[Haskell-cafe] Re: getChar + System.Cmd.system + threads causes hangups

Donn Cave donn at drizzle.com
Tue Feb 21 12:10:37 EST 2006

On Tue, 21 Feb 2006, Simon Marlow wrote:
> The reason for the deadlock is because getChar is holding a lock on 
> stdin, and System.Cmd.system needs to access the stdin Handle in order 
> to know which file descriptor to dup as stdin in the child process (the 
> stdin Handle isn't always FD 0, because of hDuplicateTo).

I was puzzled by this;  from a quick review of the source, it seems
that this is because system calls runProcessPosix, which has optional
arguments for (stdin, stdout, stderr) for the child process, and when
value is Nothing the parent's stdin etc. are used instead.

That last part doesn't seem right to me, so if it's awkward to implement,
I hope you will consider the possibility that no one needs it.  The
default value for fd 0 should be fd 0.  A process that intends to change
the default input stream on this level where the same stream should be
inherited by children, can open or dup another file onto fd 0.  Conversely,
if a program contrives to change stdin to something besides fd 0, I would
have assumed the intent was to avoid any affect on child processes.

	Donn Cave, donn at drizzle.com

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