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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] Regarding PTRACE implementation in QEMU user mode
Date: Tue, 23 Sep 2008 15:12:40 +0100
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To: qemu-devel@nongnu.org
Cc: sathish kumar <sathishs_45@yahoo.com>, rajesh-siso <rajesh.uvce@gmail.com>

> Can someone explain why explain why ptrace system call is not implemented
> in QEMU? if someone is working on it, please share some ideas.

Because it's extremely hard (read: nearly impossible) to implement properly.
ptrace allows direct access to the state of a different process. Passing 
through ptrace would give you the state of qemu itself, not the program 
you're interested in, so you have to emulate everything via some othe IPC 
mechanism (which must also be hidden from the emulated application). This 
includes signals like SIGSTOP which cannot be trapped.

If you're not extremely careful you end up with a monstrosity like Cygwin, and 
all the problems that implies.

Paul