How to end C code

Question

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

User · Accepted Answer

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  lt fstream gt   include  lt exception gt   include  lt memory gt   void inner mad         throw std  exception       void mad         auto ptr   std  make unique lt int gt         inner mad       int main         std  ofstream os  file txt        os  lt  lt   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 a exception  Throwing a 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 a 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 amp       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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

If your if statement is in Loop You can use    break     If you want to  escape some code  amp  continue to loop  Use     continue   If your if statement not in Loop You can use     return 0   Or        exit

User · Answer

return 0  put that wherever you want within int main   and the program will immediately close

User · Answer

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

User · Answer

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

User · Answer

Call the std  exit function   nbsp  nbsp

User · Answer

To break a condition use the return 0    So  in your case it would be        if x  1                return 0

User · Answer

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

User · Answer

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

User · Answer

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  amp  uncommented exit 0   the compiler doesn t assure you that the file will have the required text   int main         ofstream os  out txt        os  lt  lt   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

User · Answer

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

[c++] How to end C++ code

RAII and Stack Unwinding

Possible Ways To End

Return from main!

[Do not] throw a exception

Do throw a exception and catch it at main!

[Do not] std::exit

Other [as bad] alternatives

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