If I were to use pthreads in POSIX environments, and a context switch is about to happen, the current value of the esp register has to be stored somewhere so I can retrieve it when I context switch back to this thread, as the esp register's value will be overwritten by another thread's saved SP value. I think it is impossible to have separate esp register for every thread (correct me if I am wrong). Having said that, I would like to know in what data structure the SP value of the current thread is stored right before the context switching is hit?
I tried looking up the struct pthread*'s value casted from the value of pthread_t, but nothing was changing when, say, I call a certain function to change the current SP of the thread I am testing (i.e. compare before and after calling the testing function).
This depends entirely upon how the Posix library is implemented. If the threads are implemented by the OS, the values of all registers are stored in the thread's [process] context block before a context switch.
if the thread are implemented in a library, the registers have to be stored in some data structure managed by the library. Such a library implementation needs to save all the general registers but does not (and cannot) need to save the process-specific kernel registers.