kvm_write: fix -Wcast-qual warning in pointer arithmetic argument

[ngie]

Summary

kvm_write: fix -Wcast-qual warning in pointer arithmetic argument

Cast buf to const char * when doing arithmetic operation to match
cp's type.

MFC after: 1 week
Sponsored by: Dell EMC Isilon

[cem]

Why not just cast buf (const void *) to const char * to match cp? This is not a situation where DECONST is actually needed.

[vangyzen]

In D10082#208481, @cem wrote:

Why not just cast buf (const void *) to const char * to match cp? This is not a situation where DECONST is actually needed.

Agreed.

[ngie]

In D10082#208484, @vangyzen wrote:

In D10082#208481, @cem wrote:

Why not just cast buf (const void *) to const char * to match cp? This is not a situation where DECONST is actually needed.

Agreed.

clang 4.0.0 and gcc 6.3.0 both don't like that.

clang:

/usr/src/lib/libkvm/kvm.c:479:26: error: arithmetic on pointers to void is a GNU extension [-Werror,-Wpointer-arith]
        return ((const void*)cp - buf);
                ~~~~~~~~~~~~~~~ ^ ~~~
1 error generated.
*** Error code 1

gcc:

/usr/src/lib/libkvm/kvm.c: In function 'kvm_write':
/usr/src/lib/libkvm/kvm.c:479:26: error: pointer of type 'void *' used in subtraction [-Werror=pointer-arith]
  return ((const void*)cp - buf);
                          ^
cc1: all warnings being treated as errors
*** Error code 1

Stop.
make: stopped in /usr/src/lib/libkvm

[cem]

In D10082#208485, @ngie wrote:

clang 4.0.0 and gcc 6.3.0 both don't like that.

I said cast buf, not cp.

[ngie]

Per cem/vangyzen: don't use __DECONST (overkill); use const char * cast instead to
match buf's type.

[ngie]

In D10082#208486, @cem wrote:

In D10082#208485, @ngie wrote:

clang 4.0.0 and gcc 6.3.0 both don't like that.

I said cast buf, not cp.

Derp. I missed that..