I was learning the example and I am not a
#include <windows.h> // for EXCEPTION_ACCESS_VIOLATION
#include <excpt.h>
int filter(unsigned int code, struct _EXCEPTION_POINTERS *ep) {
puts("in filter.");
if (code == EXCEPTION_ACCESS_VIOLATION) {
puts("caught AV as expected.");
return EXCEPTION_EXECUTE_HANDLER;
}
else {
puts("didn't catch AV, unexpected.");
return EXCEPTION_CONTINUE_SEARCH;
};
}
int main()
{
int* p = 0x00000000; // pointer to NULL
puts("hello");
__try {
puts("in try");
__try {
puts("in try");
*p = 13; // causes an access violation exception;
}
__finally {
puts("in finally. termination: ");
puts(AbnormalTermination() ? "\tabnormal" : "\tnormal");
}
}
__except (filter(GetExceptionCode(), GetExceptionInformation())) {
puts("in except");
}
puts("world");
}
Output of program is:
hello
in try
in try
in filter.
caught AV as expected.
in finally. termination:
abnormal
in except
world
I am confused in why it goes to filter first and then to finally and then prints "in except".
Isn't that true if it goes to __except then it should complete the execution.
Those aren't standard C++ exceptions but Microsoft's own language extensions.
If you had continued on from the page where you found that example to read the documentation for try-finally, you would have found this:
If a handler is found, any and all __finally blocks are executed and execution resumes in the handler.
This isn't surprising - in languages that have these constructs, the finally block is always executed (that's the purpose of it).
It's just that normally, the programmer has no runtime control over where or if the exception gets caught, so you can't interrupt the processing to look behind the scenes as you did with filter.
In the micorosoft site the following statement is written :
Exiting a try-finally statement using the longjmp run-time function is considered abnormal termination. It is illegal to jump into a __try statement, but legal to jump out of one. All __finally statements that are active between the point of departure (normal termination of the __try block) and the destination (the __except block that handles the exception) must be run. This is called a local unwind.
It clearly mention that first try block is executed then _except and then at last before exiting try block it executes finally block.
Refer. https://msdn.microsoft.com/en-us/library/9xtt5hxz.aspx
Related
Is it possible to throw an exception and catch it without using a try block ?
ex:
int main()
{
throw 1;
catch(int){
std::cerr << "caught exception\n";
}
return 0;
}
According to my experience, you have to use try and catch block in a combination. Here if any error occur in the try block then only it is passed to the catch block. Thus if there is no try block then it won't know where to look for the error. Thus they need to be used in combination. I am not sure in C++ but in Java you can use catch block in singly if you extend the class to throwable.
There is a slight difference in the behavior of program control flow during an exception in flight. try-catch blocks hopefully brings the program back to the usual control flow.
When an exception is thrown at any point in the program. Normal program control flow halts. The program control flow now goes through the runtime's mechanism for exception handling. This is different from what is normal. For the normal, destructors for automatic storage always runs when control flow reaches the end of scope (lifetime) "}". For exceptions in flight, they run earlier than normal
Consider this (note the comments);
MyClass foo(){
{
MyClass c;
c.call(balhhh); //assuming this throws
// (1)
....
return c;
}
void bar(){
std::map<K,V> mp;
auto c = foo();
// (2)
mp.insert(c);
....
}
When you call bar, we can see how control flow is per-maturely leaves foo() at // (1), no other code after // (1) is executed within the scope of foo(), the same thing happens when foo() has been unwound, and we are now at // (2)
This happens throughout stack-unwinding process, And this recursively unwinds all functions... until we are back to main() where std::terminate is called.
Now, with a try-catch block, the compiler generates code that suspends that early exit behavior after program has moved to a catch block (in search for a matching handler), and if the exception is handled, control flow goes back to normal.
Consider:
MyClass foo()
try {
MyClass c;
c.call(balhhh); //assuming this throws
// (1)
....
return c;
}
catch(...){
...
}
void bar(){
std::map<K,V> mp;
auto c = foo();
// (2)
mp.insert(c);
....
}
Now, when the exception leaves, control flow leaves the norm at // (1) to search for the handler in the associated catch, if the exception was handled, the program goes back to the normal control flow. Code after // (2) will execute now...
As you can see from the explanation, try-catch blocks are the only way as dictated by C++ standards, to restore normal program control flow after an exception is thrown.
int main()
{
throw 1; //fine, program control leaves the whole of main at this point.
catch(int){ //illegal, like using an else block without an if
std::cerr << "caught exception\n";
}
return 0;
}
Even if the above code was legal, throw means control will leave the whole of the enclosing scope at the throw site.
I am working with a 3rd party C++ DLL that is running __debugbreak() in some scenario, and is not checking IsDebuggerPresent() before doing so. This results in my application "crashing" when that scenario happens outside of a debugger (e.g. end user running the application). I would like to catch this and deal with it myself, or at least ignore it.
I actually have had an unhandled exception filter in place to translate SEH to C++ exceptions for a while, so it's a little strange that it's not working.
::SetUnhandledExceptionFilter(OnUnhandledException);
I've been doing some direct testing, and the standard __try/__except works, so I could wrap every call into the DLL with this as a fallback, but seems to be that if __try/__except works, then ::SetUnhandledExceptionFilter() should also work.
__try
{
__debugbreak();
}
__except (EXCEPTION_EXECUTE_HANDLER)
{
printf("caught");
}
try/catch(...) does not work.
try
{
__debugbreak();
}
catch (...)
{
printf("caught");
}
_set_se_translator() isn't working either.
From the MSDN documentation at https://msdn.microsoft.com/en-us/library/ms679297(VS.85).aspx it states that it should function as a structured exception. I realize that is the documentation for DebugBreak() but I have tested with that as well and have the same problem, even with "catch(...)".
I am compiling with /EHa.
How can I catch the __debugbreak (asm INT 3), or at least change the behavior?
Breakpoints generate the EXCEPTION_BREAKPOINT structured exception. You cannot use try/catch to catch it because it doesn't get translated to a C++ exception, irrespective of the /EHa switch or _set_se_translator. EXCEPTION_BREAKPOINT is a special exception.
First, you should know that catch blocks and __except blocks execute only after unwinding the stack. This means that execution continues after the handler block, NOT after the call to __debugbreak(). So if you just want to skip EXCEPTION_BREAKPOINT while at the same time continue execution after the int 3 instruction. You should use a vectored exception handler. Here is an example:
// VEH is supported only on Windows XP+ and Windows Server 2003+
#define _WIN32_WINNT 0x05020000
#include <windows.h>
#include <stdio.h>
//AddVectoredExceptionHandler constants:
//CALL_FIRST means call this exception handler first;
//CALL_LAST means call this exception handler last
#define CALL_FIRST 1
#define CALL_LAST 0
LONG WINAPI
VectoredHandlerBreakPoint(
struct _EXCEPTION_POINTERS *ExceptionInfo
)
{
if (ExceptionInfo->ExceptionRecord->ExceptionCode == EXCEPTION_BREAKPOINT)
{
/*
If a debugger is attached, this will never be executed.
*/
printf("BreakPoint at 0x%x skipped.\n", ExceptionInfo->ExceptionRecord->ExceptionAddress);
PCONTEXT Context = ExceptionInfo->ContextRecord;
// The breakpoint instruction is 0xCC (int 3), just one byte in size.
// Advance to the next instruction. Otherwise, this handler will just be called ad infinitum.
#ifdef _AMD64_
Context->Rip++;
#else
Context->Eip++;
#endif
// Continue execution from the instruction at Context->Rip/Eip.
return EXCEPTION_CONTINUE_EXECUTION;
}
// IT's not a break intruction. Continue searching for an exception handler.
return EXCEPTION_CONTINUE_SEARCH;
}
void main()
{
// Register the vectored exception handler once.
PVOID hVeh = AddVectoredExceptionHandler(CALL_FIRST, VectoredHandlerBreakPoint);
if (!hVeh)
{
// AddVectoredExceptionHandler failed.
// Practically, this never happens.
}
DebugBreak();
// Unregister the handler.
if (hVeh)
RemoveVectoredExceptionHandler(hVeh);
}
In this way, the breakpoint instruction int 3 will just be skipped and the next instruction will be executed. Also if a debugger is attached, it will handle EXCEPTION_BREAKPOINT for you.
However, if you really want to unwind the stack, you have to use __except(GetExceptionCode() == EXCEPTION_BREAKPOINT ? EXCEPTION_EXECUTE_HANDLER : EXCEPTION_CONTINUE_SEARCH).
Basically, how to catch exceptions on mac/linux? That is, exceptions, that are not intrinsic to the language, like segfaults & integer division. Compiling on MSVC, __try __except is perfect because the stack handling allows to catch exceptions and continue execution lower down the stack.
Now, i would like to extend my program to other platforms (mainly the ones mentioned), but i have no idea how exception handling works on these platforms work. As far as i understand, it's handled through posix signals? And as of such, wont allow to handle exception and continue lower down the stack?
Edit: Would this be valid (pseudo code)? As i see it, i leave C++ blocks correctly and thus dont indulge myself in UB.
jmp_buf buffer;
template< typename func >
protected_code(func f) {
if(!setjmp(buffer) {
f();
}
else
{
throw std::exception("exception happened in f()"):
}
}
void sig_handler() {
longjmp(buffer);
}
int main() {
sigaction(sig_handler);
try {
protected_code( [&]
{
1/0;
}
);
}
catch(const std::exception & e) {
...
}
}
Edit 2:
Wow for some reason i never thought of just throwing a C++ exception from the signal handler, no need to use longjmp/setjmp then. It of course relies on the fact that the thread calling the signal handler is the same stack and thread that faulted. Is this defined/guaranteed somewhere?
Code example:
void sig_handler(int arg) {
throw 4;
}
int main() {
signal(SIGFPE, sig_handler);
try {
int zero = 1;
zero--;
int ret = 1/zero;
} catch(int x) {
printf("catched %d\n", x);
}
return 0;
}
In Unix, you'd catch processor faults with signal handlers, using the sigaction function to install a suitable handler for the signal that you want to handle.
(I think you mean __try ... __except rather than __try ... __catch.
Is there a way to cause a throw in C++ to dump core at the throw site if the thrown exception would be handled by a certain catch block? I would like something similar to what happens with g++ when an exception reaches the top level.
For example, I would like something like this:
try {
bar();
try {
foo();
} catch(...) {
# pragma dump_at_throw_site
}
} catch(...) {
std::cerr << "There was a problem" << std::endl;
}
This way, if any exception thrown from foo() or its callee's that reaches the call-site of foo() would cause a core dump at the throw site so one can see who threw the exception that made it to the to this level.
On the other hand, exceptions thrown by bar() would be handled normally.
Yes,it can in Windows. I don't know Linux, suppose it can also.
We can register a Exception Handler function to response the throw before the catch
Here is the code example:
#include <iostream>
#include "windows.h"
#define CALL_FIRST 1
LONG WINAPI
VectoredHandler(
struct _EXCEPTION_POINTERS *ExceptionInfo
)
{
UNREFERENCED_PARAMETER(ExceptionInfo);
std::cout <<"VectoredHandler"<<std::endl;
return EXCEPTION_CONTINUE_SEARCH;
}
int main()
{
PVOID handler;
handler = AddVectoredExceptionHandler(CALL_FIRST,VectoredHandler);
try {
throw 1;
}catch(...)
{
std::cout <<"catch (...)"<< std::endl;
}
RemoveVectoredExceptionHandler(handler);
std::cout << "end of main"<<std::endl;
return 0;
}
The outputs of code are:
VectoredHandler
catch (...)
end of main
So,you can dump core int the function VectoredHandler.
The VectoredHandler is called after the debugger gets a first chance notification, but before the system begins unwinding the stack.
And if your purpose is just to debug the problem issue, then you can rely on the debugger feature to handle the first chance exception, don't need dump the application.
For your information, you may need know What is a First Chance Exception? in windows to understand how windows dispatch the exception.
I am new to programming and am having trouble with try / catch clauses.
Here is an example from a textbook that I have:
int main( )
{
char *ptr;
try {
ptr = new char[ 1000000000 ];
}
catch( … ) {
cout << "Too many elements" << endl;
}
return 0;
}
I have tried to look online for a further explanation and the textbook does not exactly tell me what what these clauses actually do or what it is used for.
Any information would be helpful.
EDIT: The textbook I am using is:
C++: Classes and Data Structures by Jeffrey Childs
A try-catch is the C++ construct for exception handling. Google 'C++ exceptions'.
Try catch is a way of handling exceptions:
try
{
// Do work in here
// If any exceptions are generated then the code in here is stopped.
// and a jump is made to the catch block.
// to see if the exception can be handled.
// An exception is generated when somebody uses throw.
// Either you or one of the functions you call.
// In your case new can throw std::bad_alloc
// Which is derived from std::runtime_error which is derived from std::exception
}
// CATCH BLOCK HERE.
The catch block is where you define what exceptions you want to handle.
// CATCH BLOCK
catch(MyException const& e)
{
// Correct a MyException
}
catch(std::exception const& e)
{
// Correct a std::exception
// For example this would cat any exception derived from std::exception
}
You can have as many catch blocks as you like. If you exception matches any of the catch expressions in the catch statement then the associated block of code is executed. If no catch expressions matches an exception then the stack is unwound until it finds a higher level catch block and the processes is repeated (this can cause the application to exit if no matching catch block is found).
Note: If multiple catch expressions match then the lexically first one is used. Only one or none of the catch blocks will be executed. If none then the compiler will look for a higher level try/catch.
There is also a catch anything clause
catch(...)
{
// This is a catch all.
// If the exception is not listed above this will catch any exception.
}
So how does this apply to your code.
int main( )
{
char *ptr;
try
{
// This calls ::new() which can potentially throw std::bad_alloc
// If this happens then it will look for a catch block.
ptr = new char[ 1000000000 ];
// If the ::new() works then nothing happens and you pointer `ptr`
// is valid and code continues to execute.
}
catch( … )
{
// You only have one catch block that catches everything.
// So if there are any statements that generate an exception this will catch
// the excetption and execute this code.
cout << "Too many elements" << endl;
}
// As you have caught all exceptions the code will continue from here.
// Either after the try block finishes successfully or
// After an exception has been handled by the catch block.
return 0;
}
Try-catch blocks are used to trap errors in the code.
At the most basic level, errors occur because the program tries to execute an invalid instruction. That instruction (read: line of code) could be invalid for a number of reasons. In your specific instance, the instruction could be invalid if your program was not able to allocate 1,000,000,000 bytes of memory to story your ptr. The most common exception is trying to access a bad pointer, which is called a Null Pointer Exception, which occurs when you try to perform some action on an Object that either has not been created, or has been deleted (or got corrupt). You will learn to hate that exception.
Using catch(...) tells the program to execute the code inside the catch block if any error occurs inside the code within the try block. There you can handle your error and try to find someway to either fix the error condition or gracefully exit that module.
You can also catch specific errors, which you can find out more about here : http://www.cplusplus.com/doc/tutorial/exceptions/
If you already know C, try/catch achieves the same thing as setjmp/longjmp when used for error handling. Think of try as code for the if condition of setjmp and catch code for else of setjmp. This makes longjmp equivalent to throw in C++, which is used to throw an exception. In your example, probably, the new operator, which calls some memory allocation function internally, throws an exception on seeing a very large number as input by using the C++ throw operator.
void a()
{
.......
longjmp(buf,1); // <--- similar to throw
.......
}
if ( !setjmp(buf) ) // <--- similar to try
{
.......
a();
.......
}
else // <--- similar to catch
{
.......
}
try/catch is a bit more sophisticated than setjmp/longjmp, as for setjmp/longjmp you will need to declare variables which are modified in between setjmp/longjmp calls as volatile, which is not necessary for try/catch.