How to catch run time error in C and C++?

Question

Like modifying a CONST int,

Can I register a specific function to handle run time errors so that this kind of operation just fails instead of terminating the application?

Answer 1

If you mean C++, there is a certain exception class called runtime_error. You can catch it with a catch clause:

catch(std::runtime_error& e) {}

However, many things in C and C++ (like modifying a const int) result in undefined behavior. You can't catch them at runtime. You can't catch them, because no exception is thrown (technically, anything may happen, including a throwing of exception (C++ only), but that's not something you can or should hope for).

The solution is to write clean safe code. For that there are many advices listed in many books. :)

Answer 2

Modifying a const is, according to the specification, "undefined behavior" so the compiler can do anything. In practice, many implementations sometimes generate runtime errors for such code but many do not. It often depends on the nature of the program. Here is an illustration:

#include <stdio.h>
#include <string.h>

typedef int (*fn)(const char *);

extern const fn global_fn_ptr;
extern const char global_string[];
const fn global_fn_ptr = puts;
const char global_string[] = "hello";

int main(int argc, char *argv[])
{
    puts("Setting global_fn_ptr to NULL");
    *(fn *)&global_fn_ptr = NULL;
    puts("Setting string to \"bye\"");
    strcpy((char *)global_string, "bye");
    return 0;
}

On my system, I get a SIGBUS from modifying the string, but modifying the function works fine. This is due to the peculiar nature of function pointers, whose value is not always determined at runtime so the value must be stored in writable memory.

It is generally not safe to catch a SIGBUS or SIGSEGV in C++ and turn it into an exception. It is also very difficult to correctly longjmp out of a signal handler — half the code that uses this pattern in C is probably incorrect. The safest option is to let the program terminate immediately, or if you really need this kind of help from the runtime, work very carefully with C code so you can free the appropriate resources in a nonlocal exit — C++ won't do because longjmp won't call destructors.

Or you can just move to C# or Java, both of which have runtimes that do this for you and garbage collectors that clean up afterwards.

Answer 3

This is operating system specific. The language itself specifies these as undefined behaviour.

In POSIX-compliant operating systems your program can catch a SIGSEGV signal in case of a restricted memory access, SIGILL in case of an invalid instruction or SIGFPE in case of an illegal floating point operation, for example division by zero.