Set a breakpoint at line 10 and rerun the program: gdb break 10 Breakpoint 1 at 0x40055f: file test. Also, it would be even more useful if you could have that piece of information whenever your beautiful, bug-free program suddenly crashes, and you have no debugger at hand. In display : z is 10 and is stored at 0x7fffd1f1bc3c. I recommend setting a breakpoint around where you think the problem is and then stepping through the program until it breaks. Pass the number of the frame you want as an argument to the command.
Stack Frames and Backtraces Before diving into the article, let's briefly go over how function calls and parameters pass work in C. If a process fails, most operating systems write the error information to a log file to alert system operators or users that the problem occurred. Abort core dumped gdb out core. Having no stack is a good clue in itself. The program counter value is also shown--unless you use set print address off.
In this case, the external command addr2line can be used instead. Why a lot of people override the notify method and try. This entry point is encoded by the linker when the application is built, and is likely before the user entry point main or equivalent is called. . Typically, you will see several execute lines when you issue bt.
When more function calls are nested, the whole procedure is repeated, causing the stack to keep growing downwards and building a chain of stack frames see Figure 1. Breakpoint 1, main at test. There is one problem though — it may not be useful without a stack trace in certain cases. It consists of three functions declared in execinfo. By default, gdb prints the value of a parameter only if it is a scalar integer, pointer, enumeration, etc.
We will show how to examine the stack at each stage. The program counter value is omitted if it is at the beginning of the code for that line number. On early standalone or batch-processing systems, core dumps allowed a user to debug a program without monopolizing the very expensive computing facility for debugging; a printout could also be more convenient than debugging using switches and lights. This technique also can be used to understand where your program is looping in case it stops responding. This works with gdb 7.
When a crash occurs, error information is saved to a log file, and the memory state is saved to a crash dump. This gives us access to the variables in frame 1. You can print any internal data that you like, if you know the internal data structure. Exercises Continuing from the previous example, switch back to display 's frame. I have compiled the program with g++ and the -g parameter, but even simply test programs won't work: I'm not sure what's wrong. I might be a little bit unlucky in this regard — this happened to me a lot, so I decided to share a few ways for overcoming this type of situations.
Wander no more and print a backtrace. In which case, you may be a buffer overflow problem somewhere, severe enough to mash your runtime state entirely. Each frame listing gives the arguments to that function. Below is an example gdb session that can be used as a guideline on how to handle your segfault. Here is an example of a backtrace.
Actually it was not loaded by gdb so typing bt has no effect. Additional information tells exactly which instruction was executing at the time the core dump was initiated. Each frame listing gives the line number that's currently being executed within that frame. Remember these principles: break 5 means to pause at line 5. We see that main took no arguments, but display did and we're shown the value of the arguments. But one thing we can't do on our own is to pause the program in the middle of execution and take a look at the stack.
One Step Farther At this point, we have in hand a tool that is able to print the list of function calls up to the current execution point. In the example above, there is one frame on the stack, numbered 0, and it belongs to main. And it comes extremely handy when dealing with exceptions. Breakpoint 1, main at test. It is one of the most powerful tools out there.