[RFC] clang, posix: bypass static assert that does not compile with Clang

[Sebastian Huber]

On 05/10/2018 08:57, Daniel Hellstrom wrote:
> From: Jacob Hansen <jacob.hansen at gaisler.com>
>
> This commits bypasses a static assert when using the Clang compiler.
> This is done as the static assertion does not compile with Clang
> ("static_assert expression is not an integral constant expression").
>
> I am not sure this static assertion makes sense at all. SEM_FAILED
> is used as a return value on failures from running sem_open(). Is
> the test supposed to check that SEM_FAILED is defiend as NULL in the
> included libraries (newlib) ? It is defiend as "(sem_t *) 0" in newlib.
>
> The draft version of the posix standard, required sem_open to return -1, so
> maybe this is an attempt to ensure that SEM_FAILED is actually defined?
> But the compilation would failed if SEM_FAILED is not defined anyway.
>
> I guess the check would make sense if somehow the code depended on
> SEM_FAILED to be equal to NULL. But in that case I would think the code
> should be updated to remove this dependency.
>
> Looking at the source for sem_open at cpukit/posix/src/semopen.c it
> seems that it uses the SEM_FAILED define as a return value, and I do not
> see any dependencies on SEM_FAILED==NULL anywhere.
>
> The change that introduces this code is:
>
> "posix: Implement self-contained POSIX semaphores":
>    c090db7405b72ce6d0b726c0a39fb1c1aebab7ea

This static assertion ensures that POSIX_SEMAPHORE_VALIDATE_OBJECT() 
rejects semaphore pointers with a SEM_FAILED value. We could change this 
macro to test also for SEM_FAILED, but it would result in code like this

if (sem == 0 (aka NULL) || sem == 0 (aka SEM_FAILED) || ...)

See also:

https://gcc.gnu.org/ml/gcc-help/2018-10/msg00021.html

Coverity Scan crashes at this line too. GCC issues only a -Wpedantic 
warning. In C++ the static assert would be fine. I am not sure how to 
fix this.

-- 
Sebastian Huber, embedded brains GmbH

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