I want to represent extended integers and came across _BitInt()
but it doesn't work dynamically,
I'm trying to do something like this:
void fun(int n)
{
_BitInt(n)* val = malloc(n); //doesn't work
//rest of function
}
I understand that everything stored on the stack needs to have its size known at compile time, but I'm mallocing here so I don't understand why this doesn't work
CodePudding user response:
First, _BitInt(N)
isn't a standard C type. It's a Clang extension and proposed for the next C23 standard. And _BitInt(N)
isn't an extended integer type either but a fixed-width integer type. The C standard proposal also says clearly
The original proposal was to name the feature
_ExtInt
and refer to it in standards text as a “special extended integer type”. However, during committee discussion, the feature was changed to be called a “bit-precise integer type” and so the name_ExtInt
was a misnomer.
Being a fixed-width compile time type, the N
is part of the type itself (like in C templates) and not some variable that modifies the type at run time. Therefore the type BitInt(n)
can't be constructed. That also means _BitInt
isn't suitable for arbitrary-precision math (dynamic bit width integers in your words)
The fact that you allocated memory dynamically has nothing to do with the type. malloc(n)
just allocates a memory region of n-bytes and doesn't know about what you're going to store in that region. And you're also doing it wrong, n bytes of memory can store _BitInt(n * CHAR_BIT)
and not _BitInt(n)