C# pinvoke of C function returning char*

Question

This is what I came up with:

The unmanaged function:

extern  "C" __declspec(dllexport) char* callme(const char * sing) {
    char buf[10];
    sprintf(buf,"hey %s",sing);
    return buf;
}

The calling function:

class Program
{

    [DllImport("testnonclr.dll", CharSet = CharSet.Auto, CallingConvention =
        CallingConvention.Cdecl)]
    public static extern IntPtr callme([In,MarshalAs(UnmanagedType.LPStr)]
        string sing);

    static void Main(string[] args) {
        Console.Write( Marshal.PtrToStringAuto (callme("baby")  ));
        Console.ReadLine();  
    }
}

The result: a bunch of gibberish

Ok, thanks to the help of some users and more digging, this is what I ended up doing:

extern  "C" {
 __declspec(dllexport)  char* __stdcall callme(const char * sing)
{
static char *buf=NULL;
    char fubb[10];


sprintf(fubb,"hey %s",sing);
ULONG usize=strlen(fubb)+sizeof(char);
buf=(char *)::GlobalAlloc (GMEM_FIXED,usize);


strcpy(buf,fubb);

return buf;

}

and C#:

class Program
{

    [DllImport("testnonclr.dll", CharSet = CharSet.Unicode, CallingConvention =    C  
    CallingConvention.StdCall )]
   [return: MarshalAs(UnmanagedType.LPStr)]
    public static extern string callme([In, MarshalAs(UnmanagedType.LPStr)] string sing);
    static void Main(string[] args)
    {
        string rrr = callme("baby"); 
        Console.WriteLine(rrr); 


        Console.ReadLine();  
    }

}

I would start with learning C first, and then try to P/Ivoke. — Jonathon Reinhart, Dec 13 '14 at 22:22
I know the ins and outs of C memory allocation as well as anyone, within the same file though. — Mihai Pruna, Dec 14 '14 at 01:30

Jcl · Accepted Answer · 2014-12-13T21:47:01.993

2

You cannot return a pointer to a local variable since those are allocated on the stack (and the stack may get destroyed when the function returns).

You'd need to allocate memory on the heap (preferably using CoTaskMemAlloc so the CLR can free up the memory later) on your C function and return a pointer to that memory.

As a side note, you can directly return a string when marshalling (assuming it's a c null-terminated unicode string), no need to return IntPtr.

More info here: http://limbioliong.wordpress.com/2011/06/16/returning-strings-from-a-c-api/

edited Dec 13 '14 at 21:47

answered Dec 13 '14 at 21:41

Jcl

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I get Error:The global scope has no CoTaskMemAlloc – Mihai Pruna Dec 14 '14 at 01:32
You need to include objbase.h – Jcl Dec 14 '14 at 02:27
ok, I finally figured it out. thanks, used GlobalAlloc(GMEM_FIXED – Mihai Pruna Dec 14 '14 at 02:36
GlobalAlloc is not the way to do it – David Heffernan Dec 14 '14 at 09:21
someone here said they are the same thing: http://stackoverflow.com/questions/393509/usage-of-cotaskmemalloc – Mihai Pruna Dec 14 '14 at 20:29

C# pinvoke of C function returning char*

1 Answers1