CODEWITHSUNDEEP
clinuxgcc
user1783732
user1783732

Reputation: 1843

warning: comparison is always false due to limited range of data type in gcc 4.1.2

I encountered the following warning from gcc 4.1.2: 

warning: comparison is always false due to limited range of data type

the concerned C code is like:

if ( unlikely(count < 0) ) BUG();

where 'count' is unsigned.

I tried to disable the warning since I was not allowed to modify the source code: 

-Wno-type-limits

but it seems gcc 4.1.2 does not support it. 

cc1: error: unrecognized command line option "-Wno-type-limits"

Any other ways to get rid of this warning?

Upvotes: 7

Views: 23404

Answers (4)

PriyaGp
PriyaGp

Reputation: 71

The variable used in the comparison was declared as a 8 bit unsigned int.When i changed it to 32 bit unsigned it, the warning was removed. Old: uint8_t variable = 0; Fix: uint32_t variable = 0;

Upvotes: 0

Maxim Kholyavkin
Maxim Kholyavkin

Reputation: 4573

options to fix this warning:

  1. remove condition which alway true

  2. best: remove condition which alway true + add static_assert to ensure type is unsigned. (for C version of static_assert look here)

  3. for gcc prior 4.3: remove compiler option: -Wextra

  4. for gcc 4.3+ add option: -Wno-type-limits

Upvotes: 4

SeKa
SeKa

Reputation: 1835

Depending on how old the source code is, it could be written in a defensive manner for a time where compilers were not as type-safe as gcc is now.

The warning looks like it's part of the -Wextra (aka -W) warning option set, so if you want the extra warnings, that will be one of them. I personally use -Wall, which believe it or not does not include the "extra" stuff. You would think that "all" would include "extra" but I guess not...

Upvotes: 2

Jonathan Leffler
Jonathan Leffler

Reputation: 755006

An unsigned value will never be negative — hence the warning. It's not so much 'unlikely' as 'impossible'.

This usually indicates that there is a bug in the code of some sort; the code was written expecting a type that could allow negative values, but the type doesn't allow negative values. So, it is quite possible that the code will misbehave because of the mismatch in expectations.

Note that on some machines, plain char is signed and on others unsigned (and it is a type distinct from both signed char and unsigned char even though its range of values overlaps with one or the other).

Upvotes: 10

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