I am debugging a code and there are 2 issues.
the debugger showed me the inner fields of each pointer, but suddenly it just wont, I dont know what changed or what did i click, but when i try to acsses the inner fields (like writing something into the pointed variable) it indeed shows me the correct variable, so it is saved there.
As you can see last clearly points to something, but it doesnt show the inner variable that the pointer is pointing to.
10 minutes ago it showed them though.
for some reason my program runs on debugging mode but encounter some sort of an unkown infinite loop when i run it regularly. Howcome?
im using the mingw debugger (i think its called GDB) on the IDE CLion.
I have no idea about the first part of the question, but for the second part:
for some reason my program runs on debugging mode but encounter some sort of an unkown infinite loop when i run it regularly. Howcome?
This is very common.
In 99.999% of instances this happens because your program exercises undefined behavior of some sort, such as using unitialized data, accessing array out of bounds, accessing memory after it has been deallocated, etc. etc.
In the remaining 0.001% of the cases it's due to a compiler bug.
On non-Widows OSes there are tools which help find such problems quickly, such as Address and Memory Sanitizers. Looks like Address Sanitizer is also available on Windows, but only under MSVC.
Update:
what can i usually do in order to find those memory bugs that the debugger wont pickup on?
The usual techniques are:
Leave no variable uninitialized.
Add assert()ions to verify that indices are in bounds, etc.
Have a very clear model of what dynamically allocated memory is owned by which object, so it's clear that no memory is accessed after it has been deleted, etc.
if my code is lets say 1500 lines long,
That is a very small program. Learning how to debug such programs will serve you well.
Related
There is a weird issue with some code I'm working with:
As far as I know and as far as I checked there are no components in the code, that rely on anything random, like system time, etc.
Yet it is enough to just add a breakpoint in the code and the behavior of the program changes. It doesn't do anything wrong, it just behaves differently (there are several objects to be selected in the code by some value, which is 0 for all of them - it simply picks a different object where the value is 0 when adding breakpoints).
Unfortunately it's not possible to post any code, because there is way too much of it.
What could cause this kind of behavior?
Edit:
I spent some more time on the problem: It doesn't seem to be related to breakpoints themselves - the "easiest" way to produce different results is the following: When starting the debug process from a different .cpp file of the project the output varies already. When disabling "Build automatically" this doesn't happen anymore. I conclude from that, that even if there are no changes in the code, something is rebuilt.
I also narrowed it down to the class that actually causes the different behavior: It's an implementation of a fibonacci heap.
If you are not running on Windows then consider using Valgrind - these are the supported platforms. The Memcheck tool will run your program (slower) looking at every memory reference to track down use of uninitialized variables.
Memcheck can detect if your program:
Accesses memory it shouldn't (areas not yet allocated, areas that have been freed, areas past the end of heap blocks, inaccessible areas of the stack).
Uses uninitialised values in dangerous ways.
Leaks memory.
Does bad frees of heap blocks (double frees, mismatched frees).
Passes overlapping source and destination memory blocks to memcpy() and related functions.
I'm currently battling with an intermittent bug. I create a float member of my class. I initialize it to zero. And then give it a value. This variable is used several times over the course of the next few processes, and inexplicably it will sometimes change its value to a really small number and cause an error in my program. I've pinpointed the general area in my code where it happens, and I swear, there is nothing in my code that is acting upon this variable. And on top of that I'll run and compile the same exact program with the same exact code several times and this bug only pops up sometimes.
I'm thinking that one of my other arrays or pointers is occasionally stepping out of bounds (because I haven't implemented bounds checking yet) and replacing the variables value with it's own, but I have no idea which one. I was wondering if there is a way in XCode, to find out what variables are stored near or next to this variable, so I can maybe pinpoint who might be stepping on this poor little son of a gun?
You can enable "guard malloc" in XCode. Guard malloc can tell you whether your code wrote out of bounds on any allocated area. I don't know the exact way to enable it (anymore), but you'll definitely find something on the nets.
If you want to watch some memory location while debugging your code with gdb you can use watch breakpoints.
Maybe you have a corrupted memory heap. Using a tool like valgrind could help.
I'm developing a game and when I do a specific action in the game, it crashes.
So I went debugging and I saw my application crashed at simple C++ statements like if, return, ... Each time when I re-run, it crashes randomly at one of 3 lines and it never succeeds.
line 1:
if (dynamic) { ... } // dynamic is a bool member of my class
line 2:
return m_Fixture; // a line of the Box2D physical engine. m_Fixture is a pointer.
line 3:
return m_Density; // The body of a simple getter for an integer.
I get no errors from the app nor the OS...
Are there hints, tips or tricks to debug more efficient and get known what is going on?
That's why I love Java...
Thanks
Random crashes like this are usually caused by stack corruption, since these are branching instructions and thus are sensitive to the condition of the stack. These are somewhat hard to track down, but you should run valgrind and examine the call stack on each crash to try and identify common functions that might be the root cause of the error.
Are there hints, tips or tricks to debug more efficient and get known what is going on?
Run game in debugger, on the point of crash, check values of all arguments. Either using visual studio watch window or using gdb. Using "call stack" check parent routines, try to think what could go wrong.
In suspicious(potentially related to crash) routines, consider dumping all arguments to stderr (if you're using libsdl or on *nixlike systems), or write a logfile, or send dupilcates of all error messages using (on Windows) OutputDebugString. This will make them visible in "output" window in visual studio or debugger. You can also write "traces" (log("function %s was called", __FUNCTION__))
If you can't debug immediately, produce core dumps on crash. On windows it can be done using MiniDumpWriteDump, on linux it is set somewhere in configuration variables. core dumps can be handled by debugger. I'm not sure if VS express can deal with them on Windows, but you still can debug them using WinDBG.
if crash happens within class, check *this argument. It could be invalid or zero.
If the bug is truly evil (elusive stack corruption in multithreaded app that leads to delayed crash), write custom memory manager, that will override new/delete, provide alternative to malloc(if your app for some reason uses it, which may be possible), AND that locks all unused memory memory using VirtualProtect (windows) or OS-specific alternative. In this case all potentially dangerous operation will crash app instantly, which will allow you to debug the problem (if you have Just-In-Time debugger) and instantly find dangerous routine. I prefer such "custom memory manager" to boundschecker and such - since in my experience it was more useful. As an alternative you could try to use valgrind, which is available on linux only. Note, that if your app very frequently allocates memory, you'll need a large amount of RAM in order to be able to lock every unused memory block (because in order to be locked, block should be PAGE_SIZE bytes big).
In areas where you need sanity check either use ASSERT, or (IMO better solution) write a routine that will crash the application (by throwing an std::exception with a meaningful message) if some condition isn't met.
If you've identified a problematic routine, walk through it using debugger's step into/step over. Watch the arguments.
If you've identified a problematic routine, but can't directly debug it for whatever reason, after every statement within that routine, dump all variables into stderr or logfile (fprintf or iostreams - your choice). Then analyze outputs and think how it could have happened. Make sure to flush logfile after every write, or you might miss the data right before the crash.
In general you should be happy that app crashes somewhere. Crash means a bug you can quickly find using debugger and exterminate. Bugs that don't crash the program are much more difficult (example of truly complex bug: given 100000 values of input, after few hundreds of manipulations with values, among thousands of outputs, app produces 1 absolutely incorrect result, which shouldn't have happened at all)
That's why I love Java...
Excuse me, if you can't deal with language, it is entirely your fault. If you can't handle the tool, either pick another one or improve your skill. It is possible to make game in java, by the way.
These are mostly due to stack corruption, but heap corruption can also affect programs in this way.
stack corruption occurs most of the time because of "off by one errors".
heap corruption occurs because of new/delete not being handled carefully, like double delete.
Basically what happens is that the overflow/corruption overwrites an important instruction, then much much later on, when you try to execute the instruction, it will crash.
I generally like to take a second to step back and think through the code, trying to catch any logic errors.
You might try commenting out different parts of the code and seeing if it affects how the program is compiled.
Besides those two things you could try using a debugger like Visual Studio or Eclipse etc...
Lastly you could try to post your code and the error you are getting on a website with a community that knows programming and could help you work through the error (read: stackoverflow)
Crashes / Seg faults usually happen when you access a memory location that it is not allowed to access, or you attempt to access a memory location in a way that is not allowed (for example, attempting to write to a read-only location).
There are many memory analyzer tools, for example I use Valgrind which is really great in telling what the issue is (not only the line number, but also what's causing the crash).
There are no simple C++ statements. An if is only as simple as the condition you evaluate. A return is only as simple as the expression you return.
You should use a debugger and/or post some of the crashing code. Can't be of much use with "my app crashed" as information.
I had problems like this before. I was trying to refresh the GUI from different threads.
If the if statements involve dereferencing pointers, you're almost certainly corrupting the stack (this explains why an innocent return 0 would crash...)
This can happen, for instance, by going out of bounds in an array (you should be using std::vector!), trying to strcpy a char[]-based string missing the ending '\0' (you should be using std::string!), passing a bad size to memcpy (you should be using copy-constructors!), etc.
Try to figure out a way to reproduce it reliably, then place a watch on the corrupted pointer. Run through the code line-by-line until you find the very line that corrupts the pointer.
Look at the disassembly. Almost any C/C++ debugger will be happy to show you the machine code and the registers where the program crashed. The registers include the Instruction Pointer (EIP or RIP on x86/x64) which is where the program was when it stopped. The other registers usually have memory addresses or data. If the memory address is 0 or a bad pointer, there is your problem.
Then you just have to work backward to find out how it got that way. Hardware breakpoints on memory changes are very helpful here.
On a Linux/BSD/Mac, using GDB's scripting features can help a lot here. You can script things so that after the breakpoint is hit 20 times it enables a hardware watch on the address of array element 17. Etc.
You can also write debugging into your program. Use the assert() function. Everywhere!
Use assert to check the arguments to every function. Use assert to check the state of every object before you exit the function. In a game, assert that the player is on the map, that the player has health between 0 and 100, assert everything that you can think of. For complicated objects write verify() or validate() functions into the object itself that checks everything about it and then call those from an assert().
Another way to write in debugging is to have the program use signal() in Linux or asm int 3 in Windows to break into the debugger from the program. Then you can write temporary code into the program to check if it is on iteration 1117321 of the main loop. That can be useful if the bug always happens at 1117322. The program will execute much faster this way than to use a debugger breakpoint.
some tips :
- run your application under a debugger, with the symbol files (PDB) together.
- How to set Visual Studio as the default post-mortem debugger?
- set default debugger for WinDbg Just-in-time Debugging
- check memory allocations Overriding new and delete, and Overriding malloc and free
One other trick: turn off code optimization and see if the crash points make more sense. Optimization is allowed to float little bits of your code to surprising places; mapping that back to source code lines can be less than perfect.
Check pointers. At a guess, you're dereferencing a null pointer.
I've found 'random' crashes when there are some reference to a deleted object. As the memory is not necessarily overwritten, in many cases you don't notice it and the program works correctly, and than crashes after the memory was updated and is not valid anymore.
JUST FOR DEBUGGING PURPOSES, try commenting out some suspicious 'deletes'. Then, if it doesn't crash anymore, there you are.
use the GNU Debugger
Refactoring.
Scan all the code, make it clearer if not clear at first read, try to understand what you wrote and immediately fix what seems incorrect.
You'll certainly discover the problem(s) this way and fix a lot of other problems too.
I've stumbled onto a very interesting issue where a function (has to deal with the Windows clipboard) in my app only works properly when a breakpoint is hit inside the function. This got me wondering, what exactly does the debugger do (VS2008, C++) when it hits a breakpoint?
Without directly answering your question (since I suspect the debugger's internal workings may not really be the problem), I'll offer two possible reasons this might occur that I've seen before:
First, your program does pause when it hits a breakpoint, and often that delay is enough time for something to happen (perhaps in another thread or another process) that has to happen before your function will work. One easy way to verify this is to add a pause for a few seconds beforehand and run the program normally. If that works, you'll have to look for a more reliable way of finding the problem.
Second, Visual Studio has historically (I'm not certain about 2008) over-allocated memory when running in debug mode. So, for example, if you have an array of int[10] allocated, it should, by rights, get 40 bytes of memory, but Visual Studio might give it 44 or more, presumably in case you have an out-of-bounds error. Of course, if you DO have an out-of-bounds error, this over-allocation might make it appear to be working anyway.
Typically, for software breakpoints, the debugger places an interrupt instruction at the location you set the breakpoint at. This transfers control of the program to the debugger's interrupt handler, and from there you're in a world where the debugger can decide what to do (present you with a command prompt, print the stack and continue, what have you.)
On a related note, "This works in the debugger but not when I run without a breakpoint" suggests to me that you have a race condition. So if your app is multithreaded, consider examining your locking discipline.
It might be a timing / thread synchronization issue. Do you do any multimedia or multithreading stuff in your program?
The reason your app only works properly when a breakpoint is hit might be that you have some watches with side effects still in your watch list from previous debugging sessions. When you hit the break point, the watch is executed and your program behaves differently.
http://en.wikipedia.org/wiki/Debugger
A debugger essentially allows you to step through your source code and examine how the code is working. If you set a breakpoint, and run in debug mode, your code will pause at that break point and allow you to step into the code. This has some distinct advantages. First, you can see what the status of your variables are in memory. Second, it allows you to make sure your code is doing what you expect it to do without having to do a whole ton of print statements. And, third, it let's you make sure the logic is working the way you expect it to work.
Edit: A debugger is one of the more valuable tools in my development toolbox, and I'd recommend that you learn and understand how to use the tool to improve your development process.
I'd recommend reading the Wikipedia article for more information.
The debugger just halts execution of your program when it hits a breakpoint. If your program is working okay when it hits the breakpoint, but doesn't work without the breakpoint, that would indicate to me that you have a race condition or another threading issue in your code. The breakpoint is stopping the execution of your code, perhaps allowing another process to complete normally?
It stops the program counter for your process (the one you are debugging), and shows the current value of your variables, and uses the value of your variables at the moment to calculate expressions.
You must take into account, that if you edit some variable value when you hit a breakpoint, you are altering your process state, so it may behave differently.
Debugging is possible because the compiler inserts debugging information (such as function names, variable names, etc) into your executable. Its possible not to include this information.
Debuggers sometimes change the way the program behaves in order to work properly.
I'm not sure about Visual Studio but in Eclipse for example. Java classes are not loaded the same when ran inside the IDE and when ran outside of it.
You may also be having a race condition and the debugger stops one of the threads so when you continue the program flow it's at the right conditions.
More info on the program might help.
On Windows there is another difference caused by the debugger. When your program is launched by the debugger, Windows will use a different memory manager (heap manager to be exact) for your program. Instead of the default heap manager your program will now get the debug heap manager, which differs in the following points:
it initializes allocated memory to a pattern (0xCDCDCDCD comes to mind but I could be wrong)
it fills freed memory with another pattern
it overallocates heap allocations (like a previous answer mentioned)
All in all it changes the memory use patterns of your program so if you have a memory thrashing bug somewhere its behavior might change.
Two useful tricks:
Use PageHeap to catch memory accesses beyond the end of allocated blocks
Build using the /RTCsu (older Visual C++ compilers: /GX) switch. This will initialize the memory for all your local variables to a nonzero bit pattern and will also throw a runtime error when an unitialized local variable is accessed.
I'm currently working on a C++ program in Windows XP that processes large sets of data. Our largest input file causes the program to terminate unexpectedly with no sort of error message. Interestingly, when the program is run from our IDE (Code::Blocks), the file is processed without any such issues.
As the data is being processed, it's placed into a tree structure. After we finish our computations, the data is moved into a C++ STL vector before being sent off to be rendered in OpenGL.
I was hoping to gain some insight into what might be causing this crash. I've already checked out another post which I can't post a link to since I'm a new user. The issue in the post was quite similar to mine and resulted from an out of bounds index to an array. However, I'm quite sure no such out-of-bounds error is occurring.
I'm wondering if, perhaps, the size of the data set is leading to issues when allocating space for the vector. The systems I've been testing the program on should, in theory, have adequate memory to handle the data (2GB of RAM with the data set taking up approx. 1GB). Of course, if memory serves, the STL vectors simply double their allocated space when their capacity is reached.
Thanks, Eric
The fact that the code works within the IDE (presumably running within a debugger?), but not standalone suggests to me that it might be an initialisation issue.
Compiler with the warning level set to max.
Then check all your warning. I would guess it is an uninitialized variable (that in debug mode is being initialized to NULL/0).
Personally I have set my templates so that warnings are always at max and that warnings are flagged as errors so that compilation will fail.
You'd probably find it helpful to configure the O/S to create a crash dump (maybe, I don't know, still by using some Windows system software called "Dr Watson"), to which you can then attach a debugger after the program has crashed (assuming that it is crashing).
You should also trap the various ways in which a program might exit semi-gracefully without a crash dump: atexit, set_unexpected, set_terminate and maybe others.
What does your memory model look like? Are you banging up against an index limit (i.e. sizeof int)?
As it turns out, our hardware is reaching its limit. The program was hitting the system's memory limit and failing miserably. We couldn't even see the error statements being produced until I hooked cerr into a file from the command line (thanks starko). Thanks for all the helpful suggestions!
Sounds like your program is throwing an exception that you are not catching. The boost test framework has some exception handlers that could be a quick way to localise the exception location.
Are there indices in the tree structure that could overflow? Are you using indexes into the vector that are beyond the current size of the vector?
new vector...
vector.push_back()
vector.push_back()
vector[0] = xyz
vector[1] = abc
vector[2] = slsk // Uh,oh, outside vector
How large is your largest input set? Do you end up allocating size*size elements? If so, is your largest input set larger than 65536 elements (65536*65536 == MAX_INT)?
I agree the most likely reason that the IDE works fine when standalone does not is because the debugger is wiping memory to 0 or using memory guards around allocated memory.
Failing anything else, is it possible to reduce the size of your data set until you find exactly the size that works, and a slightly large example that fails - that might be informative.
I'd recommend to try to determine approximately which line of your code does causes the crash.
Since this only happen outside your IDE you can use OutputDebugString to output the current position, and use DebugView.
Really the behavior of a program compiled for debug inside and outside of IDE can be completely different. They can use a different set of runtime libraries when a program is loaded from the IDE.
Recently I was bitten by a timing bug in my code, somehow when debugging from the IDE the timing was always good an the bug was not observed, but in release mode bam the bug was there. This kinda of bug are really a PITA to debug.