Printexc(3) OCaml library Printexc(3)
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.
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 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
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.
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.
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 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 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
before.
Since 3.11.2
=== Raw backtraces ===
type raw_backtrace
The abstract type raw_backtrace stores a backtrace in a low-level for-
mat, 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.
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 Print-
exc.get_backtrace uses.
Since 4.01.0
=== 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 Perva-
sives.at_exit have already been called. Because of this you must make
sure any output channel fn writes on is flushed.
If fn raises an exception, it is ignored.
Since 4.02.0
=== Manipulation of backtrace information Those function allow to tra-
verse the slots of a raw backtrace, 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 )
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
=== Raw backtrace slots ===
type raw_backtrace_slot
This type allows direct access to raw backtrace slots, without any con-
version in an OCaml-usable data-structure. Being process-specific, they
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).
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_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
=== 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_id exn returns the internal name of the constructor
used to create the exception value exn .
Since 4.02.0
OCamldoc 2014-10-18 Printexc(3)
ocaml 4.02.1 - Generated Mon Oct 20 18:39:47 CDT 2014