Invalid conversion from uint8_t* to uint32_t - when migrating from 32 to 64bit architecture?

Question

I had a little function which converted virtual memory address to physical on a 32-bit architecture:

uint32_t VIRTBASE;

uint32_t getPhysForVirt(void* virt) {
  uint32_t offset = (uint8_t*)virt - VIRTBASE;
  return PHYSBASE + offset;
}

It compiled and worked without any single issue in last 10+ years.

I've changed the compiler to build the repo for newer architectures (now with 64bit support for the first time).

Compilation fails stating

invalid conversion from ‘uint8_t*’ {aka ‘unsigned char*’} to ‘uint32_t’ {aka ‘unsigned int’} [-fpermissive]

Now, I understand the message, but I'm not sure about the necessary steps to make this compiling without errors.

I'm only sure in that I don't want to enable -fpermissive.

Answer 1

You use the wrong types. C language has special types for casting pointers to integral values.

uintptr_t VIRTBASE;

uintptr_t getPhysForVirt(const void * restrict virt) {
  ptrdiff_t offset = (uintptr_t)virt - VIRTBASE;
  return PHYSBASE + offset;
}

If the rest of code is written the way as this function was - you have plenty work for Christmas.