6.9.8 Exception Handling

If a word detects an error condition that it cannot handle, it can throw an exception. In the simplest case, this will terminate your program, and report an appropriate error.

throw ( y1 .. ym nerror – y1 .. ym / z1 .. zn error  ) exception “throw”

If nerror is 0, drop it and continue. Otherwise, transfer control to the next dynamically enclosing exception handler, reset the stacks accordingly, and push nerror.

fast-throw ( ... wball – ... wball ) gforth-experimental “fast-throw”

Lightweight throw variant: only for non-zero balls, and does not store a backtrace or deal with missing catch.

Throw consumes a cell-sized error number on the stack. There are some predefined error numbers in Standard Forth (see errors.fs). In Gforth (and most other systems) you can use the iors produced by various words as error numbers (e.g., a typical use of allocate is allocate throw). Gforth also provides the word exception to define your own error numbers (with decent error reporting); a Standard Forth version of this word (but without the error messages) is available in compat/except.fs. And finally, you can use your own error numbers (anything outside the range -4095..0), but won’t get nice error messages, only numbers. For example, try:

-10 throw                    \ Standard defined
-267 throw                   \ system defined
s" my error" exception throw \ user defined
7 throw                      \ arbitrary number
exception ( addr u – n  ) gforth-0.2 “exception”

n is a previously unused throw value in the range (-4095...-256). Consecutive calls to exception return consecutive decreasing numbers. Gforth uses the string addr u as an error message.

There are also cases where you have a word (typically modeled after POSIX’ strerror) for converting an error number into a string. You can use the following word to get these strings into Gforth’s error handling:

exceptions ( xt n1 – n2  ) gforth-1.0 “exceptions”

Xt ( +n -- c-addr u ) converts an error number in the range 0<=n<n1 into an error message. Exceptions reserves n1 error codes in the range n2-n1<n3<=n2. When (at some later point in time) the Gforth error code n3 in that range is thrown, it pushes n2-n3 and then executes xt to produce the error message.

As an example, if the errno errors (and the conversion using strerror) was not already directly supported by Gforth, you could tie strerror in as follows:

' strerror 1536 exceptions constant errno-base
: errno-ior ( -- n )
\ n is the Gforth ior corresponding to the value in errno, so
\ we have to convert between the ranges here.
\ ERRNO is not a Gforth word, so you  would have to use the
\ C interface to access it.
  errno errno-base over - swap 0<> and ;

When you call a C function (with the C interface) and its return value indictes that an error happened, you can then perform errno-ior throw to produce an exception with the proper error message (such as “Permission denied”).

A common idiom to THROW a specific err# if a flag is true is this:

( flag ) 0<> err# and throw

Your program can provide exception handlers to catch exceptions. An exception handler can be used to correct the problem, or to clean up some data structures and just throw the exception to the next exception handler. Note that throw jumps to the dynamically innermost exception handler. The system’s exception handler is outermost, and just prints an error and restarts command-line interpretation (or, in batch mode (i.e., while processing the shell command line), leaves Gforth).

The Standard Forth way to catch exceptions is catch:

catch ( x1 .. xn xt – y1 .. ym 0 / z1 .. zn error  ) exception “catch”

Executes xt. If execution returns normally, catch pushes 0 on the stack. If execution returns through throw, all the stacks are reset to the depth on entry to catch, and the TOS (the xt position) is replaced with the throw code.

nothrow ( ) gforth-0.7 “nothrow”

Use this (or the standard sequence ['] false catch 2drop) after a catch or endtry that does not rethrow; this ensures that the next throw will record a backtrace.

The most common use of exception handlers is to clean up the state when an error happens. E.g.,

base @ >r hex \ actually the HEX should be inside foo to protect
              \ against exceptions between HEX and CATCH
['] foo catch ( nerror|0 )
r> base !
( nerror|0 ) throw \ pass it on

A use of catch for handling the error myerror might look like this:

['] foo catch
  myerror OF ... ( do something about it ) nothrow ENDOF
  dup throw \ default: pass other errors on, do nothing on non-errors

Having to wrap the code into a separate word is often cumbersome, therefore Gforth provides an alternative syntax:


This performs code1. If code1 completes normally, execution continues with code3. If there is an exception in code1 or before endtry, the stacks are reset to the depth during try, the throw value is pushed on the data stack, and execution continues at code2, and finally falls through to code3.

try ( compilation  – orig ; run-time  – R:sys1  ) gforth-0.5 “try”

Start an exception-catching region.

endtry ( compilation  – ; run-time  R:sys1 –  ) gforth-0.5 “endtry”

End an exception-catching region.

iferror ( compilation  orig1 – orig2 ; run-time  –  ) gforth-0.7 “iferror”

Starts the exception handling code (executed if there is an exception between try and endtry). This part has to be finished with then.

If you don’t need code2, you can write restore instead of iferror then:


The cleanup example from above in this syntax:

base @ { oldbase }
  hex foo \ now the hex is placed correctly
  0       \ value for throw
  oldbase base !

An additional advantage of this variant is that an exception between restore and endtry (e.g., from the user pressing Ctrl-C) restarts the execution of the code after restore, so the base will be restored under all circumstances.

However, you have to ensure that this code does not cause an exception itself, otherwise the iferror/restore code will loop. Moreover, you should also make sure that the stack contents needed by the iferror/restore code exist everywhere between try and endtry; in our example this is achived by putting the data in a local before the try (you cannot use the return stack because the exception frame (sys1) is in the way there).

This kind of usage corresponds to Lisp’s unwind-protect.

If you do not want this exception-restarting behaviour, you achieve this as follows:


If there is an exception in code1, then code2 is executed, otherwise execution continues behind the then (or in a possible else branch). This corresponds to the construct


in Gforth before version 0.7. So you can directly replace recover-using code; however, we recommend that you check if it would not be better to use one of the other try variants while you are at it.

To ease the transition, Gforth provides two compatibility files: endtry-iferror.fs provides the try ... endtry-iferror ... then syntax (but not iferror or restore) for old systems; recover-endtry.fs provides the try ... recover ... endtry syntax on new systems, so you can use that file as a stopgap to run old programs. Both files work on any system (they just do nothing if the system already has the syntax it implements), so you can unconditionally require one of these files, even if you use a mix old and new systems.

restore ( compilation  orig1 – ; run-time  –  ) gforth-0.7 “restore”

Starts restoring code, that is executed if there is an exception, and if there is no exception.

endtry-iferror ( compilation  orig1 – orig2 ; run-time  R:sys1 –  ) gforth-0.7 “endtry-iferror”

End an exception-catching region while starting exception-handling code outside that region (executed if there is an exception between try and endtry-iferror). This part has to be finished with then (or else...then).

Here’s the error handling example:

    myerror OF ... ( do something about it ) nothrow ENDOF
    throw \ pass other errors on

Programming style note:

As usual, you should ensure that the stack depth is statically known at the end: either after the throw for passing on errors, or after the ENDTRY (or, if you use catch, after the end of the selection construct for handling the error).

There are two alternatives to throw: Abort" is conditional and you can provide an error message. Abort just produces an “Aborted” error.

The problem with these words is that exception handlers cannot differentiate between different abort"s; they just look like -2 throw to them (the error message cannot be accessed by standard programs). Similar abort looks like -1 throw to exception handlers.

ABORT" ( compilation ’ccc"’ – ; run-time f –  ) core,exception-ext “abort-quote”

If any bit of f is non-zero, perform the function of -2 throw, displaying the string ccc if there is no exception frame on the exception stack.

abort ( ?? – ??  ) core,exception-ext “abort”

-1 throw.

For problems that are not that awful that you need to abort execution, you can just display a warning. The variable warnings allows to tune how many warnings you see.

WARNING" ( compilation ’ccc"’ – ; run-time f –  ) gforth-1.0 “WARNING"”

if f is non-zero, display the string ccc as warning message.

warnings ( – addr  ) gforth-0.2 “warnings”

set warnings level to


turns warnings off


turns normal warnings on


turns beginner warnings on


pedantic warnings on


turns warnings into errors (including beginner warnings)