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Subject: |
coreutils' getlimits fails to represent float limits correctly |
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Date: |
Tue, 18 Jun 2013 02:10:21 -0400 |
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User-agent: |
Mutt/1.5.21 (2010-09-15) |
I noticed a bug in the getlimits utility's printing of DBL_MAX, etc.
The precision at which the limits are printed is grossly insufficient
to represent the actual limits; if the output is converted back to a
floating point number of the appropriate type, the result will differ
greatly from the actual limit.
The format specifier used for printing these values is is %Le, which
gives 7 decimal digits. The number of digits needed to faithfully
represent the limits varies by type, but a safe fix would be replacing
printf (#TYPE"_MIN=%Le\n", (long double)TYPE##_MIN); \
printf (#TYPE"_MAX=%Le\n", (long double)TYPE##_MAX);
with:
printf (#TYPE"_MIN=%.*Le\n", DECIMAL_DIG, (long double)TYPE##_MIN); \
printf (#TYPE"_MAX=%.*Le\n", DECIMAL_DIG, (long double)TYPE##_MAX);
It would be possible to use appropriate precisions for each type, but
unfortunately I'm not aware of a convenient way to compute the number
of digits needed based on other properties of the type. FLT_DIG and
DBL_DIG are not the correct values for this; they deal with the
opposite round-trip. One solution would be to simply hard-code the
number of digits for float and double when they're IEEE types, and
otherwise use DECIMAL_DIG for all three types (for obscure systems
where float/double are not IEEE single/double). I believe the correct
number of digits for IEEE single or double would be 10 and 17,
respectively.
Rich
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--- Begin Message ---
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Subject: |
Re: bug#14650: coreutils' getlimits fails to represent float limits correctly |
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Date: |
Tue, 18 Jun 2013 10:29:32 +0100 |
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User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130110 Thunderbird/17.0.2 |
On 06/18/2013 08:47 AM, Paul Eggert wrote:
> On 06/17/2013 11:10 PM, address@hidden wrote:
>> I'm not aware of a convenient way to compute the number
>> of digits
>
> coreutils already can do that; see lib/ftoastr.h.
> Presumably coreutils should use the ftoastr module hear.
Something like the patch below?
which changes the output from:
FLT_MIN=1.175494e-38
FLT_MAX=3.402823e+38
DBL_MIN=2.225074e-308
DBL_MAX=1.797693e+308
LDBL_MIN=3.362103e-4932
LDBL_MAX=1.189731e+4932
to...
FLT_MIN=1.1754944e-38
FLT_MAX=3.4028235e+38
DBL_MIN=2.2250738585072014e-308
DBL_MAX=1.7976931348623157e+308
LDBL_MIN=3.3621031431120935063e-4932
LDBL_MAX=1.189731495357231765e+4932
Note the difference lengths of LDBL_MIN and LDBL_MAX.
cheers,
Pádraig.
diff --git a/src/getlimits.c b/src/getlimits.c
index 7c1fbe2..dfc4b12 100644
--- a/src/getlimits.c
+++ b/src/getlimits.c
@@ -21,6 +21,7 @@
#include <sys/types.h>
#include <float.h>
+#include "ftoastr.h"
#include "system.h"
#include "long-options.h"
@@ -97,6 +98,19 @@ decimal_absval_add_one (char *buf)
return result;
}
+#define PRINT_FLOATTYPE(N, T, FTOASTR, BUFSIZE) \
+static void \
+N (T x) \
+{ \
+ char buf[BUFSIZE]; \
+ FTOASTR (buf, sizeof buf, FTOASTR_LEFT_JUSTIFY, 0, x); \
+ puts (buf); \
+}
+
+PRINT_FLOATTYPE (print_FLT, float, ftoastr, FLT_BUFSIZE_BOUND)
+PRINT_FLOATTYPE (print_DBL, double, dtoastr, DBL_BUFSIZE_BOUND)
+PRINT_FLOATTYPE (print_LDBL, long double, ldtoastr, LDBL_BUFSIZE_BOUND)
+
int
main (int argc, char **argv)
{
@@ -127,8 +141,8 @@ main (int argc, char **argv)
}
#define print_float(TYPE) \
- printf (#TYPE"_MIN=%Le\n", (long double)TYPE##_MIN); \
- printf (#TYPE"_MAX=%Le\n", (long double)TYPE##_MAX);
+ printf (#TYPE"_MIN="); print_##TYPE(TYPE##_MIN); \
+ printf (#TYPE"_MAX="); print_##TYPE(TYPE##_MAX);
/* Variable sized ints */
print_int (CHAR);
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