How To Check bits in C++?

In C++ I wrote:

bool ret_is_syscall = (ret_inst_data & 0x00000000000000FF) == 0x000000000000050f;

but clion says it's always wrong, why?

I am trying to check if last 4 are 0x050f

Solution

If you want to check that the least-significant 2 bytes of ret_inst_data have exactly the value 0x050F, then you need to use a mask of 0xFFFF:

bool ret_is_syscall = (ret_inst_data & 0xFFFF) == 0x050F;

As for why your original comparison is incorrect, let's look at just the least-significant two bytes of the numbers involved.

0x050F has the bit pattern 0000 0101 0000 1111
0x00FF has the bit pattern 0000 0000 1111 1111

If we bitwise and those two patterns together, we get the bit pattern

  0000 0101 0000 1111
& 0000 0000 1111 1111
---------------------
  0000 0000 0000 1111

The binary 0000 0000 0000 1111 is 0x000F in hex.

As you can see, because the second-least-significant-byte of 0x00FF is all 0s, the result of performing a bitwise and between 0x00FF and any number will produce a result with a second-least-significant-byte of all 0s. Since the second-least-significant byte of 0x050F is not all 0s your comparison can never be true.