Strange sign-compare results

Strange sign-compare results

[Kaveh R. Ghazi]

Given this code:

 > int foo(unsigned int u, unsigned int u1, unsigned int u2,
 > 	unsigned short us1, unsigned short us2,
 > 	int i1, int i2, short s1, short s2)
 > {
 >   if (u < (s1-s2))
 >     return 0;
 > 
 >   if (u < (us1-us2))
 >     return 0;
 > 
 >   if (u < (i1-i2))
 >     return 0;
 >   
 >   if (u < (u1-u2))
 >     return 0;
 > 
 >   return 1;
 > }

compiling it (on sparc-solaris2) with -Wsign-compare (using 2.95.2,
3.0.3 (pre) or 3.1) yields:

 > foo.c:5: warning: comparison between signed and unsigned
 > foo.c:8: warning: comparison between signed and unsigned
 > foo.c:11: warning: comparison between signed and unsigned

I.e. the last compare of "u < (u1-u2)" doesn't warn.  Is this a bug or
feature?  Doesn't (u1-u2) become an integer?  Please explain. :-)

		Thanks,
		--Kaveh
--
Kaveh R. Ghazi			Engagement Manager / Project Services
ghazi@caip.rutgers.edu		Qwest Internet Solutions

Re: Strange sign-compare results

[Neil Booth]

Kaveh R. Ghazi wrote:-

> I.e. the last compare of "u < (u1-u2)" doesn't warn.  Is this a bug or
> feature?  Doesn't (u1-u2) become an integer?  Please explain. :-)

No, the type of the result is the type of the operands for most (all?)
operators in C.

The others convert the unsigned operand to signed, and therefore the
result of the subtraction is signed.

Neil.

Re: Strange sign-compare results

[Neil Booth]

neil wrote:-

> The others convert the unsigned operand to signed, and therefore the
> result of the subtraction is signed.

Ignore that last sentence; I didn't read your examples closely enough
8-)

Neil.

Re: Strange sign-compare results

[Kaveh R. Ghazi]

 > From: Neil Booth <neil@daikokuya.demon.co.uk>
 > 
 > neil wrote:-
 > 
 > > The others convert the unsigned operand to signed, and therefore the
 > > result of the subtraction is signed.
 > 
 > Ignore that last sentence; I didn't read your examples closely enough
 > 8-)
 > Neil.

I'm not sure what you mean then.  Would you say the behavior I
observed is a bug or feature?

		--Kaveh
--
Kaveh R. Ghazi			Engagement Manager / Project Services
ghazi@caip.rutgers.edu		Qwest Internet Solutions

Re: Strange sign-compare results

[Neil Booth]

Kaveh R. Ghazi wrote:-

> I'm not sure what you mean then.  Would you say the behavior I
> observed is a bug or feature?

It is the behaviour the C standard requires.

Neil.