Half a croissant, on a plate, with a sign in front of it saying '50c'
h a l f b a k e r y
Renovating the wheel

idea: add, search, annotate, link, view, overview, recent, by name, random

meta: news, help, about, links, report a problem

account: browse anonymously, or get an account and write.



C exception handling macros

Goto considered helpful
  (+7, -4)
(+7, -4)
  [vote for,

With the two C macro definitions

#define except_throw(x) goto x

#define except_catch(x) while (0) x:

"goto"s can be made to look good in C. (It turns out that Dijkstra was making a fashion statement all along.)

The first one simply works around "goto" having a bad name.

The second is used with a dangling statement or block, like "if":

except_catch (mem) return ERR_OUT_OF_MEMORY;


except_catch (hit) { printf("Ouch!\n"); damage++; }

It introduces a C statement or block that is only executed if it is entered via a jump. This lets a programmer move exception handling code away from the main algorithm, a structural benefit shared with "real", operating-system level exceptions. (Which of course are infinitely cooler if they work right, but require a lot of support.)

except_catch() statements can nest; multiple except_catch()'es inside one big one could be used to set an error variable to an error to the specific catch phrase, then continue with a generic cleanup.

jutta, Aug 13 1999

Kazlib http://users.footpr...et/~kaz/kazlib.html
library which implements clean exceptions in C. [johnmeacham, Mar 16 2002]

Please log in.
If you're not logged in, you can see what this page looks like, but you will not be able to add anything.
Short name, e.g., Bob's Coffee
Destination URL. E.g., https://www.coffee.com/
Description (displayed with the short name and URL.)

       I'd just like to reiterate that real exceptions are still preferable. The biggest practical limitation of these macros, as far as I can see, is that the exception handler must be compiled together with the code that raises the exception (or you'll get a "missing label" error at compile time). So you can't write a shared library that throws an exception on some error condition with the expectation that the caller will handle it in a manner appropriate to context. Still, it's a reasonable hack for people who want exception syntax in an exceptionless environment.
baf, Apr 11 2000

       I had UNWIND_PROTECT, CATCH, and THROW macros: CATCH(block) returns 0 for normal completion, or whatever THROW was passed if it was called.   

       UNWIND_PROTECT(block, cleanup_block) runs the block, then runs the cleanup block regardless of whether the block excepted, then passes the exception back up the stack if one happened.   

       UNWIND_PROTECT({ char *p=malloc(42); foo(p); }, { free(p);})
reece, May 04 2000

       Re: baf's objection: isn't the usual way to implement exception macros in C to use setjmp in the catch (or try) macro, and longjmp in the throw macro?
brouhaha, Jun 15 2000

       Re: setjmp/longjmp: Yes! (see, for example, van der Linden's "Expert C Programming").
acooke, Aug 10 2000

       I didn't know you could put a label there; how sneaky.   

       How would reece's postulated UNWIND_PROTECT etc. macros work?
egnor, Apr 27 2001

       This reminds me why I don't do C.
phoenix, Mar 16 2002


back: main index

business  computer  culture  fashion  food  halfbakery  home  other  product  public  science  sport  vehicle