If you look very closely at my previous post, you’ll notice that I initialize a 128-bit integer with a 64-bit value. The 128-bit unsigned integer represents the internal state of a random number generator. Why not initialize it to a 128-bit value? I was trying to keep the code simple.
A surprising feature of C compilers, at least of GCC and Clang, is that you cannot initialize a 128-bit integer to a 128-bit integer literal. You can’t directly print a 128-bit integer either, which is why the previous post introduces a function print_u128.
The code
__uint128_t x = 0x00112233445566778899aabbccddeeff;
Produces the following error message.
error: integer literal is too large to be represented in any integer type
The problem isn’t initializing a 128-bit number to a 128-bit value; the problem is that the compiler cannot parse the literal expression
0x00112233445566778899aabbccddeeff
One solution to the problem is to introduce the macro
#define U128(hi, lo) (((__uint128_t)(hi) << 64) | (lo))
and use it to initialize the variable.
__uint128_t x = U128(0x0011223344556677, 0x8899aabbccddeeff);
You can verify that x has the intended state by calling print_u128 from the previous post.
void print_u128(__uint128_t n)
{
printf("0x%016lx%016lx\n",
(uint64_t)(n >> 64), // upper 64 bits
(uint64_t)n); // lower 64 bits
}
Then
print_u128(x);
prints
0x00112233445566778899aabbccddeeff
Update. The code for print_u128 above compiles cleanly with gcc but clang gives the following warning.
warning: format specifies type 'unsigned long' but the argument has type 'uint64_t' (aka 'unsigned long long') [-Wformat]
You can suppress the warning by including the inttypes header and modifying the print_u128 function.
Here’s the final code. It compiles cleanly under gcc and clang.
#include <stdio.h>
#include <stdint.h>
#include <inttypes.h>
#define U128(hi, lo) (((__uint128_t)(hi) << 64) | (lo))
void print_u128(__uint128_t n)
{
printf("0x%016" PRIx64 "%016" PRIx64 "\n",
(uint64_t)(n >> 64),
(uint64_t)n);
}
int main(void)
{
__uint128_t x = U128(0x0011223344556677, 0x8899aabbccddeeff);
print_u128(x);
return 0;
}
