[Mono-list] Problem using p/invoke on mono 1.0.1

Marcus mathpup@mylinuxisp.com
Mon, 23 Aug 2004 02:50:59 -0500

I think that the problem has to do with some changes that were made to Mono 
regarding its handling of strings as return types from P/Invoke method. 
Behind the scenes, a char* is being returned, and the runtime converts the 
char* into a System.String. In addition, MS.NET calls CoTaskFree() on the 
char*. At one time Mono did not do this, but for compatibility with MS.NET, 
its behavior changed. Apparently, libcrypt does not want user to free the 
pointer that crypt() returns.

The work-around for this is to have your method return an IntPtr and then 
convert the IntPtr to a string manually. For example, use the following in 
the place of your WriteLine() statement:

	string encrypted = Marshal.PtrToStringAnsi(UnixCrypt(arg,arg));
	Console.WriteLine("UnixEncrypting {0} : {1}",arg,encrypted);

> I am trying to use libcrypt.so.2.3.2 with c#. This code has been working 
> for quite a while but stopped to work now.
> here is a little sample program I wrote
> <snip>
> using System;
> using System.Text;
> using System.Security.Cryptography;
> using System.Web.Security;
> using System.Runtime.InteropServices;
> namespace CryptTest
> {
>      class CryptTest
>          {
>          [DllImport ("crypt", EntryPoint="crypt", SetLastError=true)]
>          private static extern string UnixCrypt(string text, string salt);
>          [STAThread]
>          static void Main(string[] args)
>              {
>              Console.WriteLine("have {0} args",args.Length);
>              string arg = args[0];
>              Console.WriteLine("UnixEncrypting {0} : 
> {1}",arg,UnixCrypt(arg,arg));
>          }
>      }
> }
> </snip>
> When I try to run this with
> mono encrypt.exe rainer
> I get a NullReference Exception. I am a bit lost here since this code has 
> been working beautifully for months. Either my debian upgraded its 
> libcrypt version (which could easily be true) or upgrading to mono-1.0.1 
> introduced the problem (which is less likely since I remember having the 
> same issue on mono-1.0).