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

[Programmingkid]

> On Oct 23, 2017, at 12:09 PM, Stefan Hajnoczi <address@hidden> wrote:
> 
> On Sun, Oct 22, 2017 at 10:50:16PM -0400, John Arbuckle wrote:
>> Prior the Mac OS 10.7, the function strnlen() was not available. This patch
>> implements strnlen() on Mac OS X versions that are below 10.7.
>> 
>> Signed-off-by: John Arbuckle <address@hidden>
>> ---
>> v3 changes:
>> - Replaced loop with memchr()
>> 
>> v2 changes:
>> - Simplified the code to make it static inline'ed
>> - Changed the type of count to size_t
>> 
>> libfdt/libfdt_env.h | 24 ++++++++++++++++++++++++
>> 1 file changed, 24 insertions(+)
>> 
>> diff --git a/libfdt/libfdt_env.h b/libfdt/libfdt_env.h
>> index 952056c..d43b66b 100644
>> --- a/libfdt/libfdt_env.h
>> +++ b/libfdt/libfdt_env.h
>> @@ -109,4 +109,28 @@ static inline fdt64_t cpu_to_fdt64(uint64_t x)
>> #undef CPU_TO_FDT16
>> #undef EXTRACT_BYTE
>> 
>> +#ifdef __APPLE__
>> +#include <AvailabilityMacros.h>
>> +
>> +#define MAC_OS_X_VERSION_10_7 1070
> 
> Apple has already defined MAC_OS_X_VERSION_10_7 here:
> https://opensource.apple.com/source/xnu/xnu-1699.24.8/EXTERNAL_HEADERS/AvailabilityMacros.h
> 
> To avoid a compiler warning, please use:
> 
> #ifndef MAC_OS_X_VERSION_10_7
> #define MAC_OS_X_VERSION_10_7 1070
> #endif

Sounds logical.

> 
>> +/* 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).

If MAC_OS_X_VERSION_MAX_ALLOWED is equal to 1070 then this code would not be 
executed. This branch would only be taken if MAC_OS_X_VERSION_MAX_ALLOWED is 
less than 1070. So there would not be a compiler error.

> 
> 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.
> 
> Perhaps this approach works better:
> 
> # ifndef MAC_OS_X_VERSION_10_7

I think you are saying I should remove the "#ifdef __APPLE__" code and just 
check to see if MAC_OS_X_VERSION_10_7 is defined. That might work on Mac OS 
10.6 and 10.5, but it would probably cause problems with non-Mac-OS platforms.