memset指针+偏移量

Question

For example, I have:

   DWORD pointer = 0x123456;
   DWORD offset = 0xABC;

I want to add offset to pointer and set the value at the address pointed by that pointer to 1.0f . How do I give memset() a pointer and an offset as first argument?

Answer 1

DWORDs are the same as uint32_t's. Just add them together as you would with any other integer.

Also, while setting a float (I am assuming you're setting a float due to the 'f' after the '1.0'), I wouldn't use memset. Just cast the pointer to a float, and de-reference it like so:

DWORD pointer = 0x123456;
DWORD offset = 0xABC;

pointer += offset;

float* float_pointer = reinterpret_cast<float*>(pointer);
*float_pointer = 1.0f;

Answer 2

This can be straightforward pointer arithmetic, but we've got a few preliminary issues to take care of first.

1. Why is your pointer variable declared as a DWORD ? We'll need a proper pointer type eventually.
2. Why are you asking about memset , if you're trying to set a floating-point value? memset sets plain bit patterns; it's not much good for floating-point.
3. I assume your offset is supposed to be measured in bytes, not sizeof(float) .

Anyway, computing the pointer you want could go something like this:

float *fp = (float *)(pointer + offset);

[Notice that I perform the addition inside the parens, before the cast, so as to get a byte offset. If I instead wrote (float *)pointer + offset , it'd offset by `sizeof(float).]

Once we've got that float pointer, we can set the location it points to to 1.0 in the usual way:

*fp = 1.0f;

Or we could set it to 0 using memset:

memset(fp, 0, sizeof(float));