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"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

"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.)

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.)