Re: What should happen with an exception in mixed C/C++ code?

Bjorn Reese wrote:


Yes, but if the C code does not expect the callback to return control
(e.g. it is error handler which by default outputs a error message and
calls exit()) you may use setjmp/longjmp to return to your C++ code and
throw exception from there. Also, you do not have to compile with
-fexceptions. For example (rough code, not tested):

/* C code */
/* lib.h */
struct lib
{
     /*...*/
     void (*on_error)(struct lib*, char const*);
};

#ifdef __cplusplus
#define EXTERN extern "C"
#else
#define EXTERN extern
#endif

EXTERN void lib_init(struct lib*);
EXTERN void lib_do_something(struct lib* lib);

/* lib.c */
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include "lib.h"

static void
on_error_exit(struct lib*, char const* message)
{
     fputs(message, stderr);
     exit(EXIT_FAILURE);
}

void
lib_init(struct lib* lib)
{
     /*...*/
     lib->on_error = on_error_exit;
}

void
lib_do_something(struct lib* lib)
{
     /*...*/
     lib->on_error(lib, "error occured");
     assert(0); /* should never get here */
}

// C++ code
#include <setjmp.h>
#include <stdexcept>
#include "lib.h"

// "subclass" lib struct
struct lib_ext
{
     struct lib lib;
     jmp_buf jmpbuf;
     char const* last_error; // for simplicity; std::string might be
used

     lib_ext(void);
     void do_something(void);

     static void on_error(struct lib_ext* lib, char const* message);
};

lib_ext::lib_ext(void)
     : last_error(0)
{
     lib_init(& lib);
     lib.on_error = (void (*)(struct lib*, char const*)) on_error;
}

void
lib_ext::do_something(void)
{
     if (setjmp(jmpbuf))
     {
         throw std::runtime_error(last_error);
     }
     lib_do_something(& lib);
}

void
lib_ext::on_error(struct lib_ext* lib, char const* message)
{
     // might be unsafe if 'message' is constructed at run-time
     lib->last_error = message;
     longjmp(lib->jmpbuf, -1);
}

That longjmp/throw looks pretty safe for me :)

Alex Shulgin

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