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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 exception 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 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 : 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 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 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
val use_printers : exn -> string option
Printexc.use_printers e returns None if there are no registered printers 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 backtrace_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 useful 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 stable between different
programs, or even between different runs of the same binary.
A raw_backtrace_entry can be converted to a usable form using backtrace_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 marshalled. 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_entry 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 restriction 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 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 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_uncaught_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 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
val backtrace_slots_of_raw_entry : raw_backtrace_entry -> backtrace_slot array option
Returns the slots of a single raw backtrace entry, or None if this entry 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 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)
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 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