ambrop7

    #include <math.h>
    #include <stdint.h>
    #include <assert.h>
    #include <stdio.h>
    #include <inttypes.h>

    int main()
    {
        float x = (float)-INFINITY;
        uint64_t count = 1;
        while (x != (float)INFINITY) {
            float y = nextafterf(x, (float)INFINITY);
            assert(y != x);
            x = y;
            ++count;
        }
        printf("Found %" PRIu64 " floats.\n", count);
        return 0;
    }
    
    $ gcc -std=c99 -O3 a.c -lm -o a
    $ ./a
    Found 4278190081 floats.

    #pragma STDC FENV_ACCESS ON
    #include <stdio.h>
    #include <stdint.h>
    #include <fenv.h>

    int main()
    {
        fesetround(FE_DOWNWARD);
        printf("%f\n", (double)UINT64_MAX);
        fesetround(FE_UPWARD);
        printf("%f\n", (double)UINT64_MAX);
        return 0;
    }

    $ gcc -std=c99 -frounding-math x.c -lm
    $ ./a.out 
    18446744073709551616.000000
    18446744073709551616.000000

    bool double_to_uint64 (double x, uint64_t *out)
    {
        double y = trunc(x);
        if (y >= 0.0 && y < ldexp(1.0, 64)) {
            *out = y;
            return true;
        } else {
            return false;
        }
    }