Unsigned __int128 literal GCC

Question

I have a function foo(unsigned __int128). How can I pass a literal of type unsigned __int128 to the function? My attempts:

   foo(0xFFFFFFFFFFFFFF23FFFFF); //Truncated (expected)
   foo(0xFFFFFFFFFFFFFFF01FFFu); //Truncated (expected)
   foo(0xFFFFFFFFFFFFFFF23FFFFull); //Truncated
   foo((unsigned __int128)0xFF45FFFFFFFFFFFFFFFFFull); //Truncated

As you can see, all of them are truncated. How can I signal GCC, that an literal is an unsigned __int128?

Edit: The gcc manual says:

There is no support in GCC for expressing an integer constant of type __int128 for targets with long long integer less than 128 bits wide

Does that mean, that I can't write literals of type (unsigned) __int128?

Are there alternatives?

One workaround is `((unsigned __int128)0xFFFFFull<<64 | 0xFFFFFFFFF23FFFFFull)`. — yao99, Aug 09 '20 at 17:28
@yao99 `ll` not needed. `((unsigned __int128)0xFFFFFu<<64 | 0xFFFFFFFFF23FFFFFu)` is suffcient. — chux - Reinstate Monica, Aug 09 '20 at 19:49
https://stackoverflow.com/q/51538694/1918193 and several other duplicates. — Marc Glisse, Aug 10 '20 at 07:50

score 3 · Accepted Answer · answered Aug 09 '20 at 18:58

From https://gcc.gnu.org/onlinedocs/gcc/_005f_005fint128.html

6.9 128-bit Integers

As an extension the integer scalar type __int128 is supported for targets which have an integer mode wide enough to hold 128 bits. Simply write __int128 for a signed 128-bit integer, or unsigned __int128 for an unsigned 128-bit integer. There is no support in GCC for expressing an integer constant of type __int128 for targets with long long integer less than 128 bits wide.

So, no, you cannot have a literal 128-bit constant

"for targets with long long integer less than 128 bits wide" — user997112, Dec 12 '22 at 14:45

Unsigned __int128 literal GCC

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