Facilities for printing exceptions and inspecting current call stack.
Printexc.to_string e returns a string representation of the exception
Printexc.print fn x applies
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.
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.
Printexc.get_backtrace () returns a string containing the same exception backtrace that
Printexc.print_backtrace would print. Same restriction usage than
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
true if exception backtraces are currently recorded,
false if not.
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
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.
val get_raw_backtrace : unit -> Printexc.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
val print_raw_backtrace : Pervasives.out_channel -> Printexc.raw_backtrace -> unit
Print a raw backtrace in the same format
val raw_backtrace_to_string : Printexc.raw_backtrace -> string
Return a string from a raw backtrace, in the same format
val raise_with_backtrace : exn -> Printexc.raw_backtrace -> 'a
Reraise the exception using the given raw_backtrace for the origin of the exception
Current call stack
val get_callstack : int -> Printexc.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
val set_uncaught_exception_handler : (exn -> Printexc.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.
fn raises an exception, both the exceptions passed to
fn and raised by
fn will be printed with their respective backtrace.
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.
val backtrace_slots : Printexc.raw_backtrace -> Printexc.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 (
- the program is a bytecode program that has not been linked with debug information enabled (
module Slot : sig ... end
Raw backtrace slots
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).
val raw_backtrace_length : Printexc.raw_backtrace -> int
raw_backtrace_length bckt returns the number of slots in the backtrace
val get_raw_backtrace_slot : Printexc.raw_backtrace -> int -> raw_backtrace_slot
get_raw_backtrace_slot bckt pos returns the slot in position
pos in the backtrace
val convert_raw_backtrace_slot : raw_backtrace_slot -> Printexc.backtrace_slot
Extracts the user-friendly
backtrace_slot from a low-level
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) done
Printexc.exn_slot_id returns an integer which uniquely identifies the constructor used to create the exception value
exn (in the current runtime).