How can i measure sequential read/write speeds?
is there any API, library that can help me?

The general answer is, you can't, at least not meaningfully.

The longer answer is, it is very dependent on the hardware and software in question, both the firmware of the drive and the drivers of the operating system. Even if you were dealing with an isolated case - a single drive running on a dedicated real-time system - the results would depend more on the caching algorithm of the drive controller than the drive's own performance.

In practice, very few operating systems ensure real-time performance or measurement (keeping in mind that 'real-time' means bounded time, not necessarily minimum time). In a general-purpose system such as Windows or the Unices, the use of virtual memory essentially makes predictability impossible.

Well thanks for your quick answer but there are tools like "CrystalDiskInfo"
that can measure sequential read/write speeds.

Well these tools basically do a certain task and messure how fast it can be acomplished.

But these tasks are not that easy to make up. You have a lot of statistical invarieties, you have driver-issues you have to ensure the OS gives you the needed privileges and so on.

If you want to do it for yourself, or for school/university, you can get some reliable results. But I wouldn't hope to write a software as good as an established benchmark.