Description:
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External libraries sometimes allow/require their clients to supply custom memory allocators so that they can behave nicely with the client applications.
When using such libraries in PHP FFI world, custom free() must iterate through malloc()ed memory one-by-one, comparing FFI\Cdata using == operator (to my knowledge):

$someLibMemMgmtStruct->malloc = function (int $size) use (&$memList) {
    $mem = $ffi->new("char[$size]");
    $ptr = FFI::addr($mem);
    $memList[] = [$ptr, $size, $mem];
    return $ptr;
};
$someLibMemMgmtStruct->free = function (FFI\Cdata $ptr) use (&$memList) {
    for ($i = count($memList); --$i >= 0; ) {
        if ($ptr == $memList[$i][0]) {
            array_splice($memList, $i, 1);
            return;
        }
    }
};

So it would be really helpful if FFI\Cdata has an interface to get a unique ID (e.g. memory address,) so that that ID can be used as a key when malloc() is called.

$someLibMemMgmtStruct->malloc = function (int $size) use (&$memList) {
    $mem = $ffi->new("char[$size]");
    $ptr = FFI::addr($mem);
    $memList[FFI::id($ptr)] = [$ptr, $size, $mem];
    return $ptr;
};
$someLibMemMgmtStruct->free = function (FFI\Cdata $ptr) use (&$memList) {
    unset($memList[FFI::id($ptr)]);
};

As another way of implementation, I considered of prepending an extra info when malloc()ed. But FFI\Cdata didn't like negative offset to be referenced...