os_generate_error and postmortems

[Jonathan Coxhead]

    Either ...

    You can catch the error by writing a signal handler for SIGOSERROR, and in 
the handler you can do whatever you want, including longjmp() to some higher 
stack frame. The x library in OSLibSupport shows you how to do this. It's hard 
to get right, and not for the novice.

    Or ...

    You can use the error-returning version instead of the error-generating 
version, and write the code yourself ...

       os_error err = {0, "foobar"};

       xwimp_report_error (xos_generate_error (&err), ...);

    (In this case, xos_generate_error() is pretty much a NOP---it just returns 
its argument, so this is the same as

       xwimp_report_error (&err, ...);
)

    By far the best approach to this whle issue is to structure your application 
like this:

    (*) Whenever you call a SWI or any error-returning function, check the error 
return, and "goto finish" immediately if there was an error.

       if ((err = f()) != NULL)
           goto finish;

    (*) Write all your functions to return an error

       os_error *g (...)
       {
           os_error *err = NULL;

           /* code code code, as above */

       finish:
           return err;
       }

    (*) At the top level of the app, decide what you want to do with errors that 
you get:

       int main()
       {
           os_error *err = NULL;

           /* body of code */

           if (err != NULL)
               xwimp_report_error (err, ...);

           return 0;
       }

    This is the "C way" of coding up C++-style exceptions. When an error 
happens, the function that called the SWI returns, straight away. Then the 
function that called *it* returns, and so on, all the way back up the call stack 
until we finally get to the place where the error is handled. In other words, 
the stack is unwound until a handler is found---just like a "real" exception.

    This lets you separate error-generation from error-handling in a very 
flexible way.

    Hope this helps ...

    Stefan Bellon wrote:

> Hi!
> 
> I'm stumbling over the fact that os_generate_error() always produces a
> postmortem with stack trace which I don't want. How should I do it if I
> only want an error message (printed on screen when from within the CLI
> or in an error box when from within the WIMP) and a return code?
> 
> My code looks as follows:
> 
> #include <oslib/os.h>
> 
> int main(void)
> {
>     os_error err = { 0, "foobar" };
>     os_generate_error(&err);
> }
> 
> And then the following happens:
> 
> *cc -IOSLib: err.c
> Norcroft RISC OS ARM C vsn 5.55 [17 Oct 2003]
> "c.err", line 2: Warning: Non-ANSI #include <oslib/os.h>
> c.err: 1 warning, 0 errors, 0 serious errors
> *err
> 
> foobar
> 
> Postmortem requested
>   Arg2: 0x00008080 32896 -> [0xe52de004 0xe24ddc01 0xe1a0000d
> 0xe59f1040]
>   Arg1: 0x0000aae4 43748 -> [0x00727265 0x0000aae8 0x3afffff2 00000000]
> 80c8 in shared library function
> 812c in anonymous function
> 
> What am I missing here? This is with the latest OSLib 6.70.
> 
> Thanks a lot in advance!
> 
> Greetings,
> Stefan.
> 

-- 
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