When/how is global "errno" variable from reent.c to be used?

[Grant Edwards <grant.b.edwards@gmail.com>]

[Sorry for yet another errno vs. reentrancy thread.]

I've been studying errno, __errno, _impure_ptr et alia, and I think I
understand how that's supposed to work. If you don't define
_REENT_ONLY, then errno is defined as *(__errno()), where __errno()
returns &_REENT->errno, and _REENT is #defined as _impure_ptr, and
thread context switches are supposed to set _impure_ptr to point to a
per-thread struct _reent.

If you define _REENT_ONLY, then there is no 'errno' declared in
errno.h, and you've got to mannually pass a struct _reent pointer to
the _r versions of various lib[cm] functions.

But, in the application source I've been given, the LWIP stack does
not include newlib's errno.h.  Instead it declares "extern int errno"
and then uses errno as usual. That external declaration is satisfied
at link time by the global "errno" exported by libc/reent/reent.c.

That seems to be non-thread-safe and won't interoperate correctly with
newlib's library calls or with application code that includes newlib's
errno.h.

What is that global errno variable for and how is to be used?

Is there a use case for single-threaded applications that I've missed
where newlib's library calls will set/check the global 'int errno'
variable instead of _impure_ptr->errno?

Or is the LWIP configuration broken?

Even if there is a single-threaded use case for newlib where everybody
uses the global 'int errno' variable, the application in question is
multi-threaded, so it's probably still broken. The RTOS in use does
not set _impure_ptr on context switches, so I believe that switching
to using the newlib errno.h which will define errno as *(__errno())
still won't work as it should.

FWIW, the toolchain I'm using is

 gcc-arm-none-eabi-9-2020-q2-update-x86_64-linux.tar.bz2

from

 https://developer.arm.com/tools-and-software/open-source-software/developer-tools/gnu-toolchain/gnu-rm/downloads

It contains v3.3.0 newlib and newlib-nano built from

 git://sourceware.org/git/newlib-cygwin.git commit 6d79e0a58866548f435527798fbd4a6849d05bc7

The LWIP application I'm looking at uses newlib-nano.

--
Grant