Selecting the correct thread debugging library

libthread_db.so is a library used to extract thread information from programs by parsing the memory of libpthread.so which is linked into multithreaded programs. This is needed when reading from thread-local storage, such as the errno variable.

The Linux thread debugging API requires that, when debugging a program using pthreads, the debugger has a compatible libthread_db.so that is able to inspect the pthread library’s internal data structures. When loading a recording on a machine with an incompatible libthread_db.so, errors may be seen when attempting to view thread-local variables:

12% 12,345> p errno
Cannot find thread-local storage for Thread 7615.7615, shared library /lib/x86_64-linux-gnu/libc.so.6:
Remote target failed to process qGetTLSAddr request

UDB incorporates two mechanisms to ensure that a libthread_db.so that is compatible with a given recording is available.

Automatic use of libthread_db from recordings

An Undo recording includes the libthread_db.so library (if any) from the system on which the recording was made. When UDB loads the recording, it extracts the library, and calls its functions when it needs to analyze thread-local storage in the recorded program.

Warning

The functions from the loaded libthread_db.so are executed directly, and not via UDB’s replay engine, as if UDB had been executed on the machine where the recording was made.

To prevent UDB from executing the libthread_db.so from the recording, and to execute the libthread_db.so on the machine where the recording is being replayed, use the --thread-db option with the special value local, or set the UNDO_thread_db environment variable to this value. For example:

$ udb --thread-db local my_recording.undo

Added in version 8.0.

Explicit libthread_db selection

When replaying a recording, UDB uses the libthread_db.so from the recording, if any. If the recording does not contain a copy of libthread_db.so (either because the recording is too old to contain one, or because libthread_db.so was not found at record time), then UDB uses the libthread_db.so found on the machine where the recording is being replayed. However, the local libthread_db.so may not be compatible with the recording. If there is an incompatibility, UDB will produce error messages when attempting to inspect thread-local state:

Remote target failed to process qGetTLSAddr request

To use an alternative libthread_db.so, use the --thread-db option or set the environment variable UNDO_thread_db.

For example, using the command-line parameter:

$ udb --thread-db $SDK/lib/libthread_db.so my_recording.undo