Re: errno (was Re: C23 thoughts and opinions - why so conservative?)

On 5/26/2024 9:18 AM, David Brown wrote:

On 26/05/2024 01:45, Keith Thompson wrote:

David Brown <david.brown@hesbynett.no> writes:
[...]

The normal way for multi-threaded systems is to implement it as a
macro.   It might be, for example :
>
    #define errno __thread_data->_errno
>
or
>
    #define errno *errno()

>
Both of those need more parentheses -- and I'm unconfortable using the
same identifier for the macro and the function.
>

 The second example was from the footnote in the C standard's section on <errno.h>, so it can't be /that/ bad!
 But I agree with your discomfort.
 

I would expect it to immediately explode, because AFAIK the usual preprocessor behavior is to keep expanding macros in a line until there is nothing left to expand.
Well, granted, it is possible I could have misinterpreted how it was supposed to work and had never noticed...

That is precisely why it is specified in the C standards as a macro,
not an external linkage object with static or thread-local storage
duration.   (The use of errno in multi-threading C code long predates
C11 and _Thread_local.)

[...]
>
glibc and musl both have :
>
     # define errno (*__errno_location ())
>
newlib (used on Cygwin) has something similar :
>
     #define errno (*__errno())
>