I want to send a struct in C# to C++ using sockets.
For example, I use this struct:
[StructLayout(LayoutKind.sequential, Pack = 1)]
struct pos {
public int i;
public float x;
};
If I somehow convert it into bytes and send over the network, I should be able to cast it to this in c++:
struct pos {
int i;
float x;
};
... I think.
1) how do you break down a struct instance in C# to send it over the network?
2) can I safely cast it to the c++ struct once I get it?
Thanks
The marshaller helps you with converting between .NET structs and raw bytes. In this answer, I posted a simple solution, which boils down to Marshal.StructureToPtr and Marshal.PtrToStructure. In contrast to the more advanced solutions provided by Johann du Toit, this is in my opinion the best thing you can do if all you want to do is to push some structures through a byte stream.
If you do this, you can safely cast to the C++ struct if the length is correct, and your C++ struct is declared with the same packing as the C# struct (i.e. #pragma pack in VC++ or __attribute__((packed)) in GCC).
Note that this also works with fixed length C strings, but will not take care of the endianness of larger values. I found it a simple solution to provide getters and setters for the latter problem which just swap the bytes accordingly (with BitConverter).
Some elaboration on the packing:
Take the following structure:
struct MyStruct {
uint8_t a;
float b;
};
With the C# declaration with StructLayout, Pack = 1, this struct will have a size of five bytes. The C++ struct, however, may have eight bytes (or even more), depending on the default packing of the compiler, who may happily insert some padding bytes to align the float value on a 32-bit boundary (just an example). Because of this, you have to apply the very same packing to both the C# and C++ struct. In Visual C++:
#pragma pack(push, 1)
// ... struct declarations...
#pragma pack(pop)
This means all structs declared between the two pragmas will have a packing of one. In GCC:
struct x {
// ...
} __attribute__((packed));
This will do the same. You can #define __attribute__(x) on Windows platforms and #ifdef _WIN32 around the pragmas to make the code compatible with both worlds.
You can either encode it in a format like JSON (There are a lot of JSON parsers out there, check on the json.org website for a list), XML or just roll your own. You could also try already built libraries like Protobuf, which allows you to serialize your structures that you would create with a file in .proto format (And use Protobuf-Net for C#). Another option would be Thrift which provides a way to serialize but also supplies a ready to use RCP system. It support c#, c++ and a ton of other languages by default.
So it's depends on taste, take your pick :D
Related
I am looking for the best way to transfer a large amount of data from C++ (struct or a value class?) into a C# class doing as little data copying as possible. In the sample code below, I have a vector of SubClass objects that has the potential to be very large (10+ million). So I want to avoid a data copy if possible.
Should I/can I just allocate the objects in GC first and use them directly in c++ and forget about the native c++ structures? (Performance is my concern with this one.)
Or, is there some trick that I leverage what is allocated in C++ without causing a data copy?
Here is a sample of something along the lines of what I want to use as a transfer between managed and unmanaged code.
#include <string>
#include <vector>
struct SubClass {
std::string DataItem1;
// lots more here
std::string DataItem50;
};
struct Sample {
int IntValue;
std::string StringValue;
std::vector<std::string> SmallList;
std::vector<SubClass> HugeList;
};
If I can avoid getting into the weeds with pinvoke and COM classes, I would prefer it.
Following the example from Unity (who uses C#), Native plugin example uses a GC handle to transfer data from C# to C++. We can try the opposite to send data to C++ from C#.
Pin down a C# variable to allow faster copying.
using System;
using System.Collections;
using System.Runtime.InteropServices;
// vertices is a Vector3[], where Vector3 is a struct
// of 3 floats using a sequential layout attribute
void test(){
GCHandle gcVertices = GCHandle.Alloc (vertices, GCHandleType.Pinned);
}
Transfer the handle to C++ using marshaling. It's unavoidable that you have to copy something. Here copying a pointer should be good enough. More on marshaling according to Microsoft doc.
[DllImport("your dll")]
private static extern void SendHandle(IntPtr vertexHandle, int vertexCount);
SendHandle(gcVertices, vertices.Length);
Inside C++, you'll receive the handle as a pointer type to a C++ type of your choosing. In this case, vertices are a list of structs of 3 floats. The reference code decided to use float *. You just need to do pointer arithmetic properly depending on the pointed type, including the case of void *.
extern "C" __decl(dllexport) void SendHandle(float* vertices, int vertexCount);
Here the example code copies data directly from the pointer, but you can also write to the pointer's location.
for (int i = 0 ; i < vertexCount; i++)
{
// read from C# heap
float x = vertices[0];
float y = vertices[1];
float z = vertices[2];
// write to C# heap
*vertices = sqrt(x);
*(vertices + 1) = sqrt(y);
*(vertices + 2) = sqrt(z);
vertices += 3; // because it is a list of struct of 3 floats
}
Clean up the pinned handle from the C# side to resume the garbage collector.
gcVertices.Free();
As for strings, I believe the interop library has an implementation that handles pointer arithmetic and copying for you. You could probably just use a string type directly inside the exposed export function, as long as you specify how to marshal it with the MarshalAs attribute in C# and a library in C++ if you are not converting to the C type char *.
I need to do complex marshaling of several nested structures, containing variable length arrays to other structures, hence I decided to use ICustomMarshaler (see for a good JaredPar's tutorial here). But then I have a problem with a struct defined in C++ as:
typedef struct AStruct{
int32_t a;
AType* b;
int32_t bLength;
bool aBoolean;
bool bBoolean;
};
On the C# side, in the MarshalManagedToNative implementation of ICustomMarshaler I was using:
Marshal.WriteByte(intPtr, offset, Convert.ToByte(aBoolean));
offset += 1;
Marshal.WriteByte(intPtr, offset, Convert.ToByte(bBoolean));
But it was not working since I discovered that each bool in the C++ struct was taking 2 bytes. Indeed in x86 sizeof(AStruct) = 16, not 14. Ok, bool is not guaranteed to take 1 byte and so I tried with unsigned char and uint8_t but still the size is 16.
Now, I know I could use an int32 instead than a boolean, but since I care about the taken space and there are several structs containing boolean that flow to disk (I use HDF5 file format and I want to map those boolean with H5T_NATIVE_UINT8 defined in the HDF5 library that takes 1 byte), is there another way? I mean can I have something inside a struct that is guaranteed to take 1 byte?
EDIT
the same problem applies also to int16 values: depending on how many values are present because of alignment reasons the size of the struct at the end might be different from what expected. On the C# side I do not "see" the C++ struct, I simply write on the unmanaged memory by following the definition of my structs in C++. It is quite a simple process, but if I have instead to think to the real space taken by the struct (either by guessing or by measuring it) it will become more difficult and prone to errors every time I modify the struct.
This answer is in addition to what Hans Passant has said.
It might be easiest to have your structures use a fixed packing size, so you can readily predict the member layout. Keep in mind though that this could affect performance.
The rest of this answer is specific to Microsoft Visual C++, but most compilers offer their own variant of this.
To get you started, check out this SO answer #pragma pack effect and MSDN http://msdn.microsoft.com/en-us/library/2e70t5y1.aspx
What you often use is a pragma pack(push, ...) followed by a pragma pack(pop, ...) idiom to only affect packing for the structures defined between the two pragma's:
#pragma pack(push, 4)
struct someStructure
{
char a;
int b;
...
};
#pragma pack(pop)
This will make someStructure have a predictable packing of 4 byte-alignment of each of its members.
EDIT: From the MSDN page on packing
The alignment of a member will be on a boundary that is either a multiple of n
or a multiple of the size of the member, whichever is smaller.
So for pack(4) a char will be aligned on a 1-byte boundary, a short on a 2-byte, and the rest on a 4-byte boundary.
Which value is best depends on your situation. You'll need to explicitly pack all structures you intend to access, and probably all structures that are members of structures you want to access.
sizeof(AStruct) = 16, not 14
That's correct. The struct has two extra bytes at the end that are not used. They ensure that, if you put the struct in an array, that the fields in the struct are still properly aligned. In 32-bit mode, the int32_t and AType* members require 4 bytes and should be aligned to a multiple of 4 to allow the processor to access them quickly. That can only be achieved if the structure size is a multiple of 4. Thus 14 is rounded up to 16.
Do keep in mind that this does not mean that the bool fields take 2 bytes. A C++ compiler uses just 1 byte for them. The extra 2 bytes are pure padding.
If you use Marshal.SizeOf(typeof(AStruct)) in your C# program then you'll discover that the struct you declared takes 20 bytes. This is not good and the problem you are trying to fix. The bool members are the problem, an issue that goes way, way, back to early versions of the C language. Which did not have a bool type. The default marshaling that the CLR uses is compatible with BOOL, the typedef in the winapi. Which is a 32-bit type.
So you have to be explicit about it when you declare the struct in your C# code, you have to tell the marshaller that you want the 1-byte type. Which you do by declaring the struct member as byte. Or by overriding the default marshaling:
[StructLayout(LayoutKind.Sequential)]
private struct AStruct{
public int a;
public IntPtr b;
public int bLength;
[MarshalAs(UnmanagedType.U1)]
public bool aBoolean;
[MarshalAs(UnmanagedType.U1)]
public bool bBoolean;
}
And you'll now see that Marshal.SizeOf() now returns 16. Do be aware that you have to force your program in 32-bit mode, make sure that the EXE project's Platform Target setting is x86.
What is the equivalent of Python's struct.pack(fmt, v1, v2, ...) in C# .NET? I have the following struct defined in C#
[StructLayout(LayoutKind.Sequential, Pack=1)]
struct user {
public char id;
public int age;
};
And would like to unpack it in Python application by using struct.unpack('<ci',myStringedStruct)
I was planing to use BinaryWriter into MemoryStream as suggested by Jon Skeet. However, BinaryWriter only write primitive types, and I have a struct.
You can write the fields one after another to the BinaryWriter:
User user = ...
using (BinaryWriter writer = ...)
{
writer.Write((byte)user.id);
writer.Write(user.age);
}
BinaryWriter uses the endianness of the platform, i.e., little endian on x86/x64.
"c – char – string of length 1 – 1 byte" is a byte in C#. Write(byte)
"i – int – integer – 4 bytes" is an int in C#. Write(Int32)
There is no pack/unpack-like functionality built into the .NET Framework.1
1. When you're p/invoking native code, there's Interop Marshaling. But you're not p/invoking native code.
You can also use the BinaryFormatter class to serialize entire hierarchies of objects, or just a simple struct. You may have to do it manually (byte-by-byte) or use a non-binary format, such as XML to make it platform and language independent, however.
See http://msdn.microsoft.com/en-us/library/system.runtime.serialization.formatters.binary.binaryformatter.aspx
EDIT: I didn't notice the request to later unpack it using Python. This is unlikely to work with Python's unpack function. In general, I would be hesitant to use library calls to write data in binary formats for use in different applications written with different libraries. There's no guarantee of consistency, and specifications may change in the future. You're better off using a language-independent standard.
I am developing a C# application. Since I have some algorithms for least-squares fit in C/C++ that would be too cumbersome too translate, I have made the C++ code into a dll and then created a wrapper in C#.
In the C# code, I have defined a struct that is passed to the unmanaged C++ code as a pointer. The struct contains the initial guesstimates for the fitting functions, and it is also used to return the results of the fit.
It appears to me that you must define the struct in both the managed and the unmanaged code. However, someone using my source code in in the future might decide to change the fields of the struct in the C# application, without understanding that they also have to change the struct in the native code. This will result in a run-time error at best (and produce erroneous results at worse), but there will be no error message telling the developer / end user what is wrong.
From my understanding, it impossible to create a test in the unmanaged C++ DLL that checks if the struct contains the correct fields, but is it posible to have the DLL returning a struct of the correct format to the C# wrapper?
Otherwise, what ways are there to reduce the risk that some careless programmer in the future causes run-time errors that are hard to detect?
Sample code:
//C++
struct InputOutputStruct {
double a,b,c;
}
extern "C" __declspec(dllexport) void DoSomethingToStruct(InputOutputStruct* s)
{
// ... algorithm
}
//C#
using System.Runtime.InteropServices;
[StructLayout(LayoutKind.Sequential)]
public struct InputOutputStruct {
public double a,b,c;
}
[DllImport("CpluplusDll.dll")]
public static unsafe extern bool DoSomethingToStruct(InputOutputStruct* s);
class CSharpWrapper {
static void Main(string[] args)
{
InputOutputStruct s = new InputOutputStruct();
unsafe {
InputOutpustruct* sPtr = &s;
DoSomethingToStruct(sPtr);
s = *sPtr;
}
}
}
It appears to me that you must define the struct in both the managed
and the unmanaged code.
Not true. This is what C++/CLI was invented for- facilitate much easier interoperation with C++ and .NET.
but is it posible to have the DLL returning a struct of the correct format to the C# wrapper?
I don't think so, because you always need to define the structs on the C# side.
Here are a solution which may work ( never tested ):
Give each struct which is shared a unique identifier on both sides ( GUID, Macros )
Create a reflection for C++ which contains informations about the the types which are used on C# and C++ side. This could be done by using macros.
Compare the C++ structs and the C# structs on startup by using the GUIDs, reflection and macros. You can also use sizeof to compare sizes first.
That could be a lot of work
When working on C++ side you still can make a lot of things wrong when you not know about the macros
faacEncConfigurationPtr FAACAPI faacEncGetCurrentConfiguration(
faacEncHandle hEncoder);
I'm trying to come up with a simple wrapper for this C++ library; I've never done more than very simple p/invoke interop before - like one function call with primitive arguments.
So, given the above C++ function, for example, what should I do to deal with the return type, and parameter?
FAACAPI is defined as: #define FAACAPI __stdcall
faacEncConfigurationPtr is defined:
typedef struct faacEncConfiguration
{
int version;
char *name;
char *copyright;
unsigned int mpegVersion;
unsigned long bitRate;
unsigned int inputFormat;
int shortctl;
psymodellist_t *psymodellist;
int channel_map[64];
} faacEncConfiguration, *faacEncConfigurationPtr;
AFAIK this means that the return type of the function is a reference to this struct?
And faacEncHandle is:
typedef struct {
unsigned int numChannels;
unsigned long sampleRate;
...
SR_INFO *srInfo;
double *sampleBuff[MAX_CHANNELS];
...
double *freqBuff[MAX_CHANNELS];
double *overlapBuff[MAX_CHANNELS];
double *msSpectrum[MAX_CHANNELS];
CoderInfo coderInfo[MAX_CHANNELS];
ChannelInfo channelInfo[MAX_CHANNELS];
PsyInfo psyInfo[MAX_CHANNELS];
GlobalPsyInfo gpsyInfo;
faacEncConfiguration config;
psymodel_t *psymodel;
/* quantizer specific config */
AACQuantCfg aacquantCfg;
/* FFT Tables */
FFT_Tables fft_tables;
int bitDiff;
} faacEncStruct, *faacEncHandle;
So within that struct we see a lot of other types... hmm.
Essentially, I'm trying to figure out how to deal with these types in my managed wrapper?
Do I need to create versions of these types/structs, in C#? Something like this:
[StructLayout(LayoutKind.Sequential)]
struct faacEncConfiguration
{
uint useTns;
ulong bitRate;
...
}
If so then can the runtime automatically "map" these objects onto eachother?
And, would I have to create these "mapped" types for all the types in these return types/parameter type hierarchies, all the way down until I get to all primitives?
I know this is a broad topic, any advice on getting up-to-speed quickly on what I need to learn to make this happen would be very much appreciated! Thanks!
Your are on the right track with how you would need to create managed structures that represent unamanged structures for use with P/Invoke.
It is however not the best strategy for interop with unmanaged libraries, because using this API from C# would still feel like using a C API - create and initialise a structure, pass it to a function and get some other structure in return.
It is OK to use P/Invoke for an odd function call that is not otherwise exposed as .NET API, but for a full blown API wrapper I would highly recommend using managed C++(C++/CLI). It is absolutely unparalleled in creating unmanaged interop layers for .NET.
The biggest challenge would be to convert this essentially C interface into an object orientated interface where you call methods off objects, as opposed to calling global functions with structures of all-public members.
As you start writing elaborate structure graphs for P/Invoke you would yourself having to deal with quite a bit of "magic" that governs how managed primitive types convert to unmanaged types. Often, using incorrect types will cause a run-time error.
With managed C++ (IJW - It Just Works) you define managed structures, classes, interfaces in C++, which allows more natural use of the underlying library and a more native interface to your C# application.
This is a great overview of C++/CLI. Also MSDN has extensive documentation for all managed C++ features.
Yes, you would need to declare all these structures in c#. Be careful to declare members with correct sizes. For example, 'long' is 32-bit in C++, but 64-bit in C#. For a pointer or void* in C++, use IntPtr in C#, etc.