What is a Kernel thread?

Question

i am just started coding of device driver and new to threading, went through many documents for getting an idea about threads. i still have some doubts.

Answer 1

Since you're comparing kernel threads with user[land] threads, I assume you mean something like the following.

The normal way of implementing threads nowadays is to do it in the kernel, so those can be considered "normal" threads. It's however also possible to do it in userland, using signals such as SIGALRM, whose handler will save the current process state (registers, mostly) and change them to another one previously saved. Several OSes used this as a way to implement threads before they got proper kernel thread support. They can be faster, since you don't have to go into kernel mode, but in practice they've faded away.

There's also cooperative userland threads, where one thread runs until it calls a special function (usually called yield), which then switches to another thread in a similar way as with SIGALRM above. The advantage here is that the program is in total control, which can be useful when you have timing concerns (a game for example). You also don't have to care much about thread safety. The big disadvantage is that only one thread can run at a time, and therefore this method is also uncommon now that processors have multiple cores.

Kernel threads are implemented in the kernel. Perhaps you meant how to use them? The most common way is to call pthread_create.