CODEWITHSUNDEEP
multithreadinggdb
George Hilliard
George Hilliard

Reputation: 15942

Printing thread hierarchy with GDB

I have a multithreaded program that I'm trying to debug. When I run info thread in GDB, I get the following:

(gdb) info thread
  Id   Target Id         Frame 
  8    Thread 0x7fffe77fd700 (LWP 17425) "SocketWriter" 0x00007ffff7bc9b2f in pthread_cond_wait@@GLIBC_2.3.2 () from /usr/lib/libpthread.so.0
  7    Thread 0x7fffe73fc700 (LWP 17426) "SocketWriter" 0x00007ffff7bc9b2f in pthread_cond_wait@@GLIBC_2.3.2 () from /usr/lib/libpthread.so.0
  6    Thread 0x7fffe7fff700 (LWP 17423) "SocketReader" 0x00007ffff7bcc66d in read () from /usr/lib/libpthread.so.0
  5    Thread 0x7fffe7bfe700 (LWP 17424) "SocketReader" 0x00007ffff7bcc66d in read () from /usr/lib/libpthread.so.0
* 4    Thread 0x7ffff4810700 (LWP 17422) "unittest" 0x00007ffff7bcc38c in __lll_lock_wait () from /usr/lib/libpthread.so.0
  3    Thread 0x7ffff4c11700 (LWP 17421) "receiver" 0x00007ffff7bcc38c in __lll_lock_wait () from /usr/lib/libpthread.so.0
  2    Thread 0x7ffff5a3b700 (LWP 17420) "unittest" 0x00007ffff634e553 in select () from /usr/lib/libc.so.6
  1    Thread 0x7ffff7fc9780 (LWP 17419) "unittest" 0x00007ffff7bc9b2f in pthread_cond_wait@@GLIBC_2.3.2 () from /usr/lib/libpthread.so.0

It would be excellent if I could make GDB display the parent/child relationships between the threads, something like the following:

(gdb) info thread
  Id   Target Id         Frame 
  1    Thread 0x7ffff7fc9780 (LWP 17419) "unittest" 0x00007ffff7bc9b2f in pthread_cond_wait@@GLIBC_2.3.2 () from /usr/lib/libpthread.so.0
    3    Thread 0x7ffff4c11700 (LWP 17421) "receiver" 0x00007ffff7bcc38c in __lll_lock_wait () from /usr/lib/libpthread.so.0
      8    Thread 0x7fffe77fd700 (LWP 17425) "SocketWriter" 0x00007ffff7bc9b2f in pthread_cond_wait@@GLIBC_2.3.2 () from /usr/lib/libpthread.so.0
      6    Thread 0x7fffe7fff700 (LWP 17423) "SocketReader" 0x00007ffff7bcc66d in read () from /usr/lib/libpthread.so.0
      2    Thread 0x7ffff5a3b700 (LWP 17420) "unittest" 0x00007ffff634e553 in select () from /usr/lib/libc.so.6
    5    Thread 0x7fffe7bfe700 (LWP 17424) "SocketReader" 0x00007ffff7bcc66d in read () from /usr/lib/libpthread.so.0
*   4    Thread 0x7ffff4810700 (LWP 17422) "unittest" 0x00007ffff7bcc38c in __lll_lock_wait () from /usr/lib/libpthread.so.0
    7    Thread 0x7fffe73fc700 (LWP 17426) "SocketWriter" 0x00007ffff7bc9b2f in pthread_cond_wait@@GLIBC_2.3.2 () from /usr/lib/libpthread.so.0

For example, thread 3 is the parent of threads 8, 6, and 2, and thread 1 is the parent of everything.

Does such functionality exist? I have not seen reference to it, if it does.

Upvotes: 1

Views: 1417

Answers (1)

Tom Tromey
Tom Tromey

Reputation: 22529

gdb doesn't print this information because it doesn't exist in your program -- there is no way for gdb to discover it once the threads have been created.

There are maybe two ways it could be done.

First, you could set a breakpoint on the thread-creation function and record the information. This is readily done from Python. Then you can write a new command, also in Python, to format the output the way you like.

The problem with this approach is that it won't work if you "attach" to a running program. It will be too late to capture the information.

Another method is if you have extra information available in your program that describes the hierarchy. Then you can write a new command in Python that extracts this information to display things as you like.

Upvotes: 5

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