[sane-devel] New magicolor backend for inclusion in git

[Richard Ryniker]

Reinhold Kainhofer <reinhold at kainhofer.com> wrote:

>If I use "unsigned char*", then I get no warning. However, I fail to see why 
>using an additional variable makes a difference...
>
>This does not work (&b[0] is a pointer to an unsigned char, right?):
>   unsigned char b[4];
>   htole32a(&b[0], value);
>
>while this does:
>   unsigned char b[4];
>   unsigned char *bp=&b[0];
>   htole32a(bp, value);

The first argument to htole32a must be an lvalue.  It is used in the
left-hand side of an assignment by htole32a; this is the exact meaning of
lvalue.

b[0] is an lvalue.  It is certainly well-defined to write something like:

    b[0] = 2;

&b[0] looks like it should be the address of the first element of b, and
indeed it is, but it is not an lvalue: it does not denote a target (such
as a variable) to which some value may be assigned.  It is not a pointer
type, which can be dereferenced by the * operator. What is needed for the
first argument of htole32a is a pointer P that can be used in a statement
like:

    *P = 2;

The statement:

    unsigned char * bp=&b[0];

is well-defined: the variable bp is assigned a value which is the address
of the unsigned char b[0].  bp is an actual variable, therefore it is an
lvalue and an acceptable first argument to ntole32a.

A statement such as:

    *&b[0] = 2;

is invalid according to the C language grammar (even if you, a human, can
say you know what it should mean).  That is why the compiler complains
when it parses:

    htole32a(&b[0], value);

but the compiler is happy with the correct:

    htole32a(bp, value);

because that expands into:

    *bp = value;