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

Daniel Hellstrom daniel at gaisler.com
Mon Oct 8 14:07:16 UTC 2018

Daniel Hellstrom
Software Section Head
Cobham Gaisler
T : +46 (0) 31 775 8657
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daniel.hellstrom at gaisler.com

Cobham Gaisler AB, Kungsgatan 12, SE-411 19, GÖTEBORG, Sweden.
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On 2018-10-05 09:24, Sebastian Huber wrote:
> 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.
I just saw that you have pushed the 
1d39e96470b27195d35a69cc94551c403b7980bd commit that updated the 
RTEMS_STATIC_ASSERT() macro, thanks for updating this. I just wanted to 
let you know that the issue I'm seeing with clang is still there:

make[5]: Entering directory 
sparc-gaisler-rtems5-clang --pipe -DHAVE_CONFIG_H   -I.. 
-mcpu=gr712rc -O2 -g -ffunction-sections -fdata-sections -Wall 
-Wmissing-prototypes -Wimplicit-function-declaration -Wstrict-prototypes 
-Wnested-externs -MT src/libposix_a-seminit.o -MD -MP -MF 
src/.deps/libposix_a-seminit.Tpo -c -o src/libposix_a-seminit.o `test -f 
'src/seminit.c' || echo 
error: static_assert expression is not an integral constant expression
/opt/rcc-1.3-rc6-llvm/lib/clang/8.0.0/include/stddef.h:105:16: note: 
expanded from macro 'NULL'
#  define NULL ((void*)0)
note: expanded from macro 'RTEMS_STATIC_ASSERT'
     _Static_assert(cond, # msg)
note: cast from 'void *' is not allowed in a constant expression
/opt/rcc-1.3-rc6-llvm/lib/clang/8.0.0/include/stddef.h:105:16: note: 
expanded from macro 'NULL'
#  define NULL ((void*)0)
1 error generated.
Makefile:3750: recipe for target 'src/libposix_a-seminit.o' failed

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