Preformant way to get a PID from a TID

Question

This question is not about pthread IDs, but about values typically obtained by gettid or ptrace .

Does the linux kernel provide any mechanism to obtain the thread group id (usually obtained by getpid ) from a given tid in a relatively efficient manner? Something that doesn't require io or parsing?

The only method seems to open /proc/TID/status , read the file into a buffer, scan line-by-line for Tgid , and then parse the string as an unsigned integer.

I'm hoping that there is a syscall I missed that returns the tgid/pid given the pid, even if the id is returned indirectly within a some data structure.

Answer 1

There seems to be no such system call. The information can only be retrieved from /proc , and your current solution looks like the most efficient way to do so.

Answer 2

just adding to the already available answers. Indeed there is no better alternative on Linux. I was looking for something better myself, and I found this project which was attempting to offer something more usable programmatically:

https://criu.org/Task-diag

Nothing of that however is upstream, probably since too few people seem to actually care about the problem at the moment. It is mostly a concern for high performance OS-level tracing applications and real time, which are just in their infancy in Linux.

An interesting fact, other similar OSes do not have this problem (OpenBSD, QNX, ...)