INT_MIN and compiler diagnostic

**bytebro** · Feb 28 '07, 11:25 AM

Re: INT_MIN and compiler diagnostic

On 28 Feb, 10:07, p_cricket_...@y ahoo.co.in wrote:

When I compile this using gcc, I get a diagnostic on line 7:
>
[pcg@mylinux test]$gcc -ansi -pedantic -Wall -o /tmp/x /tmp/x.c
/tmp/x.c: In function `main':
/tmp/x.c:7: warning: this decimal constant is unsigned only in ISO C90

On mine (Sun sparc) using gcc with exactly the same flags I get

x.c:7: warning: decimal constant is so large that it is unsigned

The program output is:

INT_MAX = 2147483647 INT_MIN = -2147483648
x = -2147483648 y = -2147483648

So amusingly, it tells me the constant is unsigned, then prints it
signed.

Where's a language lawyer when you need one?!

**Beej Jorgensen** · Feb 28 '07, 11:45 AM

Re: INT_MIN and compiler diagnostic

<p_cricket_guy@ yahoo.co.inwrot e:

>/tmp/x.c:7: warning: this decimal constant is unsigned only in ISO C90

I think it was addressed in this thread:

Embedded groups are no longer supported

http://tinyurl.com/ytuxdv

># define INT_MIN (-INT_MAX - 1)
># define INT_MAX 2147483647

This produces the warning:

int y = -2147483648;

whereas this does not:

int y = (-2147483647 - 1);

(Interestingly, though, I guess I expected the preprocessor to calculate
the final answer before passing it to the compiler, and it doesn't.

And now, it doesn't even make sense that the preprocessor would do it.
I mean, the compiler has to do that kind of stuff anyway, right?

So why did I have it in the back of my mind that the preprocessor
sometimes did simple math? Some holdover from the olden days of not-so-
optimal compilers?)

Finally,

int y = -0x80000000;

does not produce a warning, though it is the same as -2147483648.
Apparently the rules are different for hex constants than they are for
decimal constants (c99 6.4.4.1p5).

-Beej

**pete** · Feb 28 '07, 12:45 PM

Re: INT_MIN and compiler diagnostic

bytebro wrote:

INT_MAX = 2147483647 INT_MIN = -2147483648
x = -2147483648 y = -2147483648
>
So amusingly, it tells me the constant is unsigned, then prints it
signed.
>
Where's a language lawyer when you need one?!

If INT_MAX equals 2147483647,
the the type of 2147483648 can't be type int, can it?

--
pete

**bytebro** · Feb 28 '07, 01:35 PM

Re: INT_MIN and compiler diagnostic

On 28 Feb, 12:34, pete <pfil...@mindsp ring.comwrote:

bytebro wrote:

INT_MAX = 2147483647 INT_MIN = -2147483648
x = -2147483648 y = -2147483648

>

So amusingly, it tells me the constant is unsigned, then prints it
signed.

>

Where's a language lawyer when you need one?!

>
If INT_MAX equals 2147483647,
the the type of 2147483648 can't be type int, can it?

Erm... it doesn't say "2147483648 ", it says "-2147483648", which is
exactly equal to INT_MIN, and is therefore a valid int value. The
warning is therefore misleading, no?

**Chris Dollin** · Feb 28 '07, 01:35 PM

Re: INT_MIN and compiler diagnostic

bytebro wrote:

On 28 Feb, 12:34, pete <pfil...@mindsp ring.comwrote:

>bytebro wrote:

INT_MAX = 2147483647 INT_MIN = -2147483648
x = -2147483648 y = -2147483648

>>

So amusingly, it tells me the constant is unsigned, then prints it
signed.

>>

Where's a language lawyer when you need one?!

>>
>If INT_MAX equals 2147483647,
>the the type of 2147483648 can't be type int, can it?

>
Erm... it doesn't say "2147483648 ", it says "-2147483648", which is
exactly equal to INT_MIN, and is therefore a valid int value. The
warning is therefore misleading, no?

No.

`-2147483648` isn't a literal constant. It's an expression, the
negation of `2147483648`.

--
Chris "electric hedgehog" Dollin
"People are part of the design. It's dangerous to forget that." /Star Cops/

**Dik T. Winter** · Feb 28 '07, 02:35 PM

Re: INT_MIN and compiler diagnostic

In article <1172668875.979 428.288060@8g20 00cwh.googlegro ups.com"bytebro " <keith.willis@a ah.co.ukwrites:

On 28 Feb, 12:34, pete <pfil...@mindsp ring.comwrote:

bytebro wrote:

INT_MAX = 2147483647 INT_MIN = -2147483648
x = -2147483648 y = -2147483648

So amusingly, it tells me the constant is unsigned, then prints it
signed.

Where's a language lawyer when you need one?!

If INT_MAX equals 2147483647,
the the type of 2147483648 can't be type int, can it?

>
Erm... it doesn't say "2147483648 ", it says "-2147483648", which is
exactly equal to INT_MIN, and is therefore a valid int value. The
warning is therefore misleading, no?

Does it tell you "-2147483648" is unsigned? Strange. It should
tell you the constant is unsigned, and the constant is "2147483648 ".
There are no negative constants in C.
--
dik t. winter, cwi, kruislaan 413, 1098 sj amsterdam, nederland, +31205924131
home: bovenover 215, 1025 jn amsterdam, nederland; http://www.cwi.nl/~dik/

**Richard Heathfield** · Feb 28 '07, 03:15 PM

Re: INT_MIN and compiler diagnostic

Dik T. Winter said:

<snip>

There are no negative constants in C.

"No" is a counter-example.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999

http://www.cpax.org.uk

email: rjh at the above domain, - www.

**CBFalconer** · Feb 28 '07, 03:25 PM

Re: INT_MIN and compiler diagnostic

Beej Jorgensen wrote:

>

.... snip ...

>
This produces the warning:
>
int y = -2147483648;
>
whereas this does not:
>
int y = (-2147483647 - 1);
>

.... snip ...

>
Finally,
>
int y = -0x80000000;
>
does not produce a warning, though it is the same as -2147483648.
Apparently the rules are different for hex constants than they are
for decimal constants (c99 6.4.4.1p5).

In the first case you are negating the int 2147483648, which has
already overflowed and caused un/implementation defined behaviour.
In the second, you are negating the unsigned int 0x80000000, which
follows the rules for unsigned ints, and does not overflow.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>

**bytebro** · Feb 28 '07, 03:45 PM

Re: INT_MIN and compiler diagnostic

On 28 Feb, 13:20, Chris Dollin <chris.dol...@h p.comwrote:

bytebro wrote:

On 28 Feb, 12:34, pete <pfil...@mindsp ring.comwrote:

>

If INT_MAX equals 2147483647,
the the type of 2147483648 can't be type int, can it?

>

Erm... it doesn't say "2147483648 ", it says "-2147483648", which is
exactly equal to INT_MIN, and is therefore a valid int value. The
warning is therefore misleading, no?

>
No.
>
`-2147483648` isn't a literal constant. It's an expression, the
negation of `2147483648`.

Wow. I've been mucking around with C for about 20 years now, and
that's the first time I've come across that!

The thing is, in the OP's code lines 7 and 8, it says:

7 int y = -2147483648;
8 int x = INT_MIN;

which are _completely_ equivalent (INT_MIN is _defined_ as
-2147483648), and yet line 7 generates the warning:

/tmp/x.c:7: warning: this decimal constant is unsigned only in ISO
C90

on his system, and the warning:

x.c:7: warning: decimal constant is so large that it is unsigned

on my system, but line 8 generates no warning at all for either of us.

**Nelu** · Feb 28 '07, 03:55 PM

Re: INT_MIN and compiler diagnostic

Richard Heathfield wrote:

Dik T. Winter said:
>
<snip>
>

>There are no negative constants in C.

>
"No" is a counter-example.
>

It's a string literal. Attempting to modify it may succeed (UB).
It will compile fine so it's not a good example, right? :-)

--
Ioan - Ciprian Tandau
tandau _at_ freeshell _dot_ org (hope it's not too late)
(... and that it still works...)

**Dik T. Winter** · Feb 28 '07, 04:15 PM

Re: INT_MIN and compiler diagnostic

In article <1172676891.968 236.185190@s48g 2000cws.googleg roups.com"byteb ro" <keith.willis@a ah.co.ukwrites:

On 28 Feb, 13:20, Chris Dollin <chris.dol...@h p.comwrote:

....

No.

`-2147483648` isn't a literal constant. It's an expression, the
negation of `2147483648`.

....

The thing is, in the OP's code lines 7 and 8, it says:
>
7 int y = -2147483648;
8 int x = INT_MIN;
>
which are _completely_ equivalent (INT_MIN is _defined_ as
-2147483648), and yet line 7 generates the warning:

Please check it. It will probably defined as (- 2147483647 - 1),
which is something different.
--
dik t. winter, cwi, kruislaan 413, 1098 sj amsterdam, nederland, +31205924131
home: bovenover 215, 1025 jn amsterdam, nederland; http://www.cwi.nl/~dik/

**Richard Tobin** · Feb 28 '07, 04:15 PM

Re: INT_MIN and compiler diagnostic

In article <1172676891.968 236.185190@s48g 2000cws.googleg roups.com>,
bytebro <keith.willis@a ah.co.ukwrote:

>The thing is, in the OP's code lines 7 and 8, it says:
>
7 int y = -2147483648;
8 int x = INT_MIN;
>
>which are _completely_ equivalent (INT_MIN is _defined_ as
>-2147483648)

According to the OP, on his system INT_MIN is defined as

# define INT_MIN (-INT_MAX - 1)

which produces the same result, but does not involve the literal 2147483648.
This seems to be a common trick; on my Mac it is defined as (-0x7fffffff - 1).

-- Richard
--
"Considerat ion shall be given to the need for as many as 32 characters
in some alphabets" - X3.4, 1963.

**strangerdream@gmail.com** · Feb 28 '07, 05:15 PM

Re: INT_MIN and compiler diagnostic

On Feb 28, 7:14 pm, "Dik T. Winter" <Dik.Win...@cwi .nlwrote:
<snip>

There are no negative constants in C.

But the warning message says:
warning: this decimal constant is unsigned only in ISO C90

What is a signed constant then?

**Richard Tobin** · Feb 28 '07, 05:25 PM

Re: INT_MIN and compiler diagnostic

In article <1172682091.061 155.235130@z35g 2000cwz.googleg roups.com>,
<strangerdream@ gmail.comwrote:

>There are no negative constants in C.

>
>But the warning message says:
>warning: this decimal constant is unsigned only in ISO C90
>
>What is a signed constant then?

The question is whether the constant has an unsigned type, or is
a positive value of a signed type.

-- Richard

--
"Considerat ion shall be given to the need for as many as 32 characters
in some alphabets" - X3.4, 1963.

INT_MIN and compiler diagnostic

INT_MIN and compiler diagnostic

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