Backtrace.Exn has functions for controlling and printing the backtrace of the most
recently raised exception.
When an exception is raised, the runtime "unwinds" the stack, i.e. removes stack frames, until it reaches a frame with an exception handler. It then matches the exception against the patterns in the handler. If the exception matches, then the program continues. If not, then the runtime continues unwinding the stack to the next handler.
If am_recording () = true, then the runtime, while it is unwinding the stack, keeps
track of the part of the stack that is unwound. This is available as a human-readable
string via most_recent (). Calling most_recent if am_recording () = false will
yield the empty string.
With am_recording () = true, OCaml keeps only a backtrace for the most recently
raised exception. When one raises an exception, OCaml checks if it is physically
equal to the most recently raised exception. If they are equal, then OCaml appends
the string representation of the stack unwound by the current raise to the stored
backtrace. If the exception being raised is not physically equally to the most
recently raised exception, then OCaml starts recording a new backtrace. Thus one must
call most_recent before a subsequent raise of a (physically) distinct exception,
or the backtrace is lost.
The initial value of am_recording () is determined by the setting of the
environment variable OCAMLRUNPARAM. If OCAMLRUNPARAM is set, then am_recording ()
= true iff the character "b" occurs in OCAMLRUNPARAM. If OCAMLRUNPARAM is not set,
then am_recording () is initially true.
This is the same functionality as provided by the OCaml stdlib Printexc functions
backtrace_status, record_backtraces, get_backtrace.