uint128_t does not name a type

Question

I am porting some code from C to C++. During the conversion I encountered:

uint128_t does not name a type

My compiler: gcc version 5.2.1
My operating system: Ubuntu 15.1

This compiled fine as C and I thought it would be resolved by including stdint.h but it has not. So far I have not tried anything else since there doesn't seem to be a lot of information on this error (example). uint128_t is used throughout this entire program and is essential for the build, therefore I can not remove it, and I'm not sure about using a different integer type.

Below is an example of where and how it is used.

union {
    uint16_t  u16;
    uint32_t  u32;
    uint128_t u128;
} value;

Would it be okay to define a uint128_t or should I look at my compiler?

Answer 1

GCC has builtin support for the types __int128, unsigned __int128, __int128_t and __uint128_t (the last two are undocumented). Use them to define your own types:

typedef __int128 int128_t;
typedef unsigned __int128 uint128_t;

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Alternatively, you can use __mode__(TI):

typedef int int128_t __attribute__((mode(TI)));
typedef unsigned int uint128_t __attribute__((mode(TI)));

Quoting the documentation:

TImode

“Tetra Integer” (?) mode represents a sixteen-byte integer.

Sixteen byte = 16 * CHAR_BIT >= 128.