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When writing C/C++ code, in order to debug the binary executable the debug option must be enabled on the compiler/linker. In the case of GCC, the option is -g. When the debug option is enabled, how does the affect the binary executable? What additional data is stored in the file that allows the debugger function as it does?
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The debugger lets you place breakpoints in your program. When program execution reaches one of these breakpoints, the debugger can either perform predefined actions and continue program execution or suspend program execution and return control to you.
The “Debug executable” checkbox specifies whether or not you want to run with the debugger enabled. Once running, you can use Debug > Attach to Process on a process that has been launched with debugging disabled if needed. It seems like all this does is start your app with the debugger attached.
Running an app within a debugger, also called debugging mode, means that the debugger actively monitors everything that's happening as the program runs. It also allows you to pause the app at any point to examine its state and then step through your code line by line to watch every detail as it happens.
To check if there's debug info inside the kernel object, you can add the following at the end of the objdump command: | grep debug . If this string is found, you know the kernel object contains debug information. If not, then it's a "clean" kernel object.
-g tells the compiler to store symbol table information in the executable. Among other things, this includes:
Debuggers use this information to output meaningful names for symbols and to associate instructions with particular lines in the source.
For some compilers, supplying -g will disable certain optimizations. For example, icc sets the default optimization level to -O0 with -g unless you explicitly indicate -O[123]. Also, even if you do supply -O[123], optimizations that prevent stack tracing will still be disabled (e.g. stripping frame pointers from stack frames. This has only a minor effect on performance).
With some compilers, -g will disable optimizations that can confuse where symbols came from (instruction reordering, loop unrolling, inlining etc). If you want to debug with optimization, you can use -g3 with gcc to get around some of this. Extra debug info will be included about macros, expansions, and functions that may have been inlined. This can allow debuggers and performance tools to map optimized code to the original source, but it's best effort. Some optimizations really mangle the code.
For more info, take a look at DWARF, the debugging format originally designed to go along with ELF (the binary format for Linux and other OS's).
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A symbol table is added to the executable which maps function/variable names to data locations, so that debuggers can report back meaningful information, rather than just pointers. This doesn't effect the speed of your program, and you can remove the symbol table with the 'strip' command.
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