AccessViolationException when calling Marshal.PtrToStructure

Question

Hi,

I'm getting an AccessViolationExcpetion by calling Marshal.PtrToStructure(intPtr, typeof(Servent)). Any ideas what I have done wrong? I tried this on x64.

    IntPtr intPtr = NativeMethods.GetServByName(name, "tcp");
     if (intPtr != IntPtr.Zero)
     {
        Servent servent = (Servent)Marshal.PtrToStructure(intPtr, typeof(Servent));
        result = System.Convert.ToInt32(IPAddress.NetworkToHostOrder(servent.s_port));
     }
     else
     {
        throw CreateWSAException();
     }


  [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
  internal struct Servent
  {
     public string s_name;
     public IntPtr s_aliases;
    public short s_port;
     public string s_proto;
  }

Answer 1

You probably need to specify how the string fields are laid out, else marshalling will fail to determine the correct size for the type.

Answer 2

Hi,

the problem was that the Servent-Struct is differen on x64:

struct servent { char FAR * s_name; /* official service name */ char FAR * FAR * s_aliases; /* alias list */

ifdef _WIN64

    char    FAR * s_proto;          /* protocol to use */
    short   s_port;                 /* port # */

else

    short   s_port;                 /* port # */
    char    FAR * s_proto;          /* protocol to use */

endif

};