Re: const pains with lpcode in Lpeg 0.11

[Philipp Janda <siffiejoe@...>]

Am 30.03.2013 11:50 schröbte Rob Kendrick:
> On Fri, Mar 29, 2013 at 04:27:34PM +0000, Gavin Wraith wrote:
> > In message <20130329153937.GE18728@pepperfish.net> you wrote:
> > > With Norcroft:
> > > *cc -strict -c99 -fah -Wh -Otime SSEx4nim.c
> > > Norcroft RISC OS ARM C vsn 5.69 [20 Oct 2010]
> > > "c.SSEx4nim", line 7: Warning: extern 'test' not declared in header
> > > "c.SSEx4nim", line 8: Error: assignment to 'const' object 'follow'
> > > c.SSEx4nim: 1 warning, 1 error, 0 serious errors
> > >
> > > With GCC, clang, and lcc, there is no error.  I think this is a bug in
> > > Norcroft.
>
> I had somebody else take a look at this, and they have the opposing view
> that the compiler is right and the code makes use of undefined behavior.
> I don't have the standard to hand (I'm away from home) so it's hard to
> tell.

I still think that Norcroft is wrong:

6.7.3 (Type qualifiers)
§ 8   If the specification of an array type includes any type 
qualifiers, the element type is so-qualified, not the array type. If the 
specification of a function type includes any type qualifiers, the 
behavior is undefined.118)

118) Both of these can occur through the use of typedefs.


6.5.2.2 (Function calls)
§ 4   An argument may be an expression of any object type. In preparing 
for the call to a function, the arguments are evaluated, and each 
parameter is assigned the value of the corresponding argument.81)

81) A function may change the values of its parameters, but these 
changes cannot affect the values of the arguments. On the other hand, it 
is possible to pass a pointer to an object, and the function may
change the value of the object pointed to. A parameter declared to have 
array or function type is adjusted to have a pointer type as described 
in 6.9.1.


6.9.1 (Function definitions)
§ 7   [...] In either case, the type of each parameter is adjusted as
described in 6.7.5.3 for a parameter type list; the resulting type shall 
be an object type.

§ 9   Each parameter has automatic storage duration. Its identifier is 
an lvalue, which is in effect declared at the head of the compound 
statement that constitutes the function body (and therefore cannot be 
redeclared in the function body except in an enclosed block). [...]


6.7.5.3 (Function declarators (including prototypes)
§ 7   A declaration of a parameter as ‘‘array of type’’ shall be 
adjusted to ‘‘qualified pointer to type’’, where the type qualifiers (if 
any) are those specified within the [ and ] of the array type 
derivation. [...]


What had me worried for a moment was (emphasis mine)

6.3.2.1 (Lvalues, arrays, and function designators)
§ 3   Except when it is the operand of the sizeof operator or the unary 
& operator, or is a string literal used to initialize an array, an 
expression that has type ‘‘array of type’’ is converted to an expression 
with type ‘‘pointer to type’’ that points to the initial element of the 
array object _and is not an lvalue_. [...]


Source: ISO/IEC 9899:TC3 WG14/N1256 Committee Draft


So,

    typedef unsigned char Charset[123];
    void test( const Charset follow ) { ... }

becomes

    void test( unsigned char follow[123] const ) { ... }

which becomes

    void test( unsigned char const follow[123] ) { ... }

which becomes

    void test( unsigned char const* follow ) { ... }

and the assignment should be ok.


Btw., it seems that #defining const to nothing is actually allowed.

> B.

Philipp