How do I get P/Invoking a pointer to a pointer to a struct working from a function parameter?

Question

I'm trying to P/invoke this in such a way I should be able to access the const char* members of this struct from C# (system, game, song, copyright, etc).

This is the struct as it is defined in the C++ header here: gme.h starting at line 79

struct gme_info_t
{
    /* times in milliseconds; -1 if unknown */
    int length;         /* total length, if file specifies it */
    int intro_length;   /* length of song up to looping section */
    int loop_length;    /* length of looping section */

    /* Length if available, otherwise intro_length+loop_length*2 if available,
    otherwise a default of 150000 (2.5 minutes). */
    int play_length;

    int i4,i5,i6,i7,i8,i9,i10,i11,i12,i13,i14,i15; /* reserved */

    /* empty string ("") if not available */
    const char* system;
    const char* game;
    const char* song;
    const char* author;
    const char* copyright;
    const char* comment;
    const char* dumper;

    const char *s7,*s8,*s9,*s10,*s11,*s12,*s13,*s14,*s15; /* reserved */
};

In my C# code, I have modeled this struct as follows:

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct gme_info_t
{
    public int length;
    public int introLength;
    public int loopLength;
    public int playLength;

    public int i4, i5, i6, i7, i8, i9, i10, i11, i12, i13, i14, i15;

    public string system;
    public string game;
    public string song;
    public string author;
    public string copyright;
    public string comment;
    public string dumper;

    public string s7, s8, s9, s10, s11, s12, s13, s14, s15;
}

Now, the function I'm calling is one that I have P/Invoked which has the C++ prototype as follows:

gme_err_t gme_track_info( Music_Emu const* me, gme_info_t** out, int track )

where gme_err_t is a const char*

(see gme.h, line 74, if you want a direct look at it)

(see gme.cpp , line 252 for its definition)

So, the function without all of its typedefs is as follows:

const char* gme_track_info( Music_Emu const* me, gme_info_t** out, int track )

The way this function works is that when called with a valid Music_Emu and a valid track, the result is info about the music track which is assigned to the parameter 'out'.

The const char* being returned is basically for when errors occur, so it's not the main focus. The 'out' parameter is.

I have defined the P/Invoke for this function in C# as follows:

[DllImport(DllName, CallingConvention = CallingConvention.Cdecl)]
public static extern string gme_track_info(IntPtr emuHandle, out gme_info_t trackInfo, int track);

Here is my code currently for attempting to read the copyright string in that struct.

static void Main()
{

    // Initialize the MusicEmu reference first (this works fine).
    IntPtr emuRef;
    string initEmuRef = NativeMethods.gme_open_file("Adventure Island 4.nsf", out emuRef, 48000);
    Console.WriteLine("Error Message (if any): " + initEmuRef);

    // Now get the track info.
    gme_info_t trackInfo;
    NativeMethods.gme_track_info(emuRef, out trackInfo, 0);
    Console.WriteLine("Copyright: " + trackInfo.copyright); // I get an empty string. When checked with a different NSF reader it prints "1994 Hudson Soft."

    // Keep console window up.
    Console.ReadLine();
}

Any help would be appreciated. I've tried for roughly 4 hours to make this work. I've looked all around StackOverflow (and the net in general) for possible solutions, but I haven't found any that were close to this sort of question. Most other problems were about a pointer to a pointer to a struct of arrays and such, which isn't very helpful at all for this case.

If any other information is needed, just ask, I'd be happy to provide it.

Answer 1

Your trackInfo parameter should be an out IntPtr . Then you can marshal it to a C# struct with the Marshal.PtrToStructure method.

IntPtr trackInfoPtr;
NativeMethods.gme_track_info(emuRef, out trackInfoPtr);
gme_info_t trackInfo = (gme_info_t)Marshal.PtrToStructure(trackInfoPtr, typeof(gme_info_t));