Can the thread ID of a multithreaded process be the same as the process ID of another running process?
According to the man page of pthreads the thread ID is unique within the creating process, so yes another thread or process could have the same ID. However, If it's unique within a process and a process ID is unique in the system then maybe you can use a combination of the two as a unique identifier.
Each of the threads in a process has a unique thread identifier (stored in the type pthread_t). This identifier is returned to the caller of pthread_create(3), and a thread can obtain its own thread identifier using pthread_self(3). Thread IDs are only guaranteed to be unique within a process.
The TID (as returned by the sys_gettid() system call) is unique across all threads on the system1, and for a single-threaded process the PID and TID are equal. This means that a TID will never clash with a PID from another process.
1. With the caveat that if PID namespaces are in use, TIDs and PIDs are only unique within the same PID namespace.
While the pthread ID might not be unique, in a implementation where threads map to tasks, the task id (as seen in /proc/PID/task) will in fact be unique system wide, and have a form similar to an actual PID.