Unpack（）在不同的机器上提供不同的结果

Question

I have some strange behavior with the unpack function. I have a packed string, stored as longblob in a mysql database. When I read that string and unpack it, it gives me an array, so far so good. But when I run this on another machine some of the values in the array are different.

When I dump the data from mysql, they are equal on both machines.

Unpacking is done this way:

$array = unpack("N*", $packed); 

$array should then be like this (and it is on one machine)

Array
(
    [1]  => 179848175
    [2]  => -16214255
    [3]  => 179848175
    [4]  => -16214255
    [5]  => 179848175
    [6]  => -16214255
    [7]  => 179999949
    [8]  => -16152916
    [9]  => 179999277
    [10] => -16168574
...
)

But on the other machine it is like this:

Array
(
    [1]  => 179848175
    [2]  => 427853622
    [3]  => 179848175
    [4]  => 427853622
    [5]  => 179848175
    [6]  => 427853622
    [7]  => 179999949
    [8]  => 427853423
    [9]  => 179999277
    [10] => 427853341
...
)

Every second value seems to be different.

I have tested this on three different machines, on two everything was fine, but on that one machine I get that weird output.

One machine is running PHP 5.6.3 (here it is ok), two machines are running PHP 5.5.14 (on one it is ok, on the other not)

Answer 1

The pack format N means unsigned long, which means it can't be negative. However, you are storing negative values, and they are the ones that aren't being unpacked the way you want. PHP does not have a pack format for machine-independent signed longs; it only supports packing them in machine byte order, which may not be compatible from machine to machine. Thus you'll have to make the values signed yourself.

To convert your array items into signed values:

for ($i = 1; $i <= count($array); $i++) {
    // Check for a greater-than-32-bit environment,
    // and check if the number should be negative
    // (i.e., if the high bit is set in 32-bit notation).
    if (PHP_INT_SIZE > 4 && $array[$i] & 0x80000000) {
        // A negative number was unpacked as an unsigned
        // long in a greater-than-32-bit environment.
        // Subtract the appropriate amount (max 32-bit
        // unsigned long + 1) to convert it to negative.
        $array[$i] = $array[$i] - 0x100000000;
    }
}

var_dump($array);