3 ms·

Do defers get processed if an exception is thrown? I know C doesn’t have exceptions, but real world C code often interacts with C++ code. So it is necessary imo

• kevin_thibedeau 2y ago
  GCC has the __cleanup__ attribute which works in conjunction with exceptions in C to provide an RAII mechanism.
  • eqvinox 2y ago
    No it doesn't; C functions don't set up appropriate registrations with the C++ exception / stack unwinding systems, thus C functions are simply skipped over on the way up towards the nearest exception handler. __attribute__((cleanup(…))) is purely a scope-local mechanism, it has absolutely nothing to do with exceptions.
    • leni536 2y ago
      "If -fexceptions is enabled, then cleanup_function is run during the stack unwinding that happens during the processing of the exception. Note that the cleanup attribute does not allow the exception to be caught, only to perform an action. It is undefined what happens if cleanup_function does not return normally."[1] While it's true that it -fexceptions is disabled for C by default, some C libraries need to enable it anyway if they want to interact with C++ exceptions this way. For example C++ requires that qsort and bsearch propagate the exception thrown by the comparison callback normally, so libc implementations that are also used from C++ do enable it. [1] https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attribute...
      • eqvinox 2y ago
        Ah, I forgot about -fexceptions, Thanks. Though I need to point out it's non-default and rarely used, and in particular: > some C libraries need to enable it anyway if they want to interact with C++ exceptions this way. For example C++ requires that qsort and bsearch propagate the exception thrown by the comparison callback normally -fexceptions is not needed for this, C++ exceptions will just transparently bubble through C functions without its use. I don't think I know of any project using -fexceptions… time to google…
        • leni536 2y ago
          glibc allocates in its fallback algorithm within qsort. It has to free the allocation when a C++ exception is thrown in a comparison, unless it wants to leak. I recently filed a ticket for this. They kinda have to compile at least qsort with -fexceptions, unless they use a fully in-place sorting algorithm.
          • eqvinox 2y ago
            Oof, what an ugly issue… I kinda thought (hoped?) it'd be in-place anyway and thus not need any heap interactions, but I guess it does and they have no leeway to change the (fallback) algorithm… It's a bit weird for the C++ standard to require behavior off C standard functions though, I gotta say… seems more sensible to use a C++ sort…
        • loeg 2y ago
          -fexceptions used to be the only control for emitting unwind tables, IIRC, which C codebases may have desired for backtraces. (Now there is -fasynchronous-unwind-tables.)
    • kevin_thibedeau 2y ago
      GCC has comon exception handling across its supported languages. They have exceptions as a C extension that interoperate with C++. Cleanups get called during a stack unwind because they are considered the same as a destructor.
      • eqvinox 2y ago
        The sibling post to yours has a correct description of the full picture
• ryao 2y ago
  I am reminded of this: https://news.ycombinator.com/item?id=42532979 https://news.ycombinator.com/item?id=42532979 Interactions with stack unwinding are not considered by C, so I doubt this would be. Compilers would be free to make it work, however. This is just my guess.
• cryptonector 2y ago
  `extern "C"` functions coded in C++ should really not throw! I would say that they MUST NOT throw.
  • loeg 2y ago
    In practice, they can though. So whether they should or not doesn't have much bearing on the situation.