I'm looking for a way to change the exit status of a C/C++ program during an atexit callback sequence.
I'm develping some C++ code that registers a book keeping function with atexit. If the program terminates as a result of an exit() call, this book keeping function prints a message if the data was left in an incomplete state. If this is the case, I want to make sure that the exit status of the program is nonzero. This is probably going to be true anyway, but it's also possible to call exit(0).
One solution is to call exit(-1) within the book keeping function that I've registered with atexit. This seems to work, but I think it's also undefined behavior. Does anyone know if this is correct? It's also unclear if this would terminate the atexit callback chain, which would be bad if there's another critical function registered with atexit.
On POSIX systems, it is allowed to call _exit from within an atexit handler, however doing so means that any other atexit handler is not called.
Since atexit handlers are called in the reverse order of registration, have the first registered handler read a global variable and, if it's not set to some initial sentinel value, call _exit with that value. Then any other handlers you register can modify that global if they want to override the exit value passed to exit.
For example:
#define EXIT_DEFAULT 0xffff
int exit_override = EXIT_DEFAULT;
void override_exit(void)
{
if (exit_override != EXIT_DEFAULT) _exit(exit_override);
}
void handler1(void)
{
if (some_error_condition) exit_override = 1;
}
int main()
{
atexit(override_exit);
atexit(handler1);
// do something that might call exit
return 0;
}
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
}
}
I have a program which creates a thread (I am using C++ Boost library for creating threads) when started. In the main program I have registered my cleanup function as.
atexit(cleanExit)
// Trap some signals for a clean exit.
signal(SIGQUIT, signalled)
signal(SIGABRT, signalled)
signal(SIGTERM, signalled)
signal(SIGINT, signalled)
static void signalled(int signal)
{
exit(signal)
}
static void cleanExit(void)
{
thread->interrupt();
thread->join();
}
As you can see above during the clean exit process I interrupt the thread and then wait here (in the main process) so that the thread does its clean up stuff. When I call thread->interrupt, my thread gets interrupted and I do the thread cleanUp stuff. Till here everything is working smooth and there is no problem.
But the problem comes in when I call the cleanup function in the thread. In the cleanup function I am sending some status back to a server, for that I have created a utility function, In this utility function I am accessing a "const static string" member of the class. The problem is when I access this const static string my application got just stuck. I have checked with strace and I am getting a Seg Fault. But when I change this "const static string" to "const string" my cleanup goes smooth.
QUESTION
What is the behavior of C++ "const static" once the program is terminated. Do they giveup when exit is called (which can be seen in above case) or any thoughts on this behavior.
UPDATE # 1
Here is the thread handler function. As I have mentioned above its a Boost thread.
try {
while ( true ) {
// Do your job here.
// 1: We can be interrupted.
boost::this_thread::interruption_point();
}
}
catch (boost::thread_interrupted const& )
{
// Before exiting we need to process all request messages in the queue
completeAllWorkBeforeExiting();
}
When the main program calls thread->interrupt, the thread will raise thread_interrupted exception at # 1, and catching this exception I am doing my cleanup stuff.
const does not affect when any object is destroyed.
static objects are destroyed in the order opposite to their order of creation. atexit essentially creates an anonymous static object with the given function as its destructor. That is, static object destructors and atexit callbacks occur in the order opposite their construction/registration.
It's completely unsafe to call exit from a signal handler. All the signal handler is practically allowed to do is set a flag which is later polled by the threads. As you've mentioned, the signal handler runs at interrupt time, so it might interrupt a system call. It might interrupt malloc in such a way that bypasses malloc's multithreading locks, since the reentrancy is on the same thread, not a different one as usually happens.
So that's a source of all sorts of unpredictable behavior. Without seeing more, I can't be more specific about what effect static is having, but it probably has something to do with changing the object's lifetime and/or instantiating it once per object.
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);
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;
}