Native C DLL Call.

  • Thread starter Thread starter Sudeep
  • Start date Start date
S

Sudeep

Hi,
I need to create a wrapper for following native function scenario.
typedef struct _myinfo
{
char *buf;
int max_size;
}myinfo;

void getdata(struct myinfo *obj)
buf ==> is the pointer to the buffer which is filled by the getdate
function.
max_size ==> it the max. size of buf
Before calling getdata function i need to allocate the buffer and
assign its pointer to "buf" and and assign the "max_size" to the size
of that buffer. The function would just fill up that buffer to the
"max_size".
how do i deal with this ???
Thanks
Sudeep.
 
Sudeep said:
I need to create a wrapper for following native function scenario.
typedef struct _myinfo
{
char *buf;
int max_size;
}myinfo;

void getdata(struct myinfo *obj)
buf ==> is the pointer to the buffer which is filled by the getdate
function.
max_size ==> it the max. size of buf
Before calling getdata function i need to allocate the buffer and
assign its pointer to "buf" and and assign the "max_size" to the size
of that buffer. The function would just fill up that buffer to the
"max_size".
how do i deal with this ???
Click to expand...

There are multiple ways of doing this, some more efficient than others.

If you still have the opportunity of writing additional native code, it
would be a good idea to write an extra function that takes "buf" and
"max_size" as parameters rather than struct members, because this allows for
much easier marshaling.

Otherwise, this should do:

internal static class NativeMethods {
[StructLayout(LayoutKind.Sequential)]
public struct MyInfo {
public IntPtr buf;
public int max_size;
}

[DllImport(...)]
public static extern void GetData([In] ref MyInfo obj);
}

...

public byte[] GetData(int length) {
MyInfo info = new MyInfo();
byte[] buffer = new byte[length];
try {
info.buf = Marshal.AllocHGlobal(length);
info.max_size = length;
NativeMethods.GetData(ref info);
Marshal.Copy(info.buf, buffer, 0, length);
} finally {
if (info.buf != IntPtr.Zero) Marshal.FreeHGlobal(info.buf);
}
return buffer;
}

Disclaimer: untested.

If you can use unsafe code, this can be rewritten to something with better
performance, but I wouldn't do that until you can show there's a need for
it. It's called "unsafe" for a reason.
 
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