Debugger Compatibility Mode - gavinfielder/pft GitHub Wiki
On: -d Default off.
Debuggers tend to only work well on single-threaded single processes, so "debugger compatibility mode" means "disable forking and multithreading". It also disables signal handling. -d is identical to -XTS.
By default, debugger compatibility mode is turned on automatically when running a single test e.g. ./test 42. This enables you to run lldb ./test 42 to debug a particular test.
Of course, in order for lldb to read your own libftprintf.a, you must also use the -g flag in your own Makefile...