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Printexc(3) OCamldoc 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 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 : 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 (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 exception. 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 === 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 ac- tually 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 re- striction usage than Printexc.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 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 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. 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 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 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). 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 2021-03-01 source: Printexc(3)
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