Debugging C/C++/Fortran modules

Modules written in C, C++, Fortran, or other compiled languages can be debugged using standard debuggers like gdb or lldb.

These debuggers are an alternative to peppering your code with print statements - they let you stop the code at a particular point and see what values variables have. They are especially useful if your code crashes with a "segmentation fault" or similar error, because they will stop automatically and you can see what happened.

They tend to work much better for C and C++ than Fortran, and are not suited at all to debugging python - there is a python package called pdb for that.

First, check that you have one of these debuggers:

which lldb
which gdb

if one of these commands prints out a path to a debugger then you have that one installed. If not then you can get the debuggers from a package manager.

NB: The gdb that comes with the cosmosis auto-install does not work on a mac without heroically painful work. We strongly recommend lldb on macs.

Compiling for debuggability

You can tell cosmosis to compile everything in debug mode by setting an environment variable when you run make:

make clean
COSMOSIS_DEBUG=1 make

The code will now be compiled in a way that it can be run under the debugger. This will make the code slower, so remember to make clean and make again once you've finished debugging.

Running under lldb

Run under lldb like this:

lldb python bin/cosmosis params.ini

Then start the program with:

(lldb) run

Running under gdb

Run under gdb like this:

gdb python

Then start the program with:

(gdb) set args  bin/cosmosis params.ini
(gdb) run

Debugging

The program will now run as normal, but on a crash will stop and drop you into a debug environment where you can use commands like "up" "down" and "print" to move up and down the program and print out variables. You may need to go "up" a few times from where the crash actually happens.

A full tutorial on this is beyond the scope of this page - please see this gdb tutorial and this limited lldb tutorial.