Is pid really unique [closed]

Question

Is there a case where PID does not end up being unique? Is it the best way to identify a process (or a thread)? I read that previous versions of Linux had a different approach

Thanks

Answer 1

PIDs can take about 32k values (sysctl kernel.pid_max can change it), so they are reused quickly enough. pid + start time may be better. systemd attaches per-process information on extended attributes in cgroupfs (procfs would also work) to avoid the ambiguity of a pid->attr mapping.

Threads and processes share the same namespace (you can see it in /proc/<pid>/task/<taskid>), with <pid> = <taskid> for the process' initial thread. Pid namespaces limit the list of visible pids, but they don't introduce any overlap; pids and task ids remain unique while their owner is running.

Answer 2

If you are talking about the getpid() system call then yes, PID is unique per process. Except, that is, if you are using threads on older versions of the Linux kernel. Then each thread may have its own process-id.

To quote from this discussion:

Kernel 2.4.20 uses NPTL (Native posix thread library) and this is the kernel shipped with RH9. RH8 uses Kernel 2.4.18 which doesn't implement NPTL (Meaning each thread gets its own PID and therefore a good description of it's status in /proc). NPTL is a "real" implementation of POSIX threads meaning that the threads share alot more including the PID. It is more efficient way of running threads for a couple of reasons, however, I don't know of any easy tricks to debug these kind of threads. How do you know when your thread is sleeping versus waiting on a semaphore, or which threads have died in a process with lots of threads, etc.

From the wikipedia link on NPTL:

NPTL has been part of Red Hat Enterprise Linux since version 3, and in the Linux kernel since version 2.6. It is now a fully integrated part of the GNU C Library.2

Under the covers even the 2.6.X kernels have a virtual-process for threads. You can see the thread process-ids with ps auxf:

root      2501  0.0  0.3 244448 25576 ?    Ss   Jul03   0:11 /usr/sbin/httpd
apache    2716  0.0  0.5 384776 46696 ?    S    Oct14   0:17  \_ /usr/sbin/httpd
apache    2717  0.0  0.5 382208 44304 ?    S    Oct14   0:11  \_ /usr/sbin/httpd

The following program spits out the same pid for both main and thread under Linux kernel 2.6.18. The self id returned from pthread_self() identifies the thread uniquely.

#include <pthread.h>
void foo() {
  printf("thread: pid = %d, self = %ld\n", getpid(), pthread_self());
}
main() {
  pthread_t thread;
  printf("main: pid = %d, self = %ld\n", getpid(), pthread_self());
  pthread_create(&thread, 0L, foo, 0L);
  pthread_join(thread, 0L);
}

The output is:

main: pid = 13246, self = 46912496175248
thread: pid = 13246, self = 1084229952