Can I debug (step through) code without a main() function? - c++

Assume I want to understand a larger project by just stepping through the code with a debugger. Is it possible to just jump in at an arbitrary point (given I define the correct variables)? How do I debug libraries that don't have a main?

To debug code in a library, write a little application (that has a main()) that calls the functions in the library you need to debug. Then debug that application and step into the library calls it makes.
As for just "jumping into an arbitrary location" - well, you can instruct the debugger to move the instruction pointer to wherever you please, but that's usually not what you want, because you'd be missing a lot of state that previous parts of the program will have created. Usually what you want to do is set a breakpoint in the function you are interested in and then just run the program normally until you hit the breakpoint.

The best way would be to run the application as it is and set a breakpoint whenever you want to look. The thing is that the program might do lot of initialization and other stuff, that you wouldn't be able to figure out.
Another approach would be to look out for unit tests. They are like small programs itself targeting just specific parts of the program.

Related

Breaking as the code execution enters the Dll or Lib space

I'm debugging a Dll used by OS mechanisms using microsoft visual studio(In my case it is a minidriver).
I want the execution to break as it enters the dll code I want it because I don't know when base smart card CSP calls which function.
Is there any means to do it instead of having breakpoints in all exported functions?
If not, do you feel this feature is necessary?
So I'm not sure what you want to achieve - you need the program to break without setting breakpoints, or you need just an information into which function/method from the dll program will enter first? I think you can accomplish that with timeline profiling using for example dotTrace: https://www.jetbrains.com/profiler/whatsnew/

Precompile script into objects inside C++ application

I need to provide my users the ability to write mathematical computations into the program. I plan to have a simple text interface with a few buttons including those to validate the script grammar, save etc.
Here's where it gets interesting. These functions the user is writing need to execute at multi-megabyte line speeds in a communications application. So I need the speed of a compiled language, but the usage of a script. A fully interpreted language just won't cut it.
My idea is to precompile the saved user modules into objects at initialization of the C++ application. I could then use these objects to execute the code when called upon. Here are the workflows I have in mind:
1) Testing(initial writing) of script: Write code in editor, save, compile into object (testing grammar), run with test I/O, Edit Code
2) Use of Code (Normal operation of application): Load script from file, compile script into object, Run object code, Run object code, Run object code, etc.
I've looked into several off the shelf interpreters, but can't find what I'm looking for. I considered JAVA, as it is pretty fast, but I would need to load the JAVA virtual machine, which means passing objects between C and the virtual machine... The interface is the bottleneck here. I really need to create a native C++ object running C++ code if possible. I also need to be able to run the code on multiple processors effectively in a controlled manner.
I'm not looking for the whole explanation on how to pull this off, as I can do my own research. I've been stalled for a couple days here now, however, and I really need a place to start looking.
As a last resort, I will create my own scripting language to fulfill the need, but that seems a waste with all the great interpreters out there. I've also considered taking an existing open source complier and slicing it up for the functionality I need... just not saving the compiled results to disk... I don't know. I would prefer to use a mainline language if possible... but that's not required.
Any help would be appreciated. I know this is not your run of the mill idea I have here, but someone has to have done it before.
Thanks!
P.S.
One thought that just occurred to me while writing this was this: what about using a true C compiler to create object code, save it to disk as a dll library, then reload and run it inside "my" code? Can you do that with MS Visual Studio? I need to look at the licensing of the compiler... how to reload the library dynamically while the main application continues to run... hmmmmm I could then just group the "functions" created by the user into library groups. Ok that's enough of this particular brain dump...
A possible solution could be use gcc (MingW since you are on windows) and build a DLL out of your user defined code. The DLL should export just one function. You can use the win32 API to handle the DLL (LoadLibrary/GetProcAddress etc.) At the end of this job you have a C style function pointer. The problem now are arguments. If your computation has just one parameter you can fo a cast to double (*funct)(double), but if you have many parameters you need to match them.
I think I've found a way to do this using standard C.
1) Standard C needs to be used because when it is compiled into a dll, the resulting interface is cross compatible with multiple compilers. I plan to do my primary development with MS Visual Studio and compile objects in my application using gcc (windows version)
2) I will expose certain variables to the user (inputs and outputs) and standardize them across units. This allows multiple units to be developed with the same interface.
3) The user will only create the inside of the function using standard C syntax and grammar. I will then wrap that function with text to fully define the function and it's environment (remember those variables I intend to expose?) I can also group multiple functions under a single executable unit (dll) using name parameters.
4) When the user wishes to test their function, I dump the dll from memory, compile their code with my wrappers in gcc, and then reload the dll into memory and run it. I would let them define inputs and outputs for testing.
5) Once the test/create step was complete, I have a compiled library created which can be loaded at run time and handled via pointers. The inputs and outputs would be standardized, so I would always know what my I/O was.
6) The only problem with standardized I/O is that some of the inputs and outputs are likely to not be used. I need to see if I can put default values in or something.
So, to sum up:
Think of an app with a text box and a few buttons. You are told that your inputs are named A, B, and C and that your outputs are X, Y, and Z of specified types. You then write a function using standard C code, and with functions from the specified libraries (I'm thinking math etc.)
So now your done... you see a few boxes below to define your input. You fill them in and hit the TEST button. This would wrap your code in a function context, dump the existing dll from memory (if it exists) and compile your code along with any other functions in the same group (another parameter you could define, basically just a name to the user.) It then runs the function using a functional pointer, using the inputs defined in the UI. The outputs are sent to the user so they can determine if their function works. If there are any compilation errors, that would also be outputted to the user.
Now it's time to run for real. Of course I kept track of what functions are where, so I dynamically open the dll, and load all the functions into memory with functional pointers. I start shoving data into one side and the functions give me the answers I need. There would be some overhead to track I/O and to make sure the functions are called in the right order, but the execution would be at compiled machine code speeds... which is my primary requirement.
Now... I have explained what I think will work in two different ways. Can you think of anything that would keep this from working, or perhaps any advice/gotchas/lessons learned that would help me out? Anything from the type of interface to tips on dynamically loading dll's in this manner to using the gcc compiler this way... etc would be most helpful.
Thanks!

Debugging C++ code in Visual Studio: Breaking out at a non-pre-determined time

I had a question about debugging in Visual Studio (2010 if it actually matters). Sometimes I am running an application and I want to break out of it to debug or to see where it has hung, etc. However, I find that very very often the "break-out point" seems to be in some random .c file in the standard library. I understand why this is (its executing some method somewhere), but I want to know where the last point it was in the code I have written is. Is there a way to do this?
If I try and "step", the debugger seems to always return something like "there is no code to debug for the current location" or something, which I am guess means that it is making its way through some machine code. Again, that's fine, but I want to know what the last executed call in my main.cpp file was; is there a way to get this information? The call stack doesn't seem to help either, it always has a list of non-sensicle calls and even if I can locate the latest point in the call stack that is from my main.cpp, it doesn't seem to provide any useful information (like a line number or a function name... I
think it's showing the mangled name).
What do most people do in this situation? I apologize, I know I'm a beginner, and I'm sorry I don't have a concrete example, but I feel I see this often at work.
Any help appreciated, thanks!
K
Once you've paused the program observe the call stack (Debug->Windows->Call Stack) at that point, find where the last layer of your code is and get there by clicking on the corresponding line in the call stack window.
The "Step Out" command, Shift+F11, will finish execution of the current function and break immediately after the return.
You can use Step Out a few times to step out of the system calls back to your code.
The "Step Into Just My Code" option sounds like it may do what you want, but I've never used it.

print the code of a function in a DLL

I want to print the code of a function in a DLL.
I loaded the dll, I have the name of the desired function, what's next?
Thank you!
Realistically, next is getting the code. What you have in the DLL is object code -- binary code in the form ready for the processor to execute, not ready to be printed.
You can disassemble what's in the DLL. If you're comfortable working with assembly language, that may be useful, but it's definitely not the original source code (nor probably anything very close to it either). If you want to disassemble it, loading it in your program isn't (usually) a very good starting point. Try opening a VS command line and using dumpbin /disasm yourfile.dll. Be prepared for a lot of output unless the DLL in question is really tiny.
Your only option to retrieve hints about the actual implemented functionality of said function inside the DLL is to reverse engineer whatever the binary representation of assembly happens to be. What this means is that you pretty much have to use a disassembler(IDA Pro, or debugger, e.g. OllyDbg) to translate the opcodes to actual assembly mnemonics and then just work your way through it and try to understand the details of how it functions.
Note, that since it is compiled from C/C++ there is lots and lots of data lost in the process due to optimization and the nature of the process; the resulting assembly can(and probably will) seem cryptic and senseless, but it still does it's job the exact same way as the programmer programmed it in higher level language. It won't be easy. It will take time. You will need luck and nerves. But it IS doable. :)
Nothing. A DLL is compiled binary code; you can't get the source just by downloading it and knowing the name of the function.
If this was a .NET assembly, you might be able to get the source using reflection. However, you mentioned C++, so this is doubtful.
Check out this http://www.cprogramming.com/challenges/solutions/self_print.html and this Program that prints its own code? and this http://en.wikipedia.org/wiki/Quine_%28computing%29
I am not sure if it will do what you want, but i guess it may help you.

Why does my debugger sometimes freak out and do things like not line up with my code?

When I'm using my debugger (in my particular case, it was QT Creator together with GDB that inspired this) on my C++ code, sometimes even after calling make clean followed by make the debugger seems to freak out.
Sometimes it will seem to be lined up with another piece of code's line numbers, and will jump around. Sometimes this is is off by one line, sometimes this is totally off and it'll jump around erratically.
Other times, it'll freak out by stepping into things I didn't ask it to step into, like while stepping over a function call, it might step into the string initialization routine that is part of it.
When I get seg faults, sometimes it's able to tell me where it happened perfectly, and other times it's not even able to display question marks for which functions called the code and from where, and all I see is assembly, even while running the exact same code repeatedly.
I can't seem to figure out a pattern to what causes these failures, and sometimes my debugger is perfectly well behaved.
What are the theoretical reasons behind these debugger freak outs, and what are the concrete steps I can take to prevent them?
There's 3 very common reasons
You're debugging optimized code. This rarely works - optimized code can be reordered/inlined/precomputed/etc. to the point there's no chance whatsoever to map it back to the source code.
You're not debugging, for whatever reason, the binary matching the current source code.
You've invoked undefined behavior somewhere - if whatever stuff your code did, it has messed around with the scaffolding the debugger needs to keep its sanity. This is what usually happens when you get a segfault and you can't get a sane stack trace, you've overwritten/messed with the information(e.g. stack pointers) the debugger needs to do its job.
And probably hundreds more - of the stuff I personally encounter is: debugging multithreaded code; depending on gcc/gdb versions and various other things - there's been quite a handful debugger bugs.
One possible reason is that debuggers are as buggy as any other program!
But the most common reason for a debugger not showing the right source location is that the compiler optimized the code in some way, so there is no simple correspondence between the source code and the executable code. A common optimization that confuses debuggers is inlining, and C++ is very prone to it.
For example, your string initialization routine was probably inlined into the function call, so as far as the debugger was concerned, there was just one function that happened to start with some string initialization code.
If you're tracking down an algorithm bug (as opposed to a coding bug that produces undefined behavior, or a concurrency bug), turning the optimization level down will help you track the bug, because the debugger will have a simpler view of the code.
I have the same question like yours, and I cannot solve it yet. But I have came out one problem solution which is to install a virtual machine and install Unix system in it. And debug it in Linux system. Perhaps it will work.
I have found out the reason, you should rebuild the project every time you changed your code, or the Qt will just run the old version of the code.