Printexc(3o) OCaml library Printexc(3o)
NAME
Printexc - Facilities for printing exceptions and inspecting current
call stack.
Module
Module Printexc
Documentation
Module Printexc
: sig end
Facilities for printing exceptions and inspecting current call stack.
type t = exn = ..
The type of exception values.
val to_string : exn -> string
Printexc.to_string e returns a string representation of the exception e
.
val to_string_default : exn -> string
Printexc.to_string_default e returns a string representation of the ex-
ception e , ignoring all registered exception printers.
Since 4.09
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 func-
tion application.
val catch : ('a -> 'b) -> 'a -> 'b
Printexc.catch fn x is similar to Printexc.print , but aborts the pro-
gram with exit code 2 after printing the uncaught exception. This
function is deprecated: the runtime system is now able to print un-
caught exceptions as precisely as Printexc.catch does. Moreover, call-
ing Printexc.catch makes it harder to track the location of the excep-
tion using the debugger or the stack backtrace facility. So, do not
use Printexc.catch in new code.
val print_backtrace : 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 (be-
fore in the handler, or in a when-guard during the matching of the ex-
ception 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 excep-
tion backtrace that Printexc.print_backtrace would print. Same restric-
tion usage than Printexc.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 OCAML-
RUNPARAM 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 excep-
tion. Exceptions raised by the printer are ignored.
When converting an exception into a string, the printers will be in-
voked in the reverse order of their registrations, until a printer re-
turns 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 back-
trace 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 be-
fore.
Since 3.11.2
val use_printers : exn -> string option
Printexc.use_printers e returns None if there are no registered print-
ers and Some s with else as the resulting string otherwise.
Since 4.09
Raw backtraces
type raw_backtrace
The type raw_backtrace stores a backtrace in a low-level format, which
can be converted to usable form using raw_backtrace_entries and back-
trace_slots_of_raw_entry below.
Converting backtraces to backtrace_slot s is slower than capturing the
backtraces. If an application processes many backtraces, it can be use-
ful to use raw_backtrace to avoid or delay conversion.
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
type raw_backtrace_entry = private int
A raw_backtrace_entry is an element of a raw_backtrace .
Each raw_backtrace_entry is an opaque integer, whose value is not sta-
ble between different programs, or even between different runs of the
same binary.
A raw_backtrace_entry can be converted to a usable form using back-
trace_slots_of_raw_entry below. Note that, due to inlining, a single
raw_backtrace_entry may convert to several backtrace_slot s. Since the
values of a raw_backtrace_entry are not stable, they cannot be mar-
shalled. If they are to be converted, the conversion must be done by
the process that generated them.
Again due to inlining, there may be multiple distinct raw_backtrace_en-
try values that convert to equal backtrace_slot s. However, if two
raw_backtrace_entry s are equal as integers, then they represent the
same backtrace_slot s.
Since 4.12.0
val raw_backtrace_entries : raw_backtrace -> raw_backtrace_entry array
Since 4.12.0
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 re-
striction usage than Printexc.print_backtrace .
Since 4.01.0
val print_raw_backtrace : 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 Print-
exc.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 default_uncaught_exception_handler : exn -> raw_backtrace -> unit
Printexc.default_uncaught_exception_handler prints the exception and
backtrace on standard error output.
Since 4.11
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 is Printexc.default_un-
caught_exception_handler .
Note that when fn is called all the functions registered with 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 back-
trace.
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
val backtrace_slots_of_raw_entry : raw_backtrace_entry -> back-
trace_slot array option
Returns the slots of a single raw backtrace entry, or None if this en-
try lacks debug information.
Slots are returned in the same order as backtrace_slots : the slot at
index 0 is the most recent call, raise, or primitive, and subsequent
slots represent callers.
Since 4.12
type location = {
filename : string ;
line_number : int ;
start_char : int ;
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
Since 4.02.0
Raw backtrace slots
type raw_backtrace_slot
This type is used to iterate over the slots of a raw_backtrace . For
most purposes, backtrace_slots_of_raw_entry is easier to use.
Like raw_backtrace_entry , values of this type are process-specific and
must absolutely not be marshalled, and are unsafe to use for this rea-
son (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 ele-
ments are equal, then they represent the same source location (the con-
verse 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_back-
trace_slot .
Since 4.02
val get_raw_backtrace_next_slot : raw_backtrace_slot -> raw_back-
trace_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
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 run-
time).
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
OCamldoc 2023-02-12 Printexc(3o)
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