[Mono-dev] Patch for IntPtr bug

[russell.kay at realtimeworlds.com]

Replying to myself here

Using the output from this small program

using System;

namespace ConsoleApplication4
{
    enum Ptr : long
    {
    }

    class Program
    {
        static unsafe void Main(string[] args)
        {
            IntPtr test = new IntPtr(0xffffffff);
            Ptr testLong = (Ptr)test.ToInt64();
            Console.WriteLine("test = {0}, testLong={1}", test,
testLong);
            IntPtr testAnother = (IntPtr)testLong;
            Console.WriteLine("testAnother = {0}", testAnother);
            void* p = test.ToPointer();
            Console.WriteLine("p = {0}", (long)p);
        }
    }
}

You will see that under MS.NET the output is 

test = 4294967295, testLong=4294967295
testAnother = 4294967295
p = 4294967295

note the complete absence of sign extended values.

The output from Mono on OpenSuse 11.0 Linux

test = -1, testLong=-1
testAnother = -1
p = 4294967295

So the pointer conversion from void* to long is not sign extending under
Mono, and it is different output to MS.NET.

Russell

-----Original Message-----
From: mono-devel-list-bounces at lists.ximian.com
[mailto:mono-devel-list-bounces at lists.ximian.com] On Behalf Of
russell.kay at realtimeworlds.com
Sent: 11 February 2009 15:45
To: robertj at gmx.net; mono-devel-list at lists.ximian.com
Subject: Re: [Mono-dev] Patch for IntPtr bug

We get addresses on Linux that are >2Gb from malloc in the native code,
if we go through the conversion from int (i.e.)IntPtr.ToInt64() is not
giving a sign extended value back when this happens and then a cast to
IntPtr will fail. The cast from a pointer i.e. void* or byte* will also
not sign extend the long.

The conversion is not sign extending the long (perhaps because it does
actually fit)...

My patch is correct.

Russell

-----Original Message-----
From: mono-devel-list-bounces at lists.ximian.com
[mailto:mono-devel-list-bounces at lists.ximian.com] On Behalf Of Robert
Jordan
Sent: 11 February 2009 15:24
To: mono-devel-list at lists.ximian.com
Subject: Re: [Mono-dev] Patch for IntPtr bug

russell.kay at realtimeworlds.com wrote:

> I encountered a problem when casting a long to an IntPtr, which is
> something we have ended up doing a lot (we want to keep binary
assembly
> compatibility between x86 and x64, only changing the native code on
the
> different architectures). We encountered a problem with Mono as there
> has been a check introduced into the IntPtr constructor from a long,
> this is checking the range of the long to ensure it is in range,
> unfortunately the check is incorrect. A long can easily hold from
> Int32.MinValue to UInt32.MaxValue (and not Int32.MaxValue as it
> currently in there).
> 

The patch is wrong, since casting a long > Int32.MaxValue to a
4 byte signed intptr is simply invalid.

The current check in IntPtr.cs also matches MS.NET.

Robert

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