Arch Linux

It would be useful if libstdc++.so was built with Tom Tromey's statically defined tracing probes, to improve debuggability of C++ exceptions in GDB.

The GDB 'catch' command, that stops at various kinds of events including C++ exceptions, can be set to stop only on exceptions of a specific type - but without the aforementioned probes it just stops on every exception regardless of type. Also, the special variable '$_exception' that would give you details about the exception is subsequently missing, and in general all the stuff to do with exceptions and GDB is kind of flaky.

You can check for the presence of the probes with the command:
readelf -n /lib/libstdc++.so.6.0.25

When present you'll see something like:
"""
Displaying notes found in: .note.gnu.build-id
Owner Data size Description
GNU 0x00000014 NT_GNU_BUILD_ID (unique build ID bitstring)
Build ID: 0972dabed9d271753baf023aca990613ddb217b6

Displaying notes found in: .note.stapsdt
Owner Data size Description
stapsdt 0x0000003b NT_STAPSDT (SystemTap probe descriptors)
Provider: libstdcxx
Name: catch
Location: 0x000000000008e0ff, Base: 0x000000000013bff8, Semaphore: 0x0000000000000000
Arguments: 8@%rax 8@-80(%rbx)
stapsdt 0x00000036 NT_STAPSDT (SystemTap probe descriptors)
Provider: libstdcxx
Name: throw
Location: 0x000000000008f26d, Base: 0x000000000013bff8, Semaphore: 0x0000000000000000
Arguments: 8@%rdi 8@%rsi
stapsdt 0x0000003a NT_STAPSDT (SystemTap probe descriptors)
Provider: libstdcxx
Name: rethrow
Location: 0x000000000008f328, Base: 0x000000000013bff8, Semaphore: 0x0000000000000000
Arguments: 8@%rax 8@(%rdi)
"""

That was copied from a Debian install; on my Arch install I don't have the ".note.stapsdt" section. I've also found the probes present in an Ubuntu and Fedora install.

The probes get added when this test from libstdc++-v3 configure script passes: "checking for suitable sys/sdt.h", which tries to #include <sys/sdt.h>. Systemtap (AUR [https://aur.archlinux.org/packages/systemtap]) includes this file, putting it at /usr/include/sys/sdt.h. Perhaps systemtap could be made a build dependency of gcc, but I suppose it's a big add for the sake of a single file. It looks like what Debian did was make a small systemtap-sdt-dev that is pared down to a few essentials. [https://packages.debian.org/stretch/amd64/systemtap-sdt-dev/filelist]

From my own tests in building the gcc package with the probes I've found that at minimum it just needs /usr/include/sys/sdt.h to be present and also /usr/include/sys/sdt-config.h alongside it. The dtrace executable that the Debian package has isn't strictly necessary for this purpose. So ultimately my request is, please could those two files be somehow made prerequisites of the gcc build?

I took my own built libstdc++.so.6.0.25 with the probes and installed it in /lib and rebooted and it does fix the issues I originally had with GDB.

Here are some links to posts by Tom in case you want to read more in detail:
http://tromey.com/blog/?p=687
https://patchwork.ozlabs.org/patch/223711/
https://gcc.gnu.org/ml/libstdc%2B%2B/2013-03/msg00060.html