[Mono-dev] fork() in Mono

Miguel de Icaza miguel at microsoft.com
Wed Jan 3 20:05:17 UTC 2018


Very good observation Robert.

When the original warning was added, Mono was slightly different, it shipped with an IO-layer that added a new set of problems.

The problems were that we used to keep a table of file descriptors in a shared state, and there was no support for handling this properly, so our internal fork/exec code handled this, but not the public fork code.

The existing reason for the comment no longer really applies, but the point that you make certainly will continue to apply.

´╗┐On 12/4/17, 7:06 AM, "Mono-devel-list on behalf of Robert Jordan" <mono-devel-list-bounces at lists.dot.net on behalf of robertj at gmx.net> wrote:

    On 04.12.2017 02:23, Gene Thomas wrote:
    > What happens specifically?|fork()|starts a process, it does not shut 
    > anything down?|fork()|should just work, does it not (effectively (copy 
    > on write)) copy the entire VM which then each shuts down 
    > independently?|fork()|does work in Python on Unix (multiprocessing module).
    > 
    > |fork()|is powerful, it supports using processes as an alternative to 
    > threads. One can not test for a one in a million race 
    > condition.|System.Diagnostics.Process|only replaces|fork()|then|exec()|. 
    > I wish to use|fork()|to support replicated processes, starting with a 
    > complete copy of the application's state greatly simplifies this.
    
    You may want to look at fork(2)'s specs, especially to those parts
    regarding multithreading.
    
    The forked child will start with one running thread only, etc.
    
    This means that W/out some kind of advanced runtime support
    for fork(2), you won't go very far. The GC and other service
    threads won't run anymore, etc.
    
    For toying around, you can test fork(2) even w/out Mono.Posix:
    
    ---
    using System;
    using System.Runtime.InteropServices;
    
    class Program {
    	[DllImport("libc")]
    	public static extern int fork();
    
    	public static void Main()
    	{
    		int pid = fork();
    		if (pid == -1) {
    			Console.WriteLine("fork failed");
    		} else if (pid == 0) {
    			Console.WriteLine ("child");
    		} else
    			Console.WriteLine ("parent, child's pid={0}", pid);
    	}
    }
    ---
    
    Robert
    
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