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In recent mono releases, running mono with MONO_ENABLE_SHM=1 environment variable does not seems to enable shared memory anymore.
Issue noticed in mono 4.2.2 and 4.2.3.
The following program runs fine on Microsoft .NET in Windows (first process waits on semaphore and prints "Signaled!" when waits again, the second just signals semaphore and exit), but on Mono in Linux even with MONO_ENABLE_SHM both processes wait indefinitely.
Program code to test issue:
const string name = "MySystemSemaphoreObject_123hx5bwX7";
public static void Main (string  args)
if (!Semaphore.TryOpenExisting (name, out semaphore))
semaphore = new Semaphore (0, 1, name);
while (semaphore.WaitOne ())
Console.WriteLine ("Semaphore exists, signal it.");
$ MONO_ENABLE_SHM=1 mono ShmBug.exe
Linux mylaptop 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt20-1+deb8u4 (2016-02-29) x86_64 GNU/Linux
Mono version (installed from Xamarin Debian repository):
Mono JIT compiler version 4.2.3 (Stable 18.104.22.168/832de4b Wed Mar 16 13:19:08 UTC 2016)
This functionality was removed as the implementation was very complicated and error prone on unix.
Sad news. I think this should be announced in mono release notes (couldn't find anything about that).
It is now documented in the 4.4 release notes: