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Code, create, and learn together Try out the basics of Replit with our interactive playground. Code, collaborate, compile, run, share, and deploy C and more online from your browser. Dev-C can also be used in combination with Cygwin or any other GCC based compiler. In this session, we will use Mingw -included in the default Dev-C distribution- to create console C programs. Dev-C is a Free Software distributed under the terms of the GNU General Public License (GPL). The IDE can be downloaded here.

1. Dev C 2b 2b Online Gdb Pdf
2. Dev C 2b 2b Online Gdb Tutorial

Why use a debugger?

This might sound silly, but I've heard of many programmers that claimtheydo not need a debugger. They simply don't create bugs. Well, one thingis sure - either they've no idea what they are saying, or they justneverput their code to real test. Or maybe they're indeed as gifted as theyclaim. Unfortunately, most of us tend to have bugs in our code. Wecoulduse printing commands to test our code, or we could use a debugger.Manytimes our code might seem to work correctly, because we didn't test itunder enough scenarios. Other times we know there's a bug, but by justreadingthe code we don't notice it is there. Thus, we should develop a habitof launching a debugger when we get into trouble. It shouldn't comeinsteadof making an effort to write correct code, to add many tests in thecodefor invalid function arguments, NULL pointers, etc. But when we're introuble, it's probably our best shot.

The explanations given here are specific to the 'gdb' debugger, sincethereare no real standards regarding the activation and usage of debuggers,butonce you know what features to expect from a debugger, it's not toohard to adapt your knowledge to different debuggers.

Invoking the 'gdb' debugger

Before invoking the debugger. make sure you compiled your program(all itsmodules, as well as during linking) with the '-g' flag. Otherwise, lifewillbe tough. Lets compile the 'sqrtest2.cpp'program,and then invoke 'gdb' to debug it:

Note that we run the program from the same directory it was compiledin,otherwise gdb won't find the source file, and thus won't be able toshowus where in the code we are at a given point. It is possible to ask gdbtosearch for extra source files in some directory after launching it, butfor now, it's easier to just invoke it from the correct directory.

Setting breakpoints

The problem with just running the code is that it keeps on runninguntilthe program exits, which is usually too late. For this, breakpoints areintroduced. A break point is a command for the debugger to stop theexecution of the program before executing a specific source line.We cansetbreak points using two methods:

1. Specifying a specific line of code to stop in:
   will insert a break point at line 42, in the for loop of the mainfunction.
2. Specifying a function name, to break every time it is beingcalled:
   (gdb) break main
   this will set a break point right when starting the program (as thefunction 'main' gets executed automatically on the beginning of any Cor C++ program).
3. We can get the list of all breakpoints:
   (gdb)info break
   NumType DispEnb Address What
   1 breakpoint keep y 0x08048a33 inmain at sqrtest2.cpp:42
   2 breakpoint keep y 0x08048a0c inmain at sqrtest2.cpp:41
   

Stepping a command at a time

So lets see, we've invoked gdb, set breakpoints, then typed:

Then the debugger gave something like the following:

Now we want to start running the program slowly, step by step. Thereare twooptions for that:

1. 'next' - causes thedebugger to execute the current command, andstop again, showing the next command in the code to be executed.
2. 'step' - causes thedebugger to execute the current command, andif it is a function call - break at the beginning of that function.This is useful for debugging nested code.

Now is your time to experiment with these options with our debugprogram, andsee how they work. It is also useful to read the debuggers help, usingthecommand 'help break' and 'help breakpoints' to learn how toset severalbreakpoints, how to see what breakpoints are set, how to deletebreakpoints,and how to apply conditions to breakpoints (i.e. make them stop theprogramonly if a certain expression evaluates to 'true' when the breakpoint isreached).

Dev C 2b 2b Online Gdb Pdf

Printing variables and expressions

Without being able to examine variables contents during programexecution, the whole idea of using a debugger is quite lost. You canprintthe contents of a variable with a command like this:

And then you'll get a message like:
$1 = 0
which means that 'x' contains the number '0'.
Note that this requires 'x' to be in scope, or you'll get a messagesuch as:
No symbol 'x' in current context.
Regina spektor songs album download. For example, if you break inside the 'squareroot' function and try toprint the value of 'x', you'll get this message.

You may also try to print more complex expressions, like 'xnew - xold',or', and so on. In fact, you mayalso use type casts, call functions found in the program, and whateveryour sickmind could imagine (well, almost). Again, this is a good time to trythisout.

Examining the function call stack

Once we got into a break-point and examined some variables, we mightalsowish to see 'where we are'. That is, what function is being executednow,which function called it, and so on. This can be done using the'where' command.At the gdb command prompt, just type'where',and you'll see something like this:

This means the currently executing function is 'squareroot', at file'sqrtest2.cpp', line 31. The function that called it is 'main'. We alsosee whicharguments each function had received. If there were more functions inthe callchain, we'd see them listed in order. This list is also called 'a stacktrace', since it shows us the structure of the execution stack at thispointin the program's life.

Just as we can see contents of variables in the current function, wecansee contents of variables local to the calling function, or to anyotherfunction on the stack. For example, if we want to see the contents ofvariable 'x' in function 'main', we can type the following twocommands:

The 'frame' command tells thedebugger to switch to the given stackframe('0' is the frame of the currently executing function). At that stage,anyprint command invoked will use the context of that stack frame.Of-course, ifwe issue a 'step' or 'next' command, the program willcontinue at thetopframe, not at the frame we requested to see. After all, the debuggercannot'undo' all the calls and continue from there.

Dev C 2b 2b Online Gdb Tutorial

To exit gdb type:

Debugging a crashed program

One of the problems about debugging programs, has to do withMurphy's law:A program will crash when least expected. This phrase justmeansthat after you take the program out as production code, it will crash.Andthe bugs won't necessarily be easy to reproduce. mbox 1 driver download mac Luckily, there is someaid for us, in the image of 'core files'.

A core file contains the memory image of a process, and (assuming theprogramwithin the process contains debug info) its stack trace, contents ofvariables,and so on. A program is normally set to generate a core file containingits memory image when it crashes due to signals such as SEGV or BUS.Providedthat the shell invoking the program was not set to limit the size ofthis corefile, we will find this file in the working directory of the process(eitherthe directory from which it was started, or the directory it lastswitched tousing the chdir system call).

Once we get such a core file, we can look at it by issuing thefollowingcommand:

This assumes the program was launched using this path, and the corefile isin the current directory. If it is not, we can give the path to thecorefile. When we get the debugger's prompt (assuming the core file wassuccessfullyread), we can issue commands such as 'print','where' or 'frame X'. Wecan notissue commands that imply execution (such as 'next', or the invocationoffunction calls). In some situations, we will be able to see what causedthe crash.

One should note that if the program crashed due to invalidmemory address access, this will imply that the memory of the programwas corrupt, and thus that the core file is corrupt as well, and thuscontains bad memory contents, invalid stack frames, etc. Thus, weshouldsee the core file's contents as one possible past, out of many probablepasts (this makes core file analysis rather similar to quantum theory.almost).

Getting more info about debugging

It is now probably time to go play around with your programs andyour debugger.It is suggested to try 'help'inside gdb, to learn more about itscommands.Especially the 'dir' command,that enables debugging programs whosesource codeis split over several directories.