Re: [Qemu-devel] [libfdt][PATCH v3] implement strnlen for systems that need it

[Programmingkid <programmingkidx@gmail.com>]

> On Oct 23, 2017, at 12:27 PM, Peter Maydell <peter.maydell@linaro.org> wrote:
> 
> On 23 October 2017 at 17:09, Stefan Hajnoczi <stefanha@gmail.com> wrote:
>>> +/* strnlen() is not available on Mac OS < 10.7 */
>>> +# if (MAC_OS_X_VERSION_MAX_ALLOWED < MAC_OS_X_VERSION_10_7)
>> 
>> Does this cover the case where MAC_OS_X_VERSION_MAX_ALLOWED is set to
>> <1070 on a 10.7+ build machine?  It's possible that the <string.h>
>> header would define strnlen() and your code redefines the function
>> (compiler error).
> 

I was operating under the assumption that MAC_OS_X_VERSION_MAX_ALLOWED would equal the version of the host. After making this little test program:

#include <stdio.h>
#include <AvailabilityMacros.h>

int main(int argc, char *argv[])
{
    printf("value = %d\n", MAC_OS_X_VERSION_MAX_ALLOWED);
}

It reports:
"value = 101204" on Mac OS 10.12.6 (I'm not sure why there is a 04)
and
"Value = 1068" on Mac OS 10.6.8

Is using MAC_OS_X_VERSION_MAX_ALLOWED not a reliable macro to use to test for the version of the Mac OS? The ui/cocoa.m file seems to use it and have no problems. I don't think we have to worry about MAC_OS_X_VERSION_MAX_ALLOWED being set to less than 1070 on Mac OS 10.7 and up. 

> In that case you don't want to use the strnlen() declaration
> from the header, you want the inline somehow, because even if
> the declaration is present and using it doesn't fail compile
> the definition won't be around at runtime.
> 
>> It would be best to check how <string.h>, <Availability.h>, and
>> <AvailabilityMacros.h> work to make sure that all cases are handled.  I
>> don't have access to a Mac right now, sorry.
> 
> It uses the clang 'attribute availability' syntax:
> https://clang.llvm.org/docs/AttributeReference.html#availability

This feature appears to be a clang/gcc-only feature. Using it would mean making this code compiler locked. The Device Tree Compiler project (that this code belongs to) is made by IBM personnel. They might want to be able to use other compilers including their own IBM XL C compiler to compile this project. Even if that part of the code is only to run on Mac OS X I still would like to keep the code generic enough for any compiler to be able to build the Device Tree Compiler project.