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gdb
PepeHands
PepeHands

Reputation: 1406

to which thread gdb connects by default

If I have some multithreaded process and want to trace it with gdb using attach command, to which thread it will connect (e.g. current running or main)? I know that I can discover it with info threads but I want to know which thread it will choose by default.

Upvotes: 0

Views: 195

Answers (2)

Peter Dillinger
Peter Dillinger

Reputation: 3075

On Linux, where thread ids exist in the same space as process ids, it appears you can run gdb -p tid to attach to the thread with given tid and its owning process, without knowing the pid. Because the main thread of a process has tid == pid, it makes sense that running gdb -p pid connects to the main thread.

Example code that connects gdb to the currently executing thread, e.g. for generating a pretty stack trace: https://github.com/facebook/rocksdb/pull/11150

Upvotes: 0

Matthew Fisher
Matthew Fisher

Reputation: 2336

For Linux, all of the threads are stopped by the ptrace command when gdb attaches.

It has been my experience that gdb defaults to the main thread for C/C++ applications. If you attach to a process and do a 'bt' it will list the stack for 'main'.

Information is available for all threads however. gdb can look at the thread(s) information in the /proc filesystem. The proc contains information about each thread in the tasks area. Details about the stack address is located in the stat file as well as the maps file. Details are also available regarding the register values for each thread.

Along the lines of your question, I've often wondered why stepping through a multithreaded application will cause gdb to jump from thread to thread. I think that gdb is still at the mercy of the kernel scheduler so that a step on a thread may lead to a different thread getting the CPU resource and a breakpoint being triggered.

Upvotes: 1

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