void myterminate ()
{
cout << "terminate handler called";
}
int main (void)
{
set_terminate (myterminate);
throw; // throwing an exception. So, terminate handler should be invoked
// as no one is handling this exception.
getch();
return 0;
}
But After executing this code, the output is:
terminate handler called + "Debug Error!" dialog box appears.
I am not sure why it is coming like this !!!! Please help.
Based on the MSDN documentation for set_terminate the new handler function must call exit() or abort() will be called:
The set_terminate function installs term_func as the function called by terminate. set_terminate is used with C++ exception handling and may be called at any point in your program before the exception is thrown. terminate calls abort by default. You can change this default by writing your own termination function and calling set_terminate with the name of your function as its argument. terminate calls the last function given as an argument to set_terminate. After performing any desired cleanup tasks, term_func should exit the program. If it does not exit (if it returns to its caller), abort is called.
For example:
void myterminate ()
{
cout << "terminate handler called";
exit(1);
}
According to the requirments of the standard, a function used as a terminate_handler must meet the following requirement (ISO/IEC 14882:2011 18.8.3.1):
Required behavior: A terminate_handler shall terminate execution of the program without returning
to the caller.
As your function doesn't meet this requirement you program has undefined behaviour. In order to see your custom diagnostic you should output a newline to std::cout (as this can be required on many platforms) and then terminate the program in some way, such as calling std::abort.
std::abort is used to signal an abnormal termination of the program so you can expect extra diagnostics to be reported to the user such as via the dialog box that you are seeing.
Note that using std::exit from a terminate handler is potentially dangerous as std::terminate might be called in response to an exceptional condition occurring in a function registered with std::atexit or std:: at_quick_exit. This would lead to a second attempt to call std::exit.
In summary, if you don't want an "abnormal" termination, you almost always need to catch exceptions that you throw.
You have to exit program in your terminate handler. Add the following line to the handler and it will work:
exit(-1);
Related
Is the following code
#include <stdexcept>
int main() {
throw std::runtime_error("foobar");
}
guaranteed to produce the following outout?
terminate called after throwing an instance of 'std::runtime_error'
what(): foobar
fish: Job 1, './a.out' terminated by signal SIGABRT (Abort)
Can I rely on this exact output on every compiler?
Can I rely on that what method will be called and error message will be printed will be printed?
No it is not guaranteed, it unspecified whether there is any message. From cppreference:
If an exception is thrown and not caught, including exceptions that escape the initial function of std::thread, the main function, and the constructor or destructor of any static or thread-local objects, then std::terminate is called. It is implementation-defined whether any stack unwinding takes place for uncaught exceptions.
The case that is relevant here is "exceptions that escape [...] the main function". The call to std::terminate is guaranteed though. And you can install a std::terminate_handler to print a custom message if you like.
Does C/C++ support terminating a program from a subroutine function i.e not main?
So far I only found that exit and abort allow a user to terminate current function or process.
If I'm not in main function, is there a way to terminate the whole program?
If you are not in main() and in other function then also you can call exit() or abort() it will terminate your whole process.
where exit() will do required clean up where abort() will not perform that.
exit(0) or exit(1)
If this is 0 or EXIT_SUCCESS, it indicates success.
If it is EXIT_FAILURE, it indicates failure.
ref: See here
since you're talking C++, consider std::terminate
u now, for “Does C/C++ support terminating a program from a subroutine function i.e not main?”
by default std::terminate calls abort, but this is configurable by installing a handler via std::set_terminate
void exit (int status)
Above method Terminates the process normally, performing the regular cleanup for terminating programs.
Normal program termination performs the following (in the same order):
Objects associated with the current thread with thread storage duration are destroyed (C++11 only).
Objects with static storage duration are destroyed (C++) and functions registered with atexit are called.
All C streams (open with functions in ) are closed (and flushed, if buffered), and all files created with tmpfile are removed.
And after that Control is returned to the host environment.
As it terminates the calling process, becuase your function is part of same process, so using exit() in that will terminate the program.
It can only be possible if you called that function from main function. And from that function from which you want to terminate the program return a value for terminating the program for example -1.
Example:
void main()
{
//Call to a function
int i = functionFromMain();
if(i == -1)
{
//Terminate Program
}
}
What's the difference between those three, and how shall I end program in case of exception which I can't handle properly?
abort indicates "abnormal" end to the program, and raises the the POSIX signal SIGABRT, which means that any handler that you have registered for that signal will be invoked, although the program will still terminate afterwords in either case. Usually you would use abort in a C program to exit from an unexpected error case where the error is likely to be a bug in the program, rather than something like bad input or a network failure. For example, you might abort if a data structure was found to have a NULL pointer in it when that should logically never happen.
exit indicates a "normal" end to the program, although this may still indicate a failure (but not a bug). In other words, you might exit with an error code if the user gave input that could not be parsed, or a file could not be read. An exit code of 0 indicates success. exit also optionally calls handlers before it ends the program. These are registered with the atexit and on_exit functions.
std::terminate is what is automatically called in a C++ program when there is an unhandled exception. This is essentially the C++ equivalent to abort, assuming that you are reporting all your exceptional errors by means of throwing exceptions. This calls a handler that is set by the std::set_terminate function, which by default simply calls abort.
In C++, you usually want to avoid calling abort or exit on error, since you're better off throwing an exception and letting code further up the call stack decide whether or not ending the program is appropriate. Whether or not you use exit for success is a matter of circumstance - whether or not it makes sense to end the program somewhere other than the return statement in main.
std::terminate should be considered a last-ditch error reporting tool, even in C++. The problem with std::terminate is that the terminate handler does not have access to the exception that went unhandled, so there's no way to tell what it was. You're usually much better off wrapping the entirety of main in a try { } catch (std::exception& ex) { } block. At least then you can report more information about exceptions that derived from std::exception (although of course exceptions that do not derive from std::exception would still end up unhandled).
Wrapping the body of main in try { } catch(...) { } isn't much better than setting a terminate handler, because again you have no access to the exception in question. There is at least one benefit, though: whether stack unwinding is done when an exception goes completely uncaught is implementation defined, so if you need guaranteed stack unwinding, this would be a way to get that.
std::abort and std::exit (and more: std::_Exit, std::quick_exit) are just lower level functions. You use them to tell the program what you want it to do exactly: what destructors (and if) to call, what other clean-up functions to call, what value to return, etc.
std::terminate is a higher level abstraction: it is called (by either run-time or you) to indicate that an error in the program occurred and that for some reason it is not possible to handle by throwing an exception. The necessity for that typically occurs when error occurs in the exception mechanism itself, but you can use it any time when you do not want your program to continue beyond the given error. I compiled the full list of situations when std::terminate is called in my post. It is not specified what std::terminate does, because you are in control of it. You can configure the behavior by registering any functions. The limitations you have are that the function cannot return back to the error site and it cannot exit via an exception, but technically you can even start your message pump inside. For the list of useful things that you can do inside, see my other post.
In particular, note that std::terminate is considered an exception handler in contexts where std::terminate is called due to a thrown exception that could not be handled, and you can check what the exception was and inspect it by using C++11 using std::rethrow_exception and std::current_exception. It is all in my post.
quick_exit() !
If your program is multi-threaded, then calling exit() will most likely result in a crash because global/static std::thread objects will be attempted to destruct without exiting their threads.
If you want to return an error code and exit the program (more or less) normally, call quick_exit() in multi-threaded programs.
For abnormal termination (without a possibility for you to specify the error code), abort() or std::terminate() can be called.
Note: quick_exit() has not been supported by MSVC++ until version 2015 .
terminate() is automatically called
when an exception occurs that cannot
be handled. By default, terminate()
calls abort(). You can set a custom
handle with set_terminate() function.
abort() sends the SIGABRT signal.
exit() is not necessarily a bad
thing. It successfully exits the
application, and calls atexit()
functions in LIFO order. I don't
normally see this in C++
applications, however, I do see it in
many unix based applications where it
sends an exit code at the end.
Usually a exit(0) indicates a
successful run of the application.
terminate leaves you the possibility to register what will happen when it is called. Should be one of the other two.
exit is a normal exit allowing to specify an exit status. Handlers registered by at_exit() are run
abort is an abnormal exit. The only thing which is ran is the signal handler for SIGABRT.
My advice would be not to use any of them. Instead, catch the exceptions you can't handle in main() and simply return from there. This means that you are guaranteed that stack unwinding happens correctly and all destructors are called. In other words:
int main() {
try {
// your stuff
}
catch( ... ) {
return 1; // or whatever
}
}
I would like my C++ code to stop running if a certain condition is met, but I'm not sure how to do that. So just at any point if an if statement is true terminate the code like this:
if (x==1)
{
kill code;
}
There are several ways, but first you need to understand why object cleanup is important, and hence the reason std::exit is marginalized among C++ programmers.
RAII and Stack Unwinding
C++ makes use of a idiom called RAII, which in simple terms means objects should perform initialization in the constructor and cleanup in the destructor. For instance the std::ofstream class [may] open the file during the constructor, then the user performs output operations on it, and finally at the end of its life cycle, usually determined by its scope, the destructor is called that essentially closes the file and flushes any written content into the disk.
What happens if you don't get to the destructor to flush and close the file? Who knows! But possibly it won't write all the data it was supposed to write into the file.
For instance consider this code
#include <fstream>
#include <exception>
#include <memory>
void inner_mad()
{
throw std::exception();
}
void mad()
{
auto ptr = std::make_unique<int>();
inner_mad();
}
int main()
{
std::ofstream os("file.txt");
os << "Content!!!";
int possibility = /* either 1, 2, 3 or 4 */;
if(possibility == 1)
return 0;
else if(possibility == 2)
throw std::exception();
else if(possibility == 3)
mad();
else if(possibility == 4)
exit(0);
}
What happens in each possibility is:
Possibility 1: Return essentially leaves the current function scope, so it knows about the end of the life cycle of os thus calling its destructor and doing proper cleanup by closing and flushing the file to disk.
Possibility 2: Throwing a exception also takes care of the life cycle of the objects in the current scope, thus doing proper cleanup...
Possibility 3: Here stack unwinding enters in action! Even though the exception is thrown at inner_mad, the unwinder will go though the stack of mad and main to perform proper cleanup, all the objects are going to be destructed properly, including ptr and os.
Possibility 4: Well, here? exit is a C function and it's not aware nor compatible with the C++ idioms. It does not perform cleanup on your objects, including os in the very same scope. So your file won't be closed properly and for this reason the content might never get written into it!
Other Possibilities: It'll just leave main scope, by performing a implicit return 0 and thus having the same effect as possibility 1, i.e. proper cleanup.
But don't be so certain about what I just told you (mainly possibilities 2 and 3); continue reading and we'll find out how to perform a proper exception based cleanup.
Possible Ways To End
Return from main!
You should do this whenever possible; always prefer to return from your program by returning a proper exit status from main.
The caller of your program, and possibly the operating system, might want to know whether what your program was supposed to do was done successfully or not. For this same reason you should return either zero or EXIT_SUCCESS to signal that the program successfully terminated and EXIT_FAILURE to signal the program terminated unsuccessfully, any other form of return value is implementation-defined (§18.5/8).
However you may be very deep in the call stack, and returning all of it may be painful...
[Do not] throw an exception
Throwing an exception will perform proper object cleanup using stack unwinding, by calling the destructor of every object in any previous scope.
But here's the catch! It's implementation-defined whether stack unwinding is performed when a thrown exception is not handled (by the catch(...) clause) or even if you have a noexcept function in the middle of the call stack. This is stated in §15.5.1 [except.terminate]:
In some situations exception handling must be abandoned for less subtle error handling techniques. [Note: These situations are:
[...]
— when the exception handling mechanism cannot find a handler for a thrown exception (15.3), or when the search for a handler (15.3) encounters the outermost block of a function with a noexcept-specification that does not allow the exception (15.4), or [...]
[...]
In such cases, std::terminate() is called (18.8.3). In the situation where no matching handler is found, it is implementation-defined whether or not the stack is unwound before std::terminate() is called [...]
So we have to catch it!
Do throw an exception and catch it at main!
Since uncaught exceptions may not perform stack unwinding (and consequently won't perform proper cleanup), we should catch the exception in main and then return a exit status (EXIT_SUCCESS or EXIT_FAILURE).
So a possibly good setup would be:
int main()
{
/* ... */
try
{
// Insert code that will return by throwing a exception.
}
catch(const std::exception&) // Consider using a custom exception type for intentional
{ // throws. A good idea might be a `return_exception`.
return EXIT_FAILURE;
}
/* ... */
}
[Do not] std::exit
This does not perform any sort of stack unwinding, and no alive object on the stack will call its respective destructor to perform cleanup.
This is enforced in §3.6.1/4 [basic.start.init]:
Terminating the program without leaving the current block (e.g., by calling the function std::exit(int) (18.5)) does not destroy any objects with automatic storage duration (12.4). If std::exit is called to end a program during the destruction of an object with static or thread storage duration, the program has undefined behavior.
Think about it now, why would you do such a thing? How many objects have you painfully damaged?
Other [as bad] alternatives
There are other ways to terminate a program (other than crashing), but they aren't recommended. Just for the sake of clarification they are going to be presented here. Notice how normal program termination does not mean stack unwinding but an okay state for the operating system.
std::_Exit causes a normal program termination, and that's it.
std::quick_exit causes a normal program termination and calls std::at_quick_exit handlers, no other cleanup is performed.
std::exit causes a normal program termination and then calls std::atexit handlers. Other sorts of cleanups are performed such as calling static objects destructors.
std::abort causes an abnormal program termination, no cleanup is performed. This should be called if the program terminated in a really, really unexpected way. It'll do nothing but signal the OS about the abnormal termination. Some systems perform a core dump in this case.
std::terminate calls the std::terminate_handler which calls std::abort by default.
As Martin York mentioned, exit doesn't perform necessary clean-up like return does.
It's always better to use return in the place of exit.
In case if you are not in main, wherever you would like to exit the program, return to main first.
Consider the below example.
With the following program, a file will be created with the content mentioned.
But if return is commented & uncommented exit(0), the compiler doesn't assure you that the file will have the required text.
int main()
{
ofstream os("out.txt");
os << "Hello, Can you see me!\n";
return(0);
//exit(0);
}
Not just this, Having multiple exit points in a program will make debugging harder.
Use exit only when it can be justified.
Call the std::exit function.
People are saying "call exit(return code)," but this is bad form. In small programs it is fine, but there are a number of issues with this:
You will end up having multiple exit points from the program
It makes code more convoluted (like using goto)
It cannot release memory allocated at runtime
Really, the only time you should exit the problem is with this line in main.cpp:
return 0;
If you are using exit() to handle errors, you should learn about exceptions (and nesting exceptions), as a much more elegant and safe method.
return 0; put that wherever you want within int main() and the program will immediately close.
The program will terminate when the execution flow reaches the end of the main function.
To terminate it before then, you can use the exit(int status) function, where status is a value returned to whatever started the program. 0 normally indicates a non-error state
Either return a value from your main or use the exit function. Both take an int. It doesn't really matter what value you return unless you have an external process watching for the return value.
If you have an error somewhere deep in the code, then either throw an exception or set the error code. It's always better to throw an exception instead of setting error codes.
Generally you would use the exit() method with an appropriate exit status.
Zero would mean a successful run. A non-zero status indicates some sort of problem has occurred. This exit code is used by parent processes (e.g. shell scripts) to determine if a process has run successfully.
Beyond calling exit(error_code) - which calls atexit handlers, but not RAII destructors, etc.
- more and more I am using exceptions.
More and more my main program looks like
int main(int argc, char** argv)
{
try {
exit( secondary_main(argc, argv );
}
catch(...) {
// optionally, print something like "unexpected or unknown exception caught by main"
exit(1);
}
}
where secondary_main
in where all the stuff that was originally is put --
i.e. the original main is renamed secondary_main, and the stub main above is added.
This is just a nicety, so that there isn't too much code between the tray and catch in main.
If you want, catch other exception types.
I quite like catching string error types, like std::string or char*, and printing those
in the catch handler in main.
Using exceptions like this at least allows RAII destructors to be called, so that they can do cleanup. Which can be pleasant and useful.
Overall, C error handling - exit and signals - and C++ error handling - try/catch/throw exceptions - play together inconsistently at best.
Then, where you detect an error
throw "error message"
or some more specific exception type.
If the condition I'm testing for is really bad news, I do this:
*(int*) NULL= 0;
This gives me a nice coredump from where I can examine the situation.
Dude... exit() function is defined under stdlib.h
So you need to add a preprocessor.
Put include stdlib.h in the header section
Then use exit(); wherever you like but remember to put an interger number in the parenthesis of exit.
for example:
exit(0);
If your if statement is in Loop You can use
break;
If you want to escape some code & continue to loop Use :
continue;
If your if statement not in Loop You can use :
return 0;
Or
exit();
To break a condition use the return(0);
So, in your case it would be:
if(x==1)
{
return 0;
}
In C and C++, what is the difference between exit() and abort()? I am trying to end my program after an error (not an exception).
abort() exits your program without calling functions registered using atexit() first, and without calling objects' destructors first. exit() does both before exiting your program. It does not call destructors for automatic objects though. So
A a;
void test() {
static A b;
A c;
exit(0);
}
Will destruct a and b properly, but will not call destructors of c. abort() wouldn't call destructors of neither objects. As this is unfortunate, the C++ Standard describes an alternative mechanism which ensures properly termination:
Objects with automatic storage duration are all destroyed in a program whose function main() contains no automatic objects and executes the call to exit(). Control can be transferred directly to such a main() by throwing an exception that is caught in main().
struct exit_exception {
int c;
exit_exception(int c):c(c) { }
};
int main() {
try {
// put all code in here
} catch(exit_exception& e) {
exit(e.c);
}
}
Instead of calling exit(), arrange that code throw exit_exception(exit_code); instead.
abort sends a SIGABRT signal, exit just closes the application performing normal cleanup.
You can handle an abort signal however you want, but the default behavior is to close the application as well with an error code.
abort will not perform object destruction of your static and global members, but exit will.
Of course though when the application is completely closed the operating system will free up any unfreed memory and other resources.
In both abort and exit program termination (assuming you didn't override the default behavior), the return code will be returned to the parent process that started your application.
See the following example:
SomeClassType someobject;
void myProgramIsTerminating1(void)
{
cout<<"exit function 1"<<endl;
}
void myProgramIsTerminating2(void)
{
cout<<"exit function 2"<<endl;
}
int main(int argc, char**argv)
{
atexit (myProgramIsTerminating1);
atexit (myProgramIsTerminating2);
//abort();
return 0;
}
Comments:
If abort is uncommented: nothing is printed and the destructor of someobject will not be called.
If abort is commented like above: someobject destructor will be called you will get the following output:
exit function 2
exit function 1
The following things happen when a program calls exit():
Functions registered by the atexit function are executed
All open streams are flushed and closed, files created by tmpfile are removed
The program terminates with the specified exit code to the host
The abort() function sends the SIGABRT signal to the current process, if it is not caught the program is terminated with no guarantee that open streams are flushed/closed or that temporary files created via tmpfile are removed, atexit registered functions are not called, and a non-zero exit status is returned to the host.
From the exit() manual page:
The exit() function causes normal process termination and the value of
status & 0377 is returned to the parent.
From the abort() manual page:
The abort() first unblocks the SIGABRT signal, and then raises that
signal for the calling process. This results in the abnormal termination of the process unless the SIGABRT signal is caught and the signal
handler does not return.
abort sends the SIGABRT signal. abort does not return to the caller. The default handler for the SIGABRT signal closes the application. stdio file streams are flushed, then closed. Destructors for C++ class instances are not, however (not sure on this one -- perhaps results are undefined?).
exit has its own callbacks, set with atexit. If callbacks are specified (or only one), they are called in the order reverse of their registration order (like a stack), then the program exits. As with abort, exit does not return to the caller. stdio file streams are flushed, then closed. Also, destructors for C++ class instances are called.