c - gcc 7.3 128-bit unsigned integer operation

Question

I'm confused with the usage of 128-bit integer. Please look at the test code:

uint128_t test_data = 0x00000000FFFFFFFF0101010101010101;
uint64_t test_data_h = test_data  >> 64;
uint64_t test_data_l = test_data ;
printf("test_data 0x %016llx %016llx
", test_data_h, test_data_l);

I expect the test_data_h to be 0x00000000FFFFFFFF, but the result is :

test data 0x 0000000000000000 0101010101010101

Why is this?

Answer 1

Many compilers do not support 128 bit constants.

Instead

// uint128_t test_data = 0x00000000FFFFFFFF0101010101010101;
uint128_t test_data = (((uint128_t) 0x00000000FFFFFFFF) << 64) | 0x0101010101010101;

Tip: enable all compiler warnings.