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

[Richard Damon]

Software the explicitly declares errno as
extern int errno;
is broken, and violates the C standard, and creates explicitly stated
Undefined Behavior.


On 10/16/20 11:50 AM, Grant Edwards via Newlib wrote:
> [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
>
>
>
>
>

-- 
Richard Damon