[tlug] issues with format of double (or IEEE754)

[Michal Hajek <hajek1@example.com>]

Hello :)

I appologize if this is not a sort of "only linux related question".
Still I believe there is some kind of relation to linux, since I do all
the described actions using debian testing on an i386 mashine...

I have a problem with getting a double precision number from my digital
nanovoltmeter (keithley 2182A). 

I can comunicate with it through gpib interface and the manual says [1],
that it can send 10byte data in format:
(ascii header)#0 (just 64 bits) 1110010.....

So after two characters, 64 zeros and ones come. 
The later are organised such that first 11 bits are "exponent bits"
followed by 52 "fraction bits", conforming to IEEE754 format.

So I use a function ibrd() [2] to read this data in a buffer defined in
this way:
char result_normal_plus[17]="00000000000000000";
...
ibrd(K2182Ud,result_normal_plus,10);
result_normal_plus[ibcnt]='\0';

and then I do:
presult= result_normal_plus+2;

where presult is a pointer to double, eg. defined as:
double *presult;

The problem is that if I print the number:

printf("*presult: %.12f \n",*presult);

I always get a number which is 4 orders of magnitude lower. For example:
0.00002 instead of 0.2 

Can anyone help me to understand why it is so? 
Or maybe suggest a more clever [3] way of getting the result from
nanovoltmeter? 

The compilation is done with gcc (GCC) 4.0.4 20060507 (prerelease)
(Debian 4.0.3-3), 
using -pg -Wall flags. 
(-pg is for profiler info)

For clarity, I once more rewrite the structure of my program:

...
double *presult;
char result_normal_plus[17]="00000000000000000";
...
ibrd(K2182Ud,result_normal_plus,10);
result_normal_plus[ibcnt]='\0';
presult= result_normal_plus+2;
printf("*presult: %.12f \n",*presult);
...

Thank you for any kind of help.. :)


[1] http://www.keithley.com/data?asset=15977  section 15-5
[2] http://linux-gpib.sourceforge.net/doc_html/r1811.html
[3] I have implemented reading in ASCII format and than converting with
atof() function, but this I prefer to avoid, since atof() takes more
time than reading straight in double format (or at least I think so)

Best regards
Michal