在C中使用指针进行转换

Question

I'm in an introductory course and I am curious about casting with pointers.

What would be the difference between:

*(uint32_t*)(p) 
(uint32_t)(*p)

p is a pointer.

Answer 1

*(uint32_t*)(p)

Extracts 32 unsigned bits at the memory location.

(uint32_t)(*p)

Extracts p from the memory location in its native type, and cast that type to a 32 bit unsigned int.

The results will probably be most notable if p is a floating point type. When extracted the first way you can see the resulting bitwise format of a floating point number (sign|mantissa|exponent). When extracted the second way the number is converted to an integer, probably via some form of truncation.

Here's a fun example program:

main(){
    float x = 1.25, *xp = &x;
    uint32_t x1 = (uint32_t)(*xp);
    uint32_t x2 = *(uint32_t *)(xp);
    printf("x1 = %x\nx2 = %x\n",x1,x2);
}

and the output:

x1 = 1
x2 = 3fa00000

Answer 2

In former, first you are casting p to uint32_t* and then dereferencing it.
In latter, first you are dereferencing p and then casting it to uint32_t .