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

3 Answers

In addition to the debugging and symbol information
Google DWARF (A Developer joke on ELF)

By default most compiler optimizations are turned off when debugging is enabled.
So the code is the pure translation of the source into Machine Code rather than the result of many highly specialized transformations that are applied to release binaries.

But the most important difference (in my opinion)
Memory in Debug builds is usually initialized to some compiler specific values to facilitate debugging. In release builds memory is not initialized unless explicitly done so by the application code.

Check your compiler documentation for more information:
But an example for DevStudio is:

  • 0xCDCDCDCD Allocated in heap, but not initialized
  • 0xDDDDDDDD Released heap memory.
  • 0xFDFDFDFD "NoMansLand" fences automatically placed at boundary of heap memory. Should never be overwritten. If you do overwrite one, you're probably walking off the end of an array.
  • 0xCCCCCCCC Allocated on stack, but not initialized
what command -i

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?

-g adds debugging information in the executable, such as the names of variables, the names of functions, and line numbers. This allows a debugger, such as gdb to step through code line by line, set breakpoints, and inspect the values of variables. Because of this additional information using -g increases the size of the executable.

Also, gcc allows to use -g together with -O flags, which turn on optimization. Debugging an optimized executable can be very tricky, because variables may be optimized away, or instructions may be executed in a different order. Generally, it is a good idea to turn off optimization when using -g, even though it results in much slower code.

Just as a matter of interest, you can crack open a hexeditor and take a look at an executable produced with -g and one without. You can see the symbols and things that are added. It may change the assembly (-S) too, but I'm not sure.