threads / discuss / 42221

unary minus operator applied to unsigned type, result still unsigned?

Subject: unary minus operator applied to unsigned type, result still unsigned?

## tl;dr

3 messages between May 4, 2016 and May 5, 2016.

replies: 2people: 2as markdown or json

Philip Oakley· May 4, 2016, 22:59 UTC · lore

I'm working on building Git on Visual Studio as part of the Git for Windows capability.

The MSVC compiler is reporting:

1>..\sha1-lookup.c(100) : warning C4146: unary minus operator applied to unsigned type, result still unsigned

1>..\sha1-lookup.c(316) : warning C4146: unary minus operator applied to unsigned type, result still unsigned

the two lines of code are the same, and the message suggests a bad return value:

#100: return -lo-1;

#316: return -lo-1;

Should these be protected by an appropriate brackets/calculation (-1-lo) ? Or does gcc use an alternate assumption about unary minus conversion for functions returning int?

--

Philip

Junio C Hamano· May 4, 2016, 23:19 UTC · re: Philip Oakley · lore

Re: unary minus operator applied to unsigned type, result still unsigned?

"Philip Oakley" <philipoakley@iee.org> writes:

Show 7 quoted lines

> I'm working on building Git on Visual Studio as part of the Git for Windows
> capability.
>
> The MSVC compiler is reporting:
>
> 1>..\sha1-lookup.c(100) : warning C4146: unary minus operator applied to
> unsigned type, result still unsigned

Show 15 quoted lines

> 1>..\sha1-lookup.c(316) : warning C4146: unary minus operator applied to
> unsigned type, result still unsigned
>
> the two lines of code are the same, and the message suggests a bad
> return value:
>
> #100: return -lo-1;
>
> #316: return -lo-1;
>
>
> Should these be protected by an appropriate brackets/calculation
> (-1-lo) ? Or does
> gcc use an alternate assumption about unary minus conversion for functions
> returning int?

I think -lo-1 (which is carried out as unsigned) is further casted back to signed int as part of the return from the function whose type is signed int. This may be portable in practice, but changing type of lo to signed int may be prudent.

sha1_pos() uses size_t, but returns "int", which is inconsistent (size_t could be much larger). Either using int for hi/lo/mi, or using ssize_t and returning ssize_t would make sense.

Philip Oakley· May 5, 2016, 21:39 UTC · re: Junio C Hamano · lore

Re: unary minus operator applied to unsigned type, result still unsigned?

From: "Junio C Hamano" <gitster@pobox.com>

Show 37 quoted lines

> "Philip Oakley" <philipoakley@iee.org> writes:
>
>> I'm working on building Git on Visual Studio as part of the Git for 
>> Windows
>> capability.
>>
>> The MSVC compiler is reporting:
>>
>> 1>..\sha1-lookup.c(100) : warning C4146: unary minus operator applied to
>> unsigned type, result still unsigned
>
>> 1>..\sha1-lookup.c(316) : warning C4146: unary minus operator applied to
>> unsigned type, result still unsigned
>>
>> the two lines of code are the same, and the message suggests a bad
>> return value:
>>
>> #100: return -lo-1;
>>
>> #316: return -lo-1;
>>
>>
>> Should these be protected by an appropriate brackets/calculation
>> (-1-lo) ? Or does
>> gcc use an alternate assumption about unary minus conversion for 
>> functions
>> returning int?
>
> I think -lo-1 (which is carried out as unsigned) is further casted
> back to signed int as part of the return from the function whose
> type is signed int.  This may be portable in practice, but changing
> type of lo to signed int may be prudent.
>
> sha1_pos() uses size_t, but returns "int", which is inconsistent
> (size_t could be much larger).  Either using int for hi/lo/mi, or
> using ssize_t and returning ssize_t would make sense.
>

I'm going to hold off on patching this at the moment, as there are a few more signed/unsigned mismatches that are reported but I've been ignoring (mainly comparisons), so I'll try and catch them all at the same time, especially as I now understand the standards issues.

This thread has a summary of the standards http://stackoverflow.com/questions/8026694/c-unary-minus-operator-behavior-with-unsigned-operands (includes C++ >> C) while https://msdn.microsoft.com/en-us/library/4kh09110(v=vs.90).aspx explains what is being reported.

i.e. unary minus on unsigned ints produces the 2's complement + 1 value, except for 0x8000h, where that would be out of range. It's an oddball of partially defined behaviours. -- Philip