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Re: dd hangs with SIGINT

From: James Youngman
Subject: Re: dd hangs with SIGINT
Date: Fri, 8 Apr 2005 14:21:35 +0100
User-agent: Mutt/1.3.28i

On Fri, Apr 08, 2005 at 01:34:41PM +0200, Bruno Haible wrote:
> Jim Meyering wrote:
> > Thanks for the patch, but that `cure' seems worse than the disease.
> I agree. The real good way to make this work on POSIX systems (without
> assuming multithreading) is to use fork().
>   - Let the parent process fork a child process, block the signals SIGHUP,
>     SIGQUIT, SIGPIPE in the parent, and watch the child's exit status.
>     If the exit status of the child is 128 + SIGHUP/SIGQUIT/SIGPIPE,
>     print the statistics.
>   - Let the child process to the real I/O work.

This seems a bit cumbersome, and I'm not sure how the parent knows
what the right statistics are.

I have not read the "dd" code in any great detail, but it would seem
to me that the right thing to do is set a sig_atomic_t flag in the
signal handler.  The main loop in dd_copy() should check that flag and
output the statistics (and then reset the flag).

If you want a slightly higher quality of implementation than that,
then you could improve safe_read() and full_write() to call a callback
function before restarting their read or write system call.


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