I thought .NET was supposed to make things easier, if anything, than unmanaged code.
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Use unsafe code with pointers just as you would in C and you can do the same thing as you could in C.
Nope, because you can't pin an object to a specific address in memory. Ergo I can't back a .NET instance with a memory mapped file. Memory mapped files work by mapping a portion of the process address space to your application. So you can read and write the file doing pointer ops. However, in .NET you can only get pointers to the GC heap by pinning objects. So I cannot do
var arr = new int[100000000];// backed by memory mapped file
in any variation. Even if I were to pin the array to get the pointer, the pointer wouldn't be part of that mapped address space. At best, I have to *copy* the entire object into that mapped region, which absolutely defeats the whole point of why I'd use mem mapped I/O here in the first place. There's a half measure using Span, but it only works in Standard and Core, not the DNF
When I was growin' up, I was the smartest kid I knew. Maybe that was just because I didn't know that many kids. All I know is now I feel the opposite.
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Nope, because you can't pin an object to a specific address in memory. Ergo I can't back a .NET instance with a memory mapped file. Memory mapped files work by mapping a portion of the process address space to your application. So you can read and write the file doing pointer ops. However, in .NET you can only get pointers to the GC heap by pinning objects. So I cannot do
var arr = new int[100000000];// backed by memory mapped file
in any variation. Even if I were to pin the array to get the pointer, the pointer wouldn't be part of that mapped address space. At best, I have to *copy* the entire object into that mapped region, which absolutely defeats the whole point of why I'd use mem mapped I/O here in the first place. There's a half measure using Span, but it only works in Standard and Core, not the DNF
When I was growin' up, I was the smartest kid I knew. Maybe that was just because I didn't know that many kids. All I know is now I feel the opposite.
This is what "Span" is for. Return that instead of an array and your API will be "managed" in the sense to an external user. Or you can use "GCHandle.Alloc" to pin a managed array in memory. GCHandle.Alloc Method (System.Runtime.InteropServices) | Microsoft Docs[^] Using Span is a better idea though.
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This is what "Span" is for. Return that instead of an array and your API will be "managed" in the sense to an external user. Or you can use "GCHandle.Alloc" to pin a managed array in memory. GCHandle.Alloc Method (System.Runtime.InteropServices) | Microsoft Docs[^] Using Span is a better idea though.
Another commenter pointed out Span, which I had forgotten about. It's a half measure** but gets me closer, however, it does not work in the DNF, only in Core and Standard i think. GCHandle.Alloc will not let me pin to a specific address. In order for .NET to do anything beyond span it would have to allow you to create unmanaged heaps to allocate objects on which the collector wouldn't collect. It's designed such that it can't work, and I accept that. It would just be nice if it had some kind of feature like the above to enable it. ** I don't actually want to store an array, i was using that as an example. My datastructure is complex and nested, which is precisely why i'd like to use vmem
When I was growin' up, I was the smartest kid I knew. Maybe that was just because I didn't know that many kids. All I know is now I feel the opposite.
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Well in that case for .NET Framework unsafe code is the only answer. Or just return a IntPtr and let the user decide if they want to marshal or use unsafe code if this is for an API. Thats how MS does it.
Not applicable for this.
When I was growin' up, I was the smartest kid I knew. Maybe that was just because I didn't know that many kids. All I know is now I feel the opposite.
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Another commenter pointed out Span, which I had forgotten about. It's a half measure** but gets me closer, however, it does not work in the DNF, only in Core and Standard i think. GCHandle.Alloc will not let me pin to a specific address. In order for .NET to do anything beyond span it would have to allow you to create unmanaged heaps to allocate objects on which the collector wouldn't collect. It's designed such that it can't work, and I accept that. It would just be nice if it had some kind of feature like the above to enable it. ** I don't actually want to store an array, i was using that as an example. My datastructure is complex and nested, which is precisely why i'd like to use vmem
When I was growin' up, I was the smartest kid I knew. Maybe that was just because I didn't know that many kids. All I know is now I feel the opposite.
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Not applicable for this.
When I was growin' up, I was the smartest kid I knew. Maybe that was just because I didn't know that many kids. All I know is now I feel the opposite.
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MS uses unsafe code all over the place in their own frameworks. Not sure how its an issue but ok. Even if you could address managed array to a native buffer that would make it unsafe usage.
unsafe isn't a problem. This has nothing to do with a desire not to run unsafe code Again, what it has to do with is mapping a complex managed data structure to a file using Vmem. It's not doable. At best, you resolve to something much like standard file i/o which defeats the whole reason i'd use vmem. Adding, span would help here, but it doesn't go near enough to to be a solution since these data structures aren't simple arrays.
When I was growin' up, I was the smartest kid I knew. Maybe that was just because I didn't know that many kids. All I know is now I feel the opposite.
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unsafe isn't a problem. This has nothing to do with a desire not to run unsafe code Again, what it has to do with is mapping a complex managed data structure to a file using Vmem. It's not doable. At best, you resolve to something much like standard file i/o which defeats the whole reason i'd use vmem. Adding, span would help here, but it doesn't go near enough to to be a solution since these data structures aren't simple arrays.
When I was growin' up, I was the smartest kid I knew. Maybe that was just because I didn't know that many kids. All I know is now I feel the opposite.
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Then just wrap the C API that does what you want. Anything you can do in C you can do in C# by using it like C. So it is doable just maybe not how you like.
can't do that either because it ties it to a platform. the goal is cross platform. At best i'd have to do two binaries per platform. and it would only support a limited number of them. So it doesn't fulfill the requirements. If I didn't care about being cross platform, I'd just use mixed mode C++. It's okay that this isn't doable. I accept it. It would just be nice if it was.
When I was growin' up, I was the smartest kid I knew. Maybe that was just because I didn't know that many kids. All I know is now I feel the opposite.
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That Ezra Taft Benson, while true, is one of the dumbest remarks I've seen, and is used by some groups to put down government. The point of government is to take your money and do what's best for the people as a whole. You can argue that isn't what happens in some cases, but that misses the point. Government generally is the only entity that takes risks when the reward isn't obvious. That where just about all of our technology comes from because industry won't invest without the promise of fairly quick rewards.
I have seen a similar quote attributed to Thomas Jefferson. I think it is less about government per se, but more about the scale of government.
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can't do that either because it ties it to a platform. the goal is cross platform. At best i'd have to do two binaries per platform. and it would only support a limited number of them. So it doesn't fulfill the requirements. If I didn't care about being cross platform, I'd just use mixed mode C++. It's okay that this isn't doable. I accept it. It would just be nice if it was.
When I was growin' up, I was the smartest kid I knew. Maybe that was just because I didn't know that many kids. All I know is now I feel the opposite.
You don't need multiple native or managed binaries for this. You can just wrap each platform and detect what platform you're on in the .NET runtime via RuntimeInformation class. So you would probably only need one for Win32 and one for POSIX (aka Linux, BSD and macOS). Because a .NET lib doesn't exist for what you need thats portable, you will have to make one that is.
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You don't need multiple native or managed binaries for this. You can just wrap each platform and detect what platform you're on in the .NET runtime via RuntimeInformation class. So you would probably only need one for Win32 and one for POSIX (aka Linux, BSD and macOS). Because a .NET lib doesn't exist for what you need thats portable, you will have to make one that is.
Once again, I cannot map objects to a specific process address space in .NET. No amount of marshalling or unsafe code will change that. It might be possible to create an unmanaged cross compiled library and link to the different calling conventions etc, by conditional P/Invoke, at the cost of extra complexity and adds marshalling to a class that shouldn't have it for perf reasons, and still requiring a cross compiled unmanaged binary and still therefore being limited to certain platforms, not all .NET platforms. So again, it doesn't fulfill the basic requirement because of that latter bit. I accept that it can't be done in .NET
When I was growin' up, I was the smartest kid I knew. Maybe that was just because I didn't know that many kids. All I know is now I feel the opposite.
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Once again, I cannot map objects to a specific process address space in .NET. No amount of marshalling or unsafe code will change that. It might be possible to create an unmanaged cross compiled library and link to the different calling conventions etc, by conditional P/Invoke, at the cost of extra complexity and adds marshalling to a class that shouldn't have it for perf reasons, and still requiring a cross compiled unmanaged binary and still therefore being limited to certain platforms, not all .NET platforms. So again, it doesn't fulfill the basic requirement because of that latter bit. I accept that it can't be done in .NET
When I was growin' up, I was the smartest kid I knew. Maybe that was just because I didn't know that many kids. All I know is now I feel the opposite.
You said using unsafe code wasn't an issue? So again anything you can do in C can be done in C# if you use unsafe code if thats not an issue to you. It sounds like you don't fully understand what you can do in unsafe code in C#. Cross compiling a native binary would be the same as just using unsafe code in C# and wraping the C methods you need.
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You said using unsafe code wasn't an issue? So again anything you can do in C can be done in C# if you use unsafe code if thats not an issue to you. It sounds like you don't fully understand what you can do in unsafe code in C#. Cross compiling a native binary would be the same as just using unsafe code in C# and wraping the C methods you need.
zezba9000 wrote:
So again anything you can do in C can be done in C# if you use unsafe code
This is categorically incorrect. There is no way to map a .NET object to a file backed portion of the process address space and there likely never will be in .NET. Copying the object to that space or otherwise serializing it is not the same thing at all. I'm going attempt to summarize: You cannot create an object at a specific memory location in managed code.
When I was growin' up, I was the smartest kid I knew. Maybe that was just because I didn't know that many kids. All I know is now I feel the opposite.
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zezba9000 wrote:
So again anything you can do in C can be done in C# if you use unsafe code
This is categorically incorrect. There is no way to map a .NET object to a file backed portion of the process address space and there likely never will be in .NET. Copying the object to that space or otherwise serializing it is not the same thing at all. I'm going attempt to summarize: You cannot create an object at a specific memory location in managed code.
When I was growin' up, I was the smartest kid I knew. Maybe that was just because I didn't know that many kids. All I know is now I feel the opposite.
You don't need to if you use pointers in C# just like you would in C. AKA "unsafe code". Unsafe code doesn't require the use of managed objects. Use int* instead of int[] then it will work. More info: Unsafe code and pointers - C# Programming Guide | Microsoft Docs[^]
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You don't need to if you use pointers in C# just like you would in C. AKA "unsafe code". Unsafe code doesn't require the use of managed objects. Use int* instead of int[] then it will work. More info: Unsafe code and pointers - C# Programming Guide | Microsoft Docs[^]
I'm sorry but yes you do if you want that region to be backed by a memory mapped file. otherwise you're just copying/serializing objects into that space at best. to back an object with a memory mapped region of the process address space you'd have to be able to pin an object to a specific address - not really possible in a garbage collected system without some sort of significant work around.
When I was growin' up, I was the smartest kid I knew. Maybe that was just because I didn't know that many kids. All I know is now I feel the opposite.
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I'm sorry but yes you do if you want that region to be backed by a memory mapped file. otherwise you're just copying/serializing objects into that space at best. to back an object with a memory mapped region of the process address space you'd have to be able to pin an object to a specific address - not really possible in a garbage collected system without some sort of significant work around.
When I was growin' up, I was the smartest kid I knew. Maybe that was just because I didn't know that many kids. All I know is now I feel the opposite.
No you don't have to copy memory if you use a native pointer to a memory mapped file. In C# a pointer like "int*" is the same as a pointer in C/C++. There is no difference. That pointer in C# will directly be pointing to your native memory buffer you allocated from a native Win32 or POSIX API. Pointers in C# can point to literally anything just as they can in C/C++. That includes managed and unmanaged memory locations. For example this psuedo C# code would work:
[DllImport("nativeLib")]
static unsafe extern int* CreateNativeMemoryMappedFile();unsafe void WriteMemoryMappedFile()
{
int* nativeMemory = CreateNativeMemoryMappedFile();
nativeMemory[0] = 123;// this is writing directly from the native buffer (not a copy)
if (nativeMemory[1] == 321) return;// this is reading directly from the native buffer (not a copy)
} -
No you don't have to copy memory if you use a native pointer to a memory mapped file. In C# a pointer like "int*" is the same as a pointer in C/C++. There is no difference. That pointer in C# will directly be pointing to your native memory buffer you allocated from a native Win32 or POSIX API. Pointers in C# can point to literally anything just as they can in C/C++. That includes managed and unmanaged memory locations. For example this psuedo C# code would work:
[DllImport("nativeLib")]
static unsafe extern int* CreateNativeMemoryMappedFile();unsafe void WriteMemoryMappedFile()
{
int* nativeMemory = CreateNativeMemoryMappedFile();
nativeMemory[0] = 123;// this is writing directly from the native buffer (not a copy)
if (nativeMemory[1] == 321) return;// this is reading directly from the native buffer (not a copy)
}you cannot create objects at that pointer. ergo, no vmem backed objects. you can do pointer ops, but then you're copying object data from the object (on GC heap) to the unmanaged heap. If I do a memcpy, or pointer based assignment in a for loop, either way i'm copying the data. and doing so, even if i did, is really no different than calling (filestream).Write(...) which would be more direct. there is no way around it because of how a GC works. if you think I'm wrong show me how to *create an object* at a specific address in .NET - because that's the only way to back an object with vmem, short of creating a custom CLI host in C(++) or some other unmanaged language and AFAICT would have to back an entire appdomain with vmem for it to work.
When I was growin' up, I was the smartest kid I knew. Maybe that was just because I didn't know that many kids. All I know is now I feel the opposite.
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you cannot create objects at that pointer. ergo, no vmem backed objects. you can do pointer ops, but then you're copying object data from the object (on GC heap) to the unmanaged heap. If I do a memcpy, or pointer based assignment in a for loop, either way i'm copying the data. and doing so, even if i did, is really no different than calling (filestream).Write(...) which would be more direct. there is no way around it because of how a GC works. if you think I'm wrong show me how to *create an object* at a specific address in .NET - because that's the only way to back an object with vmem, short of creating a custom CLI host in C(++) or some other unmanaged language and AFAICT would have to back an entire appdomain with vmem for it to work.
When I was growin' up, I was the smartest kid I knew. Maybe that was just because I didn't know that many kids. All I know is now I feel the opposite.
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Quote:
if you think I'm wrong show me how to *create an object* at a specific address in .NET
Invoke the C method from C# you would use to allocation memory at that specific vmem address. Keep a reference to that address in C# as a pointer as you would in C. No difference from doing it in C. Again if you can address it in C you can address it in C# using pointers. Can you show me what method you're invoking in C that allocates memory at a specific address? Then I can give an example and it would remove the confusion. I can only guess at what methods you're trying to use.
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Quote:
if you think I'm wrong show me how to *create an object* at a specific address in .NET
Invoke the C method from C# you would use to allocation memory at that specific vmem address. Keep a reference to that address in C# as a pointer as you would in C. No difference from doing it in C. Again if you can address it in C you can address it in C# using pointers. Can you show me what method you're invoking in C that allocates memory at a specific address? Then I can give an example and it would remove the confusion. I can only guess at what methods you're trying to use.
what C method? the method to create the object? that's not a .NET object. I can already do all this mapping in C. There's no point in any of that.
When I was growin' up, I was the smartest kid I knew. Maybe that was just because I didn't know that many kids. All I know is now I feel the opposite.