GCC 4 and signedness

Magnus Therning magnus at therning.org
Thu Sep 1 07:31:35 UTC 2005

On Thu, Sep 01, 2005 at 03:46:42PM +1000, James Livingston wrote:
>On Wed, 2005-08-31 at 23:44 -0400, Dave Walker wrote:
>> I just updated breezy today, and noticed that GCC4 was being installed. 
>> Since it has been installed almost all the projects I have been working 
>> on, are getting at least 10
>> "pointer targets in passing argument 1 of 
>> ‘gaim_uri_list_extract_filenames’ differ in signedness"
>> or similar. My question is why is it complaining that, in an example of 
>> strcmp, I am comparing unsigned char* with a char*? The point is I don't 
>> care the sign, all I care about is if they are the same. Or am I missing 
>> a very simple thing about signedness?
>Comparing signed and unsigned values is normally a mistake, and
>potentially the source of some bugs. GCC 4 changed it so that warning
>about it is now the default. If you're sure that there are no problems
>with comparing things of different signedness, you can just stick some
>casts in.

There have been a few spectacular security holes due to signedness.
Having GCC4 warn about it is an improvement.

>If you compare their direct representations they are both 0xFF, and
>hence the same. However if they undergo type promotion to a wider type
>(e.g. a short) their representations will be 0x00FF and 0xFFFF, and
>hence not the same. Because type promotion can happens automatically
>(and AFAIK arbitrarily), the results of the comparison may depend on
>what the compiler feel like doing.

This type of implicit type conversion (or coercion) may seem arbitrary,
but it really isn't[1]. There seems to be an appendix on it in Bjarne's
book on C++, and I believe any good C or C++ book should have at least
a chapter devoted to the topic.


1. http://publib.boulder.ibm.com/infocenter/macxhelp/index.jsp?topic=/com.ibm.vacpp6m.doc/language/ref/clrc06convns.htm

Magnus Therning                    (OpenPGP: 0xAB4DFBA4)
magnus at therning.org

Software is not manufactured, it is something you write and publish.
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     -- Wade Henessy
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