A MIT licensed C/C++ uint128/256 type library, originally adapted from calccrypto's uint128/256 library turned into a primarily C first library and opt out C++ features when compiling in C++ mode.

The overarching motivation of the library can be summed up as follows

Single header, drop in - no build configuration required.
Minimal dependencies (only depends on stdbool.h when compiling in C).
Zero allocations (in other libs typically when converting to strings).
Ability to rename the API via the pre-processor for tighter integration into C code bases
C++ features are optional and can be opt out via the pre-processor, i.e. operator overloading and constructors.

Usage

Include intc.h and defined INTC_IMPLEMENTATION in one and only one file to enable the implementation in that translation unit.

#define INTC_IMPLEMENTATION
#include "intc.h"
#include <stdio.h>

int main(int, char**)
{
#if defined(__cplusplus)
    intc_u256 value = 32;
    value += 32;
    if (value == 64)
        value *= 1'000'000'000'000;
    intc_u256_string string = intc_u256_readable_int_str(value);
    printf("%.*s\n", string.size, string.str); // 64,000,000,000,000
#else
    intc_u256 value = INTC_U64_TO_U256(32);
    value = intc_u256_add_u64(value, 32);
    if (intc_u256_eq_u64(value, 64))
        value = intc_u256_mul(value, INTC_U64_TO_U256(1'000'000'000'000));
    intc_u256_string string = intc_u256_readable_int_str(value);
    printf("%.*s\n", string.size, string.str); // 64,000,000,000,000
#endif
    return 0;
}

Tests

Tests are adapted from calccrypto's test suite and can be built by running build_tests.bat for Windows and build_tests.sh for Unix. For Windows MSVC's cl compiler must be available on the path and GCC must be available on Unix.