Re: How do I find out what C's long double is?

On 18 Mar 2008 12:03:05 GMT, nmm1@xxxxxxxxxxxxx (Nick Maclaren) wrote:

Number of bits in storage unit: sizeof(double)*CHAR_BIT
Number of bits in value (assuming FLT_RADIX == 2):
1+(int)ceil(log2(DBL_MAX_EXP-DBL_MIN_EXP))+DBL_MANT_DIG

Very nice; thanks (and obvious now that you've pointed it out).

There's a fly in the ointment, though, which is that it's not quite
correct. The test prog below produces this output on my
Centos4.4/x86_64:

FLT_MAX_EXP is 128
FLT_MIN_EXP is -125
FLT_MANT_DIG is 24
FLT bits: 33
DBL_MAX_EXP is 1024
DBL_MIN_EXP is -1021
DBL_MANT_DIG is 53
DBL bits: 65
LDBL_MAX_EXP is 16384
LDBL_MIN_EXP is -16381
LDBL_MANT_DIG is 64
LDBL bits: 80

After some digging about, it seems that x_MANT_DIG is the logical
number of mantissa digits, including the missing MS significand bit,
so it should be (x_MANT_DIG - 1). But '80' is correct on my box, the
reason being that the x86 double extended format apparently has an
explicit MS significand bit (although I suspect that the silicon may
actually only be 79 bits?)

After a (very cursory) look at the C99 spec there doesn't seem to be a
way to find out whether the MS significand bit is implicit or
explicit, so I guess I'll have to live with this.

Any thoughts?

Thanks

-Dom

----------------------------
#include <stdio.h>
#include <float.h>
#include <math.h>

int main(void) {

printf("FLT_MAX_EXP is %d\n", FLT_MAX_EXP);
printf("FLT_MIN_EXP is %d\n", FLT_MIN_EXP);
printf("FLT_MANT_DIG is %d\n", FLT_MANT_DIG);
printf(
"FLT bits: %d\n",
1+(int)ceil(log2(FLT_MAX_EXP-FLT_MIN_EXP))+FLT_MANT_DIG);

printf("DBL_MAX_EXP is %d\n", DBL_MAX_EXP);
printf("DBL_MIN_EXP is %d\n", DBL_MIN_EXP);
printf("DBL_MANT_DIG is %d\n", DBL_MANT_DIG);
printf(
"DBL bits: %d\n",
1+(int)ceil(log2(DBL_MAX_EXP-DBL_MIN_EXP))+DBL_MANT_DIG);

printf("LDBL_MAX_EXP is %d\n", LDBL_MAX_EXP);
printf("LDBL_MIN_EXP is %d\n", LDBL_MIN_EXP);
printf("LDBL_MANT_DIG is %d\n", LDBL_MANT_DIG);
printf(
"LDBL bits: %d\n",
1+(int)ceil(log2(LDBL_MAX_EXP-LDBL_MIN_EXP))+LDBL_MANT_DIG);

return 0;
}

.

Relevant Pages

  • Re: How do I find out what Cs long double is?
    ... Dom Fulton writes: ... |> There's a fly in the ointment, though, which is that it's not quite ... |> number of mantissa digits, including the missing MS significand bit, ... |> explicit, so I guess I'll have to live with this. ...
    (comp.arch.arithmetic)
  • Re: How do I find out what Cs long double is?
    ... number of mantissa digits, including the missing MS significand bit, ... reason being that the x86 double extended format apparently has an ... explicit, so I guess I'll have to live with this. ...
    (comp.arch.arithmetic)
  • Re: How do I find out what Cs long double is?
    ... number of mantissa digits, including the missing MS significand bit, ... reason being that the x86 double extended format apparently has an ... explicit MS significand bit (although I suspect that the silicon may ...
    (comp.arch.arithmetic)