How to write in a single file with multiple threads?

You answers to other posts are somewhat out-of-line with your original question - it seems you want to write to one different file per thread.

The easy answer is.. just do it - the file system is thread-safe. The only snag may be performance with only one disk. Concurrently and rapidly writing small chunks of data to many files may result in a lot of disk-thrashing as the file system tries to cope with the distributed files/directories with only one rotor arm. Modern hard disks with large caches and clever controllers will mitigate this to some extent, but you may have an issue. Try it and see!

Your problems will be increased if you plan to continually create and destroy threads, open and close data files for every incoming network read. If you can possibly avoid this, do so.

If the disk cannot keep up, you could implement some clever 'lazy-writing' algorithm of your own to increase the size of the disk writes and so reduce the number, or buy an SSD, or both.