*When* does a C++ exception blow up C code -

ok, need -fexceptions allow c++ exception propagate through c code. tried compare results both c++ , c, and, can tell, compiled routines identical @ assembly, without option [1]. here test:

#include <unistd.h>  typedef int (*callback)(void* param,void* buffer,int n);  void write_wrapper(int fd,const void* buffer,int n) {     const char* temp=(const char*)buffer;     while(n!=0)         {         int k=write(fd,temp,n);         n-=k;         temp+=k;         } }  int test(callback cb,void* cb_param) {     char buffer[1024];     int n=0;             {         n=cb(cb_param,buffer,1024);         write_wrapper(stdout_fileno,buffer,n);         }     while(n!=1024); }  int test2(callback cb,void* cb_param) { char more_stack_space_please[1024]={0}; cb(cb_param,more_stack_space_please,1024); write_wrapper(stdout_fileno,more_stack_space_please,1024); test(cb,cb_param); } 

here, caller free resources, there should no leak though callback function throws exception.

even though example appears work (tested simple c++ driver)

#include "lib.h" #include <cstdio>  class resource     {     public:         resource()             {fprintf(stderr,"a resource\n");}          ~resource()             {fprintf(stderr,"not resource\n");}     };  int main()     {     try         {         resource foo;         test([](void* cb_param,void* buffer, int n)->int             {             resource bar;             throw "test";             },nullptr);         test2([](void* cb_param,void* buffer, int n)->int             {             throw "test2";             },nullptr);         }     catch(const char* err)         {         fprintf(stderr,"error: %s\n",err);         return -1;         }     return 0;     } 

, have got uncaught exception errors gtk callbacks. thoughts:

• gtk shared library, maybe affects linker, , not code generator
• gtk (and glib) contains lot strange hacks, may screw things up

[1] https://godbolt.org/g/hfsnjb

the problem question, asks defined behavior undefined result.

the c standard not describe behavior when exception thrown through c function, because shouldn't happen.

the c++ standard not explain mechanism uncaught exception dropping through c function because shouldn't happen.

a practical example

windows visual c++ uses fs: segment register thread local storage, , particular slot in thread-local data segment create linked list of catch frames.

when c++ exception thrown, linked list inspected destructors of stack objects, , suitable catch frame.

the c compiler may not aware of c++ usage of these resources, , able re-use slots different purpose. if slots used incompatible function, crash happen.

a particular platform , compiler may support this, @ whim of platform such.

gcc c compiler, not throw exceptions. can create code exception aware supports -fexception.

gnu compiler : using exceptions suggests compiling c code -fexceptions , says

in particular, unwinding frame no exception handling data cause runtime abort.

although context of statement not describe whether due being debugged clause, or due exception mechanism identifying illegal state , causing abort.

the actual implementation hardware , os specific, not specified in question, , specific answer, these should specified.

• c++ not have compatibility abi. calling through c++ objects compiled different compilers not give guarantee of working.
• exceptions thrown between different modules not recommended, implementation on compiler has produced code not intra-version compatible.
• assuming have control on both c , c++ code, ensure c safe enabling exceptions , possibly compiling c c++ (with extern "c" wrapper, ensuring c++ compilation unit.