Bug 36052 - Using stackalloc zeros out the size variable when passed to another function
Summary: Using stackalloc zeros out the size variable when passed to another function
Alias: None
Product: Runtime
Classification: Mono
Component: JIT (show other bugs)
Version: 4.0.0
Hardware: Macintosh Mac OS
: --- normal
Target Milestone: ---
Assignee: Bugzilla
Depends on:
Reported: 2015-11-19 11:11 UTC by Eric Erhardt
Modified: 2015-11-19 14:19 UTC (History)
3 users (show)

See Also:
Is this bug a regression?: ---
Last known good build:

The project.json file that is needed to run the program with dnx. (665 bytes, application/octet-stream)
2015-11-19 11:11 UTC, Eric Erhardt
The C# code file that demonstrates the bug. (725 bytes, text/plain)
2015-11-19 11:12 UTC, Eric Erhardt

Description Eric Erhardt 2015-11-19 11:11:41 UTC
Created attachment 13890 [details]
The project.json file that is needed to run the program with dnx.

When using "stackalloc char[bufferSize]" and then passing bufferSize into another function, the Mono runtime will clear/zero out the bufferSize variable.  This can be demonstrated with the following code:

using System;

namespace DNXTest
    public class Program
        public unsafe static void Main(string[] args)
            Program p = new Program();
            string typeName = typeof(Program).Name;
            int bufferSize = 45;
            fixed (char* value = typeName)
                char* buffer = stackalloc char[bufferSize];
                p.EncodeIntoBuffer(value, typeName.Length, buffer, bufferSize);
        private unsafe void EncodeIntoBuffer(char* value, int valueLength, char* buffer, int bufferLength)
            Console.WriteLine("bufferLength is " + bufferLength);

I expect to get output that says "bufferLength is 45" which is what I get when running on the coreclr, but instead on the Mono runtime I get "bufferLength is 0".

I am using Mono version on a MacBook Pro OS X El Capitan.

I am using 'dnx run' to run the program, not mcs/mono.  (Although, the code doesn't work on mcs/mono either - I am getting an unexpected NullReferenceException when running it through mcs/mono.)

To set up a repro:

1. Install Mono on the Mac.
2. Install the ASP.NET 5 RC from http://get.asp.net
3. In a terminal, make sure you are using the mono runtime by running: dnvm use 1.0.0-rc1-final -r mono
4. Copy the attached project.json file into a folder.  Also add the above code into the same folder, giving the file the name Program.cs.
5. In a terminal, navigate to the folder with Program.cs and project.json.
6. dnu restore
7. dnx run

Note: This bug was found through investigating the following higher-level issues:
Comment 1 Eric Erhardt 2015-11-19 11:12:42 UTC
Created attachment 13891 [details]
The C# code file that demonstrates the bug.
Comment 2 Zoltan Varga 2015-11-19 14:19:38 UTC
Fixed in mono master 27432be3ec4c65ba618b18389561b57e2b2716cb. Thanks for the report.

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