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Date: Thu, 07 Nov 2013 23:35:01 +0100
From: Jens Gustedt <jens.gustedt@...ia.fr>
To: musl@...ts.openwall.com
Subject: Re: [PATCH v2] shadow: Implement putspent
Am Mittwoch, den 06.11.2013, 11:36 -0500 schrieb Rich Felker:
> On Wed, Nov 06, 2013 at 02:20:54PM +0100, Jens Gustedt wrote:
> > in the context of the actual function that would certainly overkill,
> > but generally it is not a good idea to mix user strings and string
> > literals without consting them. So in a general context I'd go for
> > something like
> >
> > #define STR(S) ((char const*)((S) ? (S) : ""))
> >
> > or even
> >
> > #define STR(S) ((S) ? (char const*){ (S) } : "")
> >
> > to have a better type check for the argument
>
> I disagree with this change. The type of string literals is char *,
> not const char *, so it's not a type consistency issue. Even if it
> were, the ?: operator handles the type correctly anyway. My view is
> that casts are a code smell, and no-op casts are harmful in that they
> obfuscate the correctness of types (since the reader has to question
> whether the cast is hiding a type mismatch).
The second variant isn't a casţ but a conversion and it just checks if
S is assignment compatible with `char const*`. A completely type safe
variant then would be
#define STR(S) ((S) ? (char const*){ (S) } : (char const[]){ 0 })
which wouldn't imply any conversion. (And which a compiler *may*
realize by using a static object for the empty string.)
Just to be clear for the reason I mentioned it: the original macro has
the danger that it is used in places where it is passed as char* to a
function which then modifies the string.
updatefunction(STR(toto));
The user who calls first the macro and then the function with his own
variable thinks that he is passing a modifiable string "toto" to the
function. The bug only appears when by accident someday "toto" is 0,
and a then a string literal is passed into the function.
Jens
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