Difference between PID and TID

Question

What is the difference between PID and TID?

The standard answer would be that PID is for processes while TID is for threads. However, I have seen that some commands use them interchangeably. For example, htop has a column for PIDs, in which PIDs for threads of the same process are shown (with different values). So when does a PID represent a thread or a process?

Solution 1

It is complicated: pid is process identifier; tid is thread identifier.

But as it happens, the kernel doesn't make a real distinction between them: threads are just like processes but they share some things (memory, fds...) with other instances of the same group.

So, a tid is actually the identifier of the schedulable object in the kernel (thread), while the pid is the identifier of the group of schedulable objects that share memory and fds (process).

But to make things more interesting, when a process has only one thread (the initial situation and in the good old times the only one) the pid and the tid are always the same. So any function that works with a tid will automatically work with a pid.

It is worth noting that many functions/system calls/command line utilities documented to work with pid actually use tids. But if the effect is process-wide you will simply not notice the difference.

Solution 2

Actually, each thread in a Linux process is Light Weight Process (LWP). So, people may call thread as a process... But there is surely a difference. Each thread in a process has a different thread ID (TID) and share the same process ID (PID).

If you are working with pthread library functions, then these functions don't use these TIDs because these are kernel/OS level thread IDs.

Solution 3

Just to add to other answers, according to man gettid:

The thread ID returned by this call is not the same thing as a POSIX thread ID (i.e., the opaque value returned by pthread_self(3)).

So there are two different things one could mean by TID!

Solution 4

pid and tid are the same except when a process is created with a call to clone with CLONE_THREAD (per the man pages of gettid). In this case, you get a unique thread id but all threads belonging to the same thread group share the same process id.

However, I also recall reading (though I cant find the source) that the values returned from getpid may be cached.

[UPDATE] See the NOTES section here for a discussion on the effects of caching pids.

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Comments

[jørgensen]

And to make it even more interesting: there is also the name tgid (thread group identifier) appearing in sources — you can already guess it is synonymous with pid.

[wick]

tid from gettid() is not the same as pthread_t tid, See man gettid

[Stephen]

What is the meaning of "fds" as used in this answer?

[rodrigo]

@Stephen fds stands for file descriptors, that is, those small integers that represent open files, sockets or similar objects.