Re: pgsql: Further code review for pg_lsn data type.

On 2014-02-20 08:25:01 +0200, Heikki Linnakangas wrote:
> On 02/20/2014 02:56 AM, Andres Freund wrote:
> >On 2014-02-19 15:10:52 +0000, Robert Haas wrote:
> >>Change input function error messages to be more consistent with what is
> >>done elsewhere. Remove a bunch of redundant type casts, so that the
> >>compiler will warn us if we screw up. Don't pass LSNs by value on
> >>platforms where a Datum is only 32 bytes, per buildfarm. Move macros
> >>for packing and unpacking LSNs to pg_lsn.h so that we can include
> >>access/xlogdefs.h, to avoid an unsatisfied dependency on XLogRecPtr.
> >
> >Hm, won't
> >#define DatumGetLSN(X) ((XLogRecPtr) DatumGetInt64(X))
> >#define LSNGetDatum(X) (Int64GetDatum((int64) (X)))
> >possibly truncate the value if it's larger than 2^(63-1) as int is
> >signed but XLogRecPtr is unsigned?
>
> No. Casting between unsigned and signed integers of same width doesn't lose
> information. For example with 16-bit integers, casting unsigned 40000 to
> signed gives -25536. Casting signed -25536 back to unsigned gives back
> 40000.

Are you sure?

6.3.1.3 Signed and unsigned integers, paragraph 3:
"Otherwise, the new type is signed and the value cannot be represented
in it; either the result is implementation-defined or an
implementation-defined signal is raised."

Afaik unsigned to signed always safe, but not the other way round?

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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