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?

You can change htop settings to view processes and threads in a different color. It also helps to use the Tree view. — pztrick, Oct 20 '13 at 02:17

score 112 · Accepted Answer · answered Jan 09 '12 at 11:43

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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.

answered Jan 09 '12 at 11:43

rodrigo

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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*. – jørgensen Jan 09 '12 at 12:05
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*tid* from gettid() is not the same as pthread_t *tid*, See `man gettid` – wick May 07 '16 at 13:21
What is the meaning of "fds" as used in this answer? – Stephen Oct 22 '17 at 20:39
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@Stephen _fds_ stands for _file descriptors_, that is, those small integers that represent open files, sockets or similar objects. – rodrigo Oct 22 '17 at 20:42

score 5 · Answer 2 · edited Aug 21 '17 at 11:57

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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.

edited Aug 21 '17 at 11:57

Mateusz Piotrowski

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answered Jan 09 '12 at 11:13

sandeep

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score 3 · Answer 3 · edited Aug 21 '17 at 12:02

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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!

edited Aug 21 '17 at 12:02

Mateusz Piotrowski

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answered May 07 '16 at 13:18

wick

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ezpz · Answer 4 · 2010-12-23T10:00:57.747

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.

Difference between PID and TID

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