[lldb-dev] Process spawning and shutdown

Zachary Turner zturner at google.com
Mon Sep 22 11:31:53 PDT 2014

BTW, I'm writing this up into a larger message that explains the big
picture and how everything ties together, I play to post that in a separate
thread.  Maybe today, if I finish it, otherwise tomorrow.

On Mon, Sep 22, 2014 at 11:30 AM, Zachary Turner <zturner at google.com> wrote:

> The only way to wait for a process to exit is calling
> WaitForSingleObject() with the handle. You can specify a timeout to that
> function, and INFINITE is one possible value.  However, it seems like the
> primary use case for LLDB waiting for a process to exit is to fire off the
> exit callback asynchronously.  In that case Windows, like other platforms,
> will probably need to create a background thread, then
> WaitForSingleObject() on the background thread, then fire the callback
> after it returns.
> What I was thinking is have the HostProcess provide a method called
> BlockUntilExit() which can be used anywhere (even from the main thread)
> should the caller desire it, but when launching a process we would create a
> thread for on each platform that just calls this method on the HostProcess,
> then issues the callback.  This would unify alot of code currently spread
> across the different Host.[cpp/mm] files.
> One thing that still eludes me though, is that when processes are launched
> using Applescript, no monitoring thread is created, and it looks at least
> like the callback is never invoked on these processes.  Is this by design,
> a bug, or am I misreading the code?
> On Mon, Sep 22, 2014 at 11:15 AM, Greg Clayton <gclayton at apple.com> wrote:
>> So you can get the exit status of a process using
>> BOOL WINAPI GetExitCodeProcess(_In_ HANDLE hProcess, _Out_  LPDWORD
>> lpExitCode);
>> Is there a way to wait for the process to exit without polling? Or do you
>> have to keep calling GetExitCodeProcess() until STILL_ACTIVE is not
>> returned? I am guessing you can wait on the hProcess handle somehow?
>> Anyway, it seems like we want the following when launching processes:
>> 1 - a way to wait for a process to finish with an optional timeout (which
>> defaults to infinite)
>> 2 - a way to say, no need to wait for the process to finish
>> For 1, we do what we are currently doing. The process monitor callback
>> for windows will need to wait for the process to exit, then call
>> GetExitCodeProcess() when done and return the value. On unix machines, we
>> will reap the process and return the exit status.
>> For 2, windows doesn't need to do anything, but we still need to reap the
>> process on unix to avoid zombies.
>> Not sure if I missed any scenarios?
>> Greg
>> > On Sep 22, 2014, at 9:33 AM, Zachary Turner <zturner at google.com> wrote:
>> >
>> > Yes, but closing the handle is semantically a different operation than
>> reaping a process.  You can close a handle to a process while the process
>> is still running and it doesn't affect anything except that it frees up
>> space in your process's handle table.  You can also not close a handle, and
>> if the process exits nothing bad will come of it.  So closing the handle is
>> more tied to the lifetime of the instantiated HostProcess , and independent
>> of the actual process running on ths system.
>> >
>> > On Sun, Sep 21, 2014 at 11:40 PM, Matthew Gardiner <mg11 at csr.com>
>> wrote:
>> > On Wed, 2014-09-17 at 00:27 -0700, Zachary Turner wrote:
>> >
>> > > I know Join isn't really the right word, but there's no concept of
>> > > reaping a process on Windows.
>> >
>> > However, on Windows, isn't there still a requirement to CloseHandle that
>> > handles returned by CreateProcess?
>> >
>> > Matt
>> >
>> >
>> >
>> >
>> >
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