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fingolfin merged 6 commits into from
Mar 25, 2026
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Error handling: split stack frames over two lines; adjust more manual examples #6263

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011dec8
error: split stack frames over two lines
fingolfin Mar 11, 2026
04e6294
avoid setjmp/longjmp clobbering
fingolfin Mar 11, 2026
4a0e38f
Adjust some more stacktrace / break loop relevant docs
fingolfin Mar 12, 2026
bbe95f1
Update some comments
fingolfin Mar 13, 2026
6cc08be
fixup! error: split stack frames over two lines
fingolfin Mar 13, 2026
6b15d58
Avoid a trailing space
fingolfin Mar 13, 2026
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96 changes: 39 additions & 57 deletions doc/ref/debug.xml
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Expand Up @@ -915,41 +915,33 @@ to terminate the calculation, or enter <B>Return</B><C>;</C> to continue it.
gap> dive:= function(depth) if depth>1 then dive(depth-1); fi; return; end;
function( depth ) ... end
gap> dive(100);
gap> OnBreak:= function() Where(1); end; # shorter traceback
function( ) ... end
gap> OnBreak:= function() Where(2); end;; # shorter traceback
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@fingolfin fingolfin Mar 11, 2026

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the "1" showed 2 level due to a bug that I fixed in #6257, so I've now changed it to 2 levels to more closely match was the example showed before

gap> dive(6000);
recursion depth trap (5000)
at
dive( depth - 1 );
called from
dive( depth - 1 ); called from
...
Entering break read-eval-print loop ...
you can 'quit;' to quit to outer loop, or
you may 'return;' to continue
Error, recursion depth trap (5000)
*[1] dive( depth - 1 );
@ *stdin*:1
[2] dive( depth - 1 );
@ *stdin*:1
... at *stdin*:4
you may 'return;'
brk> return;
gap> dive(11000);
recursion depth trap (5000)
at
dive( depth - 1 );
called from
dive( depth - 1 ); called from
...
Entering break read-eval-print loop ...
you can 'quit;' to quit to outer loop, or
you may 'return;' to continue
Error, recursion depth trap (5000)
*[1] dive( depth - 1 );
@ *stdin*:1
[2] dive( depth - 1 );
@ *stdin*:1
... at *stdin*:5
you may 'return;'
brk> return;
recursion depth trap (10000)
at
dive( depth - 1 );
called from
dive( depth - 1 ); called from
...
Entering break read-eval-print loop ...
you can 'quit;' to quit to outer loop, or
you may 'return;' to continue
brk> return;
gap>
Error, recursion depth trap (10000)
*[1] dive( depth - 1 );
@ *stdin*:1
[2] dive( depth - 1 );
@ *stdin*:1
... at *stdin*:5
you may 'return;'
brk> return;gap>
]]></Log>
<P/>

Expand Down Expand Up @@ -979,39 +971,29 @@ gap> dive := function(depth)
> fi;
> end;;
gap> SetRecursionTrapInterval(1000);
gap> OnBreak:= function() Where(2); end;; # shorter traceback
gap> dive(100);
Depth 100
gap> dive(2500);
recursion depth trap (1000)
at
dive( depth - 1 );
called from
dive( depth - 1 ); called from
...
Entering break read-eval-print loop ...
you can 'quit;' to quit to outer loop, or
you may 'return;' to continue
Error, recursion depth trap (1000)
*[1] dive( depth - 1 );
@ *stdin*:4
[2] dive( depth - 1 );
@ *stdin*:4
... at *stdin*:12
you may 'return;'
Comment thread
fingolfin marked this conversation as resolved.
brk> return;
recursion depth trap (2000)
at
dive( depth - 1 );
called from
dive( depth - 1 ); called from
...
Entering break read-eval-print loop ...
you can 'quit;' to quit to outer loop, or
you may 'return;' to continue
Error, recursion depth trap (2000)
*[1] dive( depth - 1 );
@ *stdin*:4
[2] dive( depth - 1 );
@ *stdin*:4
... at *stdin*:12
you may 'return;'
brk> GetRecursionDepth();
0
brk> return;
gap> SetRecursionTrapInterval(-1);
Error, SetRecursionTrapInterval: <interval> must be a small integer greater than 5 (n\
ot the integer -1)
not in any function
Entering break read-eval-print loop ...
you can 'quit;' to quit to outer loop, or
you can replace <interval> via 'return <interval>;' to continue
brk> return 0;
gap> SetRecursionTrapInterval(2^50);
gap> dive(20000);
Depth 20000
gap> dive(2000000);
Expand Down
6 changes: 1 addition & 5 deletions doc/ref/grpperm.xml
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Expand Up @@ -652,11 +652,7 @@ gap> h:=Group(permutationlist);
<permutation group with 5 generators>
gap> StabChainOptions(h).random:=1;;
gap> Size(h);
exceeded the permitted memory (`-o' command line option) at
mlimit := 1; called from
SCRMakeStabStrong( S.stabilizer, [ g ], param, orbits, where, basesize,
base, correct, missing, false ); called from
SCRMakeStabStrong( S.stabilizer, [ g ], param, orbits, where, basesize,
exceeded the permitted memory (`-o' command line option)
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fingolfin marked this conversation as resolved.
...
]]></Log>
<P/>
Expand Down
19 changes: 7 additions & 12 deletions doc/ref/language.xml
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Expand Up @@ -1556,24 +1556,19 @@ If the expression <A>expr</A> is a function call then this function must
return a value. If the function does not return a value an error is
signalled and you enter a break loop (see <Ref Sect="Break Loops"/>).
As usual you can leave the break loop with <C>quit;</C>.
If you enter <C>return <A>return-expr</A>;</C> the value of the expression
<A>return-expr</A> is assigned to the variable,
and execution continues after the assignment.
Comment thread
fingolfin marked this conversation as resolved.
<P/>
<Log><![CDATA[
gap> f1:= function( x ) Print( "value: ", x, "\n" ); end;;
gap> f2:= function( x ) return f1( x ); end;;
gap> f2( 4 );
value: 4
Function Calls: <func> must return a value at
return f1( x );
called from
<function>( <arguments> ) called from read-eval-loop
Entering break read-eval-print loop ...
you can 'quit;' to quit to outer loop, or
you can supply one by 'return <value>;' to continue
brk> return "hello";
"hello"
Error, Function Calls: <func> must return a value
*[1] f1( x )
@ *stdin*:2
<function "f2">( <arguments> )
called from read-eval loop at *stdin*:3
type 'quit;' to quit to outer loop
brk>
]]></Log>
<P/>
In the above example, the function <C>f2</C> calls <C>f1</C> with argument
Expand Down
17 changes: 6 additions & 11 deletions doc/ref/methsel.xml
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Expand Up @@ -125,19 +125,15 @@ objecty, a "No Method Found" error will be returned.
<Log><![CDATA[
gap> XCons(IsFullTransformationMonoid,4);
Error, no method found! For debugging hints type ?Recovery from NoMethodFound
Error, no 1st choice method found for `XCons' on 2 arguments called from
<function "HANDLE_METHOD_NOT_FOUND">( <arguments> )
called from read-eval loop at line 8 of *stdin*
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
Error, no 1st choice method found for `XCons' on 2 arguments
called from read-eval loop at *stdin*:8
type 'quit;' to quit to outer loop
brk> quit;
gap> XCons(IsNilpotentGroup,4);
Error, no method found! For debugging hints type ?Recovery from NoMethodFound
Error, no 1st choice method found for `XCons' on 2 arguments called from
<function "HANDLE_METHOD_NOT_FOUND">( <arguments> )
called from read-eval loop at line 9 of *stdin*
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
Error, no 1st choice method found for `XCons' on 2 arguments
called from read-eval loop at *stdin*:8
type 'quit;' to quit to outer loop
brk>
]]></Log>

Expand Down Expand Up @@ -435,4 +431,3 @@ the mathematical term <Q>centre</Q>.
<!-- %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% -->
<!-- %% -->
<!-- %E -->

108 changes: 62 additions & 46 deletions doc/ref/mloop.xml
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Expand Up @@ -476,24 +476,20 @@ indicate that you are in a break loop.
<P/>
<Log><![CDATA[
gap> 1/0;
Rational operations: <divisor> must not be zero
not in any function
Entering break read-eval-print loop ...
you can 'quit;' to quit to outer loop, or
you can replace <divisor> via 'return <divisor>;' to continue
Error, Rational operations: <divisor> must not be zero
not in any function at *stdin*:2
type 'quit;' to quit to outer loop
]]></Log>
<P/>
If errors occur within a break loop &GAP; enters another break loop at a
<E>deeper level</E>. This is indicated by a number appended to <C>brk</C>:
<P/>
<Log><![CDATA[
brk> 1/0;
Rational operations: <divisor> must not be zero
not in any function
Entering break read-eval-print loop ...
you can 'quit;' to quit to outer loop, or
you can replace <divisor> via 'return <divisor>;' to continue
brk_02>
Error, Rational operations: <divisor> must not be zero
not in any function at *stdin*:2
type 'quit;' to quit to outer loop
brk_2>
]]></Log>
<P/>
There are two ways to leave a break loop,
Expand All @@ -511,7 +507,7 @@ Note that &GAP; code between <C>quit;</C> and the end of the input line
is ignored.
<P/>
<Log><![CDATA[
brk_02> quit;
brk_2> quit;
brk>
]]></Log>
<P/>
Expand Down Expand Up @@ -750,25 +746,30 @@ same as <Ref Func="Where"/> while also showing the arguments and local
variables of each function.
<P/>
<Log><![CDATA[
gap> StabChain(SymmetricGroup(100)); # After this we typed ^C
user interrupt at
bpt := S.orbit[1];
called from
SiftedPermutation( S, (g * rep) ^ -1 ) called from
StabChainStrong( S.stabilizer, [ sch ], options ); called from
StabChainStrong( S.stabilizer, [ sch ], options ); called from
StabChainStrong( S, GeneratorsOfGroup( G ), options ); called from
StabChainOp( G, rec(
) ) called from
...
Entering break read-eval-print loop ...
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
gap> StabChain(SymmetricGroup(1000)); # After this we typed ^C
Comment thread
fingolfin marked this conversation as resolved.
Error, user interrupt
*[1] genlabels := Filtered( genlabels, function ( x )
return pnt ^ sgs[x] = pnt;
end );
@ GAPROOT/lib/stbc.gi:426
[2] StabChainBaseStrongGenerators( dom{[ 1 .. Length( dom ) - 1 ]}, sgs, () )
@ GAPROOT/lib/gpprmsya.gi:2006
[3] StabChainOp( G, rec(
) )
@ GAPROOT/lib/stbc.gi:28
[4] StabChainImmutable( arg[1] )
@ GAPROOT/lib/stbc.gi:18
<function "StabChain">( <arguments> )
called from read-eval loop at *stdin*:2
you can 'return;'
brk> Where(2);
called from
SiftedPermutation( S, (g * rep) ^ -1 ) called from
StabChainStrong( S.stabilizer, [ sch ], options ); called from
...
*[1] genlabels := Filtered( genlabels, function ( x )
return pnt ^ sgs[x] = pnt;
end );
@ GAPROOT/lib/stbc.gi:426
[2] StabChainBaseStrongGenerators( dom{[ 1 .. Length( dom ) - 1 ]}, sgs, () )
@ GAPROOT/lib/gpprmsya.gi:2006
... at *errin*:1
]]></Log>
<P/>
Note that the variables displayed even in the first line of the
Expand Down Expand Up @@ -836,10 +837,14 @@ Error, !
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
brk> Where();
*[1] Error( "!\n" ); at *stdin*:3 called from
[2] test( n + 1 ); at *stdin*:3 called from
[3] test( n + 1 ); at *stdin*:3 called from
[4] test( n + 1 ); at *stdin*:3 called from
*[1] Error( "!\n" );
@ *stdin*:4
[2] test( n + 1 );
@ *stdin*:4
[3] test( n + 1 );
@ *stdin*:4
[4] test( n + 1 );
@ *stdin*:4
<function "test">( <arguments> )
called from read-eval loop at *errin*:1
brk> n;
Expand All @@ -848,20 +853,28 @@ brk> DownEnv();
brk> n;
3
brk> Where();
[1] Error( "!\n" ); at *stdin*:3 called from
*[2] test( n + 1 ); at *stdin*:3 called from
[3] test( n + 1 ); at *stdin*:3 called from
[4] test( n + 1 ); at *stdin*:3 called from
[1] Error( "!\n" );
@ *stdin*:4
*[2] test( n + 1 );
@ *stdin*:4
[3] test( n + 1 );
@ *stdin*:4
[4] test( n + 1 );
@ *stdin*:4
<function "test">( <arguments> )
called from read-eval loop at *errin*:5
brk> DownEnv( 2 );
brk> n;
1
brk> Where();
[1] Error( "!\n" ); at *stdin*:3 called from
[2] test( n + 1 ); at *stdin*:3 called from
[3] test( n + 1 ); at *stdin*:3 called from
*[4] test( n + 1 ); at *stdin*:3 called from
[1] Error( "!\n" );
@ *stdin*:4
[2] test( n + 1 );
@ *stdin*:4
[3] test( n + 1 );
@ *stdin*:4
*[4] test( n + 1 );
@ *stdin*:4
<function "test">( <arguments> )
called from read-eval loop at *errin*:8
brk> DownEnv( -2 );
Expand Down Expand Up @@ -894,10 +907,13 @@ gap> fun := function() local x; x := 3; bar(); end;
function( ) ... end
gap> fun();
Error, !!
called from
bar( ); called from
<function>( <arguments> ) called from read-eval-loop
Entering break read-eval-print loop ...
Stack trace:
*[1] Error( "!!\n" );
@ *stdin*:4
[2] bar( );
@ *stdin*:7
<function "fun">( <arguments> )
called from read-eval loop at *stdin*:8
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
brk> x;
Expand Down
10 changes: 5 additions & 5 deletions doc/tut/group.xml
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Expand Up @@ -927,11 +927,12 @@ gap> Size( Image( hom, DerivedSubgroup(s4) ) );
<P/>
<Log><![CDATA[
gap> PreImage( hom, (1,2,3) );
Error, <map> must be an inj. and surj. mapping called from
Error, <map> must be an injective and surjective mapping
*[1] ErrorNoReturn( "<map> must be an injective and surjective ", "mapping" );
@ GAPROOT/lib/mapping.gi:262
<function "PreImage">( <arguments> )
called from read-eval loop at line 4 of *stdin*
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
called from read-eval loop at *stdin*:7
type 'quit;' to quit to outer loop
brk> quit;
]]></Log>
<P/>
Expand Down Expand Up @@ -1258,4 +1259,3 @@ Chapter&nbsp;<Ref Chap="Group Homomorphisms" BookName="ref"/>.
<!-- %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% -->
<!-- %% -->
<!-- %E -->

4 changes: 2 additions & 2 deletions doc/tut/introduc.xml
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Expand Up @@ -253,8 +253,8 @@ We will no longer mention the <B>Return</B> key from now on.
<Index>break loops</Index>
Sometimes a syntax error will cause &GAP; to enter a <E>break loop</E>. This
is indicated by the special prompt <C>brk></C>. If another syntax error occurs
while &GAP; is in a break loop, the prompt will change to <C>brk_02></C>,
<C>brk_03></C> and so on. You can leave the current break loop and exit to the
while &GAP; is in a break loop, the prompt will change to <C>brk_2></C>,
<C>brk_3></C> and so on. You can leave the current break loop and exit to the
next outer one by either typing <C>quit;</C> or by hitting <B>Ctrl-D</B>.
Eventually &GAP; will return to its normal state and show its normal
prompt <C>gap></C> again.
Expand Down
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