Re: Need a library to write/read floats and doubles in IEEE754
On Mar 6, 7:30 am, Jack Klein <jackkl...@spamcop.net> wrote:
On Thu, 06 Mar 2008 04:01:51 GMT, Pavel
<dot_com_yahoo@paultolk_reverse.yourself> wrote in comp.lang.c++:
I'm having trouble understanding what you are asking for.
Does anyone know a (preferably open-source) multi-platform C
or C++ library that would be able to write and read C/C++
doubles and floats to/from streambuf, char array or similar
device in IEEE 754 with reasonably optimal precision and
performance?
Assuming that your platform uses an IEEE format for doubles
and floats, about the fastest possible way to write them would
be to a binary file with fwrite().
That would be true if it worked, but it doesn't.
Just saying that doubles and floats are IEEE isn't enough to
specify the format, and while both my Linux PC's and my Sparcs
use IEEE, the format is different on each. What *usually* works
(although formally undefined behavior) is to convert the address
of the double to a uint64_t* (or the address of the float to a
uint32_t*), and output that in the usual way, e.g.:
dest.put( value >> 56 ) ;
dest.put( value >> 48 ) ;
dest.put( value >> 40 ) ;
dest.put( value >> 32 ) ;
dest.put( value >> 24 ) ;
dest.put( value >> 16 ) ;
dest.put( value >> 8 ) ;
dest.put( value ) ;
To be totally generic, of course, you need to extract the
different parts of the double, and reassemble them. Something
like:
bool isNeg = source < 0 ;
if ( isNeg ) {
source = - source ;
}
int exp ;
if ( source == 0.0 ) {
exp = 0 ;
} else {
source = ldexp( frexp( source, &exp ), 53 ) ;
exp += 1022 ;
}
uint64_t mant = static_cast< uint64_t >( value ) ;
dest.put( (isNeg ? 0x80 : 0x00) | exp >> 4 ) ;
dest.put( ((exp << 4) & 0xF0) | ((mant >> 48) & 0x0F) ) ;
dest.put( mant >> 40 ) ;
dest.put( mant >> 32 ) ;
dest.put( mant >> 24 ) ;
dest.put( mant >> 16 ) ;
dest.put( mant >> 8 ) ;
dest.put( mant ) ;
(Interestingly enough, when I measured, I found that it wasn't
as slow as it looks. Note that it should work even if the host
doesn't use IEEE, e.g. an IBM or Unisys mainfram, but I've not
been able to actually test it there.)
The purpose is to exchange serialized doubles and floats
between C/C++ and Java programs where Java serialization
rules are used.
I have no idea what Java serialization of floating point types
looks like. Are you talking about writing to a text file or a
binary file? The IEEE 754 standard does not specify a text
representation.
Java uses a binary format: for floating point, it's basically
IEEE, output in big endian, the same as XDR, but (I think)
without the alignment requirements. (Formally, I don't think
that IEEE specifies where in the "word" the bits for the sign,
exponant and mantissa are placed, but practically, I've never
heard of an implementation where the order---from high bit to
low---wasn't sign, exponent, mantissa.)
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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