I'm trying to access some Ghostscript functions like so:
[DllImport(@"C:\Program Files\GPLGS\gsdll32.dll", EntryPoint = "gsapi_revision")]
public static extern int Foo(gsapi_revision_t x, int len);
public struct gsapi_revision_t
{
[MarshalAs(UnmanagedType.LPTStr)]
string product;
[MarshalAs(UnmanagedType.LPTStr)]
string copyright;
long revision;
long revisiondate;
}
public static void Main()
{
gsapi_revision_t foo = new gsapi_revision_t();
Foo(foo, Marshal.SizeOf(foo));
This corresponds with these definitions from the iapi.h header from ghostscript:
typedef struct gsapi_revision_s {
const char *product;
const char *copyright;
long revision;
long revisiondate;
} gsapi_revision_t;
GSDLLEXPORT int GSDLLAPI
gsapi_revision(gsapi_revision_t *pr, int len);
But my code is reading nothing into the string fields. If I add 'ref' to the function, it reads gibberish. However, the following code reads in the data just fine:
public struct gsapi_revision_t
{
IntPtr product;
IntPtr copyright;
long revision;
long revisiondate;
}
public static void Main()
{
gsapi_revision_t foo = new gsapi_revision_t();
IntPtr x = Marshal.AllocHGlobal(20);
for (int i = 0; i < 20; i++)
Marshal.WriteInt32(x, i, 0);
int result = Foo(x, 20);
IntPtr productNamePtr = Marshal.ReadIntPtr(x);
IntPtr copyrightPtr = Marshal.ReadIntPtr(x, 4);
long revision = Marshal.ReadInt64(x, 8);
long revisionDate = Marshal.ReadInt64(x, 12);
byte[] dest = new byte[1000];
Marshal.Copy(productNamePtr, dest, 0, 1000);
string name = Read(productNamePtr);
string copyright = Read(copyrightPtr);
}
public static string Read(IntPtr p)
{
List<byte> bits = new List<byte>();
int i = 0;
while (true)
{
byte b = Marshal.ReadByte(new IntPtr(p.ToInt64() + i));
if (b == 0)
break;
bits.Add(b);
i++;
}
return Encoding.ASCII.GetString(bits.ToArray());
}
So what am I doing wrong with marshaling?
