package ocaml-base-compiler

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Facilities for printing exceptions and inspecting current call stack.

val to_string : exn -> string

Printexc.to_string e returns a string representation of the exception e.

val print : ('a -> 'b) -> 'a -> 'b

Printexc.print fn x applies fn to x and returns the result. If the evaluation of fn x raises any exception, the name of the exception is printed on standard error output, and the exception is raised again. The typical use is to catch and report exceptions that escape a function application.

val catch : ('a -> 'b) -> 'a -> 'b

Printexc.catch fn x is similar to Printexc.print, but aborts the program with exit code 2 after printing the uncaught exception. This function is deprecated: the runtime system is now able to print uncaught exceptions as precisely as Printexc.catch does. Moreover, calling Printexc.catch makes it harder to track the location of the exception using the debugger or the stack backtrace facility. So, do not use Printexc.catch in new code.

val print_backtrace : Pervasives.out_channel -> unit

Printexc.print_backtrace oc prints an exception backtrace on the output channel oc. The backtrace lists the program locations where the most-recently raised exception was raised and where it was propagated through function calls.

If the call is not inside an exception handler, the returned backtrace is unspecified. If the call is after some exception-catching code (before in the handler, or in a when-guard during the matching of the exception handler), the backtrace may correspond to a later exception than the handled one.

  • since 3.11.0
val get_backtrace : unit -> string

Printexc.get_backtrace () returns a string containing the same exception backtrace that Printexc.print_backtrace would print. Same restriction usage than print_backtrace.

  • since 3.11.0
val record_backtrace : bool -> unit

Printexc.record_backtrace b turns recording of exception backtraces on (if b = true) or off (if b = false). Initially, backtraces are not recorded, unless the b flag is given to the program through the OCAMLRUNPARAM variable.

  • since 3.11.0
val backtrace_status : unit -> bool

Printexc.backtrace_status() returns true if exception backtraces are currently recorded, false if not.

  • since 3.11.0
val register_printer : (exn -> string option) -> unit

Printexc.register_printer fn registers fn as an exception printer. The printer should return None or raise an exception if it does not know how to convert the passed exception, and Some s with s the resulting string if it can convert the passed exception. Exceptions raised by the printer are ignored.

When converting an exception into a string, the printers will be invoked in the reverse order of their registrations, until a printer returns a Some s value (if no such printer exists, the runtime will use a generic printer).

When using this mechanism, one should be aware that an exception backtrace is attached to the thread that saw it raised, rather than to the exception itself. Practically, it means that the code related to fn should not use the backtrace if it has itself raised an exception before.

  • since 3.11.2

Raw backtraces

type raw_backtrace

The abstract type raw_backtrace stores a backtrace in a low-level format, instead of directly exposing them as string as the get_backtrace() function does.

This allows delaying the formatting of backtraces to when they are actually printed, which may be useful if you record more backtraces than you print.

Raw backtraces cannot be marshalled. If you need marshalling, you should use the array returned by the backtrace_slots function of the next section.

  • since 4.01.0
val get_raw_backtrace : unit -> raw_backtrace

Printexc.get_raw_backtrace () returns the same exception backtrace that Printexc.print_backtrace would print, but in a raw format. Same restriction usage than print_backtrace.

  • since 4.01.0
val print_raw_backtrace : Pervasives.out_channel -> raw_backtrace -> unit

Print a raw backtrace in the same format Printexc.print_backtrace uses.

  • since 4.01.0
val raw_backtrace_to_string : raw_backtrace -> string

Return a string from a raw backtrace, in the same format Printexc.get_backtrace uses.

  • since 4.01.0
val raise_with_backtrace : exn -> raw_backtrace -> 'a

Reraise the exception using the given raw_backtrace for the origin of the exception

  • since 4.05.0

Current call stack

val get_callstack : int -> raw_backtrace

Printexc.get_callstack n returns a description of the top of the call stack on the current program point (for the current thread), with at most n entries. (Note: this function is not related to exceptions at all, despite being part of the Printexc module.)

  • since 4.01.0

Uncaught exceptions

val set_uncaught_exception_handler : (exn -> raw_backtrace -> unit) -> unit

Printexc.set_uncaught_exception_handler fn registers fn as the handler for uncaught exceptions. The default handler prints the exception and backtrace on standard error output.

Note that when fn is called all the functions registered with Pervasives.at_exit have already been called. Because of this you must make sure any output channel fn writes on is flushed.

Also note that exceptions raised by user code in the interactive toplevel are not passed to this function as they are caught by the toplevel itself.

If fn raises an exception, both the exceptions passed to fn and raised by fn will be printed with their respective backtrace.

  • since 4.02.0

Manipulation of backtrace information

These functions are used to traverse the slots of a raw backtrace and extract information from them in a programmer-friendly format.

type backtrace_slot

The abstract type backtrace_slot represents a single slot of a backtrace.

  • since 4.02
val backtrace_slots : raw_backtrace -> backtrace_slot array option

Returns the slots of a raw backtrace, or None if none of them contain useful information.

In the return array, the slot at index 0 corresponds to the most recent function call, raise, or primitive get_backtrace call in the trace.

Some possible reasons for returning None are as follow:

  • none of the slots in the trace come from modules compiled with debug information (-g)
  • the program is a bytecode program that has not been linked with debug information enabled (ocamlc -g)
  • since 4.02.0
type location = {
  1. filename : string;
  2. line_number : int;
  3. start_char : int;
  4. end_char : int;

The type of location information found in backtraces. start_char and end_char are positions relative to the beginning of the line.

  • since 4.02
module Slot : sig ... end

Raw backtrace slots

type raw_backtrace_slot

This type allows direct access to raw backtrace slots, without any conversion in an OCaml-usable data-structure. Being process-specific, they must absolutely not be marshalled, and are unsafe to use for this reason (marshalling them may not fail, but un-marshalling and using the result will result in undefined behavior).

Elements of this type can still be compared and hashed: when two elements are equal, then they represent the same source location (the converse is not necessarily true in presence of inlining, for example).

  • since 4.02.0
val raw_backtrace_length : raw_backtrace -> int

raw_backtrace_length bckt returns the number of slots in the backtrace bckt.

  • since 4.02
val get_raw_backtrace_slot : raw_backtrace -> int -> raw_backtrace_slot

get_raw_backtrace_slot bckt pos returns the slot in position pos in the backtrace bckt.

  • since 4.02
val convert_raw_backtrace_slot : raw_backtrace_slot -> backtrace_slot

Extracts the user-friendly backtrace_slot from a low-level raw_backtrace_slot.

  • since 4.02
val get_raw_backtrace_next_slot : raw_backtrace_slot -> raw_backtrace_slot option

get_raw_backtrace_next_slot slot returns the next slot inlined, if any.

Sample code to iterate over all frames (inlined and non-inlined):

(* Iterate over inlined frames *)
let rec iter_raw_backtrace_slot f slot =
  f slot;
  match get_raw_backtrace_next_slot slot with
  | None -> ()
  | Some slot' -> iter_raw_backtrace_slot f slot'

(* Iterate over stack frames *)
let iter_raw_backtrace f bt =
  for i = 0 to raw_backtrace_length bt - 1 do
    iter_raw_backtrace_slot f (get_raw_backtrace_slot bt i)
  • since 4.04.0

Exception slots

val exn_slot_id : exn -> int

Printexc.exn_slot_id returns an integer which uniquely identifies the constructor used to create the exception value exn (in the current runtime).

  • since 4.02.0
val exn_slot_name : exn -> string

Printexc.exn_slot_name exn returns the internal name of the constructor used to create the exception value exn.

  • since 4.02.0

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