[libcxx] r174642 - Belt and suspenders when calling sysconf

[Dave Zarzycki]

To be clear, I am *not* objecting to this patch. It is clearly safe in this specific case.

My point is that replacing "== -1" with "< 0" is a common POSIX anti-pattern. If negative values do not make sense, then arguably an assert() is appropriate if one wants to be paranoid.

davez



On Feb 7, 2013, at 2:04 PM, Howard Hinnant <hhinnant at apple.com> wrote:

> Fwiw, Marshall discussed this change with me before committing.  That being said, I have no objection to reverting it either.  And either way we go, this code does not execute on Apple platforms (#ifdef'd out).
> 
> Howard
> 
> On Feb 7, 2013, at 1:54 PM, Dave Zarzycki <zarzycki at apple.com> wrote:
> 
>> Marshall,
>> 
>> FYI — While safe in this specific example, this is not a correct transform to make in the general case. POSIX very clearly specifies that -1 is the error code for this and similar APIs. This matters for APIs like mmap() which can return "negative" values that not an error.
>> 
>> davez
>> 
>> 
>> On Feb 7, 2013, at 1:48 PM, Marshall Clow <mclow at qualcomm.com> wrote:
>> 
>>> Author: marshall
>>> Date: Thu Feb  7 12:48:09 2013
>>> New Revision: 174642
>>> 
>>> URL: http://llvm.org/viewvc/llvm-project?rev=174642&view=rev
>>> Log:
>>> Belt and suspenders when calling sysconf
>>> 
>>> Modified:
>>>  libcxx/trunk/src/thread.cpp
>>> 
>>> Modified: libcxx/trunk/src/thread.cpp
>>> URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/src/thread.cpp?rev=174642&r1=174641&r2=174642&view=diff
>>> ==============================================================================
>>> --- libcxx/trunk/src/thread.cpp (original)
>>> +++ libcxx/trunk/src/thread.cpp Thu Feb  7 12:48:09 2013
>>> @@ -69,7 +69,9 @@ thread::hardware_concurrency() _NOEXCEPT
>>>   long result = sysconf(_SC_NPROCESSORS_ONLN);
>>>   // sysconf returns -1 if the name is invalid, the option does not exist or
>>>   // does not have a definite limit.
>>> -    if (result == -1)
>>> +    // if sysconf returns some other negative number, we have no idea
>>> +    // what is going on. Default to something safe.
>>> +    if (result < 0)
>>>       return 0;
>>>   return static_cast<unsigned>(result);
>>> #else  // defined(CTL_HW) && defined(HW_NCPU)
>>> 
>>> 
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>> 
>> 
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