* [9fans] endiness and IEEE floats
@ 2004-12-23 16:45 Steve Simon
2004-12-23 17:00 ` Nigel Roles
2004-12-23 17:12 ` Russ Cox
0 siblings, 2 replies; 4+ messages in thread
From: Steve Simon @ 2004-12-23 16:45 UTC (permalink / raw)
To: 9fans
I am reading data files, they are always little endian
(Microsoft) but they include IEEE floats. How do I read them
portably?
This is what I came up with, It assumes the endianess
of doubles is the same as vlongs, which I think is fair...
double
gf64(Biobuf *b) // get 64 bit IEEE double
{
uvlong n;
double d;
n = gi64(b); // get 64 bit int
memcpy(&d, &n, sizeof(n));
return d;
}
anyone got somthing better?
-Steve
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [9fans] endiness and IEEE floats
2004-12-23 16:45 [9fans] endiness and IEEE floats Steve Simon
@ 2004-12-23 17:00 ` Nigel Roles
2004-12-23 17:14 ` Russ Cox
2004-12-23 17:12 ` Russ Cox
1 sibling, 1 reply; 4+ messages in thread
From: Nigel Roles @ 2004-12-23 17:00 UTC (permalink / raw)
To: Fans of the OS Plan 9 from Bell Labs
Steve Simon wrote:
>I am reading data files, they are always little endian
>(Microsoft) but they include IEEE floats. How do I read them
>portably?
>
>This is what I came up with, It assumes the endianess
>of doubles is the same as vlongs, which I think is fair...
>
>double
>gf64(Biobuf *b) // get 64 bit IEEE double
>{
> uvlong n;
> double d;
> n = gi64(b); // get 64 bit int
> memcpy(&d, &n, sizeof(n));
> return d;
>}
>
>anyone got somthing better?
>
>-Steve
>
>
>
if you are going to assume size and endianness matches, then
#define bf64(b) ((double)gi64(b))
would suffice.
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [9fans] endiness and IEEE floats
2004-12-23 16:45 [9fans] endiness and IEEE floats Steve Simon
2004-12-23 17:00 ` Nigel Roles
@ 2004-12-23 17:12 ` Russ Cox
1 sibling, 0 replies; 4+ messages in thread
From: Russ Cox @ 2004-12-23 17:12 UTC (permalink / raw)
To: Fans of the OS Plan 9 from Bell Labs
> I am reading data files, they are always little endian
> (Microsoft) but they include IEEE floats. How do I read them
> portably?
>
> This is what I came up with, It assumes the endianess
> of doubles is the same as vlongs, which I think is fair...
>
> double
> gf64(Biobuf *b) // get 64 bit IEEE double
> {
> uvlong n;
> double d;
> n = gi64(b); // get 64 bit int
> memcpy(&d, &n, sizeof(n));
> return d;
> }
>
> anyone got somthing better?
That's what I do (well, different problem but same sort of solution)
to implement isNaN etc. "portably" in libfmt. You could also fiddle
with the bytes, but that requires detecting the byte order.
gf64(Biobuf *b)
{
uchar buf[8];
static uchar big1[4] = { 0, 0, 0, 1 };
Bread(b, buf, 8);
if(*(ulong*)big1 == 1) /* running on big endian machine */
reverse(buf, 8);
return *(double*)buf;
}
Ken came up with clever way to do doing integer conversion:
gi32(Biobuf *b)
{
int i;
u64int leorder = 0x0706050403020100, to;
uchar lebuf[8];
uchar *o, *t, *f;
Bread(b, lebuf, 8);
o = (uchar*)&leorder;
f = lebuf;
t = (uchar*)&to;
for(i=0; i<8; i++)
t[o[i]] = f[i];
}
The kernel and user space use this trick to convert to and
from the big-endian 64-bit numbers in /dev/bintime.
When the 64-bit support was worse, it was a huge win.
It's probably still a win but less so.
Russ
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [9fans] endiness and IEEE floats
2004-12-23 17:00 ` Nigel Roles
@ 2004-12-23 17:14 ` Russ Cox
0 siblings, 0 replies; 4+ messages in thread
From: Russ Cox @ 2004-12-23 17:14 UTC (permalink / raw)
To: Fans of the OS Plan 9 from Bell Labs
> if you are going to assume size and endianness matches, then
>
> #define gf64(b) ((double)gi64(b))
>
> would suffice.
that would be true if gf64 and gi64 were returning pointers,
but they are returning the actual values.
#define bf64(b) (*(double*)&gi64(b))
would work except that you can't take the address
of the return value.
^ permalink raw reply [flat|nested] 4+ messages in thread
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2004-12-23 16:45 [9fans] endiness and IEEE floats Steve Simon
2004-12-23 17:00 ` Nigel Roles
2004-12-23 17:14 ` Russ Cox
2004-12-23 17:12 ` Russ Cox
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