Re: exp sets the errno to ERANGE error .

[Szabolcs Nagy <Szabolcs dot Nagy at arm dot com>]

On 07/05/2019 08:51, Umesh Kalappa wrote:
> Hi everyone ,
> 
> we have situations ,where the following program sets the errno to
> 34(range error) in the version 2.28
> 
> $cat test.c
> int main() {
> printf("errno %d\n", errno);
> printf("%f\n", exp(-1421.0));
> printf("errno %d\n", errno);
> }
> 
> according the C11 standard the exp should not set the errno to ERANGE
> when it underflow to zero .
> 
> so we think that  glibc (atleast 2.28) is not consistent with the C
> standard and any comments regrading the same .

the c standard allows ERANGE for uflow.

"If the result underflows, the function returns an
 implementation-defined value whose magnitude is no
 greater than the smallest normalized positive number
 in the specified type; if the integer expression
 math_errhandling & MATH_ERRNO is nonzero, whether
 errno acquires the value ERANGE is implementation-defined;"

http://port70.net/~nsz/c/c11/n1570.html#7.12.1p6

glibc documents its behaviour as

"When underflow occurs, the underflow exception is raised,
 and zero (appropriately signed) or a subnormal value, as
 appropriate for the mathematical result of the function and
 the rounding mode, is returned. errno may be set to ERANGE,
 but this is not guaranteed; it is intended that the GNU C
 Library should set it when the underflow is to an appropriately
 signed zero, but not necessarily for other underflows."

https://www.gnu.org/software/libc/manual/html_node/Math-Error-Reporting.html

> 
> Thank you  and looking forward for any suggestions / comments.
> ~Umesh
>