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

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.

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())