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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 while the runtime is unwinding the stack, it keeps track of the part of the stack that is unwound. This is available as a backtrace via most_recent (). Calling most_recent if am_recording () = false will yield the empty backtrace.
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 it is, 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.