Getting time of thread owned by another process

Question

I need to monitor the time of the threads. To do this, I need to get the threads' clock IDs. Since I need to keep track of other processes' threads, I don't have a pthread_t identifier and can't use pthread_getcpuclockid. I looked at the implementation of pthread_getcpuclockid and implemented a similar function in my program (it uses the glibc internal macro MAKE_THREAD_CPUCLOCK). And I use clock_gettime to get the thread times. Everything works fine when I'm tracking my own child threads. But if I try to track another process's thread, clock_gettime returns -EINVAL. The clock ID is correct (I checked it inside the program being monitored). Is there way to watch other processes' threads times?