[PD] "sorry: only 5 args typechecked; use A_GIMME"

[IOhannes m zmoelnig]

On 2017-02-06 07:42, Chris Chronopoulos wrote:
> Code is attached. Any suggestions? What's this typechecking/A_GIMME

Pd has special, "optimized" functions to check the types of arguments.

basically, if you have a callback function (and from a pure "C"
perspective, the class-methods of an objects are callback functions),
you must correctly call that function.
that is: if the callback function expects a symbols and then a float
("void foo_mess(t_myobject*x, t_symbol*s, t_float f)"), you must
actually call it with a symbol and then a float ("foo_mess(x, s2,
2.3)"), to ensure that the compiler will put the correct bits at the
right positions.

now in Pd, an external can wish for "any" function signature of the
callback it wishes for, but it is the task of Pd (core) to actually call
that callback function correctly.
since "any" signature is pretty broad and it is tedious to write special
callback callers for all the possibilities, it offers a (slightly less
convenient) generic solution for "non-standard" callbacks: pass a list
of atoms (aka A_GIMME)
messages with more that 5 atoms are "non-standard", and therefore you
must use A_GIMME.

the function signature of an A_GIMME is:
  int argc, t_atom*argv
you then can iterate over the <argc> atoms in the <argv> array, and use
them whoever you please (mainly: check that each element is of correct
type, and use atom_getfloat() resp atom_getsymbol() to acccess the values).

i suspect (without having looked at your code), that you were simply
changing the type to A_GIMME when registering the callback (in the
class_addmothod() function), but then didn't bother to change the
function signature of the actual callback function.

so if your callback signature is:
 void foo_mess(t_myobject*x, t_symbol*s, t_float f);
and you register it with
 class_addmethod(x, (t_method)foo_mess, gensym("foo"), A_GIMME, 0);

then the system will end up calling foo_mess with the following arguments:
- the first 4 bytes are the address to the object (good)
- the next 4 bytes are an int value (bad because you are expecting the 4
bytes to be the address of a symbol)
- the next 4 bytes being the address of some t_atom-array (bad, because
you are expecting it to be 4 bytes of an ieee758 floating point number)
(for the sake of simplicity the above example assumes a 32bit system)

if you then happen to dereference so-acquired symbol, it will hopefully
go kaboong (hopefully insofar as it will go kaboong without launching
the missilies first)


mfgasdr
IOhannes

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