Is std::uncaught_exceptions useful for avoiding all exceptions?

Question

I need to catch segmentation fault and other unknown exceptions in my application. But I do not know how I can do that! Can I use std::uncaught_exceptions for this aim?

Answer 1

Can I use std::uncaught_exceptions for this aim?

Consider this code:

int main(int argc, char* argv[])
{
    int *val = NULL;
    *val = 1;
    std::cout << "uncaught: " << std::uncaught_exceptions() << std::endl;
    return 0;
}

This will likely cause a segmentation fault and nothing will be output.

I need to catch segmentation fault and other unknown exceptions in my application. But I do not know how I can do that!

Exception handling in C++ can be done through the try-catch block, and you could use the std::signal function to catch certain errors like SIGSEGV, SIGFPE, or SIGILL, example:

#include <iostream>
#include <exception>
#include <csignal>
#include <cstdio>

extern "C" {
    void sig_fn(int sig)
    {
        printf("signal: %d\n", sig);
        std::exit(-1);
    }
}

int main(int argc, char* argv[])
{
    int *val = NULL;
    std::signal(SIGSEGV, sig_fn);
    try {
        *val = 1;
    } catch (...) {
        std::cout << "..." << std::endl;
    }
    if (std::uncaught_exception()) {
        std::cout << "uncaught" << std::endl;
    }
    std::cout << "return" << std::endl;
    return 0;
}

But you should note that this type of exception handling is really meant to do clean-up and shutdown, not necessarily catch and release; take this code for example:

#include <iostream>
#include <exception>
#include <csignal>
#include <cstdio>

extern "C" {
    void sig_fn(int sig)
    {
        printf("signal: %d\n", sig);
    }
}

int main(int argc, char* argv[])
{
    int *val = NULL;
    std::signal(SIGSEGV, sig_fn);
    while (true) {
        try {
            *val = 1;
        } catch (...) {
            std::cout << "..." << std::endl;
        }
    }
    if (std::uncaught_exception()) {
        std::cout << "uncaught" << std::endl;
    }
    std::cout << "return" << std::endl;
    return 0;
}

This code will cause and catch the segmentation fault forever!

If you are trying to catch a segmentation fault, you need to investigate why the segmentation fault (or any error for that matter) happened in the first place and correct that issue; using the above code as an example:

int *val = NULL;
if (val == NULL) {
    std::cout << "Handle the null!" << std::endl;
} else {
    *val = 1;
}

For further reading: here is a SO Q&A on what a segfault is, as well, here is the Wiki on it, and MIT has some tips on handling and debugging segfaults too.

Hope that can help.