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From: Julian Cowley <julian@lava.net>
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To: "M. Warner Losh" <imp@bsdimp.com>
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cc: freebsd-hackers@FreeBSD.ORG
cc: dmw@unete.cl
Subject: Re: [Hackers] Re: any way to reset errno?
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M. Warner Losh wrote:
> In message: <20050206132124.GA746@daemon.unete.cl>
>             Daniel Molina Wegener <dmw@unete.cl> writes:
> :    Any way to reset errno?
> 
> errno = 0;
> 
> Routines that return an error status in errno generally don't set it
> to 0 to mean no error.

Which implies errno should never need to be set to zero since the 
convention is to only look at errno if a system call fails.  The only 
time errno needs to be explicitly set (to any value) is when preserving 
the error value between system calls.  Such as:

if (write(fd, buf, len) < 0) {
	int saved_errno;

	saved_errno = errno;
	close(fd); /* ignore any error */
	errno = saved_errno;
	return -1;
}