; 


$bytes_parsed = $bytes_parsed + 1; 


$parse_info = unpack("x$bytes_parsed/A".$str_len."str", $parse_str); 


$str = $parse_info["str"]; 


$bytes_parsed = $bytes_parsed + strlen($str); 





return array($str, $bytes_parsed); 


}

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unpack

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