Question

how to Generate a unique sequence ID to be stored in unsigned long ?
Getting seconds elapsed after 1970 would have been good idea but requirement is within a second the id might be updated , so second wont be unique !

Answer 1

If your requirements are to pick something pseudo random, fast, reliably unique, and not used for requirement for crytographic security purposes, I offer up the following

On Windows X86:

__rdtsc() – is about as good of a sequential number as it gets. XOR the upper 32-bits of the return value with the lower 32-bits. As the lower 32-bits will cycle every couple of seconds

#include <Windows.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <intrin.h>
#include <stdint.h>

uint32_t GetPseudoRandomNumber()
{
    uint64_t t = _time64(NULL);
    uint64_t cpu = __rdtsc();
    uint32_t result;
    cpu = cpu ^ t;
    result = (cpu >> 32) ^ (cpu & 0xffffffff);
    return result;
}

uint32_t GetPseudoRandomNumber2()
{
    GUID guid = {};
    uint32_t* pValue = (uint32_t*)&guid;
    uint32_t result;

    CoCreateGuid(&guid);

    result = pValue[0] ^ pValue[1] ^ pValue[2] ^ pValue[3];

    return result;
}

Other sources of entropy include GetTickCount() (unique to the millisecond)

On Linux:
Just read 4 bytes from /dev/urandom