gcc - How Does The Debugging Option -g Change the Binary Executable?

Question

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?

Answer 1

-g tells the compiler to store symbol table information in the executable. Among other things, this includes:

• symbol names
• type info for symbols
• files and line numbers where the symbols came from

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).