Send signal to one specific thread
long sys_tgkill(pid_t tgid, pid_t pid, int sig);
tgid
the thread group ID of the thread
pid
the PID of the thread
sig
signal to be sent
This syscall also checks the tgid and returns -ESRCH even if the PID exists but it's not belonging to the target process anymore. This method solves the problem of threads exiting and PIDs getting reused.