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Date: Fri, 20 Jan 2006 00:59:40 +0000
From: Cliff Hones <cliff AT hones DOT org DOT uk>
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Subject: Re: lseek + read = ENOENT
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Sam Steingold wrote:
> I cannot read the last 4-byte word in a file using lseek + read:
> 
> /* file "foo" exists and is large enough - say, 4 MB */
>   int fd = open("foo",O_RDONLY|O_BINARY);
>   uint32 data;
> /* this succeeds and correctly returns the size of file "foo" minus 4 */
>   lseek(fd,-sizeof(data),SEEK_END);
> /* this returns 0 -- instead of the expected 4 -- and sets errno to ENOENT */
>   read(fd,&data,sizeof(data));
> 
> if I run this under gdb and type
>   lseek(fd,-sizeof(data),SEEK_END);
>   read(fd,&data,sizeof(data));
> several times, eventually read() starts to return 4 and set data to the
> value I actually wrote into "foo" last.
> 
> I observe this on linux, cygwin and solaris -- what am I doing wrong?

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.

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

-- Cliff



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