c++ - warning: right shift count >= width of type on 32 bit machine

Question

I have the following function:

void func(unsigned long v)
{
  char max_byte = 0xFF;
  char buffer[8];

  buffer[0] = static_cast<char>((v)       & max_byte);
  buffer[1] = static_cast<char>((v >> 8)  & max_byte);
  buffer[2] = static_cast<char>((v >> 16) & max_byte);
  buffer[3] = static_cast<char>((v >> 24) & max_byte);
  buffer[4] = static_cast<char>((v >> 32) & max_byte);
  buffer[5] = static_cast<char>((v >> 40) & max_byte);
  buffer[6] = static_cast<char>((v >> 48) & max_byte);
  buffer[7] = static_cast<char>((v >> 56) & max_byte);
}

which takes an unsigned long argument and insert its 8 bytes to char buffer ( don't try to figure out why. it is a concise version of a meaningful function).

This code compiles well on 64 bit but on 32 bit I get the following warning:

warning: right shift count >= width of type

referring to lines:

  buffer[4] = static_cast<char>((v >> 32) & max_byte);
  buffer[5] = static_cast<char>((v >> 40) & max_byte);
  buffer[6] = static_cast<char>((v >> 48) & max_byte);
  buffer[7] = static_cast<char>((v >> 56) & max_byte);

I think I understand the warning but I'm not sure what should I do to be able to compile it smoothly on 32 bit as well.

Answer 1

unsigned long is only guaranteed to have 32 bits. See here. You need to use unsigned long long to have 64 bits guaranteed.

Even better would be to use a fixed width integer, i.e. uint64_t. They are defined in header <cstdint> (or <stdint.h>).