CODEWITHSUNDEEP
cmemory-management
Joe
Joe

Reputation: 47619

Possible to cast to pointer of given type determined at runtime in C?

I'm fairly sure the answer to this is 'no, don't be stupid', but as they say, there are no stupid questions (only stupid people asking them).

I want to calculate the offset of an array. I have a void * handle to it and the sizeof(element). I don't have an x* pointer (where x is the type of the element).

Is there any way I can cast the void * pointer to one of a given size so I can do pointer arithmetic based on that size? At the moment I'm casting to a char * (assuming a char is 1 byte*) and then multiplying the sizeof value by the offset. If I could somehow cast the pointer, then I could just use the array syntax, which would be cleaner.

As I say, I'm 99% sure this isn't possible and I'm basically asking for language-level type representation which C doesn't have. But worth an ask.

*And I'd rather not because it's not always true.

Upvotes: 2

Views: 172

Answers (3)

eruciform
eruciform

Reputation: 7736

You can't do introspection in C, but that doesn't prevent you from casting anything to anything.

void *v = whatever;
char *c = (char*)v;
c[0] = stuff;
element *e = (element*)v;
e[42].foo = bar;

If you want arbitrary size at runtime, then I don't believe there is a standardized way of doing it. Some compilers support:

char c[x];

where x is a variable, but I wouldn't trust it on all architectures.

You can definitely write a couple simple accessors for your pointers to at least separate out the pointer-math:

void* pointerAtPos( void*p, int offset, int width )
{
  return (void*)&( ((char*)p)[ offset * width ] );
}

Be careful of address alignment.

Upvotes: 2

caf
caf

Reputation: 239051

As several people have said in comments, sizeof(char) is guaranteed to be 1. The relevant part of the C99 standard is in section 6.5.3.4, describing the sizeof operator:

When applied to an operand that has type char, unsigned char, or signed char, (or a qualified version thereof) the result [of the sizeof operator] is 1.

This means that casting the void * to char * and adding N * sizeof(element) is correct and idiomatic.

(C is statically typed, so you can't cast to a type determined at runtime).

Upvotes: 4

user206545
user206545

Reputation:

In C, casting is a compile time-only operation. As such, it isn't possible to determine the actual type of array element at run time.

Upvotes: 0

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