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if a C routine tried to pass you -1, it would appear as hex FFFFFFFF. If a value is passed that does not fit the type you have chosen for a given parameter, a Euphoria type-check error may occur (depending on type_check)
No error will occur if you declare all parameters as atom.

Normally, as in the case of WndProc() above, Windows initiates these call-backs to your routines. It is also possible for a C routine in any .dll to call one of your Euphoria routines. You just have to declare the C routine properly, and pass it the call-back address.

Here's an example of a WATCOM C routine that takes your call-back address as its only parameter, and then calls your 3-parameter Euphoria routine:

/* 1-parameter C routine that you call from Euphoria */ 
unsigned EXPORT APIENTRY test1( 
    LRESULT CALLBACK (*eu_callback)(unsigned a, 
    unsigned b, 
    unsigned c)) 
{ 
    /* Your 3-parameter Euphoria routine is called here 
    via eu_callback pointer */ 
    return (*eu_callback)(111, 222, 333); 
} 

The C declaration above declares test1 as an externally-callable C routine that takes a single parameter. The single parameter is a pointer to a routine that takes 3 unsigned parameters - i.e. your Euphoria routine.

In WATCOM C, "CALLBACK" is the same as "__stdcall". This is the calling convention that's used to call Windows API routines, and the C pointer to your Euphoria routine should be declared this way too, or you'll get an error when your Euphoria routine tries to return to your .DLL.

If you need your Euphoria routine to be called using the __cdecl convention, you must code the call to call_back as:

myroutineaddr = call_back({'+', id}) 

The plus sign and braces indicate the __cdecl convention. The simple case, with no braces, is __stdcall.

In the example above, your Euphoria routine will be passed the three values 111, 222 and 333 as arguments. Your routine will return a value to test1. That value will then be immediately returned to the caller of test1 (which could be at some other place in your Euphoria program).

A call-back address can be passed to the UNIX signal() function to specify a Euphoria routine to handle various signals (e.g. SIGTERM). It can also be passed to C routines such as qsort, to specify a Euphoria comparison function.