If I got the C99 restrict keyword right, qualifying a pointer with it is a promise made that the data it references won’t be modified behind the compiler’s back through aliasing.
By contrast, the way I understand the const qualifier is as compiler-enforced documentation that a given object won’t be modified behind the back of a human being writing code. The compiler might get a hint as a side effect, but as a programmer I don’t really care.
In a similar way, would it be appropriate to consider a restrict qualifier in a function prototype as a requirement that the user ensures exclusive access (“avoid aliasing”, or maybe something stronger) for the duration of the call? Should it be used as “documentation”?
Also, is there something to understand in the fact that restrict qualifies a pointer rather than the data it points to (as const does) ?
EDIT: I originally believed that restrict could have implications with threaded code, but this seems wrong so I remove references to threads from the question to avoid confusing readers.
The best ‘intuition’ to have about the restrict keyword is that its a guarantee (by the programmer to the compiler) that, for the lifetime of the pointer, memory accessed via that pointer will ONLY be accessed via that pointer and not via another pointer or reference or global address. So its important that its on a pointer as its a property of both the pointer and the memory, tying the two together until the pointer goes out of scope.