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问题:
I need to fill a byte[] with a single non-zero value. How can I do this in C# without looping through each byte in the array?
Update: The comments seem to have split this into two questions -
- Is there a Framework method to fill a byte[] that might be akin to
memset - What is the most efficient way to do it when we are dealing with a very large array?
I totally agree that using a simple loop works just fine, as Eric and others have pointed out. The point of the question was to see if I could learn something new about C# :) I think Juliet's method for a Parallel operation should be even faster than a simple loop.
Benchmarks: Thanks to Mikael Svenson: http://techmikael.blogspot.com/2009/12/filling-array-with-default-value.html
It turns out the simple for loop is the way to go unless you want to use unsafe code.
Apologies for not being clearer in my original post. Eric and Mark are both correct in their comments; need to have more focused questions for sure. Thanks for everyone's suggestions and responses.
回答1:
You could use Enumerable.Repeat:
byte[] a = Enumerable.Repeat((byte)10, 100).ToArray();
The first parameter is the element you want repeated, and the second parameter is the number of times to repeat it.
This is OK for small arrays but you should use the looping method if you are dealing with very large arrays and performance is a concern.
回答2:
Actually, there is little known IL operation called Initblk (English version) which does exactly that. So, let's use it as a method that doesn't require "unsafe". Here's the helper class:
public static class Util { static Util() { var dynamicMethod = new DynamicMethod("Memset", MethodAttributes.Public | MethodAttributes.Static, CallingConventions.Standard, null, new [] { typeof(IntPtr), typeof(byte), typeof(int) }, typeof(Util), true); var generator = dynamicMethod.GetILGenerator(); generator.Emit(OpCodes.Ldarg_0); generator.Emit(OpCodes.Ldarg_1); generator.Emit(OpCodes.Ldarg_2); generator.Emit(OpCodes.Initblk); generator.Emit(OpCodes.Ret); MemsetDelegate = (Action<IntPtr, byte, int>)dynamicMethod.CreateDelegate(typeof(Action<IntPtr, byte, int>)); } public static void Memset(byte[] array, byte what, int length) { var gcHandle = GCHandle.Alloc(array, GCHandleType.Pinned); MemsetDelegate(gcHandle.AddrOfPinnedObject(), what, length); gcHandle.Free(); } public static void ForMemset(byte[] array, byte what, int length) { for(var i = 0; i < length; i++) { array[i] = what; } } private static Action<IntPtr, byte, int> MemsetDelegate; }
And what is the performance? Here's my result for Windows/.NET and Linux/Mono (different PCs).
Mono/for: 00:00:01.1356610 Mono/initblk: 00:00:00.2385835 .NET/for: 00:00:01.7463579 .NET/initblk: 00:00:00.5953503
So it's worth considering. Note that the resulting IL will not be verifiable.
回答3:
A little bit late, but the following approach might be a good compromise without reverting to unsafe code. Basically it initializes the beginning of the array using a conventional loop and then reverts to Buffer.BlockCopy(), which should be as fast as you can get using a managed call.
public static void MemSet(byte[] array, byte value) { if (array == null) { throw new ArgumentNullException("array"); } const int blockSize = 4096; // bigger may be better to a certain extent int index = 0; int length = Math.Min(blockSize, array.Length); while (index < length) { array[index++] = value; } length = array.Length; while (index < length) { Buffer.BlockCopy(array, 0, array, index, Math.Min(blockSize, length-index)); index += blockSize; } }
回答4:
Building on Lucero's answer, here is a faster version. It will double the number of bytes copied using Buffer.BlockCopy every iteration. Interestingly enough, it outperforms it by a factor of 10 when using relatively small arrays (1000), but the difference is not that large for larger arrays (1000000), it is always faster though. The good thing about it is that it performs well even down to small arrays. It becomes faster than the naive approach at around length = 100. For a one million element byte array, it was 43 times faster. (tested on Intel i7, .Net 2.0)
public static void MemSet(byte[] array, byte value) { if (array == null) { throw new ArgumentNullException("array"); } int block = 32, index = 0; int length = Math.Min(block, array.Length); //Fill the initial array while (index < length) { array[index++] = value; } length = array.Length; while (index < length) { Buffer.BlockCopy(array, 0, array, index, Math.Min(block, length-index)); index += block; block *= 2; } }
回答5:
If performance is critical, you could consider using unsafe code and working directly with a pointer to the array.
Another option could be importing memset from msvcrt.dll and use that. However, the overhead from invoking that might easily be larger than the gain in speed.
回答6:
This simple implementation uses successive doubling, and performs quite well (about 3-4 times faster than the naive version according to my benchmarks):
public static void Memset<T>(T[] array, T elem) { int length = array.Length; if (length == 0) return; array[0] = elem; int count; for (count = 1; count <= length/2; count*=2) Array.Copy(array, 0, array, count, count); Array.Copy(array, 0, array, count, length - count); }
Edit: upon reading the other answers, it seems I'm not the only one with this idea. Still, I'm leaving this here, since it's a bit cleaner and it performs on par with the others.
回答7:
If performance is absolutely critical, then Enumerable.Repeat(n, m).ToArray() will be too slow for your needs. You might be able to crank out faster performance using PLINQ or Task Parallel Library:
using System.Threading.Tasks; // ... byte initialValue = 20; byte[] data = new byte[size] Parallel.For(0, size, index => data[index] = initialValue);
回答8:
Or use P/Invoke way:
[DllImport("msvcrt.dll", EntryPoint = "memset", CallingConvention = CallingConvention.Cdecl, SetLastError = false)] public static extern IntPtr MemSet(IntPtr dest, int c, int count); static void Main(string[] args) { byte[] arr = new byte[3]; GCHandle gch = GCHandle.Alloc(arr, GCHandleType.Pinned); MemSet(gch.AddrOfPinnedObject(), 0x7, arr.Length); }
回答9:
All answers are writing single bytes only - what if you want to fill a byte array with words? Or floats? I find use for that now and then. So after having written similar code to 'memset' in a non-generic way a few times and arriving at this page to find good code for single bytes, I went about writing the method below.
I think PInvoke and C++/CLI each have their drawbacks. And why not have the runtime 'PInvoke' for you into mscorxxx? Array.Copy and Buffer.BlockCopy are native code certainly. BlockCopy isn't even 'safe' - you can copy a long halfway over another, or over a DateTime as long as they're in arrays.
At least I wouldn't go file new C++ project for things like this - it's a waste of time almost certainly.
So here's basically an extended version of the solutions presented by Lucero and TowerOfBricks that can be used to memset longs, ints, etc as well as single bytes.
public static class MemsetExtensions { static void MemsetPrivate(this byte[] buffer, byte[] value, int offset, int length) { var shift = 0; for (; shift < 32; shift++) if (value.Length == 1 << shift) break; if (shift == 32 || value.Length != 1 << shift) throw new ArgumentException( "The source array must have a length that is a power of two and be shorter than 4GB.", "value"); int remainder; int count = Math.DivRem(length, value.Length, out remainder); var si = 0; var di = offset; int cx; if (count < 1) cx = remainder; else cx = value.Length; Buffer.BlockCopy(value, si, buffer, di, cx); if (cx == remainder) return; var cachetrash = Math.Max(12, shift); // 1 << 12 == 4096 si = di; di += cx; var dx = offset + length; // doubling up to 1 << cachetrash bytes i.e. 2^12 or value.Length whichever is larger for (var al = shift; al <= cachetrash && di + (cx = 1 << al) < dx; al++) { Buffer.BlockCopy(buffer, si, buffer, di, cx); di += cx; } // cx bytes as long as it fits for (; di + cx <= dx; di += cx) Buffer.BlockCopy(buffer, si, buffer, di, cx); // tail part if less than cx bytes if (di < dx) Buffer.BlockCopy(buffer, si, buffer, di, dx - di); } }
Having this you can simply add short methods to take the value type you need to memset with and call the private method, e.g. just find replace ulong in this method:
public static void Memset(this byte[] buffer, ulong value, int offset, int count) { var sourceArray = BitConverter.GetBytes(value); MemsetPrivate(buffer, sourceArray, offset, sizeof(ulong) * count); }
Or go silly and do it with any type of struct (although the MemsetPrivate above only works for structs that marshal to a size that is a power of two):
public static void Memset<T>(this byte[] buffer, T value, int offset, int count) where T : struct { var size = Marshal.SizeOf<T>(); var ptr = Marshal.AllocHGlobal(size); var sourceArray = new byte[size]; try { Marshal.StructureToPtr<T>(value, ptr, false); Marshal.Copy(ptr, sourceArray, 0, size); } finally { Marshal.FreeHGlobal(ptr); } MemsetPrivate(buffer, sourceArray, offset, count * size); }
I changed the initblk mentioned before to take ulongs to compare performance with my code and that silently fails - the code runs but the resulting buffer contains the least significant byte of the ulong only.
Nevertheless I compared the performance writing as big a buffer with for, initblk and my memset method. The times are in ms total over 100 repetitions writing 8 byte ulongs whatever how many times fit the buffer length. The for version is manually loop-unrolled for the 8 bytes of a single ulong.
Buffer Len #repeat For millisec Initblk millisec Memset millisec 0x00000008 100 For 0,0032 Initblk 0,0107 Memset 0,0052 0x00000010 100 For 0,0037 Initblk 0,0102 Memset 0,0039 0x00000020 100 For 0,0032 Initblk 0,0106 Memset 0,0050 0x00000040 100 For 0,0053 Initblk 0,0121 Memset 0,0106 0x00000080 100 For 0,0097 Initblk 0,0121 Memset 0,0091 0x00000100 100 For 0,0179 Initblk 0,0122 Memset 0,0102 0x00000200 100 For 0,0384 Initblk 0,0123 Memset 0,0126 0x00000400 100 For 0,0789 Initblk 0,0130 Memset 0,0189 0x00000800 100 For 0,1357 Initblk 0,0153 Memset 0,0170 0x00001000 100 For 0,2811 Initblk 0,0167 Memset 0,0221 0x00002000 100 For 0,5519 Initblk 0,0278 Memset 0,0274 0x00004000 100 For 1,1100 Initblk 0,0329 Memset 0,0383 0x00008000 100 For 2,2332 Initblk 0,0827 Memset 0,0864 0x00010000 100 For 4,4407 Initblk 0,1551 Memset 0,1602 0x00020000 100 For 9,1331 Initblk 0,2768 Memset 0,3044 0x00040000 100 For 18,2497 Initblk 0,5500 Memset 0,5901 0x00080000 100 For 35,8650 Initblk 1,1236 Memset 1,5762 0x00100000 100 For 71,6806 Initblk 2,2836 Memset 3,2323 0x00200000 100 For 77,8086 Initblk 2,1991 Memset 3,0144 0x00400000 100 For 131,2923 Initblk 4,7837 Memset 6,8505 0x00800000 100 For 263,2917 Initblk 16,1354 Memset 33,3719
I excluded the first call every time, since both initblk and memset take a hit of I believe it was about .22ms for the first call. Slightly surprising my code is faster for filling short buffers than initblk, seeing it got half a page full of setup code.
If anybody feels like optimizing this, go ahead really. It's possible.
回答10:
You could do it when you initialize the array but I don't think that's what you are asking for:
byte[] myBytes = new byte[5] { 1, 1, 1, 1, 1};
回答11:
Looks like System.Runtime.CompilerServices.Unsafe.InitBlock now does the same thing as the OpCodes.Initblk instruction that Konrad's answer mentions (he also mentioned a source link).
The code to fill in the array is as follows:
byte[] a = new byte[N]; byte valueToFill = 255; System.Runtime.CompilerServices.Unsafe.InitBlock(ref a[0], valueToFill, (uint) a.Length);
回答12:
Tested several ways, described in different answers. See sources of test in c# test class
回答13:
The Array object has a method called Clear. I'm willing to bet that the Clear method is faster than any code you can write in C#.
