Will C++ exceptions safely propagate through C code?

Question

My guess is that this is compiler dependent. However, throwing an exception in the callback would be a very bad idea. Either it will flat-out not work, or the C code in the SQLite library will be unable to handle it. Consider if this is some code in SQLite:

{
  char * p = malloc( 1000 );
  ...
  call_the_callback();  // might throw an exception
  ...
  free( p );
}

If the exception "works", the C code has no possible way of catching it, and p will never be freed. The same goes for any other resources the library may have allocated, of course.

There is already a protocol for the callback to abort the API call. From the docs:

If an sqlite3_exec() callback returns non-zero, the sqlite3_exec() routine returns SQLITE_ABORT without invoking the callback again and without running any subsequent SQL statements.

I'd strongly recommend you use this instead of an exception.

SQLite is expecting you to return a SQLITE_ABORT on error and a 0 return code for no error. So you ought to wrap all your C++ callback in a try catch. Then in the catch return a SQLite SQLITE_ABORT error code, otherwise a zero.

Problems will occur if you bypass returning through SQLite as it will not free up/complete whatever code it does after you return back from your callback. This will cause untold problems potentially some of which maybe very obscure.

That was a really interesting question and I tested it out myself out of curiosity. On my OS X w/ gcc 4.2.1 the answer was YES. It works perfectly. I think a real test would be using gcc for the C++ and some other (MSVC?, LLVM?) for the C part and see if it still works.

My code:

callb.h:

#ifdef __cplusplus
extern "C" {
#endif

typedef void (*t_callb)();
void cfun(t_callb fn);

#ifdef __cplusplus
}
#endif

callb.c:

#include "callb.h"

void cfun(t_callb fn) {
 fn();
}

main.cpp:

#include <iostream>
#include <string>
#include "callb.h"

void myfn() {
  std::string s( "My Callb Except" );
  throw s;
}

int main() {
  try {
    cfun(myfn); 
  }
  catch(std::string s) {
    std::cout << "Caught: " << s << std::endl;
  }
  return 0;
}