Vala needs a way to specify immutable pointers that should be directly mapped into C with the 'const' attribute.

It was discussed in the IRC to use the 'immutable' attribute from the Vala side. Here's an example:

Vala code: immutable uint8 *ptr;
C code: const unsigned char *ptr;

Actually gcc show warnings when trying to cast a const pointer to a non-const one. It is harmless, unless you try to write something there...but vala should take care about this kind of access types.