Develop Reference

How to marshall byte* from C.dll in C#

I have two functions that I am trying to call from C# that shares similar signatures:
BOOL Read (BYTE Len, BYTE* DataBuf)
BOOL Write (BYTE Len, BYTE* DataBuf)
From the Doc: DataBuf Destination of transmitted data
What shall I use in C# call?
byte[]
myByteArr[0]
P/Invoke Assistant suggested System.IntPtr
Don't have the hardware to test yet, but i am trying to get as many of calls right for when we have.
Thanks.

For the read function you use:
[Out] byte[] buffer
For the write function you use:
[In] byte[] buffer
[In] is the default and can be omitted but it does not hurt to be explicit.
The functions would therefore be:
[DllImport(filename, CallingConvention = CallingConvention.Cdecl)]
static extern bool Read(byte len, [Out] byte[] buffer);
[DllImport(filename, CallingConvention = CallingConvention.Cdecl)]
static extern bool Write(byte len, [In] byte[] buffer);
Obviously you'll need to allocate the array before passing it to the unmanaged functions.
Because byte is blittable then the marshaller, as an optimisation, pins the array and passes the address of the pinned object. This means that no copying is performed and the parameter passing is efficient.

It'll probably be IntPtr obtained from a byte[] array. Older questions should definitely have covered that: How to get IntPtr from byte[] in C#

How to get the memory address of a file

In a WCE app I’m looking for a way to copy a file (I only have to file name/path of it) to a specific memory address.
The file is rather large’ish, ~40MB, so with limited resources, I was hoping to avoid reading the whole file into memory (byte array), by using the answer from this post:
Copy data from from IntPtr to IntPtr
[DllImport("kernel32.dll", EntryPoint = "CopyMemory", SetLastError = false)]
public static extern void CopyMemory(IntPtr dest, IntPtr src, uint count);
static void Main()
{
const int size = 200;
IntPtr memorySource = Marshal.AllocHGlobal(size);
IntPtr memoryTarget = Marshal.AllocHGlobal(size);
CopyMemory(memoryTarget,memorySource,size);
}
That leaves me to 2 problems.
First of all: How do I assign a memory address to an IntPtr?, kind of like: int* startAddr = &0x00180000.
And secondly: How do I obtain the memory address of a file?
With these two questions answered, my code would look something like:
[DllImport("kernel32.dll", EntryPoint = "CopyMemory", SetLastError = false)]
public static extern void CopyMemory(IntPtr dest, IntPtr src, uint count);
private unsafe void CopyFile()
{
try
{
fixed (Int32* startAddr = /*0x00180000*/)
{
fixed(Int32* fileAddr = /*Memory Address of file*/)
{
CopyMemory(new IntPtr(startAddr), new IntPtr(fileAddr), (uint)new FileInfo("File name").Length);
}
}
}
catch { }
}
Would that be a valid way of going about it?
Any help would be greatly appreciated. Thanks in advance!!
Update:
CopyMemory is not the way of going about it. So please disregard.
Also, Sorry for not being clearer. Basically I want to move a file to the start of a partition on disk. I thought that IntPtr could also point to a disk address, but in retrospect I can see that of course it can not.
Anyway sorry for the confusion.

Files don't have addresses in memory. So what you are asking for is something of a non-sequitur.
So far as I can tell, you don't need any unsafe code at all. You don't need to call CopyMemory. And you don't need to load the entire file in one go and then copy. What you can do is as follows:
Use a standard file stream to read the file.
Read the file in small chunks into a small buffer. Just to be concrete, you might choose to read the file in 8KB chunks.
Each time you read a chunk of the file, copy it to the unmanaged memory location with a call to Marshal.Copy.
The code might look like this:
static void CopyStreamToMemory(Stream stream, IntPtr addr, int bufferSize)
{
byte[] buffer = new byte[bufferSize];
long bytesLeft = stream.Length - stream.Position;
while (bytesLeft > 0)
{
int bytesToCopy = (int)Math.Min(bufferSize, bytesLeft);
stream.Read(buffer, 0, bytesToCopy);
Marshal.Copy(buffer, 0, addr, bytesToCopy);
addr += bytesToCopy;
bytesLeft -= bytesToCopy;
}
}

Unmanaged file access (WriteFile) slower than managed (FileStream)

I have an application that heavily reads and write to files (a custom format), I was told to improve performance by using direct unmanaged code.
Before attempting in the real application I made a small tests just to see how the performance gains would be, but for my surprise, the unmanaged versions seems to be like 8x slower than using simply filestream.
Here is the managed function:
private int length = 100000;
private TimeSpan tspan;
private void UsingManagedFileHandle()
{
DateTime initialTime = DateTime.Now;
using (FileStream fileStream = new FileStream("data2.txt", FileMode.Create, FileAccess.ReadWrite))
{
string line = "ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890123";
byte[] bytes = Encoding.Unicode.GetBytes(line);
for (int i = 0; i < length; i++)
{
fileStream.Write(bytes, 0, bytes.Length);
}
fileStream.Close();
}
this.tspan = DateTime.Now.Subtract(initialTime);
label2.Text = "" + this.tspan.TotalMilliseconds + " Milliseconds";
}
Here is the unmanaged way:
public void UsingAnUnmanagedFileHandle()
{
DateTime initialTime;
IntPtr hFile;
hFile = IntPtr.Zero;
hFile = FileInteropFunctions.CreateFile("data1.txt",
FileInteropFunctions.GENERIC_WRITE | FileInteropFunctions.GENERIC_READ,
FileInteropFunctions.FILE_SHARE_WRITE,
IntPtr.Zero,
FileInteropFunctions.CREATE_ALWAYS,
FileInteropFunctions.FILE_ATTRIBUTE_NORMAL,
0);
uint lpNumberOfBytesWritten = 0;
initialTime = DateTime.Now;
if (hFile.ToInt64() > 0)
{
string line = "ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890123";
byte[] bytes = Encoding.Unicode.GetBytes(line);
uint bytesLen = (uint)bytes.Length;
for (int i = 0; i < length; i++)
{
FileInteropFunctions.WriteFile(hFile,
bytes,
bytesLen,
out lpNumberOfBytesWritten,
IntPtr.Zero);
}
FileInteropFunctions.CloseHandle(hFile);
this.tspan = DateTime.Now.Subtract(initialTime);
label1.Text = "" + this.tspan.TotalMilliseconds + " Milliseconds";
}
else
label1.Text = "Error";
}
[DllImport("kernel32.dll", SetLastError = true)]
public static extern bool CloseHandle(IntPtr hObject);
[DllImport("kernel32.dll", SetLastError = true)]
public static extern unsafe IntPtr CreateFile(
String lpFileName, // Filename
uint dwDesiredAccess, // Access mode
uint dwShareMode, // Share mode
IntPtr attr, // Security Descriptor
uint dwCreationDisposition, // How to create
uint dwFlagsAndAttributes, // File attributes
uint hTemplateFile); // Handle to template file
[DllImport("kernel32.dll")]
public static extern unsafe int WriteFile(IntPtr hFile,
// byte[] lpBuffer,
[MarshalAs(UnmanagedType.LPArray)] byte[] lpBuffer, // also tried this.
uint nNumberOfBytesToWrite,
out uint lpNumberOfBytesWritten,
IntPtr lpOverlapped);
The iteration using FileStream takes about 70 ms in my computer.
The one using WriteFile takes about 550ms.
I tested several times and with several amount of iterations and the difference in performance is consistent.
I have no idea why the unmanaged code is being slower then the managed code.
EDIT
Thank you very much for your explanations, guys . I thought there was something "magical" undergoing FileStream and you have explained it so well.
So, I know now there's no easy path to gain performance in this part, and I would like to ask you for opinion for other simple ways to gain speed. The file is random access in the real application, and size could range from 1MB to 1GB.

Your unmanaged calls write the data to disk as soon as possible while FileStream is buffered (ie does most operations in-memory and should call the underlying unmanaged calls much less often)
There are constructors on FileStream that let you control the buffer size if you want to tweak performance further.

Well, FileStream is jut a wrapper around CreateFile/WriteFile. It's written by bunch of smart guys. So I see no logical explanation at all why you assume that your one should be faster :P.
As already stated, FileStream probably does extra-buffering before calling WriteFile() thus minimizing unmanaged method calls. And this is important - only make unmanaged calls when they are necessary. They cost. Buffer sizes are usually multiple of disk sector size. You can experiment with different sizes, though this is OS dependent, and most likely will yield other results on other computers.
But it's also important to know that WriteFile() does internal buffering too. It's not like you call WriteFile() and bam it's written to file. It will be flushed to HDD once it's time.
I think there is unnecessary byte[] marshaling going on. Eg when you call WriteFile(), system makes copy of your buffer. It should be avoidable by unsafe() keyword and little bit of hacking.
There is also FILE_FLAG_SEQUENTIAL_SCAN that can't be accessed through FileStream(afaik) and it should let system know that you're gonna do file writes/reads only sequentially. This might give some performance boost theoretically.

The difference is because the calls to WriteFile are synchronous while the writes to the FileStream are not.
By default CreateFile will create a synchronous file handle, so the calls to WriteFile do not return until the data is written. If you add FILE_FLAG_OVERLAPPED to the CreateFile call the un-managed implementation will take approximately the same time as the managed.
See the documenation for Synchronous and Asynchronous I/O Handles section of the CreateFile defini

Get an image of process memory

My goal is to create a method that will take a process handle and return an array of bytes representing that process's memory. Here's what I have:
[DllImport("Kernel32.dll")]
public static extern bool ReadProcessMemory(IntPtr hProcess, IntPtr lpBaseAddress, byte[] lpBuffer, UInt32 nSize, ref UInt32 lpNumberOfBytesRead);
public static byte[] MemRead(IntPtr handle, IntPtr address, UInt32 size, ref UInt32 bytes)
{
byte[] buffer = new byte[size];
ReadProcessMemory(handle, address, buffer, size, ref bytes);
return buffer;
}
I don't know what to pass to the wrapper method as arguments. I can find a handle and the bytes is an output variable, but what about address and size? Where can I get this data from?

Use VirtualQuery to find out if an address has actually been allocated before calling MemRead.
Start with zero as the address and 64K as the page size and then simply increment the pointer with 64K on every iteration until you reach the maximum size of memory on your system.

Directly reading large binary file in C# w/out copying

I am looking for the most efficient/direct way to do this simple C/C++ operation:
void ReadData(FILE *f, uint16 *buf, int startsamp, int nsamps)
{
fseek(f, startsamp*sizeof(uint16), SEEK_SET);
fread(buf, sizeof(uint16), nsamps, f);
}
in C#/.NET. (I'm ignoring return values for clarity - production code would check them.) Specifically, I need to read in many (potentially 10's to 100's of millions) 2-byte (16-bit) "ushort" integer data samples (fixed format, no parsing required) stored in binary in a disk file. The nice thing about the C way is that it reads the samples directly into the "uint16 *" buffer with no CPU involvement, and no copying. Yes, it is potentially "unsafe", as it uses void * pointers to buffers of unknown size, but it seems like there should be a "safe" .NET alternative.
What is the best way to accomplish this in C#? I have looked around, and come across a few hints ("unions" using FieldOffset, "unsafe" code using pointers, Marshalling), but none seem to quite work for this situation, w/out using some sort of copying/conversion. I'd like to avoid BinaryReader.ReadUInt16(), since that is very slow and CPU intensive. On my machine there is about a 25x difference in speed between a for() loop with ReadUInt16(), and reading the bytes directly into a byte[] array with a single Read(). That ratio could be even higher with non-blocking I/O (overlapping "useful" processing while waiting for the disk I/O).
Ideally, I would want to simply "disguise" a ushort[] array as a byte[] array so I could fill it directly with Read(), or somehow have Read() fill the ushort[] array directly:
// DOES NOT WORK!!
public void GetData(FileStream f, ushort [] buf, int startsamp, int nsamps)
{
f.Position = startsamp*sizeof(ushort);
f.Read(buf, 0, nsamps);
}
But there is no Read() method that takes a ushort[] array, only a byte[] array.
Can this be done directly in C#, or do I need to use unmanaged code, or a third-party library, or must I resort to CPU-intensive sample-by-sample conversion? Although "safe" is preferred, I am fine with using "unsafe" code, or some trick with Marshal, I just have not figured it out yet.
Thanks for any guidance!
[UPDATE]
I wanted to add some code as suggested by dtb, as there seem to be precious few examples of ReadArray around. This is a very simple one, w/no error checking shown.
public void ReadMap(string fname, short [] data, int startsamp, int nsamps)
{
var mmf = MemoryMappedFile.CreateFromFile(fname);
var mmacc = mmf.CreateViewAccessor();
mmacc.ReadArray(startsamp*sizeof(short), data, 0, nsamps);
}
Data is safely dumped into your passed array. You can also specify a type for more complex types. It seems able to infer simple types on its own, but with the type specifier, it would look like this:
mmacc.ReadArray<short>(startsamp*sizeof(short), data, 0, nsamps);
[UPATE2]
I wanted to add the code as suggested by Ben's winning answer, in "bare bones" form, similar to above, for comparison. This code was compiled and tested, and works, and is FAST. I used the SafeFileHandle type directly in the DllImport (instead of the more usual IntPtr) to simplify things.
[DllImport("kernel32.dll", SetLastError=true)]
[return:MarshalAs(UnmanagedType.Bool)]
static extern bool ReadFile(SafeFileHandle handle, IntPtr buffer, uint numBytesToRead, out uint numBytesRead, IntPtr overlapped);
[DllImport("kernel32.dll", SetLastError=true)]
[return:MarshalAs(UnmanagedType.Bool)]
static extern bool SetFilePointerEx(SafeFileHandle hFile, long liDistanceToMove, out long lpNewFilePointer, uint dwMoveMethod);
unsafe void ReadPINV(FileStream f, short[] buffer, int startsamp, int nsamps)
{
long unused; uint BytesRead;
SafeFileHandle nativeHandle = f.SafeFileHandle; // clears Position property
SetFilePointerEx(nativeHandle, startsamp*sizeof(short), out unused, 0);
fixed(short* pFirst = &buffer[0])
ReadFile(nativeHandle, (IntPtr)pFirst, (uint)nsamps*sizeof(short), out BytesRead, IntPtr.Zero);
}

You can use a MemoryMappedFile. After you have memory-mapped the file, you can create a view (i.e. a MemoryMappedViewAccessor) which provides a ReadArray<T> method. This method can read structs from the file without marshalling, and it works with primitive types lie ushort.

dtb's answer is an even better way (actually, it has to copy the data as well, no gain there), but I just wanted to point out that to extract ushort values from a byte array you should be using BitConverter not BinaryReader
EDIT: example code for p/invoking ReadFile:
[DllImport("kernel32.dll", SetLastError=true)]
[return:MarshalAs(UnmanagedType.Bool)]
static extern bool ReadFile(IntPtr handle, IntPtr buffer, uint numBytesToRead, out uint numBytesRead, IntPtr overlapped);
[DllImport("kernel32.dll", SetLastError=true)]
[return:MarshalAs(UnmanagedType.Bool)]
static extern bool SetFilePointerEx(IntPtr hFile, long liDistanceToMove, out long lpNewFilePointer, uint dwMoveMethod);
unsafe bool read(FileStream fs, ushort[] buffer, int offset, int count)
{
if (null == fs) throw new ArgumentNullException();
if (null == buffer) throw new ArgumentNullException();
if (offset < 0 || count < 0 || offset + count > buffer.Length) throw new ArgumentException();
uint bytesToRead = 2 * count;
if (bytesToRead < count) throw new ArgumentException(); // detect integer overflow
long offset = fs.Position;
SafeFileHandle nativeHandle = fs.SafeFileHandle; // clears Position property
try {
long unused;
if (!SetFilePositionEx(nativeHandle, offset, out unused, 0);
fixed (ushort* pFirst = &buffer[offset])
if (!ReadFile(nativeHandle, new IntPtr(pFirst), bytesToRead, out bytesToRead, IntPtr.Zero)
return false;
if (bytesToRead < 2 * count)
return false;
offset += bytesToRead;
return true;
}
finally {
fs.Position = offset; // restore Position property
}
}

I might be a bit late to the game here... but the fastest method I found was using a combination of the previous answers.
If i do the following:
MemoryMappedFile mmf = MemoryMappedFile.CreateFromFile(somePath);
Stream io = mmf.CreateViewStream();
int count;
byte[] byteBuffer = new byte[1024 << 2];
ushort[] dataBuffer = new ushort[buffer.Length >> 1];
while((count = io.Read(byteBuffer, 0, byteBuffer.Length)) > 0)
Buffer.BlockCopy(buffer, 0, dataBuffer, 0, count);
This was ~2x faster than the accepted answer.
For me, the unsafe method was the same as the Buffer.BlockCopy without the MemoryMappedFile. The MemoryMappedFile cut down on a bit of time.