A hint on marshalling a byte array into a void* pointer


B

Beorne

I have to pass a byte array as an input parameter to a function in a
propertary dll.
The c++ signature of the original function is the following:

---------- C++ ----------

int ncWrite(unsigned long ObjHandle, unsigned long DataSize, void*
DataPtr)

---------- end C++ ----------
From C# I have defined this function as:

---------- C# ----------

[DllImport("Nican.dll")]
unsafe static extern int ncWrite(uint ObjHandle, uint DataSize,
IntPtr DataPtr);

---------- end C# ----------

And I use it as:

---------- C# ----------

byte[] DataWrite = {0x23, 0x23, 0x30, 0x03, 0x78, 0xEC, 0xFF, 0xFF };
int myStatus;

IntPtr ptr = Marshal.AllocHGlobal(8);
Marshal.Copy(DataWrite, 0, ptr, 8);
myStatus = ncWrite(WriteAddress, 8 , ptr);
Marshal.FreeHGlobal(ptr);

---------- end C# ----------

This works, but having to handle unmanaged code I''m worried by memory
leaks (this function is called thousdands of times per minute).
Is it the usage correct? Have I choosen the right Marshal methods
(Marshal.AllocHGlobal, Marshal.Copy, Marshal.FreeHGlobal)?

Thank you very much
 
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C

Christof Nordiek

Beorne said:
From C# I have defined this function as:

---------- C# ----------

[DllImport("Nican.dll")]
unsafe static extern int ncWrite(uint ObjHandle, uint DataSize,
IntPtr DataPtr);

---------- end C# ----------

And I use it as:

---------- C# ----------

byte[] DataWrite = {0x23, 0x23, 0x30, 0x03, 0x78, 0xEC, 0xFF, 0xFF };
int myStatus;

IntPtr ptr = Marshal.AllocHGlobal(8);
Marshal.Copy(DataWrite, 0, ptr, 8);
myStatus = ncWrite(WriteAddress, 8 , ptr);
Marshal.FreeHGlobal(ptr);

---------- end C# ----------

This seems to be OK. You should add a try/finally to get sure the memory
gets cleared in case of an exception.

IntPtr ptr = Marshal.AllocHGlobal(8);
try
{
Marshal.Copy(DataWrite, 0, ptr, 8);
myStatus = ncWrite(WriteAddress, 8, ptr);
}
finally
{
Marshal.FreeHGlobal(ptr);
}

Christof
 
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S

Stephen Martin

You don't really need to manually marshal the data. Change your C#
declaration to:

[DllImport("Nican.dll")]
unsafe static extern int ncWrite(uint ObjHandle, uint DataSize, byte[]
DataPtr);

and then just pass in DataWrite as is. The runtime marshaller will pin your
array and then pass in a pointer to the unmanaged function.



Beorne said:
I have to pass a byte array as an input parameter to a function in a
propertary dll.
The c++ signature of the original function is the following:

---------- C++ ----------

int ncWrite(unsigned long ObjHandle, unsigned long DataSize, void*
DataPtr)

---------- end C++ ----------
From C# I have defined this function as:

---------- C# ----------

[DllImport("Nican.dll")]
unsafe static extern int ncWrite(uint ObjHandle, uint DataSize,
IntPtr DataPtr);

---------- end C# ----------

And I use it as:

---------- C# ----------

byte[] DataWrite = {0x23, 0x23, 0x30, 0x03, 0x78, 0xEC, 0xFF, 0xFF };
int myStatus;

IntPtr ptr = Marshal.AllocHGlobal(8);
Marshal.Copy(DataWrite, 0, ptr, 8);
myStatus = ncWrite(WriteAddress, 8 , ptr);
Marshal.FreeHGlobal(ptr);

---------- end C# ----------

This works, but having to handle unmanaged code I''m worried by memory
leaks (this function is called thousdands of times per minute).
Is it the usage correct? Have I choosen the right Marshal methods
(Marshal.AllocHGlobal, Marshal.Copy, Marshal.FreeHGlobal)?

Thank you very much
 

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