get_count_rate function err qualifier
If the function get_count_rate() is called without the err qualifier (so in the most fundamental way possible) it runs into a Variable Uninitialized Error. Variable counts_sub_err remains uninitialised if err is not added. However, the bkg differentiation part of the function will always try to use the variable.
Maybe this can be resolved by removing the err qualifier all together, and making the function return the error all the time (might cause backwards compatibility)? Or add an if clause in the bkg part or assign NULL to counts_sub_err if err is not present.