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How do I convert the comp_t data type to a float in C, given its encoding?

I need to convert the Linux C comp_t data type to a float, or something else I can print out. The description of this type is

/*
* comp_t is a 16-bit "floating" point number with a 3-bit base 8
* exponent and a 13-bit fraction. See linux/kernel/acct.c for the
* specific encoding system used.
*/

typedef __u16 comp_t;

I have looked up the encoding function, but still do not know how I can decode it (i.e., convert to a standard data type):

224 /*
225 * encode an unsigned long into a comp_t
226 *
227 * This routine has been adopted from the encode_comp_t() function in
228 * the kern_acct.c file of the FreeBSD operating system. The encoding
229 * is a 13-bit fraction with a 3-bit (base 8) exponent.
230 */
231
232 #define MANTSIZE 13 /* 13 bit mantissa. */
233 #define EXPSIZE 3 /* Base 8 (3 bit) exponent. */
234 #define MAXFRACT ((1 << MANTSIZE) - 1) /* Maximum fractional value. */
235
236 static comp_t encode_comp_t(unsigned long value)
237 {
238 int exp, rnd;
239
240 exp = rnd = 0;
241 while (value > MAXFRACT) {
242 rnd = value & (1 << (EXPSIZE - 1)); /* Round up? */
243 value >>= EXPSIZE; /* Base 8 exponent == 3 bit shift. */
244 exp++;
245 }
246
247 /*
248 * If we need to round up, do it (and handle overflow correctly).
249 */
250 if (rnd && (++value > MAXFRACT)) {
251 value >>= EXPSIZE;
252 exp++;
253 }
254
255 /*
256 * Clean it up and polish it off.
257 */
258 exp <<= MANTSIZE; /* Shift the exponent into place */
259 exp += value; /* and add on the mantissa. */
260 return exp;
261 }

What would be a function to convert it back? THANKS!

nikhiledu

A possible solution is to extract the fractional components for the mantissa and the exponent from the bits encoded in the comp_t value supplied and to create a double number from these fractional components. This algorithm is coded in the function comp_t_to_double() as given below.


/* This function converts from comp_t to double value for exponent base 10. */
double comp_t_to_double(comp_t value)
{
	double dval = 0;
	int exp = 0;
 
	/*
	* Get the fraction components.
	*/
	exp = value >> MANTSIZE;        /* Get the exponent (first 3 bits). */
	dval = (value & MAXFRACT);        /* Get the mantissa (last 13 bits). */
 
	for(int i=0; i < exp; i++) {
		dval /= 10;
	}
 
	return dval;
}

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Infinity08

>> What would be a function to convert it back?

Right from the docs :

comp_t c;
unsigned long v = (c & 0x1fff) << (((c >> 13) & 0x7) * 3);

converts the comp_t value c to an unsigned long value v.

ASKER CERTIFIED SOLUTION

Infinity08

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