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From: BGB <cr88192@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: errno (was Re: C23 thoughts and opinions - why so conservative?)
Date: Sun, 26 May 2024 09:48:58 -0500
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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())
>>
> 
> 
>