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From: ericblake AT comcast DOT net (Eric Blake)
To: Cliff Hones <cliff AT hones DOT org DOT uk>, cygwin AT cygwin DOT com
Subject: Re: lseek + read = ENOENT
Date: Fri, 20 Jan 2006 02:48:41 +0000
Message-Id: <012020060248.13997.43D04F890009515C000036AD22070009530A050E040D0C079D0A@comcast.net>
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> This seems to be a bug in gcc.  The off_t argument to lseek is a 64-bit
> type, but instead of being sign-extended to 64 bits, the value passed
> (-sizeof(data)) passed is only extended to 32-bits, so is actually +4294967292.

No, it is not a bug in gcc.  Read a good book on C, please.

> 
> If you write:
>    int n = -sizeof(data);
>    lseek(fd, n, SEEK_END);
> it works as expected.

Mostly right, because there you are promoting a signed
32-bit number to a signed 64-bit number, which
sign-extends.  However, that approach is risky - if you
have a file that is bigger than 2 GB, you will not get the
correct result, because negation of an unsigned greater
than 2GB results in a positive signed 32-bit value less
than 2GB, instead of the intended negative 64-bit value
with absolute value greater than 2GB.

The safer fix is to call:
lseek(fd, -(off_t)sizeof(data), SEEK_END);

That is, perform the negation after the sign extension,
instead of beforehand, since you know that 32-bit
unsigned to 64-bit signed 0-extends, but that
sizeof(data) is intended to be positive anyway, and
64-bit signed negation of a positive number is
guaranteed to be safe.

--
Eric Blake

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