question about Glibc extensions

[Rich Felker]

On Wed, May 20, 2020 at 10:22:55PM +0200, Andreas Schwab wrote:
> On Mai 20 2020, Martin Sebor wrote:
> 
> > For readlink, using EINVAL to also mean "buffer size is greater
> > than SSIZE_MAX" implies programs cannot interpret that error value
> > alone as meaning the path doesn't refer to a symbolic link.
> 
> You cannot do that anyway.  There is no one-to-one mapping between error
> conditions and error numbers.  The only guarantee is that a defined
> error condition results in a unique error number, not the other way
> round.

POSIX does have something to say about this:

    "Implementations may generate error numbers listed here under
    circumstances other than those described, if and only if all those
    error conditions can always be treated identically to the error
    conditions as described in this volume of POSIX.1-2017."

but just before that, it also says:

    "Implementations may support additional errors not included in
    this list, may generate errors included in this list under
    circumstances other than those described here, or may contain
    extensions or limitations that prevent some errors from
    occurring."

https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_03