[musl] sqrt does not emit errno=EDOM

[musl] sqrt does not emit errno=EDOM

[Paul Zimmermann]

       Hi,

it seems musl does not set errno=EDOM for sqrt(x) when x is negative.
Here on cfarm27 (Alpine Linux):

$ cat /tmp/e.c
#include <stdio.h>
#include <math.h>
#include <errno.h>
 
int
main()
{
  errno = 0;
  float x = -1.0f;
  float y = sqrtf (x);
  printf ("y=%a errno=%d\n", y, errno);
}

$ gcc /tmp/e.c -lm
$ ./a.out 
y=-nan errno=0

Is this a known issue?

Best regards,
Paul

Re: sqrt does not emit errno=EDOM

[Damian McGuckin]

Hi Paul,

On Thu, 3 Jul 2025, Paul Zimmermann wrote:

> it seems musl does not set errno=EDOM for sqrt(x) when x is negative.
> Here on cfarm27 (Alpine Linux):
>
> $ cat /tmp/e.c
> #include <stdio.h>
> #include <math.h>
> #include <errno.h>
>
> int
> main()
> {
>  errno = 0;
>  float x = -1.0f;
>  float y = sqrtf (x);
>  printf ("y=%a errno=%d\n", y, errno);
> }
>
> $ gcc /tmp/e.c -lm
> $ ./a.out
> y=-nan errno=0
>
> Is this a known issue?

I think it is a policy issue:

 	Errors are reported by raising floating-point status flags ie.
 	math_errhandling is set to MATH_ERREXCEPT on all platforms (errno
 	is not set by math functions and on platforms without IEEE 754
 	exception semantics there is no error reporting).

Later - Damian

Re: sqrt does not emit errno=EDOM

[Rich Felker]

On Thu, Jul 03, 2025 at 09:44:12AM +0200, Paul Zimmermann wrote:
>        Hi,
> 
> it seems musl does not set errno=EDOM for sqrt(x) when x is negative.
> Here on cfarm27 (Alpine Linux):
> 
> $ cat /tmp/e.c
> #include <stdio.h>
> #include <math.h>
> #include <errno.h>
>  
> int
> main()
> {
>   errno = 0;
>   float x = -1.0f;
>   float y = sqrtf (x);
>   printf ("y=%a errno=%d\n", y, errno);
> }
> 
> $ gcc /tmp/e.c -lm
> $ ./a.out 
> y=-nan errno=0
> 
> Is this a known issue?

It's intentional that none of musl's math library sets errno.
Implementations have the option to do either or both of setting errno
or raising fenv exception flags as long as this is reflected in the
value of math_errhandling.

Generally, the modern view seems to be that setting errno is bad
practice. It makes it so that the math functions can never be treated
as pure functions by the compiler (note: fenv also breaks purity but
the application gets to signal whether it wants FENV_ACCESS or not,
and if not, they're pure).

Note that on soft float archs without fenv, omitting errno is actually
nonconforming. It was originally my hope that we'd eventually get soft
fenv working on these, but they seem to be of waning relevance these
days anyway. I don't see adding errno as an appropriate fix for this.
Applications generally don't want it, and any such effort to add it
would be better spent on workout out a way to do soft fenv.

Rich

Re: sqrt does not emit errno=EDOM

[Paul Zimmermann]

thank you Rich for clarifying the Musl position with respect to errno.

Paul

> Date: Thu, 3 Jul 2025 16:05:17 -0400
> From: Rich Felker <dalias@libc.org>
> Cc: musl@lists.openwall.com, maxence.ponsardin@inria.fr
> 
> On Thu, Jul 03, 2025 at 09:44:12AM +0200, Paul Zimmermann wrote:
> >        Hi,
> > 
> > it seems musl does not set errno=EDOM for sqrt(x) when x is negative.
> > Here on cfarm27 (Alpine Linux):
> > 
> > $ cat /tmp/e.c
> > #include <stdio.h>
> > #include <math.h>
> > #include <errno.h>
> >  
> > int
> > main()
> > {
> >   errno = 0;
> >   float x = -1.0f;
> >   float y = sqrtf (x);
> >   printf ("y=%a errno=%d\n", y, errno);
> > }
> > 
> > $ gcc /tmp/e.c -lm
> > $ ./a.out 
> > y=-nan errno=0
> > 
> > Is this a known issue?
> 
> It's intentional that none of musl's math library sets errno.
> Implementations have the option to do either or both of setting errno
> or raising fenv exception flags as long as this is reflected in the
> value of math_errhandling.
> 
> Generally, the modern view seems to be that setting errno is bad
> practice. It makes it so that the math functions can never be treated
> as pure functions by the compiler (note: fenv also breaks purity but
> the application gets to signal whether it wants FENV_ACCESS or not,
> and if not, they're pure).
> 
> Note that on soft float archs without fenv, omitting errno is actually
> nonconforming. It was originally my hope that we'd eventually get soft
> fenv working on these, but they seem to be of waning relevance these
> days anyway. I don't see adding errno as an appropriate fix for this.
> Applications generally don't want it, and any such effort to add it
> would be better spent on workout out a way to do soft fenv.
> 
> Rich
>